Plume emissions accompanying 248 nm laser ablation of graphite invacuum: Effects of pulse durationFrederik Claeyssens, Micheal N. R. Ashfold, Emmanuel Sofoulakis, Carmen G. Ristoscu, Demetrios Anglos et al. Citation: J. Appl. Phys. 91, 6162 (2002); doi: 10.1063/1.1467955 View online: http://dx.doi.org/10.1063/1.1467955 View Table of Contents: http://jap.aip.org/resource/1/JAPIAU/v91/i9 Published by the American Institute of Physics. Related ArticlesPlasma-accelerated flyer-plates for equation of state studies Rev. Sci. Instrum. 83, 073504 (2012) Generation of magnetized collisionless shocks by a novel, laser-driven magnetic piston Phys. Plasmas 19, 070702 (2012) A very sensitive ion collection device for plasma-laser characterization Rev. Sci. Instrum. 83, 063305 (2012) Angular emission of ions and mass deposition from femtosecond and nanosecond laser-produced plasmas J. Appl. Phys. 111, 123304 (2012) Experimental reduction of laser imprinting and Rayleigh–Taylor growth in spherically compressed, medium-Z-doped plastic targets Phys. Plasmas 19, 062704 (2012) Additional information on J. Appl. Phys.Journal Homepage: http://jap.aip.org/ Journal Information: http://jap.aip.org/about/about_the_journal Top downloads: http://jap.aip.org/features/most_downloaded Information for Authors: http://jap.aip.org/authors

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JOURNAL OF APPLIED PHYSICS VOLUME 91, NUMBER 9 1 MAY 2002

Plume emissions accompanying 248 nm laser ablation of graphitein vacuum: Effects of pulse duration

Frederik Claeyssensa) and Micheal N. R. Ashfoldb)

School of Chemistry, University of Bristol, Bristol, BS8 1TS U.K.

Emmanuel Sofoulakis,c) Carmen G. Ristoscu,d) Demetrios Anglos, and Costas Fotakisc)

Foundation of Research and Technology—Hellas (FO.R.T.H.), Institute of Electronic Structure andLaser, P.O. Box 1527, GR711 10 Heraklion, Crete, Greece

~Received 18 January 2002; accepted for publication 13 February 2002!

We report a comparative study of the ultraviolet laser ablation of graphite, in vacuum, usingnanosecond~34 ns!, picosecond~5 ps!, and femtosecond~450 fs! pulses of 248 nm radiation,focusing on the plume characteristics as revealed by wavelength, time- and spatially resolved opticalemission spectroscopy. Nanosecond pulsed ablation gives a distinctively different optical emissionspectrum from that observed with the two shorter pulse durations. Emissions attributable toelectronically excited C* , C1* and C2* fragments are identified in the former, while the spectraobtained when using the shorter duration, higher intensity pulses contain additional lines attributableto C21* species but none of the C* emission lines. As before@Claeyssenset al., J. Appl. Phys.89,697 ~2001!#, we consider that each atomic emission is a step in the radiative cascade that followswhen an electron recombines with a Cn1 species~wheren is one charge state higher than that of theobserved emitter! formed in the original ablation process. Broadband visible radiation attributable toblackbody emission from larger particulates is also observed following ablation with any of thethree laser pulse durations. Time gated imaging studies allow estimation of the velocity distributionsof various of these emitting species within the plume, and their variation with incident laser fluenceand/or intensity. The deduced multicomponent structure of the plume emission following excitationwith short duration laser pulses is rationalized in terms of contributions from both nonthermal andthermal mechanisms for material ejection from the target. Use of longer duration~nanosecond! laserpulses offers the opportunity for additional laser-plume interactions, which we suggest areresponsible for much of the observed emission in the nanosecond pulsed laser ablation of graphite.© 2002 American Institute of Physics.@DOI: 10.1063/1.1467955#

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I. INTRODUCTION

Ablation of graphite using nanosecond pulses of UVser radiation@using, for example, an excimer laser operatiat 193 nm~ArF! or 248 nm~KrF!# is an established route tproducing thin films of diamond-like-carbon~DLC!1–4 and,in the presence of appropriate nitrogen containing baground gas, carbon nitride (CNx) films.5,6 The characteristicsof the deposited films can depend crucially on the properof the ablation plume, e.g., the degree of ionization, andvelocity distributions of the various species contained witthe plume. Hence, the many reported investigationsplumes generated by UV ablation of graphite using nanosond UV pulses together with, for example, maspectrometry,7 wavelength, time, and spatially resolved opcal emission spectroscopy~OES!2,8–14 and ion probemeasurements.15–18 In contrast, graphite ablation usinshorter duration~pico- or femtosecond! laser pulses has re

a!Present address: School of Physics and Astronomy, University of Birmham, Edgbaston, Birmingham, B15 2TT UK.

b!Author to whom correspondence should be addressed; electronicmike.ashfold@bris.ac.uk

c!Also at: Department of Physics, University of Crete.d!Present address: Lasers Department, National Institute for Laser, Pl

and Radiation Physics, Bucharest V, RO-76900, Romania.

6160021-8979/2002/91(9)/6162/11/$19.00

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ceived much less attention. Both DLC19 and CNx films20

have been deposited following femtosecond pulsed laserlation of graphite, and a recent study21 has explored properties of the irradiated spot on the target following Ti-sapphlaser irradiation, with pulse durations in the range 120 fs–ps. Studies of the plume composition under such ablaconditions remain rare, however. Ion probes have been uto investigate the charged components of the ablation plufollowing irradiation of graphite using 100 fs Ti-sapphirlaser pulses,19 and Rodeet al.22 have reported OES studieof the ablation plume accompanying graphite ablation w60 ps Nd:yttrium–aluminum–garnet~YAG! laser pulses (l51064 nm). The present work presents the first comparastudy of UV laser ablation of graphite, in vacuum, usinanosecond~34 ns!, picosecond~5 ps!, and femtosecond~450 fs! pulses of 248 nm radiation, focusing on the plumcharacteristics as revealed by wavelength, time- and spatresolved OES. These techniques allow quantitative detenations of the velocity distributions of the various emittinspecies within the plume, and estimates of the relative degof ionization of the ablated material. The observationsdiscussed within the framework of recent mechanistic thries of material ablation following excitation with these vedifferent pulse durations.

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II. EXPERIMENT

The ablation apparatus comprises a high vacuum stless steel chamber equipped with several side arms, eacwhich is sealed by a quartz window, permitting optical cotact with the target. Chamber evacuation involves use oturbomolecular pump, backed by a rotary pump, whichgether maintain the chamber at a base pressure o31027 Torr. The incident laser beam is directed through oof the side arms, at 45° to the target surface normal, anbrought to a focus at the target using a biconvex lens~offocal length 30 cm!. For future reference, we define the suface normal as thex axis. The viewing ports all lie in thexyplane and allow optical emission accompanying the ablaplume to be monitored along axes parallel and perpendicto the surface normal; images shown in the present wwere all recorded in the latter configuration. The target uin this work was a 2 in. diameter disk of highly orientepyrolytic graphite~Poco Graphite Inc., DFP-3-2 grade!. Thetarget was rotated during the experiments in order to mmize pit formation.

Nanosecond ablation experiments employed a LPXexcimer laser~Lambda-Physik! operating on KrF@34 ns fullwidth at half maximum~FWHM! pulse duration#. Subpico-second laser pulses at 248 nm were generated using aexcimer pumped dye laser system producing 450 fs pufrom a distributed feedback dye laser at 496 nm which, afrequency doubling, were amplified in a KrF excimer lascavity ~Lambda-Physik!.23 Inserting a multiple reflection etalon in the dye laser system enabled the production of 5pulses at 248 nm. The temporal profiles of both pulses hbeen established by autocorrelation techniques.24 The outputsof the nanosecond excimer laser and the amplified frequedoubled dye laser have markedly different spatial profilbeam divergences, etc., with the result that these two souyield very different spot sizes on the target. Characteristicthe various excitation pulses, summarized in Table I, hilight the similar average incident fluences but very differeaverage intensities provided by the three different excitasources.

Emission accompanying the ablation plume was focuinto a quartz fiber bundle the exit of which abutted the etrance slit~width set at 30mm! of a 0.32 m spectrograph~TRIAX-320, Jobin Yvon/Spex! equipped with three holographic gratings~user selectable, with 600, 1800, and 24lines/mm, providing spectral resolutions of, respectively, 00.15, and 0.1 nm!. The resulting wavelength dispersed emsion, in the wavelength range 360–900 nm, was recorusing an intensified charge coupled device~i-CCD! ~DH520-

TABLE I. Characteristics of the various 248 nm laser excitation schemused in the present work.

Pulseduration

Spot size on target~mm2!

Average fluence~J cm22!

Average intensity~W cm22!

34 ns 0.932 6.3 23108

10.3 33108

5 ps 0.3330.66 3.6 731011

450 fs 0.3330.66 2.1 531012

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18F, Andor Technology!. The evolving spatial distributionsof selected emitting species were monitored by usingi-CCD camera in time-gated detection mode, decouplingfrom the spectrometer and focussing the optical emiss~either total emission, or just that fraction that is transmittthrough an appropriate narrow bandpass interference fi!onto the 10243256 diode array with a;2:1 magnification,yielding a viewing window of;832 cm2 and a spatial resolution of ;84 mm pixel21.

III. RESULTS

A. Wavelength dispersed optical emissionspectroscopy

Figure 1 shows wavelength resolved emission spectrthe ablation plume induced by~a! ns ~10.3 J cm22!, ~b! ps,and~c! fs 248 nm laser excitation. Spectrum~a! was obtainedby viewing at a distanced55 mm from the interaction vol-ume on the target along the surface normal while thoseplayed in ~b! and ~c! were recorded by monitoring muccloser to the target, atd50.5 mm. Spectra taken at largerdin the case of the ps and fs pulse showed relative line insities seemingly the same as those shown in~b! and ~c!, butwith progressively poorer signal to noise—reflecting tcomparative weakness of the OES in the short pulse ablaexperiments. All are dominated by the strong C1(4 f 1;2F°→3d1;2D) emission atl;426.7 nm. Features evident ithe ps and fs spectra are all assignable to C1 emissions,except for three lines appearing at 406.9, 418.7, and 46nm as shown in Table II. Inserting a long bandpass filterfront of the entrance slit readily confirms that the weak fetures apparent atl;750 nm in ~b! and ~c! are also secondorder reflections~of the C1 transitions at 392.2 and 426.

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FIG. 1. Wavelength dispersed spectra of the plume emission accompan248 nm PLA of graphite in vacuum using~a! ns, ~b! ps, and~c! fs laserpulses as listed in Table I, incident at 45° to the surface normal. Spec~a! was recorded by monitoring at a distanced55 mm from the focal spot,while ~b! and~c! were recorded atd50.5 mm. The boxed region around th426.8 nm C1* feature in each spectrum is displayed on a four-fold copressed vertical scale. Discernable C* , C1* , and C21* features are indi-cated by the combs festooned above spectra~a!, ~b!, and ~c!, respectively,while features that appear in second order are indicated by the openangles.

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6164 J. Appl. Phys., Vol. 91, No. 9, 1 May 2002 Claeyssens et al.

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TABLE II. Persistent lines~from Ref. 39! in the wavelength dispersed optical emission arising in the 248ablation of graphite in vacuum using ns, ps, and fs laser pulses, at fluences as listed in Table I. Linesto be ‘‘present’’ in any given spectrum are those with observed intensities.1% that of the most intense line inthat spectrum.

Wavelength~nm!

Emitter and emitting transition Pulse duration

C* C1* C21* 34 ns 5 ps 450 fs

383.7 3p;2D→4p;2P° 3

387.3 4f ;2,4G→3d;4F° 3 3 3

392.2 4s;2S→3p;2P° 3 3 3

396.9 6p;1D→3s;1P° 3

397.0 4f ;2,4D→3d;4D° 3

406.9 5g;3G→4f ;3F° 3 3

407.7 4f ;2,4F→3d;4D° 3 3 3

418.9 5g;1G→4f ;1F° 3 3

426.8 4f ;2F°→3d;2D 3 3 3

437.4 4f ;4D→3d;4P° 3

441.2 4f ;2F→3d;2D° 3

462.0 4f ;2G→3d;2F° 3

464.7 3p;3P°→3s;3S 3 3

477.0 4p;3P→3s;3P° 3

493.2 4p;1S→3s;1P° 3

505.2 4p;1D→3s;1P° 3

514.6 3p;4P→3s;4P° 3 3

538.1 4p;1P→3s;1P° 3

589.4 4p;2P°→3d;2D 3 3 3

658.2 3p;2P°→3s;2S 3 3 3

658.7 4d;1P°→3p;1P 3

723.5 3d;2D→3p;2P° 3 3

833.4 3p;1S→3s;1P° 3

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nm!. The features attributable to electronically excited C21

ions are not evident in the wavelength dispersed spectrumthe plume emission accompanying ns laser ablation. As TII shows, all of the additional peaks evident in~a! are attrib-utable to electronically excited neutral C atoms~e.g., theC(2p13p1;1S→2p13s1;1P1°) transition at 833.4 nm!. AsTable II also shows, all of the more intense emission linessingly ionized C1 ions ~and in the case of ns laser ablatioof neutral C atoms! expected in the monitored wavelengrange are observed, indicating a lack of specificity inexcited state production process. The radiative lifetimesthe various emitting states are all much shorter than the tscales over which their emission is observed, implying tthere must be a mechanism by which the observed emitlevels are populated post-ablation. Such behaviors parobservations made previously in the case of 193 nm pulaser ablation~PLA! of graphite in vacuum and, as in thastudy,11 we conclude that the emitting levels are populateda result of collisionally assisted recombination between etrons and atomic species carrying a positive chargehigher than that of the observed species, and subsequenlisional and/or radiative cascade through a manifold of coparatively long-lived Rydberg states.

Weak emission from electronically excited C2 radicals intheir d3Pu state~most noticeably, the origin transition of thd–a Swan band system at;516.5 nm! was also discernablewithin ;1 mm of the target when ablating with all threpulse durations. As Fig. 2~a! illustrates, the wavelength dispersed spectra recorded very close to the target when ups and fs pulse durations also reveal weak, broad emissio

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the range 360–450 nm, which could be due to electronicexcited C3* radicals and/or to emission from hot electro~Brehmstrahlung!. Once corrected for the wavelength depedence of the grating reflectivity and detector sensitivity, thespectra also show a clear rising background to longer walengths. This we associate with blackbody emission frlarger ejecta. This latter emission can be studied more cleby gating the camera on only after an initial 0.1ms timedelay during which time all atomic emission has escapedviewing zone. As Fig. 2~b! shows, the ‘‘long time’’ emissionfrom the region close to the target shows hints of persisC2* Swan band emission atop a rising base line whichreproduced by the Planck distribution function withT;29006200 K. Such a value is plausible, given that tmelting/sublimation temperature of graphite is estimatedbe ;4300 K,25 and the opportunities for radiative coolinduring the observation period. Given the persistent C2*emission, the lack of 360–450 nm emission in these ‘‘latime’’ spectra might be considered to favor assignmentthis latter feature in the emission spectra accompanyingablation of graphite in terms of Brehmstrahlung rather thC3* fragments.

The present results are in good accord with the findinof previous OES studies of 248 nm ns laser ablationgraphite by Germainet al.8,15,16 and by Yamagataet al.12,13

The former report OES recorded atd510 mm from the tar-get. As here, they found the wavelength dispersed spectto be dominated by C* emissions at low fluences,F, but thatthese were supplemented by C1* and C21* emissions asFexceeded, respectively, 8 and 17 J cm22.8,15,16They also re-

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6165J. Appl. Phys., Vol. 91, No. 9, 1 May 2002 Claeyssens et al.

port weak C2(d–a) emissions at lowF and smalld. Yama-gataet al.12,13 report weak C1* emission at somewhat lowefluences ~3 J cm22!, and both studies observe that thC2(d–a) emission becomes dominant if the ablation is cried out in a low pressure of N2 background gas~reinforcingthe view that these species arise as a result of collisioprocesses!. Clearly, at least in the case of ns excitationprecise details like the C* /C1* /C21* /C2* emission ratiosare likely to depend on a number of factors, not justincident fluence, but also the pulse duration, the size ofviewing column and its distance from the interaction volumand the quality of the background vacuum. We also notethis stage the complementary study of Pappaset al.17 whoinvestigated the ns ablation of graphite in vacuum at 248using mass spectrometric rather than OES detection. Tworkers found the C2 fraction in the ablation plume to increase with increasingF up to ;5 J cm22, and thereafter todecline—presumably as a result of increased laser-plumeteractions.

The overall intensity of the optical emission measurfollowing ps and fs 248 nm laser excitation of graphitevacuum is appreciably weaker than that recorded usinglaser excitation~consistent with the much greater numberphotons per pulse in the latter case!. The relative intensitiesof the various C1* peaks evident in Figs. 1~b! and 1~c! arestrikingly similar, and seemingly insensitive to viewing ditance. These spectra also show many similarities with threported by Rodeet al.,22 who also investigated graphite ablation in vacuum using ps laser pulses. They employehigh repetition rate~76 MHz!, short pulse~60 ps!, Nd–YAG

FIG. 2. Expanded view of the wavelength dispersed spectrum~corrected forthe wavelength dependent reflectance of the grating and the quantumciency of the i-CCD detector! of the plume emission arising in the fs 248 nPLA of graphite in vacuum monitored~a! at t;0 and~b! with a long ~0.1ms! time delay prior to gating ‘‘on’’ the detector.

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laser operating at 1064 nm, and an incident fluence ontarget estimated at 0.17 J cm22 ~corresponding intensity;2.83109 W cm22!. Their emission spectrum shows C1*and C21* emissions, weak radiation from the strong C*transition at 248 nm, but no emission attributable to C2* .This may reflect the position of their viewing column relativto the target, which is not specified. As here, the temperaof the target and of particulates in the plume was estimaby fitting the underlying continuum emission in termsblackbody emission, yielding a temperature in the ran2500–3500 K. Small differences between these andpresent observations might well be explicable simplyterms of differences in the detailed experimental setup. Tbeing so, it is tempting to suggest that the OES followilaser ablation of graphite in vacuum with short~,100 ps!laser pulse durations is largely independent of the prepulse duration or the laser wavelength.

B. Time resolved imaging of specific speciesemissions

The left hand column in Fig. 3 shows i-CCD cameimages of the ablation plume resulting from 248 nm ns laablation of graphite in vacuum, at a fluence of 10.3 J cm22,recorded using a nominal 20 ns time gate delayed bt5380 ns.~Note that the actual ‘‘active’’ time of the i-CCDwhen triggered with such 20 ns pulses is actually measuto be ;5.6 ns.! t50 for the delays quoted in the presework are referenced to the gate setting at which emissresulting from the laser pulse impacting onto the targefirst visible. From top to bottom, the four images show~a!the total OES, and the respective fractions passed by:~b! anarrow band interference filter centered at 427 nm~whichtransmits the intense C1* emission feature at 426.8 nm!; ~c!a long wavelength bandpass filter that only transmits emsions withl.780 nm~all of which are associated with neutral C* transitions!; and ~d! an interference filter with transmission peak at 520 nm and FWHM 20 nm, that passes bC2(d–a)Dv50 emission and the weak C* and C1* emis-sions that fall within this transmission window. 0 mm definthe front face of the target. In each case, the laser is incidin the xy plane at 45° to the surface normal~x! and the OESis viewed along they axis, i.e., the images in Fig. 3 arsquashed two-dimensional~2D! projections~in the xz plane!of the time-gated three-dimensional~3D! plume emission.The white arrow~shown in the top left hand panel only, buapplicable to all of the displayed images! indicates the pro-jection of the laser propagation axis onto the viewing plaThe center and right hand columns in Fig. 3 show the cosponding images recorded using the 5 ps and 450 fs lpulses, respectively, again viewed with a 20 ns time gatethe intensifier—the midpoint of which, in these cases, wdelayed tot5120 ns. Note the lack of detectable emissifrom neutral C* fragments in these latter columns.

As previously,11 i-CCD images such as these can be alyzed to gain some measure of the velocity distributionsthe various emitting species. All of these images appear smetric about the surface normal. Figure 4 shows cuts althe x axis, atz50, for ~a! the C1* , ~b! the C* , and~c! the

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6166 J. Appl. Phys., Vol. 91, No. 9, 1 May 2002 Claeyssens et al.

contaminated C2* images obtained in the ns laser ablatiexperiments (F510.3 J cm22), plotted in the form of emis-sion intensity,I, versus distance from the target,d. The C1*and C* data both show single peaked behavior which,shown in the respective figures~solid lines!, can be fittedwell using split Gaussian functions in which the half widhalf maximum~HWHM! of the leading and trailing edgeare both adjustable parameters. Such processing enabletimation of the distance,d, at which the integrated intensitattains half the total measured signal. Analyses of many simages, recorded at different time delays, and subseqplotting of d vs t gives a straight line, the gradient of whicprovides a measure of the mean velocities,vx , of the variousemitting species as shown in Fig. 5. The dashed lines in thplots show thet dependence ofd0.25 andd0.75 ~the d valuescorresponding to the HWHM values of the slow and fahalves of the split Gaussian fitting function!; their difference~and the way this difference evolves witht! provide mea-sures of the velocity dispersion alongx, dvx .

Figure 6 shows analogousI vs d slices through C1*images recorded during~a! ps and~b! fs laser ablation of

FIG. 3. i-CCD images~a! the total OES and@~b!–~d!# wavelength selectedcomponents of the OES arising in the 248 nm laser ablation of graphitvacuum using ns~F510.3 J cm22—left hand column!, ps ~center column!,and fs~right hand column! laser pulses. Each was recorded using a nom20 ns time gate on the intensifier of the camera, delayed by, respectivt5380, 120, and 120 ns. The image labeled~b! in each case is of emissiontransmitted by a narrow band interference filter centred at 427 nm~whichtransmits the intense C1* emission feature at 426.8 nm!, image~c! in thecase of ns laser ablation is due to emission passed by a long wavelband pass filter that only transmits atl.780 nm~all of which is associatedwith neutral C* transitions!, while images~d! are from emissions occurringat l5520620 nm ~which can contain contributions from C2* , C* , andC1* species!. 0 mm defines the front face of the graphite target, andshort horizontal line to the left of each image indicates the position oflaser focus.

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graphite, in vacuum, at 248 nm~the original images appeain Fig. 3!, and the corresponding fits to the data using sGaussian functions. Analysis of many such images, obtaiat various delay times in the range 40,t,320 ns, yieldsdvalues~andd0.25 andd0.75 values! as a function oft. These,in turn, are plotted in Figs. 6~c! and~d!; the mean velocities,vx , and the velocity dispersions,dvx , derived from theseanalyses and from the ns laser ablation studies~at two dif-

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FIG. 4. I vs d plots obtained from cuts along the surface normal~x!, at z50, through the images shown in~a! Fig. 3~b!, ~b! Fig. 3~c!, and ~c! Fig.3~d!—left hand column in each case. The solid curves in~a! and~b! are bestfits to the distributions obtained using split Gaussian functions in whichHWHM of both the leading and trailing edges are adjustable parameThe solid curves in~c! illustrate the deconvolution of this slice into components associated with C1* and C* species and, by difference, C2*fragments.

FIG. 5. Plots ofd vs t derived from analysis ofI vs d plots ~such as shownin Fig. 4! derived from C* and C1* images recorded for 248 nm ns laseablation of graphite in vacuum (F510.3 J cm22) at different delay timest.The gradients of the lines of best fit give a measure of the propagavelocities, vx , along the surface normal, for these two types of emittispecies. The dashed lines show the correspondingt dependences ofd0.25 andd0.75, thed values corresponding to the HWHM values of the slow and fhalves of the split Gaussian fitting function used to fit the measuredI vs dplots.

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ferent incident fluences! are collected together in Table IIIWe note that the mean propagation velocities so derivedboth the C* and C1* species arising in the ns ablationgraphite, in vacuum, are in good accord with the values~;11and 35–39 km s21, respectively! derived for these species ianalogous studies from Geohegan’s group,2,9 and with values~2161 and 4261 km s21! obtained in our study of graphit

FIG. 6. I vs d plots obtained from cuts along the surface normal~x!, at z50, through the C1* images arising in the~a! ps and~b! fs 248 nm laserablation of graphite, in vacuum@Fig. 3~b!—center and right hand imagesrespectively#. The solid curves are best fits to the distributions obtain

using split Gaussian functions.~c! and~d! are plots ofd vs t that result fromanalysis of many such C1* images; as in Fig. 5, the solid and dashed linprovide measures ofvx andd vx .

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ablation at 193 nm.11 The dvx values ~and their errors!quoted in Table III are simply the sum of the HWHM valuederived by fitting the slow and fast halves of theI vs d slicesin terms of the split Gaussian function. As in the previostudies, the emitting ions are found to propagate with a mhigher mean velocity than the emitting neutrals. Increasthe incident fluence in the ns ablation studies is foundhave little effect on the expansion properties of the emittspecies in the plume, though the C1* /C* emission ratio in-creases markedly. This is most probably a reflection ofincreased intensity which, for any given pulse duratioscales with the fluence. Large increases in the incident insity have a dramatic effect on the C1* /C* emission ratio: noC* emission is observed when the ablation is performed wps or fs laser pulses. The mean propagation velocity ofC1* species is also seen to increase, by as much as 5upon switching from ns to fs laser pulses, even thoughTable I showed, the incident fluence has decreased fivef

It is also possible to analyze the image intensity disbutions along an axis perpendicular tox ~e.g., alongz! at

distances close tod. Figures 7~a! and 7~b! provide illustra-tive examples of such slices, for the case of the C1* and C*emissions arising in ns laser ablation, respectively. Theand all other such slices, have a centrosymmetric appearand can readily be fitted in terms of normal Gaussian disbutions. Figures 7~c! and 7~d! show how the FWHM of theseC1* and C* distributions expand witht. The gradients ofsuch plots provide a species specific measure of the velodispersion,dv' , within the plume along axes orthogonalthe surface normal. Values so derived, for each excitatimescale, are also listed in Table III. Inspection of the valuso derived showsdv' to increase proportionately withvx asthe incident intensity increases, suggesting that the emitC1* ions are ejected into a rather constant solid angle thalargely independent of the pulse duration.

Analysis of i-CCD images obtained by monitoring emision transmitted by the bandpass filter centered at 520@row ~d! in Fig. 3# requires extra care, since these include njust the C2* emission of interest but also C* and C1* emis-sions that fall within the transmission window. We startassuming that all emitting species of a given type~e.g., C* !have the same velocity distribution and that it is therefovalid to use theI vs d plots for C1* and C* species recordedat any givent @such as those shown in Figs. 4~a! and 4~b!# asbasis functions for deconvoluting the contribution eachthese species makes to theI vs d plots obtained by monitor-ing 520 nm emission at that samet. We obtain the mostplausible C2* I vs d profiles by attributing all of the moredistant ~i.e., faster! emission to C1* and C* species, andsubtracting the maximum possible amount of each of thbasis functions. As the deconvolution in Fig. 4~c! shows, theresidual signal, which we associate with C2* species, peaksat or very close to the target. The same conclusion reswhen the maximum possible contribution attributable to C1*emission is deconvolved from theI vs d slices through im-ages of the total emission accompanying ps and fs lasercitation. Given the extent, and arbitrariness, of the processinvolved in reaching this result, however, we elect notdefine a velocity for these slow moving distributions of C2*

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6168 J. Appl. Phys., Vol. 91, No. 9, 1 May 2002 Claeyssens et al.

Do

TABLE III. Mean propagation velocities,nx , along the surface normal, and velocity dispersions~both along,d vx , and perpendicular,dv' , to the surface normal! of electronically excited C* and C1* species formed inthe ns~at two different fluences,F!, ps and fs laser ablation of graphite at 248 nm, in vacuum. Quoted erepresent the standard deviation from five separate measurements.

Pulse duration

C* emission C1* emission

vx d vx dv' vx d vx dv'

34 ns (F56.3 J cm22) 16.060.4 25.060.5 31.562.4 41.260.5 12.960.8 40.363.034 ns (F510.3 J cm22) 16.860.9 17.262.8 31.362.0 43.360.3 15.361.1 46.466.8

5 ps 51.261.0 16.062.1 57.166.2450 fs 63.261.1 14.264.1 63.668.0

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species. These findings are also in good accord with prevOES studies of graphite ablation at 248 nm. For examGeoheganet al.2,9 observed not just fast C1* and slower C*emissions in their i-CCD imaging studies of the ns laablation of graphite at 248 nm, but also a slow moving coponent (n;3 km s21) which they attributed to C2* species.This latter component was seen to be supplemented, ahighest fluences studied~;17 J cm22!, by jets of C2* mol-ecules arising from hydrodynamic instabilities.

Most of these observations can be accommodated wiexisting models for the interaction between laser radiatand a solid target, and with the resulting ejecta. Mass sptrometric studies of particles leaving a graphite target folloing ns laser excitation at 248 nm, at fluences only just abthe ablation threshold~0.3–0.7 J cm22!, reveal the presencof neutral Cn clusters withn<5 and, predominantly, C, C2 ,and C3 species.7 The observed cluster size distributioncompatible with a thermally driven ejection mechanism, bthe translational energy distributions are not~being far toohigh!. At higher incident fluences~and intensities!, many ofthese molecular species can be expected to fragmentresult of photodissociation processes—consistent withobservation of Pappaset al.17 that the relative yield of C2fragments arising in the ns 248 nm laser ablation of grapincreases with fluence up toF;5 J cm22 but thereafter de-clines sharply. Pressure and temperature gradients, whicgreatest in the focal volume immediately adjacent to theget surface, encourage acceleration of the ablated mataway from the heated zone until settling to the obserdistributions of terminal velocities.26 Multiphoton processesbecome increasingly important at higher intensities. As Ppaset al.17 have pointed out, the KrF laser output overlawith the 3s12p1(1P1°)←2p2(1S0) transition of atomic car-bon ~247.93 nm! and, as a consequence, any metastaC(1S0) atoms in the ablation plume will have a high propesity for two photon resonant ionization. As in the case of Alaser excitation@which shows accidental overlap with th3s12p1(1P1°)←2p2(1D2) transition of atomic C#, we sug-gest that this process acts as an efficient, localized seefurther absorption~due to electron-neutral and, particularlelectron-ion inverse Brehmstrahlung! and ionization.11 Re-calling our assertion that the C* and C1* emissions are actually signatures of entities that were~prior to their recom-bining with an electron! C1 and C21 ions, such anexplanation is seen to be wholly consistent with the progrsively greater degrees of ionization deduced in OES spe

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recorded using ns, ps, and fs laser pulses. Such a modelprovides a qualitative rationale for the deduced plume strture, namely a fast C1* ion emission signal, followed by aslower distribution of C* emission. Material at the front edgof the expanding plume receives the greatest incident laintensity, and thus has the greatest propensity for ionizatCoulombic interactions, and the ambipolar expansion of ioand electrons in the plasma, serve to supplement the effof the near-target pressure and temperature gradients,lead to the higher terminal velocities measured for the mhighly charged ejected material.

Lifetime arguments preclude radiative cascade as a psible mechanism for the observed persistence of the2*emissions. Binary collisions involving hot C atoms and sloC atoms have been proposed as the source of the C2* emis-sion observed during graphite sputtering induced by 532radiation.27 Geoheganet al.2 have shown that the region olocalized C2* emission also contains emission from highcarbon clusters and even blackbody radiation from smgraphite particles, again consistent with the view thatobserved C2* species are formed via collisional processesthe relatively high pressure region close to the target surfaOther possible contributors include unimolecular, or cosionally induced, fragmentation of larger Cn clusters.

Larger particulates are visible to the bare eye as briincandescent tracks within the ablation plume. As Fig.shows, for the case of fs laser ablation of graphite,vacuum, at 248 nm, the trajectories of individual particulacan be studied more quantitatively by i-CCD imaging withwide ~50 ms! intensifier time gate, suitably delayed with respect to the laser-target interaction that all C* , C1* , etc.,species have either decayed within, or escaped from, theservation volume. Clearly, these particulates are associwith the laser ablation process; all have a well-directed fward velocity and all originate from the laser interaction voume on the target surface. It is tempting to surmise that thparticulates are the carrier of the continuum emission evidin wavelength dispersed emission spectra~e.g., Fig. 2! andattributed to blackbody emission. Figures 8~d! and 8~e!,which clearly show many of the ejected particulatesbounding from a substrate surface positioned atd;3 cm,serve to confirm that these are indeed solid particles whpresumably, cool radiatively during flight through thvacuum. We can gain a measure of the average expanproperties of these particulates by observing the accumulsum of images resulting from many~typically 200! laser

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6169J. Appl. Phys., Vol. 91, No. 9, 1 May 2002 Claeyssens et al.

shots taken with a sufficiently small~1 ms! intensifier timegate that each particulate shows as just a well-defined prather than a track. Three such accumulated images,served during fs pulsed laser ablation of graphite, in vacuat 248 nm and at different delay times, are shown in FigThese summed i-CCD camera images each show a con

FIG. 7. ~a! and~b! showI vs z plots obtained from cuts perpendicular to th

surface normal, atd, through the ns images shown in the left hand columof Figs. 3~b! and 3~c!, respectively, together with fits~solid lines! to aGaussian function.~c! and~d! illustrate how the FWHM of each distributionexpands witht; the gradients of these plots yield a value for the velocdispersions,dn' , of the emitting C* and C1* species along an axis orthogonal to the surface normal.

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ous distribution that can be analyzed in the same way asearlier C* and C1* images to yieldd values as a functionof t.

Unlike the C* and C1* images analyzed previously, thresultingd vs t plots shown in Fig. 10 are nonlinear. This cabe understood as follows: scanning electron microsc~SEM! images of the target surface, postablation, revrounded features, consistent with local melting and subquent resolidification within the irradiated region rather thany explosive melting or phase explosion.28 Further evidenceof a local phase transition is provided by Raman spectrcopy, which indicates a substantialsp3 fraction in the irradi-ated region of the target after ablation.28 Analogous findingshave been reported previously, following laser irradiationgraphite with pulse durations varying from 120 ns to 20 ps21

Proceeding with the assumption that the observed partlates arise as a result of thermal evaporation from the tasurface, the initial velocity distribution of the particulateshould be Maxwellian, with a mean speed proportionalAT/M—whereT is the melting/sublimation temperature ographite ~;4300 K!25 and M is the particulate mass. Thparticulates will be formed with a range of masses. Tlighter fraction can be expected to ‘‘escape’’ detection firstboth as a result of flight from the viewing zone and becauhaving less heat capacity, their rate of radiative cooling wbe faster than that of the larger particulates. Given thatdiative cooling will cause the associated blackbody emissto shift to longer wavelengths, where the i-CCD cameracomes progressively less sensitive, this effect will also teto discriminate against detection of the lighter particulatThus the average mass of theobservedparticulates can beexpected to increase witht, consistent with the progressivdecrease in the gradients of thed vs t plots shown in Fig. 10.Clearly, the evolution ofd with t is similar whether ablationis induced with 5 ps or 450 fs pulses of 248 nm laser radtion. In both cases the initial slope at smallt ~correspondingto nx;160 m s21! is consistent with formation of Cn particu-

FIG. 8. i-CCD images of the total emission resulting from 450 fs PLAgraphite, in vacuum, at 248 nm detected using a 50ms time gate on theintensifier of the camera, delayed by, respectively,t5(a) 40,~b! 60, and~c!80 ms. Images~d! and ~e!, recorded with a 500ms time-gate opening att550ms, illustrate particulates rebounding from a strategically positionsubstrate surface.

ense or copyright; see http://jap.aip.org/about/rights_and_permissions

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6170 J. Appl. Phys., Vol. 91, No. 9, 1 May 2002 Claeyssens et al.

lates with a meann;275 ~assumingT;4300 K!. Consistentwith previous reports,1,4 SEM images show films grownfrom this ablated material to be generally smooth, on anometer scale, but to contain some embedded particuthat are very much larger than this.

The angular distribution of the total emission at agiven delay time can also be calculated, by taking radslices through the images as shown in the cartoon in11~a! and summing the total signal along each radius.distributions measured for a given pulse duration were foto show a very similar angular form, so those presentedFigs. 11~b! and 11~c! are the average distributions for aimages recorded when exciting with~b! 5 ps and~c! 450 fspulses of 248 nm laser radiation. The observation thatvelocity distribution is forward peaked might appear to cotradict the preceding discussion in terms of a thermal partlate ejection mechanism, for which a more isotropic anguscattering would be predicted. To rationalize this apparinconsistency, we note that~a! the angular analysis has beeperformed on an image that is a squashed 2D projectiothe true 3D plume and~b! the particulates are ejected frommicroscopically rough surface. Both of these factors wconspire to enhance the apparent, or in the latter case,

FIG. 9. Integrated i-CCD images~200 shots! of the total emission fromlarge particulates resulting from 450 fs pulsed laser ablation of graphitevacuum, at 248 nm. Images were recorded using a 1ms intensifier gatewidth at respective time delays,t5(a), 40,~b! 80, and~c! 130 ms.

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yield of forward directed material. Additionally, of coursewe recognize that these macroparticles are ejected simneously with, and into a stream of, light, fast moving atomspecies. As shown earlier, the velocity distributions of thelatter species are forward peaked along the surface norany momentum transfer between atoms and macropartin the early stages of the expansion should thus also conute a net forward velocity to the latter.

in

FIG. 10. Plots ofd vs t derived from analysis ofI vs d plots from images~such as shown in Fig. 9! of the total particulate emission recorded following ~a! 5 ps and~b! 450 fs 248 nm laser ablation of graphite in vacuumdifferent delay timest.

FIG. 11. Plots showing the way the total signal intensity from particulaarising in ~b! 5 ps and~c! 450 fs 248 nm laser ablation of graphite ivacuum vary with the angleu, defined in the cartoon~a!. The solid linesthrough the data are best fits to a cosq u distribution withq54.5 @in ~b!# and6 @in ~c!#.

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6171J. Appl. Phys., Vol. 91, No. 9, 1 May 2002 Claeyssens et al.

IV. DISCUSSION

The optical emission accompanying ns pulsed laserlation of graphite, in vacuum, at 248 nm is distinctively dferent from that arising in ps or fs ablation at this samwavelength. The observation of C21* emission~which weinterpret as implying the initial presence of C31 ions! and thelack of C* emissions in the short pulse experiments indicaa much higher plasma temperature in the latter cases. Tobservations can be rationalized by recognizing thatdominant laser-material interactions giving rise to the oserved optical emission are different in the ns and ps/fsperiments. In the ns case, the bulk of the plasma is formedinteraction of laser photons with gas phase species~ions,atoms, etc.! ejected from the target during the laser irradtion process; plume excitation and ionization is mainlyresult of multiphoton absorption, ionization, and inverse Bhmstrahlung, in the gas phase, induced by the laser liSuch a mechanism cannot account for most of the optemission observed when using ps or fs laser pulses, howethe time scale of the laser irradiation in such cases is justshort for their to be significant material ejection and hyddynamic flow during the ablation pulse. In these cases, thfore, there is little justification for attributing the observeemission to laser-plume interactions; particles mustejected from the lattice in highly excited, ionized states.

The literature contains at least two models—the twtemperature model and the plasma-annealing model—offer descriptions of the material excitation and ablation pcesses occurring on ps/fs timescales. The former29 treats heatconduction by the electrons and the lattice separately. Lexcitation appears as a source term in the equation descrelectron excitation only, but the two equations are coupleda term that depends on the temperature difference betwthe two sub-systems and the strength of the electron-phocoupling. In this model, the electron distribution is assumto equilibrate on a short~,1 ps! time scale, giving an essentially immediate rise in the electron temperature while tlattice temperature remains at a low~ambient! temperature.The hot electrons start to diffuse within the target materiaresponse to the electron temperature gradient, and the latemperature rises, on a longer~1–10 ps! time scale, depending on the electron-phonon coupling strength. Within tframework of the two temperature model, therefore, ablatis pictured as a fast thermal evaporation. The model has bused extensively to account for observed melting threshin metals,30–32 but studies of short, intense, pulsed lasercitation of both semiconductors33 and graphite34 suggest thatthese target materials begin to ‘‘melt’’ on time scales thatmuch faster than those compatible with the two-temperamodel. In the context of the present study, it is also hardsee how this model can support the high degree of ionizaobserved in the plasma, since the level of ionization isrectly related to the surface temperature.

The plasma-annealing model35 has been used36,37 to ac-count for the very fast disordering of the lattice found bofor carbon34 and semiconductors like silicon33 at ionic tem-peratures much lower than the melting temperature ofrespective lattices. The laser source term in this mo

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couples directly into the electronic distribution, promotinthe electrons into highly excited states. High levels of eltron excitation are presumed to alter the interatomic foracting within the solid, with the target responding to the locdifferences in bonding interactions by melting on a timscale that is too fast to allow appreciable energy transbetween the excited ions and the lattice. Such a modelvides a reasonable qualitative rationale for the optical emsion we observe in the case of ps/fs ablation of graphite.high photon flux is assumed to promote a large numbeelectrons into antibonding states, thereby inducing a hdegree of excitation in the lattice, i.e., a very fast~;90 fs!,34

electronically induced phase transition. The resulting ‘‘liuid’’ state is considered to be very analogous to a higdensity plasma, with a high degree of ionizationcompatible with the presence of C21 and C31 ions in theplume as revealed by the present OES measurementswith the observation of ‘‘suprathermal’’ ions~perceived asbeing highly charged ions! in the plume arising in the ul-trafast~;100 fs! pulsed laser ablation of graphite at wavlengths;800 nm.19

i-CCD imaging and wavelength dispersed emissspectroscopy both show persistent C2* emission close to thetarget surface, irrespective of the incident pulse duration.have speculated that, in all three cases, unimolecular ancollisionally induced fragmentations of larger Cn clustersrepresent possible formation routes for the observed C2*species, as do collisional recombination processes—probinvolving one or more excited C atoms or ions. Given tlower emission intensities and the lack of C* emission inboth the ps and fs ablation of graphite, however, the lasuggestion would almost certainly require that there bsignificant yield of ground state~and thus non-emitting! car-bon atoms in the ablation plume~and thus available to contribute to C2 radical formation!. Reliable estimation of thefraction of ground state material in the plume, and its depdence on parameters like fluence, intensity, and wavelenremains one of the outstanding experimental challengethe field of pulsed laser ablation and deposition. If we mathe seemingly reasonable assumption that at least part oobserved C2* emission is of associative origin, however,follows that the yield of ground state atoms is likely to bquite substantial. That, in turn, would suggest that the twtemperature model of material ablation also plays a signcant role in the pulsed UV laser ablation of graphite. Thmodel assumes initial electronic excitation by the laser radtion followed, on a longer~typically 1–5 ps! time scale, byenergy transfer into the lattice and subsequent thermdriven ejection of material—mainly in its ground state, orfast decaying excited states, as dictated by the Boltzmdistribution appropriate for the prevailing target surface teperature. Any such material ejected after cessation oflaser pulse must remain in these ground~or low lying ex-cited! states and will thus largely escape detection by OEThus, we conclude that both of the models traditionally usto describe material ablation contribute to explaining theserved composition and characteristics of the ablation pluarising in the pulsed laser ablation of graphite.

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Additionally, at late time scales, we identify a slow component associated with ejection of large particles fromtarget. Both the trajectories of these particulates, and ttemperature as deduced by analysis of the accompanblackbody radiation, indicate that these are solid macroticles ejected from the target. Analysis of these emitting pticulates arising in both ps and fs laser ablation of graphshow them to have very similar, forward peaked, velocdistributions. Consistent with other recent studies,38 we sug-gest rapid pressure buildup within the target during laserradiation, and subsequent ejection of macroscopic fragmeas the most plausible formation mechanism for these partlates.

Most applications for DLC films seek coatings that ahard ~consistent with highsp3 fraction! and smooth. Theconsensus view is that formation of such films~at room tem-perature! is encouraged by use of an ablation flux containcarbon cations with reasonably high~;100 eV! kinetic en-ergy though, as commented previously, no studies to dhave succeeded in quantifying the relative abundance orvelocity distribution of the ground state carbon atoms withsuch plumes. The velocity corresponding to this favorednetic energy~;40 km s21 in the case of a carbon atom!matches well with that of the C1* emitters in the presenstudies, particularly those arising in the case of the nslaser ablation experiments, but is considerably faster tthat of the C* emitters and, by inference, the nascent neucarbon atoms. The present work suggests that use of hiintensity, shorter pulse durations offer few, if any, advantafrom the viewpoint of DLC film deposition. In comparisowith ns ablation, the material removal rate~and thus thedeposition rate! per pulse is smaller, particulate ejection rmains a problem and, as Qianet al.19 have shown, the filmquality ~judged by itssp3 content! declines. Indeed, one othe main outcomes of the present work is the suggestionfor optimal film formation, it is probably advantageoususemilder ablation conditions~i.e., less intense, longer duration, less tightly focused pulses! than those employed inmost ns UV ablation experiments to date. Finally, givenevident intensity dependences highlighted here, we wocomment that the detailed spatial and temporal profile offocussed laser beam on the target surface~especially in thecase of the short pulse laser! must influence the local material ejection process and, possibly, the eventual qualityany deposited film. Such factors merit further investigatiand are the subject of current research at FO.R.T.H.

ACKNOWLEDGMENTS

M. N. R. A. and F. C. are grateful to the EPSRC for taward of, respectively, a Senior Research Fellowship anstudentship. This work was supported in part by the Ultviolet Laser Facility operating at FO.R.T.H. under the Improving Human Potential~IHP!—Access to Research Infrastructures program~Contract No. HPRI-CT-1999-00074!. C.G. R. has been supported by a Marie Curie Fellowship ofEuropean Commission Human Potential program~ContractNo. HPMT-GH-00-00177-01!. The authors would also like

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to thank A. Egglezis and A. Klini for assistance with thshort pulse laser and the vacuum system, respectively.

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