far-uv photochemical bond cleavage of n-amyl nitrite ... december 17, 2011 r 2011 american chemical...

Published: December 17, 2011

r 2011 American Chemical Society 810 dx.doi.org/10.1021/jp209727g | J. Phys. Chem. A 2012, 116, 810–819

ARTICLE

pubs.acs.org/JPCA

Far-UV Photochemical Bond Cleavage of n-Amyl Nitrite: Bypassing aRepulsive SurfaceMichael P. Minitti,§,† Yao Zhang,† Martin Rosenberg,‡ Rasmus Y. Brogaard,‡ Sanghamitra Deb,†

Theis I. Sølling,‡ and Peter M. Weber*,†

†Department of Chemistry, Brown University, Providence, Rhode Island 02912, United States‡Department of Chemistry, University of Copenhagen, Universitetsparken 5, DK-2100 Copenhagen, Denmark§SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States

bS Supporting Information

1. INTRODUCTION

The homolytic, photochemical bond cleavage of a singleσ-bond is of central importance to chemistry.1�3 Pioneeringspectroscopic4 and time-resolved5�8 studies have clarified muchof the molecular dynamics in small molecules. Yet in manymedium sized or large molecular systems, the complexities of thehomolytic bond cleavage and in particular the details of theultrafast dynamics, remain to be uncovered. A particularlyimportant group of molecules are alkyl nitrites, RONO, whereR represents an aliphatic group. Photochemical dissociation ofalkyl nitrites produces NO radicals, which are important regula-tors of ozone and leading contributors in the formation ofsmog9,10 and alkoxy radicals, which are central intermediates inthe atmospheric degradation of volatile organic compounds.11

The photodissociation of alkyl nitrites has been investigated innumerous ways,9,12�15 in most cases by inducing the photo-chemistry with radiation in the near or mid UV. Only few studieshave been reported with excitation wavelength in the deepUV.16�18 From these studies we learn that the excitationwavelength is an important determinant of the vibrationalquantum state of the photolytically generated NO radicals. Evenso, they reveal little about the time-resolved dynamics thattakes place during the molecular dissociation. It is precisely thatintramolecular dynamics, however, that determines the outcomeof the reaction.

In the present study, we focus on the strongly absorbing,highly excited S2 state of amyl nitrite, C5H11ONO (in thefollowing abbreviated RO�NO). The fast dissociation dynamics

is initiated using ultrashort laser pulses at 207 nm, and theprogress of the reaction is studied using ultrafast pump�probephotoionization mass spectrometry and photoelectron spectros-copy. According to the calculated potential energy surfaces(PESs) along the RO�NO stretch coordinate,15 one expectsthat excitation at 207 nm prepares amyl nitrite on its steeplyrepulsive S2 state and that it rapidly dissociates on that PES intothe alkoxy and NO products. Contrary to this expectation, wefind that the ejection of the NO fragment does not happen on theS2 PES but is, instead, preceded by an internal conversion to thelower S1 PES. After lingering in the S1 state for a short amount oftime, the NO is ejected with a kinetic energy significantly lessthan what would be expected if the dissociation were to occur inthe S2 state. To support this conclusion, we draw on evidencefrom the multiphoton ionization photoelectron spectra, from thetime-resolved appearance of the NO fragments seen in both thephotoelectron spectra and the mass spectra, and from the kineticenergy of the ejected NO radicals. While our discovery contra-dicts long-held beliefs and earlier interpretations, it is in factconsistent with previously measured data.

2. EXPERIMENTAL AND COMPUTATIONAL DETAILS

Amyl nitrite was seeded in 1 bar of helium carrier gas andpassed through a 96 μm nozzle followed by a 150 μm skimmer,

Received: October 10, 2011Revised: December 12, 2011

ABSTRACT: We have investigated the deep-UV photoin-duced, homolytic bond cleavage of amyl nitrite to form NOand pentoxy radicals. One-color multiphoton ionization withultrashort laser pulses through the S2 state resonance gives riseto photoelectron spectra that reflect ionization from the S1 state.Time-resolved pump�probe photoionization measurementsshow that upon excitation at 207 nm, the generation of NOin the v = 2 state is delayed, with a rise time of 283 (16) fs. Thetime-resolved mass spectrum shows the NO to be expelled witha kinetic energy of 1.0 eV, which is consistent with dissociation on the S1 state potential energy surface. Combined, theseobservations show that the first step of the dissociation reaction involves an internal conversion from the S2 to the S1 state, which isfollowed by the ejection of the NO radical on the predissociative S1 state potential energy surface.

811 dx.doi.org/10.1021/jp209727g |J. Phys. Chem. A 2012, 116, 810–819

The Journal of Physical Chemistry A ARTICLE

before entering a differentially pumped chamber where it wasphotoexcited and ionized. The details of the Brown Universitytime-of-flight photoelectron/photoion spectrometer have beendescribed elsewhere.19�21 Briefly, the spectrometer was outfittedwith a tunable (750�850 nm), regenerative amplifier laser sys-tem (Positive Light, Spitfire) operating at 5 kHz. The amplifiedfundamental output (828 nm) was frequency upconverted to thesecond, third, and fourth harmonics. In the time-resolvedexperiments, the fourth harmonic (4ω, 207 nm) was used asthe pump pulse and the second harmonic (2ω, 414 nm) as theprobe pulse. The time zero of the laser pulse overlap wasestablished by monitoring the two-color signal resulting fromfourth harmonic excitation and second harmonic ionization ofdimethylpiperazine, and the cross-correlation between the pulseswas determined to be 280 (20) fs. The pulse energies (powerdensities at the focus) of the 4ω and 2ω were approximately1.4 μJ (1.5 � 1011 W/cm2) and 20 μJ (2.0 � 1010 W/cm2),respectively. Some experiments were also performed using thethird harmonic (3ω) of the output in which case the wavelengthwas 266 nm. Both photoelectrons and ions were detected usingmultichannel plate detectors, and the time-of-flight spectra wereacquired using ultrafast timing electronics. Amyl nitrite waspurchased from TCI Europe (purity >96%), and the purity wasverified by both 1H and 13C NMR spectroscopy. To reduce theamount of clustering observed in the molecular beam, the amylnitrite sample was placed in a temperature controlled bath of0 �C. All calculations were performed using the Gaussian03 suiteof programs.22 All structures were optimized to local energyminimum structures at the B3LYP/6-31+G(d,p) level. From thecalculated structures, the total energies were calculated using theG3(MP2) composite ab initio method,23 where the geometryoptimizations and frequency calculations were carried out at theB3LYP/6-31+G(d,p) level (G3(MP2)//B3LYP/6-31+G(d,p)).The 0 K heats of formation (ΔHf) were calculated as describedby Nicolaides et al.24

3. RESULTS

The calculated potential energy surfaces along the RO�NObond stretch of amyl nitrite are shown in Figure 1. Radiation at207 nm is resonant with the S2 state, which is steeply repulsivealong this coordinate. Very similar PESs have been reported inthe closely related n-butyl nitrite and isoamyl nitrite systems.9,25

The S2 state is of (π, π*) character, while the S1 state has (n, π*)character, and according to calculations the πf π* and nf π*transitions involve promotion of an electron from the HOMO� 1 to the LUMO, and from the HOMO to the LUMO,respectively.9,15 The vertical excitation energies, calculated byRosenberg et al. to be 3.6 and 6.2 eV for excitation to the S1 andS2 states, respectively, agree well with the experimental gas phaseabsorption spectrum (inset in Figure 1), which shows peakmaxima at 355 nm (3.5 eV) and 220 nm (5.6 eV).15

3.1. Multiphoton Ionization Photoelectron Spectra. Thesingle color,multiphoton ionization photoelectron spectra, obtainedwith pulses at 414 nm (2.99 eV), 400 nm (3.10 eV), 266 nm(4.66 eV), and 207 nm (5.99 eV), respectively, are shown inFigure 2. The spectra are rather featureless but neverthelessinformative. Their broad, unstructured appearance, featuringphotoelectron kinetic energies over a large part of the spectrum,are consistent with an ionization via a short-lived (on the timescale of the laser pulses) intermediate valence state.26 This isbecause during the laser ionization process, a large amount of

energy is converted from electronic to vibrational degrees offreedom. This energy remains in the molecule after ionization sothat ions are born with significant internal energies and thatelectrons are ejected over fair ranges of kinetic energies. Since thepotential of the ionic ground state is calculated to be very shallow,vibrational fine structure cannot be observed.Figure 2a,b shows the photoelectron kinetic energy spectra

obtained with 2ω pulses of 400 and 414 nm. Starting with thephoton energy, the signal rises steadily with decreasing kineticenergy, with most intensity around 0 eV. If the pathway toionization were to involve the excitation to the S2 PES, whichrequires two 2ω photons, followed by the ionization from the S2state, which would require a further two 2ω photons, the totalphoton energy deposited in the molecule would be 12.0 and12.4 eV for the two wavelengths, respectively. One would expectthat vertical ionization27 would lead to electron kinetic energiesof 12.0 � 10.6 = 1.4 eV and 12.4 � 10.6 = 1.8 eV, respectively.The spectrum recorded using 2.99 eV photons has a shoulder atabout 1.5 eV, which may seem like a good match, but thespectrum recorded using 3.10 eV photons shows a peak at thesame energy, which does not match the 1.8 eV electron kineticenergy predicted based on ionization out of the S2 state. It istherefore unlikely that the shoulder and weak peak in the twospectra reflect the ionization via the S2 state to the verticalionization energy. Instead, the almost constant position of thosepeaks with changing photon energy rather suggests a Rydbergstate resonance.28 Specifically, three photons could excite anelectron to a Rydberg state with a binding energy of about 1.5 eV,from where a 2.99 or 3.10 eV photon would eject it with kineticenergies of 1.5 or 1.6 eV, respectively. The binding energy of1.5 eV is in the range where 4s Rydberg states are frequentlyobserved.29 In this explanation, the width of the Rydberg peakwould stem from structural dispersion of the molecule incurredon account of the excitation process.30,31

The most intense signal in the 2ω spectra is at very low kineticenergies, implying that ionization leaves the molecule withsubstantial internal energy. We suggest that upon excitation intothe S2 state, a rapid internal conversion to the S1 state converts

Figure 1. Calculated PESs (TD-B3LYP/6-31+G(2df,p)//B3LYP/6-31+G(d,p)) as a function of the RO�NO bond length in the calculatedcis�trans conformation (B3LYP/6-31+G(d,p)) from ref 14. The insetshows the UV absorption spectrum and the 4ω laser wavelength used toinduce the dissociation.



electronic energy to vibrational energy, which remains in themolecule upon ionization. Given that the S1 state is at 3.6 eV,ionization to the adiabatic ionization threshold14 should require9.8� 3.6 = 6.2 eV. This is just about at the energy of two 2ω pho-tons, explaining why the photoelectron kinetic energy distribu-tions peak around zero energy. The broad, uniformly slopingspectra are therefore the wings of very broad Franck�Condonenvelopes. The 2ω ionization spectra are, therefore, consistentwith an internal conversion during the laser pulse to the S1 statebut not with ionization out of the S2 state.The photoelectron spectrum (Figure 2c) for ionization with

3ω photons (266 nm, 4.66 eV) also shows electron ejection overa very broad range. Absorption of a 3ω photon excites themolecule to the low energy range of the S2 state, and ionizationout of this state requires two additional 3ω photons. The total,three-photon energy is thus 14.0 eV, and vertical ionization tothe ion ground state at 10.6 eV would produce electrons with14.0� 10.6 = 3.4 eV. The spectrum has no intensity maximum atthat energy. Instead, the maximum photoelectron kinetic energyis at about 2.2 eV, about 1.2 eV lower than expected from verticalionization. This difference in kinetic energy must be absorbed inthe molecule during the ionization. Excitation of amyl nitrite intothe S2 state with a 4.66 eV photon, followed by rapid internalconversion to the S1 state at 3.6 eV, deposits 1.06 eV of energy

into vibrations. Thus, invoking relaxation from the S2 to the S1state gives a good match to the 3ω photoelectron kinetic energyspectrum. The width of the spectrum could arise because manyvibrational modes are excited in the internal conversion, lendingFranck�Condon intensity to a wide range of ionization transi-tions. Thus, the 3ω spectrum is inconsistent with ionization outof S2 but does conform with an ionization mechanism thatincludes rapid internal conversion from the S2 to the S1 state.The photoelectron spectrum recorded with 4ω photons

(5.99 eV), Figure 2d, shows no discernible features other thanthe rise as the photoelectron kinetic energy approaches zero.This spectrum is quite similar to the one recorded with 2.99 eVphotons (Figure 2a), except that the shoulder at 1.5 eV electronkinetic energy is missing. While in the 2.99 eV spectrum twophotons are required for the final ionization step, in the 5.99 eVspectrum one photon suffices. With that adjustment, our inter-pretation of the 5.99 eV spectrum is like the one of the 2.99 eVspectrum, and it supports the notion that rapid internal conver-sion takes place during ionization. The absence of the 1.5 eVshoulder in the 2ω spectrum is easily explained in the contextof the model proposed above: the 4s Rydberg state resonancecan be accessed with three 2ω photons but not at all with 4ωphotons.As the excitation to the S2 state involves the promotion of an

electron from the HOMO � 1 orbital, it could be reasoned thatthe ionization out of the S2 state should prepare the molecularion in its excited state.32 We estimate this energy to be higherthan the lowest ionization energy by the energy difference be-tween the S2 and S1 states, i.e., 6.2 � 3.6 = 2.6 eV. This wouldplace the HOMO � 1 excited state ion at a vertical excitationenergy of 10.6 + 2.6 = 13.2 eV. For most spectra of Figure 2, thisenergy is out of reach with the minimum number of photons.Higher order processes might be possible but would be muchweaker. Only the spectrum with three 3ω photons has a totalphoton energy of 14.0 eV, enough to reach the excited ion. Theelectrons from the ionization with three 3ω photons would beexpected at a kinetic energy of 14.0 � 13.2 = 0.8 eV. However,there is no corresponding peak at that energy. Thus, we concludethat amyl nitrite ionizes to the electronic ground state of the ionand that there is no support from this conjecture for an ionizationmechanism out of the S2 state.Finally, one might object to our conclusion by arguing that

dissociation along the repulsive S2 state PES quickly reduces thetotal electronic energy so that the required ionization energyincreases during the dissociation. The washed-out, broad photo-electron kinetic energy spectrum would then be the reflection ofthe ionization at different dissociation points, but this seemsquite implausible since during the dissociation the electronicenergy is converted to kinetic energy of the emerging fragments.In the ionization transition, the momentum would need toremain conserved. However, the ion state is weakly bound (seeFigure 1), and its well does not support vibrational states havingthe requisite kinetic energy. An ionization transition is thereforepossible only very early in the dissociation process, and if it wereto happen, it would have decreasing signal intensity for lowerelectron kinetic energies. Consequently, this conjecture can alsonot explain the observed spectra.In summary, the one-color multiphoton ionization photoelec-

tron spectra of amyl nitrite are inconsistent with an ionizationpathway where the molecules are ionized out of the S2 state.However, all spectra are consistent with ionization out of theS1 state.

Figure 2. One-color multiphoton ionization photoelectron spectraobtained with, (a) 414 nm (2.99 eV) photons, (b) 400 nm (3.10 eV)photons, (c) 266 nm (4.66 eV) photons, and (d) 207 nm (5.99 eV)photons.



3.2. Time-Resolved Photoelectron Spectra. The time-re-solved photoelectron spectrum (Figure 3a), obtained by excitingand ionizing with 207 nm (5.99 eV) and 414 nm (2.99 eV)photons, respectively, is plotted with the electron binding energyas the vertical axis. The binding energy is the energy required toeject an electron from a high lying orbital. The spectrum shows avery short-lived transient with a spectrum that matches that ofthe 2.99 eV spectrum of Figure 2. At positive time delays, there isa persistent peak at a binding energy of 2.76 eV. The ionizationenergy of the NO radical is 9.264 eV,33,34 and NO is known tohave its C2Σ+ excited state at an energy of 6.493 eV above theground state (origin transition).35 Together, this implies that theelectron binding energy of the C2Σ+ state is 9.264 � 6.493 =2.771 eV. This is in close agreement with our observed value of2.76 eV and within the spectral bandwidth of the femtosecondlaser pulses. The C2Σ+ state is likely reached in a two-photonprocess, even though the two-photon energy of 5.99 eV is, byitself, not sufficient to reach the level because the NO radicalsmay be vibrationally and/or rotationally excited. Finke et al. havefound that photofragmentation at 193 nm heavily favors forma-tion of NO in the v = 1 and v = 2 vibrational levels, with arotational distribution that peaks at J00 = 13.5 for v = 2.16 Twovibrational quanta, each with 0.236 eV, suffice to enable the

excitation of NO to the C2Σ+ state: the electronic transitionenergy would be reduced to 6.493 eV� 2 3 0.236 eV = 6.021 eV,which is within 31 meV of the laser center wavelength. An exactresonance can arise from the rotational envelope of the transitionor made possible by the fairly broad (typically 30 meV) band-width of the 414 nm femtosecond laser pulses. We note that inour previous experiments with 266 nm (4.66 eV) excitation and400 nm (3.10 eV) probe, the NO was not detected.14 This islikely due to a mismatch between the probe pulse energy and thevibronic transitions of NO, and possibly because at longer photo-lysis wavelength, less vibrationally excited NO is generated.36

The 4ω + 2ω two-color photoelectron spectrum is comprisedof a broad transient with a short lifetime, and a spectrally narrowpeak that, after an initial rise, approaches a constant value.Because the spectral features are so distinct, it is possible todeconvolve the two components of the transient signal. Figure 3bshows the resulting temporal evolutions of the NO photoelec-tron signal and the transient broad signal integrated over electronbinding energies from 2.5 to 2.95 eV. The broad, transient signalcan be fitted to a cross-correlation function (presumably reflect-ing various ionization paths with different 4ω and 2ω photoncombinations) and a rapid decay. The rise of the NO signal iswell fitted to an exponential rise. A simultaneous fit gives a timeconstant of 283 (16) fs for both processes.37 This value impliesthat there is a time lapse between excitation at 207 nm and thecreation of free NO radicals. This conclusion is also supported bythe one-color multiphoton experiments. If NO was generated onthe S2 state PES, one would expect it to appear within a few tensof femtoseconds, and certainly within the duration of the laserpulses. The one-color spectra should therefore show the NOsignal. The fact that we do not observe such a signal indicates alsothat the NO is created on a time scale slower than the laser pulsesand therefore slower than the dissociation time if dissociationwere on the S2 PES. We therefore conclude that both the singlecolor photoelectron spectra and the time-resolved photoelectronspectra show that NO is generated on a time scale longer than thepulse duration and that a dissociation on the repulsive S2 statePES is unlikely.It may be asked why, at long delay times, there is no apparent

photoelectron signal from the alkoxy radical that is also formed inthe decomposition. On the basis of the values for smaller alkoxyradicals,38 the vertical ionization energy of the 1-pentoxy can beestimated to be about 9.2 eV.14 There are several good reasonsfor its absence in our spectra. First, NO has a resonant excitedstate corresponding to the energy of two 414 nm photons,whereas the RO may not have such a resonance. Thus, theionization of NO could be much more facile than that of the1-pentoxy radical. Second, the ionization of NO is possible withthree photons because many of the NO radicals are generated inthe v = 2 vibrational state; in the alkoxy radical, such a boost fromvibrations may not be possible, and possibly, four instead of threephotons would be required, which would imply a weaker signal.Finally, the ionization of RO leads to RO+, which according toour calculations is not a stable species. A geometry optimizationleads to a rearranged valence isomer of RO+ as shown inScheme 1. Its photoelectron signal would therefore be extremelybroad and its intensity diluted and thus not distinguishable fromthe background.3.3. Time-Resolved Mass Spectrometry. The one-color and

two-color photoionization mass spectra of amyl nitrite showfragments at manym/z values. For all ionization wavelengths, thepatterns look qualitatively similar to the one previously published

Figure 3. Time-resolved photoelectron spectrum with binding energy(ionizing photon energy minus the electron kinetic energy) in thevertical axis and delay time in the horizontal axis (a). Temporal evolu-tions of the characteristic NO photoelectron peak at 2.76 eV and thebroad transient from 2.5 to 2.95 eV (b). The symbols in panel b arethe experimental data after deconvolution of the two components, whilethe solid lines are fits as described in the text.



for the 3ω ionization.14 In the time-resolved mass spectrumrecorded with 4ω pump and 2ω probe, the fragments havecharacteristic temporal profiles. Analysis of the ion decomposi-tion patterns is possible with results similar to those of ourprevious investigation with 266 nm excitation.14 Since the focusof the present article is the neutral state dynamics, the reader isreferred to the Supporting Information for a detailed descriptionof the ion-state decomposition patterns of amyl nitrite.Important for the present discussion is that during the

ionization process, the ions are generated with sufficient internalenergy to fragment. This is already evident from the absence ofthe molecular ion in the mass spectrum. Given the large amountof fragmentation, we surmise that several eV are inserted in themolecule during the ionization. This is consistent with thephotoelectron spectra in Figure 2. The rise and decay timesresult from the competition between ion decay after ionization,and the fragmentations upon further excitation of the ion by aprobe pulse photon. Ions born with different internal energieshave different such absorption and decay pathways. Since ourexperiment does not measure these events dispersed by internal

ion energy, we cannot fully separate all those dynamical pro-cesses. We can, however, understand those transients that speci-fically relate to the generation of NO radicals and NO+ ions, aswell as the corresponding RO+ counterions and RO counter-radicals. However, before we discuss this, we need to take a closerlook at the ejection of the NO radicals.3.4. Recoil of the NO Fragments. A close inspection of the

time-resolved mass spectrum in Figure 4 reveals that the peak atm/z 30 (NO+ ion) has a unique shape: at very early delay times,there is an intense transient at the expected mass of NO+, butwithin a hundred femtoseconds, this peak splits into twocomponents that, after a slight initial decay, remain at constantintensity for the remainder of the delay times. Them/z 30 peak isthe only peak in the two-color experiment with this distinctiveappearance. In the previous experiments with 266 nm (4.66 eV)excitation and 400 nm (3.10 eV) probe pulses, we observedno such splitting of the NO peak at positive delay times.14 Thesplitting of the NO peak in the present work can therefore notbe attributed to a process involving the already ionized amylnitrite molecule, i.e., the types of processes that are implicated inthe analysis of the other mass peaks.Instead, we attribute the distinctive appearance of the NO

mass peak to the dynamics of the ejection of the NO radical fromamyl nitrite on a repulsive PES of the neutral molecule. Thisejection creates NO radicals with significant kinetic energy. Sincethe direction of the transition dipole moment of the S0f S2excitation is almost parallel to the RO�NO dissociation co-ordinate9 and since the distribution of the molecules ispolarized,16 many of the NO radicals are born with a velocity

Figure 4. (a) Time-resolvedmass spectrum in the vicinity of theNO fragment at massm/z 30. The splitting of the NOmass peak at positive delay timesarises from the kinetic energy acquired by the NO radicals during the photodissociation. In the contour plot, the 207 and 414 nm one-color spectra aresubtracted from the time-dependent two-color spectrum. (b) The profile of the NO peak in the mass domain at long delay time, averaged from 2 to 5 ps.The dots are the experimental data, while the solid curve is the fit using the model described in the text. (c) The temporal evolutions of the twocomponents of the mass 30 peak: blue, the component of NOmolecules emitted with negligible kinetic energy; green, the component of NOmoleculesemitted with 1 eV of kinetic energy. Both temporal profiles are from the two-dimensional fit in which the two components have time-of-flightdistributions as described in the text. (d) The residual of the two-dimensional fit, i.e., the computed minus experimental (panel a) components. Theresiduals comprise at most about 10% of the total signal, with the largest deviations around time zero.

Scheme 1. Computed Rearrangement of the Alkoxy Cation



component directed toward the mass spectrometer detector.Others are ejected in the opposite direction, and, subsequently,their direction is reversed by the electric acceleration field (9854V/m). The difference in flight times between the two peaks atapparent masses of 29.5 and 30.5 is 143 ns. Fitting the experi-mental time-of-flight spectrum to a polarized generation of NOradicals results in a kinetic energy of Ekin(NO) = 1.0 eV with β =1.3, Figure 4b.39 Details regarding the mathematical procedureare described in the Supporting Information provided with thisarticle. Given the masses of the NO fragment (m/z 30) and the1-pentoxy fragment (m/z 87), one calculates the total kineticenergy release as 1.0 eV � (1 + 30/87) = 1.34 eV.It is instructive to compare our values for the NO kinetic

energy and anisotropy to previously measured values for amylnitrite and other alkyl nitrites, Table 1. In most molecules andmeasurements, kinetic energies between 1.0 and 1.4 eV weremeasured and were found to be in good agreement with ourresults. However, in almost all those measurements, the excita-tion energy was lower than in our experiment. The two measure-ments most comparable to ours are the studies of methyl nitriteby Keller et al.53,54 and that of t-butyl nitrite by Finke et al.,18 bothof which were performed with 193 nm radiation. Our kineticenergy and anisotropy are in reasonably good agreement with thevalues for methyl nitrite by Keller et al. However, in t-butylnitrite, Finke et al. found the total kinetic energy releaseassociated with NO radicals in the v = 2 vibrational state to be0.59( 0.2 eV with β = 0.57( 0.05. Those numbers do not agreewell with ours. While it is unclear if the particular discrepancyis due to the different molecular systems or to experimentalinadequacies of one or the other measurement, part of theexplanation may be that our data can be well modeled using asingle kinetic energy, while Finke et al. modeled their data with awide distribution. Their value of 0.59 eV refers to the maximumof the distribution; their highest kinetic energies were measuredto be about 1 eV, which is in better agreement with our result.In the photolysis, the energy of the photon is partitioned into the

bond dissociation energy, D0, the translational energy of the frag-ments, Etrans, and vibrational and rotational energies, Evib and Erot.

There also can be an electronic excitation of the alkoxy radical,Eel. Conservation of energy requires

Ehν ¼ D0 þ Etrans þ Evib þ Erot þ Eel

According to our calculations, the dissociation energy of amylnitrite is 1.7 eV, which is in excellent agreement with experi-mentally determined values for smaller alkyl nitrites.40,41 Thisvalue is also in reasonable agreement with the value of D0 =14 308 cm�1 or 1.8 eV, reported by Schwartz-Lavi for t-butylnitrite.36 Our experiments discussed here measure the vibrationalenergy content of the NO radicals to be two quanta of 0.236 eVor 0.472 eV. Finke et al. estimate that in t-butyl nitrite, the alkoxyproduct would be vibrationally excited by only a few quanta ofvibrational energy in the C�O stretch vibration of 600 cm�1,which could account for about 0.15 eV (for two quanta ofvibrational energy). Further, the NO rotational distributionwas measured by Finke et al. to peak at J00 = 13.5 or about0.04 eV. Combining those numbers from Finke et al. with ourobserved kinetic energy of 1.34 eV, we thus account for D0 +Etrans + Evib + Erot = 1.7 + 1.34 + (0.47 + 0.15) + 0.04 = 4.2 eV.This leaves us 2.3 eV short of the photon energy, 5.99 eV.Facing a similar energy discrepancy with slightly different

numbers (Ehν = 6.42 eV; Etrans = 0.59 eV), Finke et al. draw onthe previous work by Ebata et al. that showed methoxy andethoxy radicals to fluoresce when created by dissociation of thenitrites.42 By proposing that the butoxy radicals are also born intheir excited electronic states, i.e., by taking Eel = 3.4 eV, Finkeet al. indeed can account for themissing energy. However, tracingback the record of alkoxy radical fluorescence reminds us ofthe systematic study of a series of alkyl nitrites by Ohbayashiet al.,17 who see fluorescence from CH3ONO, C2H5ONO, andC3H7ONO, but specifically note that “No emission was observedwhen t-C4H9ONO was photolyzed.” Therefore, it appears thatFinke’s proposal that the missing energy is in the form of elec-tronic excitation of the alkoxy is in doubt. The same doubt wouldalso be harbored when inspecting the PESs (Figure 1), whichin the larger alkyl nitrites converge to the ground state of thefragments.

Table 1. Comparison of Kinetic Energy Release, Anisotropy Parameters, and Lifetimes of Small Alkyl Nitrites in the S1 State andthe S2 State

S1 state S2 state

alkyl λ (nm) Etrans (eV) β τ (fs) λ (nm) Etrans (eV) β τ (fs)

Me53,54 350 1.0 �0.70 193, 248 1.7�2.0 1.40

Me55 248 1.7

Me13 351 125 ( 50

Me56 199, 205 25 ( 15

Et57 347 0.7 �0.70

Et54 248 1.4

i-Pr54 248 1.2

t-Bu54 248 1.3

t-Bu58 248�250 1.4 ∼1

t-Bu18 193 0.59 0.57

n-Bu13 351 125 ( 20

i-Bu13 351 130 ( 20

i-Am13 351 115 ( 30

n-Ama 207 1.34 1.3 283 (16)aThis work.



The missing energy is readily found in the model proposed byus: energy is converted from electronic to vibrational degrees offreedom during the electronic relaxation from the S2 to the S1state. With a photon energy of 5.99 eV and the vertical S1state energy of 3.6 eV, 2.4 eV of energy must be converted tovibrational energy. This energy will remain in the fragmentsafter dissociation. Since we specifically observe NO in its v = 2vibrational state, we conclude that the alkoxy fragment mustretain much more energy than a few quanta of CO vibration aspostulated by Finke et al. Replacing the vibrational energies of0.15 and 0.472 eV by the 2.4 eV of energy inserted into vibrationsduring the energy redistribution process accounts for a totalenergy of D0 + Etrans + Evib + Erot = 1.7 + 1.34 + 2.4 + 0.04 =5.48 eV. This value is close to the photon energy of 5.99 eV,supporting the notion that internal conversion from the S2 to theS1 state precedes the dissociation.3.5. Dynamics of NO Ejection. The NO radicals can be

released from amyl nitrite at several points in our experiment. AsFigure 4a shows, radicals that are created near time zero emergewithout much kinetic energy and are therefore observed at theexact mass, while those created at positive delay times are ejectedwith a large kinetic energy and observed as the split mass peak.Our experiment measures the time-of flight distributions at themass spectrometer detector. To model the time dependence ofNO ejection, we fit the split time-of-flight distribution at delaytimes larger than 2 ps to the kinetic energy release model de-scribed above. The NO molecules released without kineticenergy are modeled using a time-of-flight distribution with awidth taken from the m/z 29 peak, centered at the mass of NO.By keeping these two distributions constant and multiplyingthem with the exponentially rising and decaying time evolutionfunctions, we can now obtain the optimal fits in the two dimen-sions of the mass and the time delay between the pump and theprobe pulse, Figure 4c. In the fit, we only vary the time constantsof the rise and decay times. The 2-dimensional residuals, plottedin Figure 4d, are very small for most of the contour plot, butreach about 10% of the total signal around zero delay time. Thissuggests that the simple model captures most, but not all, of theNO release. Especially right around time zero, other processesappear to contribute a small amount of signal. The fits to the timedependence results in a rise of the high kinetic energy NO signalwith a rise time of 66 fs. The low kinetic energy NO signal isdescribed similar to the other mass peaks, with double exponen-tial decays of 86 fs and 1.7 ps, respectively.

4. DISCUSSION

The time-resolved multiphoton ionization photoelectron ex-periments provide us with a host of experimental results that,combined, provide a detailed view of the dissociation dynamics ofamyl nitrite. To summarize, we have the following experimentalobservations that wewish to reconcile into a self-consistentmodel:(1) The one-color multiphoton ionization photoelectron

spectra show that much energy is inserted into vibrationalmodes during the ionization, consistent only with a rapidinternal conversion from the S2 to S1 state during theionization process.

(2) The time-resolved photoelectron spectrum shows evidencefor an intermediate state with a 283 fs lifetime and that NOradicals are created on a time scale that matches this decay.

(3) The time-resolved mass spectra are dominated by frag-ment dynamics that happens on the PES of the ion state.

The time constant associated with the transient signal ofthe RO+ (m/z 87) is especially fast, showing a decay timeof 380 fs.

(4) The dynamics of the NO+ (m/z 30) ions is particularlycomplex. Some NO species are ejected with 1.0 eV ofkinetic energy and are seen rising with a time constantof 66 fs. Other NO ions are generated with a negligibleamount of kinetic energy and with a time dependence thatis more similar to those of the other ions (1.7 ps).

To discuss these observations, we turn to Scheme 2, whichsummarizes the excitation and dissociation pathways. We pro-pose that amyl nitrite is excited by the 207 nm pulse into the S2band but finds itself on the S1 state PES from where it predis-sociates with a decay time τ1. The dissociation creates RO 3 andhigh kinetic energy (1.0 eV) NO 3 radicals. The latter can beionized by the probe pulse in a three-photon process via the C2Σ+

state to generate NO+ ions and electrons. The lifetime of the S1state, τ1= 283 fs, is observed as the decay of the photoelectronsignal of the transient amyl nitrite molecule and in the rise of theNO photoelectrons. The observation of a faster, 66 fs, rise of thehigh kinetic energy NO+ ions does not match the 283 fs timescale. We will return to this discrepancy later.

The RO+ and NO+ ions that are created via the RONO•+

pathway are generated either in the fragmentation on the ion

Scheme 2. Proposed Scheme of the Photodissociation andPhotoionization of Amyl Nitrite When Exciting with 4ωPulses and Probing with 2ω Pulsesa

aThe excitation at 207 nm (4ω) produces an excited state that rapidlyconverts to S1. The predissociation with a time constant τ1 = 283 fsproduces NO 3 and pentoxy radicals, RO 3 . The time constant τ1 isobserved as a rise in the NO photoelectron signal and the decay of thephotoelectron signal from amyl nitrite out of S1 (red fonts). The amylnitrite ions RONO•+ decay with a time constant τ2, which is observed asthe decay of the RO+ and NO+ ions (blue fonts), but which can varydepending on the particular ion generated.



ground state PES, which should give a rise with increasing probedelay, or upon further excitation of the ion followed by fragmen-tation, which should give a decay. The relative signal strengthsdepend on the probability of generating those fragments com-pared to pathways that generate other ion fragments. We findthat the pathway via the electronically excited (RONO•+)* domi-nates so that both the RO+ and the NO+ signal are observed with adecaying time dependence (380 fs and 1.7 ps, respectively).

Themodel in Scheme 2 accounts for the complexities of all theobserved phenomena. However, there is an apparent inconsis-tency in the time constants: from the photoelectron signals ofNO and amyl nitrite in the S1 state, we find a time constant ofτ1 = 283 fs., but the ion signal of the fast NO

+ rises with 66 fs. Asan explanation, we note that NO+ is generated in several ways. Thesignal of NO+ around time zero is a complex superposition ofseveral contributions, some of which are still poorly understood.Of particular note is the generation of NO+ after absorption of aprobe photon by the RONO•+ species generated from directionization with the pump pulse: it would be reasonable to assumethat the dissociation into RO 3 and NO+ also involves motionsalong a repulsive excited ion state PES. This process could,therefore, also generate NO+ with high kinetic energy, a processthat would be particularly intense right around time zero. Ourexperiments cannot completely disentangle the effects of such apathway from the ones on the S1 state PES. Amodel that assumesthat the NO+ ions generated through this ion pathway are alsogenerated with 1.0 eV of kinetic energy and with an exponentialdecay of 257 fs results in a residual contour plot that are similar tothe one of Figure 4d but with greatly reduced amplitude: errorsin the range of (1.5% of the total signal are now obtained, buteven while this suggests that this more elaborate model canaccount for almost the entire signal quantitatively, we do notthink that we have enough experimental evidence to affirm thisparticular pathway and those parameters. This exercise doesshow, though, that inclusion of an ion pathway can greatlyimprove the fits of our data and resolve the discrepancy in thetime constants. We conclude that the time-resolvedNO+ signal isnot a good observable to determine the reaction dynamics, andthat our best experimental value for the predissociation out ofthe S1 state is τ1 = 283 fs.4.1. Electronic Relaxation. Alkyl nitrites have repulsive

electronic PESs along the S1 and S2 states on which RO�NOdissociation can happen. The preponderance of evidence pre-sented here shows that even when excited in the absorptionregion of the S2 state, amyl nitrite reacts on the S1 PES: thephotoelectron spectra show the signature of electronic relaxationbefore the molecule breaks apart; the photoelectron spectra ofthe NO fragment show a delayed ejection; and the translational,kinetic energy of the fragments match a path along the S1 PES. Itis clear, then, that even though the dissociation on the PES of theS2 state would presumably be very rapid indeed, the moleculeinstead converts to the S1 state.The framework for understanding electronic relaxation phe-

nomena is well established43�45 and can inform the discussionsabout the dissociation pathway of amyl nitrite. Built upon thepredissociative PES of the S1 state are a dense multitude ofvibrational states. Given the short lifetime of the S1 state and theabundance of low frequency vibrations in the conformationallyflexible amyl nitrite, the molecule is in the statistical limit, whereit is impossible to spectrally separate individual molecular eigen-states. At the energy corresponding to the S2 absorption band,there is a dense set of vibrational states belonging to vibrations of

the S1 state. The S2 state itself has vibrational states, all of whichcouple to the vibrational states of the lower electronic PES. Themolecular eigenstates, which are spectrally unresolvable due totheir high density, are therefore all of mixed character, i.e., theyhave a component that is of the S2 state parentage and a com-ponent that is of the S1 state parentage. The S2 state componentprovides the oscillator strength from the electronic ground state,but given the high density of states with S1 state parentage, the S2state contribution is diluted over a large number of states; each ofthe states therefore has a small component only from the S2 state.The nature of the state that is prepared by optical excitation

depends on the coherence properties of the exciting radiation.A true S2 state is excited only with a laser pulse that has a coherencebandwidth that is wide enough to cover all the eigenstatesthat have a component of a particular S2 vibronic state. In stablemolecules, the dependence of the electronic excitation on thecoherence properties of the exciting radiation has been well esta-blished.46�48 In amyl nitrite, where evidently there is a strongcoupling between the S2 and S1 states, the spectral width requiredto create a true S2 state is probably very large. The peak in the UVabsorption spectrum that corresponds to the S2 state has a fwhmof 51 nm or 10600 cm�1,which would correspond to an S2 statelifetime of 0.5 fs. This is a lower bound since most likely part ofthe absorption envelope is due to the Franck�Condon envelopeof the vibronic transitions. In comparison, the absorption to theS1 state has a width of about 3000 cm�1. Even if we were tostrip such a contribution from the envelope of the transition tothe S2 state, it would still lead us to conclude a lifetime in thefemtosecond regime.The dominance of the internal conversion from the S2 to the

S1 state can now be seen to result from an optical excitation thatfails to coherently couple enough eigenstates to prepare a statethat resembles the S2 state. This is even though broadbandfemtosecond pulses were used in our experiment. The excitationtherefore results in a state that never has a true S2 state identity.Rather, one prepares the molecule in a set of states that, from theoutset, has largely S1 state character. Not surprisingly, this set ofstates quickly relaxes in the bath of other vibrational states of theS1 state, in particular the vibrations of the flexible alkyl chain.Consequently, all observations find that the dissociation of amylnitrite proceeds on the S1 state PES.

5. PERSPECTIVES

It is a commonly held misconception that if a molecule isexcited with a wavelength that matches the optical absorption toa particular state, that state is actually prepared and one actuallyfollows the dynamics of that state. Using a combination of photo-electron and mass spectroscopic techniques with ultrashort laserpulses, we have shown that the photoinduced cleavage of NOfrom amyl nitrite takes place on the S1 state PES even whenexcited with a 207 nm laser pulse into the S2 absorption band.All evidence points to the molecule residing in the S1 state beforethe fragmentation, which proceeds by ejecting NO radicals on arelatively slow time scale of 283 fs.

Our experiments have focused on amyl nitrite, and the opticalexcitation was at only one particular wavelength. There arecharacteristic differences in the behavior of smaller nitrites. Speci-fically, smaller alkoxy nitrites fluoresce upon such excitation.It would be interesting and important to systematically explorethose systems with the ultrafast time-resolved methodologyapplied here.



Finally, the present work demonstrates yet again how sensitivethe molecular dynamics can be to the exact electronic excitation.In recent studies, we have shown that the dynamics of moleculesexcited to high-lying electronic states can be very different fromthat of the lower states.49�52 In the present case, we find theopposite: even though the dissociation on the S2 state wasthought to be very fast, the reaction dynamics is, in the end,quite the same as that on the lower excited state PES. It can besurmised that this may be a result of the strongmixing on accountof theHOMO� 1 nature of the excitation to the S2 state. Furtherexplorations will seek to address this point.

’ASSOCIATED CONTENT

bS Supporting Information. Detailed description of theion-state decomposition patterns of amyl nitrite as well as athorough explanation behind the mathematical procedure usedto fit the experimental time-of-flight spectrum of NO. Thismaterial is available free of charge via the Internet at http://pubs.acs.org.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

’ACKNOWLEDGMENT

The Brown part of the project was supported by the Divisionof Chemical Sciences, Geosciences, and Biosciences, the Officeof Basic Energy Sciences, the U.S. Department of Energy byGrant No. DE-FG02-03ER15452.

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’NOTE ADDED AFTER ASAP PUBLICATION

This article was published ASAP on January 7, 2012, with minortext errors in section 3.4. The correct version was reposted onJanuary 19, 2012.

far-uv photochemical bond cleavage of n-amyl nitrite ... december 17, 2011 r 2011 american chemical...

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