Spectrochimica Acta Part A 61 (2005) 2115–2120

Continuum and discrete pulsed cavity ring down laser absorptionspectra of Br2 vapor

Ramesh C. Sharmaa,∗, Hong-Yi Huangb, Wang-Ting Chuangb, King-Chuen Linb

a Laser Chemical Physics Laboratory, Engineering Faculty, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japanb Institute of Atomic and Molecular Sciences, Academia Sinica and Chemistry Department, National Taiwan University, Taipei 106, Taiwan

Received 17 June 2004; accepted 19 August 2004

Abstract

The absorption cross-sections at room temperature are reported for the first time, of Br2 vapor in overlapping bound–free and bound–boundtransition of A3�1u← X�g

+, X1�1u← X1�g+ and B3�0u← X1�g

+, using cavity ring down spectroscopy (CRDS) technique. We reportedhere, the A3�1u← X1�g

+, transition is included along with the two stronger X1�1u← X1�g+ and B3�0u← X1�g transitions of Br2. We

obtained discrete absorption cross-section in the rotational structure, the continuum absorption cross-sections, and were also able to measuret 3 1 + 1 1 + 3 1 + Smo©

K

1

nsttsttatacr

mm

uan-tion

ationasterem-

ond)lex-for

entssspec-e it isross-

udiesc-ndsoxon

1d

he absorption cross-section in separate contribution of A�1u← X �g , �1u← X �g , and B�0u← X �g transitions using CRDethod to use quantum yield of Br* (2P1/2). We obtained absorption cross-section order 10−19 cm2 and detection 1013 molecule cm−3 (1 mTorr)f Br2. The absorption cross-sections are increasing with increasing excitation energy in the wavelength region 510–535 nm.2004 Elsevier B.V. All rights reserved.

eywords:Cavity ring down spectrum; Laser spectroscopy; Bound–free and bound–bound transitions; Detection limit

. Introduction

Cavity ring down spectroscopy (CRDS) is a laser basedew direct absorption technique, that has potential for the ab-olute/quantitative detection of atomic and molecular absorp-ion spectroscopy. The direct absorption technique is advan-age in a variety, as wavelength dependent absorption cross-ections can be extracted for the measurements. The absorp-ion technique is based on the measurement of the photon life-ime in an optical cavity after excitation with a light source,nd is independent of light source intensity fluctuations. It is

he most straightforward and accurate means of determiningbsolute vibronic band intensities and for probing states thatould not be detected by laser induced fluorescence (LIF) andesonance enhanced multiphoton ionisation (REMPI).

Laser induced fluorescence, for example is excellent forolecules that fluorescence but is obviously inapplicable toolecules with low fluorescence quantum yield, because fast

∗ Corresponding author.E-mail address:sharma@chemsys.t.u-tokyo.ac.jp (R.C. Sharma).

inter system crossing (ISC), dramatically reduces the qtum yield in such molecule. ISC also influences detecscheme[1] based resonance enhance multiphoton ionis(REMPI). ISC depletes the excited state on a time scale fthan the nanosecond (ns) duration of the laser, usuallyployed in such experiment. Use of ultrafast (femtoseclaser may alleviative this problem, but the cost and compity of such system would render the method unsuitablefield development.

The new CRDS technique is used here for measuremof absolute absorption cross-section of Br2 vapour. CRDS ialso superior to other popular techniques such as FT-IRtroscopy and laser photoacoustic spectroscopy, becausmore sensitive and can provide absolute absorption csection and is self-calibrated technique[2,3]. Pulsed CRDSwas first demonstrated by O’Keefe and Deacon[4]. The Br2vapour has been the subject of several high-resolution ston the B3�1u← X1�g

+ system and the rotational fine struture as well as the intensity distribution in the vibronic bahave been understood through the research work of C[5], Horsley and Barrow[6], Barrow et al.[7] and Coxon[8].

386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.oi:10.1016/j.saa.2004.08.011

2116 R.C. Sharma et al. / Spectrochimica Acta Part A 61 (2005) 2115–2120

Le Roy et al.[9] who estimated the intensity contribution tothe continuum resulting from the1�1u←X1�g

+, and B3�1u← X1�g

+ transition in the wavelength range 320–500 nm.The visible and ultraviolet absorption spectra electronic

states of Br2 molecule have attractive scientific interest, halo-genated hydrocarbon are rapidly being added to the atmo-sphere due to their essential use as aerosol propellants, refer-ents and fire retardants[10]. Farman et al.[11] have reportedas ozone concentration occur over Antarctica, the chemistryof the Antarctica stratosphere was greatly perturbed withlarge abundance of ozone destroying halogen BrO[12]. Theconventional absorption cross-section of Br2 has been car-ried out[13,14]. The absorption intensity measurements wasreported by Lindemann and Wiesenfeld[15] to estimate rela-tive contribution of B3�0u←X1�g

+ transitions. Sharma andThakur[16] have studied photoacoustic spectrum towards thedissociation limit of B state. Smedley et al.[17] have studiedabsorption structure by photodissociation of Br2 and to gen-erate quantum yield in the wavelength region 510–590 nm.

In the present work we focused on the absorption dis-crete ro-vibrational with isotope separation structure of theB3�0u← X1�g

+ transition. The B3�0u← X1�g+ absorp-

tion overlapping with states A3�1u← X�g+ and1�1u←

X1�g+ transitions. Using new simple straightforward CRDS

technique to measure absolute absorption cross-section incontinuum absorption as well as discrete absorption in over-lhb y di-a

2. Experimental

The cavity ring down spectroscopy (CRDS) techniquewas used to measure the continuum and discrete absorptioncross-section of Br2 vapour at room temperature.Fig. 2 il-lustrates the experimental arrangement. A dye laser (Scan-mate) was pumped by the 355 nm of a pulsed Nd-YAG laser(Spectra-Physics GCR) with 10 Hz repetition rate. The pulseduration was about 7–8 ns. The dye laser was operated withcoumarin 503 dye and generated 3–4 mJ P−1 with scan rateof 0.002 nm s−1 in 500–540 nm wavelength region. Modematching between the incident beam and the cavity field dis-tribution was achieved with the help of a 2X Galilean tele-scope with two quartz lenses with 5 and 10 cm focal lengthsand a 70�m pinhole.

The 510 nm laser beam entering the ring down cavity hasnearly 1 mJ energy, 3 mm diameter. The resonance cavity wasbuilt 55 cm from mirrors M1 and M2 (CVI Laser Corpo-ration) with 1 m radius of curvature. The reflectivity of themirrors was∼99.9% at 510 nm. A PMT is used to record thedecay waveform. Special attention was paid to the acquisitionmode of the PMT to avoid signal saturation. As was experi-mentally verified, the signal amplitude with 50� impedancedoes not exceed 50 mV. To make the receiver system less sen-sitive to the ring down resonator alignment, a converging lensand quartz diffuser plate before the PMT were used. Signalsw and

Fig. 2. Experimental setup of cavity ring down laser absorption spectra.

apping transitions, and limit of detection of the Br2 moleculeas been carried out. The spectral feature of Br2 molecule cane understood with the help of the partial potential energgram (Fig. 1).

Fig. 1. Partial potential energy curves of Br.

2

ere digitized by a digital oscilloscope (Lecroy 9450A)

R.C. Sharma et al. / Spectrochimica Acta Part A 61 (2005) 2115–2120 2117

Fig. 3. Temporal profile of: (a) empty cavity decay time curve without Br2,(b) decay time in total continuum absorption of Br2 in overlapping transitionsand (c) decay time discrete resonant absorption in ro-vibronic transitions ofBr2.

transmitted to a PC through a GPIB interface. The recordeddecay waveforms were approximated by an exponential func-tion to extract the decay timeτ fitted by Levenberg Marquartalgorithm. The temporal profile is given (Fig. 3). A small flowof purge gas introduce into the volume between the adjustableand fixed flanges ensured that potentially considerable or cor-rosive vapour Br2 inside the main cavity volume did not comeinto contact with the mirrors. A small stainless steel apertureof 1/8 or 1/4 in. diameter mounted between the purge volumeand the main cavity increased the linear flow velocity betweenthe volumes and minimized flow back into the purge volume.The absorption spectra of Br2 are measured at room temper-ature. In the experiment the vapour pressure was 1–30 mTorrof Br2.

2.1. Principal of CRDS method for measurement ofabsolute absorption cross-section

The method is based on measurements of the time rateof decay a pulse of light, an optical cavity with couplingoptics and a photo detection system. A stable optical cavityis formed by two concave mirrors of high reflectivity (R).These mirrors serve also as side windows of the sample gascell. A laser pulse enters the cavity through one of the mirrors.The pulse is spatially shaped via a pinhole to form TEMm

idet ors,w s.T ni-tl sig-n ntial

decay:

I = I0 exp(− t

τ

)(1)

This decay arises from losses in the mirrors coating and ab-sorption by the gas sample contained between the mirrors,when light is tuned away from the resonance or the cell isempty, the ring down time (RDT)τ0 is determined by

τ0 = L

c(1− R)(2)

for τ0, the temporal profile is given inFig. 3a,L is the cavitylength between two mirrors,c the velocity of light,R thereflectivity of mirrors. When there are gas molecules presentinside the cell, the ring down timeτ1 (Fig. 3b) for continuumabsorption (ν), is then determined also by absorption of themolecule

τ(ν) = L

c(1− R)+

∫σ(ν)N(x) dx

τ1(ν) = L

c(1− R)+ σ(λ)N ls (3)

τ1(ν) is the ring down time at frequencyν,σ(ν) the absorptioncross-section of the particular molecule that absorb light atf alt the ctionc

T fromt

ityo iti Brc1 ss-s

2

intl angeb eo RD)s fromE n-t , thes

00ode of the cavity.The small amount of light that is now trapped ins

he cavity reflects back and forth between the two mirrith a small (∼1 − R) transmitting through the mirrorhe resultant transmission of the circulating light is mo

ored at the output mirror as a function of time (t) and al-ow the decay time of the cavity to be determined. Thisal has an envelope that is simple a first-order expone

requency (ν),N the number of density, which is proportiono the absolute concentration,L (=ls) the absorption lengqual to cavity length. Continuum absorption cross-sean be calculated from the following equation:

1

τ1(ν)− 1

τ0(ν)= cσ(ν)N (4)

he discrete absorption cross-section can be measuredemporal profile is given as (Fig. 3c)

1

τ2(ν)− 1

τ1(ν)= cσ(ν)N (5)

We also measured limit of detection (LOD) sensitivf this technique of Br2. The minimum detection of lim

n our experiment nearly 1 mTorr vapour pressure of2,alibrated molecular density of Br2 at 1 mTorr is 3.6×013 molecule cm−3. The fitted discrete absorption croection at 519.5 nm is given inFig. 4.

.2. Analysis of cavity ring down spectra of Br2 vapour

Cavity ring down spectra of Br2 vapour are recordedhe wavelength region 516–518 nm is given (Fig. 5). Simi-arly CRDS spectra were recorded in the wavelength retween 510–516 and 518–535 nm. InFig. 5a shows base linf the spectra, which corresponds to cavity ring decay (Cpectra without sample in the cavity. This spectra comesq.(2). The base line ofFig. 5b corresponds to the total co

inuum absorption in the wavelength region 516–518 nmpectra also comes from the temporal profile (Fig. 3b). The

2118 R.C. Sharma et al. / Spectrochimica Acta Part A 61 (2005) 2115–2120

Fig. 4. Plot of absorption coefficient vs. number density of Br2 at laserwavelength 519.5 nm, yielding a slop indicative of the absorption cross-section (1.3× 10−19 cm2).

continuum signal is calculated fromFig. 3a and b, absorptioncoefficient of continuum absorption is

αcon= 1

c

[1

τ1− 1

τ0

](6)

hereτ1 and τ0 are decay time of cavity at continuum ab-sorption and without absorption, respectively. InFig. 5c, thediscrete absorption of the spectra is in the wavelength range516–518 nm. The intensity of the spectra directly related tothe absorption coefficient, which is related to

αd = 1

c

[1

τ2− 1

τ1

](7)

where�k= 1/τ2− 1/τ1, τ2 andτ1 are decay time of the cav-ity at resonant absorption and continuum absorption, respec-

F –518 nc h regio n5

tively, and�k is the change in decay time, which correspondsto spectral intensity andαd the discrete absorption coefficient.The fitted discrete absorption cross-section at 519.5 nm is ofthe order of 10−19 cm2 shown inFig. 4.

The temporal profiles are given inFig. 3a–c. Fig. 3ais the cavity decay timeτ0 without resonant absorption,Fig. 3b the decay time at continuum absorption in over-lapping transitions,Fig. 3c the decay time at discrete ro-vibrational resonant absorption. The CRDS spectrum isgiven in Fig. 5a–c is directly related to the temporalprofiles.

The discrete ro-vibronic transition in the cavity ring downspectrum is easily identified in the light of the ro-vibrationalanalysis of the corresponding optical spectrum by Coxon[5]. The natural abundance of79Br is almost equal to thatof 81Br and hence the proportional79Br2, 81Br2, 79,81Br2 inthe bromine vapour would be in the ratio 1:1:2.

In our experiment discrete spectra are assigned using dataCoxon[5], which belongs to B3�0u← X1�g

+ ro-vibronictransition of Br2 molecule and continuum spectra are theoverlapping B3�0u← X1�g

+, A3�1u← X1�g+, and1�1u

← X1�g+ transitions. Specially, we assigned here Br2 spec-

trum in the wavelength range 516–518 nm is given inFig. 5.In the B3�0u← X1�g

+ transitions at ro-vibronic transitionv′ = 34–39,v′′ = 0, rotationally assigned using the molecularconstant of Coxon[5] in P and R branches. P branch is fromJ mn

it ed ge5 nt← isg

ig. 5. (a) Cavity ring down spectrum in the wavelength region 516orresponds to total continuum absorption spectrum in the wavelengt16–518 nm.

m without Br2, (b) is the base line of the ro-vibronic spectrum of Br2, whichn 516–518 nm and (c) cavity ring down spectrum of Br2 in the wavelength regio

′′ = 1–37 and R branch is fromJ′′ = 1–43 rotational quantuumbers.

We also observed isotopic splitting79,79Br2 and81,81Br2n our present work at vibronic bandv′ = 35–39,v′′ = 0 inhe B 3�0u← X1�g

+ transition of Br2 molecule using thata from Coxon[5], the spectra in the wavelength ran16–518 nm is given inFig. 5. The continuum absorptio

ransitions easily understood in the B3�0u←X1�g+, A3�1u

X1�g+ and 1�1u ← X1�g

+ potential energy curveiven (Fig. 1).

R.C. Sharma et al. / Spectrochimica Acta Part A 61 (2005) 2115–2120 2119

Table 1The continuum absorption cross-section of A3�1u, B3�0u and1�1u states of Br2 molecule

λ (nm) φ∗Br σcon (×10−19 cm2) σB–X (×10−19 cm2) σ�–X (×10−19 cm2) σA–X (×10−19 cm2)

510.5 85.04 0.98 0.84 0.046 0.12511.5 81.84 0.84 0.69 0.043 0.11512.5 78.64 0.75 0.58 0.040 0.10513.5 76.08 0.54 0.41 0.038 0.092514.5 73.68 0.39 0.29 0.035 0.068515.5 71.28 0.32 0.23 0.033 0.058516.5 68.88 0.28 0.19 0.031 0.056517.5 66.48 0.22 0.15 0.029 0.047518.5 65.68 0.22 0.15 0.027 0.046519.5 65.28 0.20 0.13 0.025 0.043520.6 63.40 0.17 0.11 0.023 0.041523.7 50.20 0.12 0.060 0.019 0.041525.5 42.40 0.11 0.048 0.017 0.040529.0 34.75 0.087 0.030 0.013 0.038531.0 29.84 0.056 0.017 0.011 0.028533.0 25.28 0.046 0.012 0.009 0.025534.5 24.48 0.039 0.010 0.0084 0.021

We have measured absorption coefficient in the wave-length region 516–518 nm of Br2 vapour using intensity ofthe signal in discrete and continuum absorption. The inten-sity of the spectral signal is equal to∼�k = 1/τ2 − 1/τ1 and�k= 1/τ1− 1/τ0, absorption coefficientα = �k/c, from Eqs.(2), (6) and (7), wherec is the velocity of light, these spec-tra are also rotationally assigned using molecular constant[5], rotationally assigned P and R branch ro-vibronic transi-tions, similarly we recorded in the wavelength range between510–516 and 518–535 nm.

In the present work, we measured density of Br2 vapourusing calibrated pressure gauge, the density of Br2 moleculenearly∼3.6× 1013 molecule cm−3 at 1 mTorr. The absorp-tion cross-sectionσ�–X is used[9]. Total continuum absorp-tion cross-sectionsσcon of Br2 at discrete wavelengths aregiven inTable 1using Eq.(4). The total continuum absorp-tion cross-section (σcon) corresponds to B3�0u ← X1�g,A3�1u← X1�g

+, and1�1u← X1�g+ transitions, i.e.σcon

= σB–X + σA–X + σ�–X. The A3�1u state and1�1u repulsivestate dissociate into two ground states Br(2P3/2) + Br(2P3/2)atoms, the stable B3�0u state is dissociated into Br* (2P1/2) +Br(2P3/2) atoms at excitation energy of nearly 19 579 cm−1

[16].The continuum absorption cross-section of the B3�0u,

state�B–X can be obtained from Eq.(8)

σ

φ

u -s nt bytiaE -c n ofb

predissociation rate is decreases for the near vibrational levelsof B3�0u statev′ = 40–45 and since this rate is much smallerthan the rate at which the Br* (2P1/2) and Br(2P3/2) atoms sep-arate upon excitation continuum region of the B3�0u

+ state.It is very unlikely that appreciable mixing of the two con-tinuums will occur. Similarly, the predissociation rate of thebound state B3�0u by the1�1u, A3�1u continuum states is ofthe order of magnitude smaller than the1�1u state, so cross-ing from the B3�0u state to the A3�1u state is also expectedto be relatively weak. Smedley et al.[17] reported that thewavelength 510.7 nm corresponds to the energy needed toexcite molecules initially inv′′ = 0,J′′ = 0 of the ground stateto the continuum region just above the B3�0u state dissocia-tion threshold. At the excitation energyλ≥ 510.7 nm,v′′ = 0,J′′ = 0 threshold for Br* , dissociation to products Br* (2P1/2),Br(2P3/2) commonly occur for molecules which are initiallyin thermally excitedv′′, J′′ levels.

Fs

B–X = σconφ∗Br (8)

∗Br is the quantum yield for Br* (2P1/2) production which issed from Smedley et al.[17], σcon the total continuum aborption cross-section, we measured in our experimehe decay time and absorption spectrum is given (Fig. 5), us-ng the equationσcon = αcon/N, αcon is the total continuumbsorption coefficient andN be the number density of Br2.vidence for weak mixing of the B3�0u and1�1u, states exites from the known rotationally induced predissociatioound B3�0u state overlapping with repulsive1�1u, state the

ig. 6. Continuum absorption cross-sectionσB–X, σA–X andσ�–X in tran-itions of Br2 in the wavelength region 510–535 nm.

2120 R.C. Sharma et al. / Spectrochimica Acta Part A 61 (2005) 2115–2120

The continuum absorption cross-section of A state is ob-tained fromσA–X = σcon − σB–X − σ�–X. The continuumabsorption cross-section is given inTable 1. The calculatedcontinuum absorption cross-sectionsσcon, σB–X, σA–X andσ�–X are plotted inFig. 6. The absorption cross-section isdecreasing with increasing wavelength range 510–535 nm.The rapid decrease in the population of vibrationally excitedstate which have sufficient energy to access the B3�0u stateabsorption cross-section.

3. Conclusion

We have measured the cavity ring down continuum anddiscrete absorption spectrum, the ro-vibrational resolved ofthe B3�0u← X1�g

+, transitions of Br2 vapor to use ear-lier data. We have measured absorption cross-section in thecontinuum and discrete structure to use new CRDS tech-nique in temporal profile as well as spectral signals in thewavelength region 510–535 nm. We have reported for thefirst time absorption cross-section to use CRDS methodin individual contribution of the B3�0u, A3�1u and 1�1ustates to the continuum absorption in the spectral regionto use calibrated molecular number density in our experi-ment. The continuum absorption cross-sections of Brarei Weh r of∼B

Acknowledgements

This work was financially supported by National ScienceCouncil of ROC under contract no. NSC 92-2113-M-002-046.

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