Time-dependent pump-probe spectra of NeBr 2Jose A. Cabrera, Craig R. Bieler, Benjamin C. Olbricht, Wytze E. van der Veer, and Kenneth C. Janda

Citation: The Journal of Chemical Physics 123, 054311 (2005); doi: 10.1063/1.1990118 View online: http://dx.doi.org/10.1063/1.1990118 View Table of Contents: http://scitation.aip.org/content/aip/journal/jcp/123/5?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Real-time dissociation dynamics of the Ne 2 Br 2 van der Waals complex J. Chem. Phys. 133, 014305 (2010); 10.1063/1.3456550 Time and frequency resolved dynamics of Ar Br 2 J. Chem. Phys. 127, 164309 (2007); 10.1063/1.2794332 Photodissociation of NeBr 2 (B) below and above the dissociation limit of Br 2 (B) J. Chem. Phys. 115, 2566 (2001); 10.1063/1.1386648 Quantum calculations on the vibrational predissociation of NeBr 2 : Evidence for continuum resonances J. Chem. Phys. 112, 2265 (2000); 10.1063/1.480791 Intramolecular vibrational redistribution and fragmentation dynamics of I 2 Ne n (n=2–6) clusters J. Chem. Phys. 111, 239 (1999); 10.1063/1.479269

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Time-dependent pump-probe spectra of NeBr2

Jose A. Cabrera,a� Craig R. Bieler,b� Benjamin C. Olbricht,Wytze E. van der Veer, and Kenneth C. JandaDepartment of Chemistry and Institute of Surface and Interface Science, University of California,Irvine, California 92697-2025

�Received 24 March 2005; accepted 8 June 2005; published online 9 August 2005�

Time- and frequency-resolved pump-probe measurements on NeBr2 have been performed to bettercharacterize its fragmentation dynamics on the B electronic state for vibrational levels in the energyregion of the transition from direct vibrational predissociation to intramolecular vibrationalrelaxation dynamics. Above ��=20 of the Br2 stretching mode, it was observed that the dependenceof lifetime on the vibrational quantum number deviates from the energy-gap law by leveling off inthe range of 10 ps���20 ps. In addition to measuring the appearance of Br2 product state levels,we were able to monitor the decay of the initially excited NeBr2 via B→E transitions of thecomplex. These transitions are shifted 20 cm−1 to lower energy from the free Br2 resonances,indicating an E state Ne–Br2 bond energy of 82 cm−1. Measurements of NeBr2 vibrationalpredissociation via the ��=−2 channel were also performed for ��=27, 28, and 29. The closing ofthe ��=−1 channel leads to an increase in the lifetimes of these vibrational levels. A newNd:yttrium aluminum garnet pumped dual optical parametric oscillator/optical parametric amplifiersystem is described that allows us to conveniently record time-delayed pump-probe spectra with2-cm−1 spectral resolution and 15-ps time resolution. © 2005 American Institute of Physics.�DOI: 10.1063/1.1990118�

I. INTRODUCTION

Since the first observation of vibrational predissociationin the HeI2 molecule by Smalley et al.,1 the series of noblegas-halogen molecules has become a model system forstudying the dynamics of small- and medium-sized clusters.2

Although these molecules exude apparent simplicity, a widevariety of phenomena can be studied within the series, andnew complications continue to arise. The present study ismotivated by recent theoretical indications that intramolecu-lar vibrational relaxation �IVR� may occur even for such asimple process as ��=−1 vibrational predissociation ofNeBr2.3–5 This paper reports direct, real time measurementsof the evolution of the pump-probe spectra of NeBr2 to in-vestigate the appearance of these IVR processes.

That IVR could occur for a triatomic van der Waals mol-ecule was first demonstrated for the ��=−2 dissociation ofArCl2 in the electronic state formed by attaching the Ar atomto the B electronic state of Cl2.6 �For convenience, this stateis often referred to as the B electronic state of ArCl2, and wewill also employ this shorthand.� The experimental evidencefor this phenomenon was a highly structured Cl2 productrotational distribution that suggested the involvement of spe-cific resonances in the dissociation mechanism. This inter-pretation was later confirmed by theoretical work thatshowed that highly excited vibrational levels in theCl2 ��=−1 manifold of states serves as the doorway be-tween the initially excited zero-order bright state and the��=−2 dissociative continuum.7 Evidence for IVR in the

��=−1 regime for rare-gas-halogen van der Waals mol-ecules was found for the highest Br2 vibrational levels ofHeBr2. Theory and experiment suggest that ��=−1 dissocia-tion proceeds via an IVR mechanism in which the doorwaystates are van der Waals vibrational resonances in the con-tinuum of the Br2 ��=−1 channel of the initially excitedstate.8 That is, a quantum of the Br2 stretching motion firstcouples to a long-lived van der Waals vibrational resonance,presumably involving bending or internal rotational motion,above the dissociation limit for that channel before the mol-ecule dissociates. The predictions of the theory are consistentwith the observed line shapes and rotational distributions, butthe data are not distinctive enough to be completely defini-tive. Direct excitation to analogous metastable resonanceshas recently been reported for HeICl.9

More recently, theoretical work on NeBr2 suggests thatintermediate resonances are important for the dissociationdynamics for vibrational levels well below the closing of the��=−1 channel. Stephenson and Halberstadt calculated non-Lorentzian excitation line shapes for Br2 vibrational levelsabove ��=20.3 �The ��=−1 channel closes at ��=28.10�They also predict that the product rotational distribution ofthe Br2 fragment will change for different excitation energieswithin a single vibronic band due to a changing mixture ofbright and dark state contributions to the excited-state wavefunction within the resonance width. The calculations alsopredict that there is a significant Br2 isotope effect on boththe excitation line shapes and the rotational product statedistributions. Each of these phenomena suggests an IVRmechanism analogous to the one calculated for high vibra-tional levels of HeBr2,8 but for levels 20 quanta lower in theBr2 stretch manifold. Miguel et al.4 performed hybrid

a�Electronic mail: jcabrera@uci.edub�Present address: Department of Chemistry, Albion College, Albion, MI.

THE JOURNAL OF CHEMICAL PHYSICS 123, 054311 �2005�

0021-9606/2005/123�5�/054311/8/$22.50 © 2005 American Institute of Physics123, 054311-1

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quantum/classical calculations that allow the dissociation dy-namics to be classified as being due to two distinct mecha-nisms: direct vibrational predissociation �VP� and intramo-lecular vibrational relaxation followed by evaporativecooling �IVR-EC�. Their calculations suggest that for ��=10, the ��=−1 dissociation of NeBr2 proceeds completelyby direct VP; for ��=20, the ��=−1 channel is 65% due todirect VP and 35% due to IVR-EC; and for ��=27 all of the��=−1 dissociation occurs via IVR-EC. Interestingly, belowthe closing of the ��=−1 threshold, all dissociation that pro-ceeds with loss of more than one Br2 quantum is due to thedirect VP mechanism. Each of these predictions is based onthe assumption of a very simple atom-atom potential to de-scribe the B electronic state of NeBr2.

Recently, the NeBr2 IVR dynamics were studied theo-retically from the region of the closing of the ��=−1 chan-nel to the Br2 B state dissociation limit.5 The calculated spec-tra become increasingly congested with higher energy, butvery narrow resonances appear near the dissociation limit.These narrow resonances were interpreted as “horse-shoetype” resonances, which are named after the shape of theassociated periodic orbits.11 A classical study12 of these reso-nances showed that they are indeed due to periodic orbits ofthe horse-shoe type. Another interesting development in rare-gas-halogen spectroscopy is the direct probing of linear con-formers of several species. Loomis13 has recently observed alinear conformer of NeBr2, confirming predictions by ab ini-tio calculations.14 Although the dynamics near the Br2 disso-ciation limit and of the linear isomer are also of great inter-est, we concentrate here on the T-shaped isomer near the topof the ��=−1 dissociation regime. We did not search for, norinadvertently locate, resonances due to linear isomers.

The present studies are made possible by a new opticalparametric oscillator �OPO� system with a 15-ps pulse dura-tion �full width at half maximum �FWHM�� and a spectralbandwidth less than 2 cm−1. The time resolution of our ex-periments is similar to those reported by Gutmann et al.15 forHeI2 and NeI2, but the design of our laser allows the entirepump or probe spectrum to be recorded in the frequencydomain as a function of the delay between the two pulses.The temporal and frequency resolution, as well as the con-venient wavelength tunability gives us increased ability toinvestigate the time development of the pump-probe spectraand the ability to directly observe the appearance and disap-pearance of NeBr2 excited states. In this paper we reportpump-probe measurements of the NeBr2 B state dynamicsfor ��=16 to ��=29.

II. EXPERIMENTAL SECTION

The pump-probe method used to study the dissociationdynamics of the NeBr2 complex is analogous to that firstapplied to rare-gas-halogen van der Waals molecules bySkene and Lester.16 Briefly, the NeBr2 complex is created ina pulsed free jet expansion and is excited by a “pump” laserto the desired Br2 stretching vibrational level ���� associatedwith the B�3�0u

+� state of the Br2. A “probe” laser then ex-cites either the NeBr2 B state or its free Br2 dissociationproducts to the E�0g

+� state. Fluorescence from the E state is

detected as the experimental signal. This scheme allows awide variety of phenomena to be observed, as illustrated inFig. 1.

In these experiments, a picosecond laser is employed toexplore the time evolution of the dynamics that proceed onthe B electronic surface. The time-resolved pump-probetechnique allows for three types of spectra to be recorded. Torecord the excitation spectrum of the complex, the pumplaser is scanned while the probe laser is fixed on a specificBr2 B→E transition. To determine the product state distribu-tions for a particular excitation of the complex, the probelaser is scanned while the pump laser is positioned on aNeBr2 X→B transition. Time-dependent studies can be per-formed by fixing both pump and probe lasers on appropriatetransitions and adjusting the time delay between the pulses.These experiments reveal the lifetimes of the NeBr2 statesand the rates for forming different product channels.

The apparatus used in these studies has been describedpreviously.17 For these studies, a neon/helium/bromine gasmixture is obtained by passing a 20% neon, 80% heliummixture at 30 bars over a natural isotopic mixture of bromineheld at −20 °C in a stainless-steel trap. This maintains theBr2 partial pressure at about 20 mbars. The gas mixture isexpanded through a 150-�m-diameter pulsed-solenoid valveinto the vacuum chamber.

The tunable picosecond laser pulses are generated by alaser system described below. The variable time delay be-

FIG. 1. A schematic of the various transitions and dynamical processesinvolved in these experiments. The wells on the left side of the figure rep-resent the van der Waals Ne–Br2 bonds, and the flat part of the curves on theright side of the figure represents the energy of free Br2 states as labeled.The vertical arrows represent transitions employed in Fig. 4: Br2 doubleresonance, transition �d� followed by transitions �e� or �g�; NeBr2 doubleresonance, transition �a� followed by transitions �b� or �c�; or pump-probespectroscopy, transition �a� followed by transitions �h� or �i�. Also shown arethe arrows marked VP to schematically denote the direct vibrational disso-ciation process and IVR-EC to denote intramolecular vibrational relaxationof NeBr2 followed by evaporative cooling. To avoid congestion not all fea-tures observed in Fig. 4 are illustrated.

054311-2 Cabrera et al. J. Chem. Phys. 123, 054311 �2005�

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tween the pump and probe pulses is achieved by sending theprobe laser through a delay line comprised of a prismmounted on a computer-controlled linear translation stage.The stage has an accuracy of 5 �m. Typically a 1-mm stepsize between data points is used corresponding to a temporaldelay of 6.7 ps. The laser pulses are combined by a dichroicmirror and pass collinearly through a baffle arm into thevacuum chamber. The beams intersect the free jet approxi-mately 1.0 cm downstream from the nozzle orifice. Typicalfree jet rotational temperatures of 4 K were achieved, asmeasured from the bandwidth of the Br2 spectrum.

The beams of independently tunable laser radiation weregenerated with a synchronously pumped picosecond OPO-optical parametric amplifier �OPA� system illustrated in Fig.2.18,19 A mode-locked Ekspla PL2143B Nd:yttrium alumi-num garnet �YAG� laser produces 15-ps pulses at 1064 nmwith a repetition rate of 10 Hz. Light is “leaked’ from oneend of the cavity forming a train of 10 pulses. This pulsetrain is amplified and the third harmonic, 355 nm, is gener-ated using two KD2PO4, deuterated potassium dihydrogenphosphate �DKDP�, crystals. The UV pulse train synchro-nously pumps two OPO’s. A single pulse of the third har-monic from the Nd:YAG laser pumps two OPA’s. The cavityof each OPO contains two type-I �-barium borate �BBO�crystals which are placed in a walk-off compensating ar-rangement. Each cavity is terminated on one side by a flatmirror, and on the other side with a grazing incidence gratingand tuning mirror assembly. The tuning mirror directs thediffracted first-order reflection back into the cavity via thegrating. Light is coupled out of the oscillator via the zeroth-order reflection of the grating, and is directed via two mirrorsinto the OPA.

Each OPA employs a single BBO crystal wherein thepump pulse is merged with the oscillator beam. Each OPO/OPA independently produces 15-ps pulses whose wavelengthcan be adjusted from 410 to 2000 nm by choosing the signalor idler wave. The visible output pulse energy of each systemcan reach 4.5 mJ depending on the pump energy and thegenerated wavelength. If UV radiation is required, UV pulseenergies of up to 1 mJ can be obtained with a potassiumdihydrogen phosphate �KDP� or a BBO second-harmonicgenerator. Tuning each laser requires the angular adjustmentof three BBO crystals and the tuning mirror. In addition theangle of the second-harmonic generation �SHG� crystal mustbe controlled. Thus, nine angles need to be continuously ad-

justed for the experiments described below. This is achievedvia computer-controlled stepper motors. A software programwritten in LABVIEW builds a calibration table for the positionof each component versus the wavelength. Fine tuning of thepositions for each wavelength is obtained through a polyno-mial interpolation.

The duration of the Nd:YAG pulses prior to third-harmonic generation is measured with a single-shot autocor-relator. The round beam from the Nd:YAG laser is elongatedby a cylindrical telescope and splits into two parts which areoverlapped on a DKDP crystal. Wave mixing will only takeplace where the incident beams overlap both in space andtime. The volume where harmonic generation effectivelytakes place is thus defined by the duration of the pulse. Thewidth of this volume is measured by imaging the resulting532-nm radiation with a cylindrical lens on a charge-coupleddevice �CCD� diode array. The line camera can thus recordthe correlation profile of each consecutive shot from thepump laser. The Gaussian fit of the recorded signal yields apulse width, FWHM, of 15 ps. If no saturation takes place,the pulse will be shortened by third-harmonic generation bya factor �3. Considering the high pulse intensities in thislaser system, �up to 5 GW/cm2 at the OPA crystal� somesaturation is expected and an upper limit of 15 ps is expectedfor the duration of the third-harmonic generation �THG�pulse.

III. RESULTS

The experiments reported here can be classified intothree types depending on whether the pump laser wave-length, the probe-laser wavelength, or the time delay is ad-justed during the experiment. Figure 3 illustrates the case ofexcitation spectra obtained by detecting the Br2 , B state, ��=15 level with short �top� and long �bottom� delays betweenthe two pulses. The intensity of the Br2 double-resonancepeak, analogous to transition �d� followed by transition �e� inFig. 1, is about the same in the two spectra since the radia-tive lifetime of free Br2 is much longer than the delay. Incontrast, the intensity of the peak due to excitation of NeBr2,to ��=16, analogous to transition �a� followed by transition�h� in Fig. 1, is much more intense in the long delay spec-trum since the lifetime for NeBr2 vibrational predissociationis 84±8 ps. Also, note that the spectral resolution of the lasersystem is such that the probe laser selects only the 79Br2isotopomer, greatly simplifying the excitation spectrum. TheFWHM of the Br2 excitation peak is 3.0 cm−1 due to a con-volution of the laser bandwidth with the 4-K Br2 rotationaldistribution. Finally, we note that in the spectra recorded forthis study, there were no features observed that can be as-signed to a linear NeBr2 isomer. This is probably due to ourrelatively high cluster temperature.

Figure 4 shows four examples of probe-laser scans withvarying delay times between the pump and probe pulses. Thepump laser is set to excite Ne79Br2 to the ��=22 level, tran-sition �a� of Fig. 1, and the probe laser is tuned over a portionof the B→E spectrum. For t=−100 ps �the negative signindicates that the probe pulse occurs before the pump pulse�no transitions are observed, as expected. For t=0 ps, the two

FIG. 2. Overview schematic of the OPO/OPA system. The components andlight path are described in the text.

054311-3 Spectra of NeBr2 J. Chem. Phys. 123, 054311 �2005�

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lasers are coincident and transitions originating from NeBr2

��=22 double resonance, transitions �b� and �c� of Fig. 1,and from Br2, ��=21, 20, and 19 ���=−1, −2, and −3� prod-uct states are observed. For clarity, Fig. 1 only shows thedetection of ��=21, transitions �h� and �i�. Probing ��=20and 19 employs analogous transitions starting at these lower

vibrational levels. The lifetime of the NeBr2 ��=22 level is19 ps. So, significant product state population already ap-pears during the time the two laser pulses are coincident.However, as the pump-probe delay is increased, the NeBr2

double resonance disappears and the product state transitionsbecome more intense. This gives information to characterizethe appearance and disappearance rates of the double-resonance level and appearance rate for the free Br2 prod-ucts.

The bond energy between Ne and Br2 in the observedvibrational level of the E state can be extracted from datasuch as that shown in Fig. 4. The E state bond energy isequal to the B state bond energy plus the redshift of theNeBr2 B→E transition from that of free Br2. The B statebond energies have been previously determined.10 Thisanalysis yields a bond energy value of 82±3 cm−1 for thebond between Ne and Br2 in the �=2 level of the E state.The error bar on this value is due to the uncertainty in theband origin measurement using a picosecond light source.

Figure 5 shows a typical time-delay study, in this casefor the NeBr2 ��=16 level. Figure 5�a� shows the Br2

double-resonance signal, which is used to extract both the t=0 calibration and a Gaussian fit to the pump-probe autocor-relation width, 35 ps. Figure 5�b� shows NeBr2 , B state, ��=16 double-resonance intensity along with a fit to extract thelifetime of the level, 74±8 ps. Figure 5�c� shows the appear-ance of Br2 ��=15 product intensity and a fit to the data thatyields a NeBr2 ��=16 lifetime of 84±8 ps. The two valuesagree within the error limits, which are discussed in moredetail below.

Since the extraction of lifetimes is an important part of

FIG. 3. Excitation spectra with the probe laser positioned on the Br2, B,��=15→E, �=1 transition with �a� a 13-ps delay between the pump and theprobe pulses and �b� a 700-ps delay. The intense peak in each scan is due toBr2 double resonance. In �a� the weaker Br2 product state signal is juststarting to appear while in �b� it has had time to grow to its maximumintensity.

FIG. 4. Probe spectra with the pump laser positioned on the Ne79Br2 X,��=0→B, ��=22 transition for several values of the pump-probe delay. Allbut one of the probe transitions are well resolved and unambiguously as-signed as labeled. The exception is the probe transition at 32 092 cm−1

which is mainly due to NeBr2 double resonance at t=0 and product Br2 fort=27 ps. The �� values refer to the Br2 stretching level in the B state. See thetext for more details.

FIG. 5. Three time-delay scans for NeBr2, B state, ��=16. �a� Free Br2 B,��=16 double resonance and laser autocorrelation curve. The width of thelaser autocorrelation curve is 35-ps FWHM. �b� Appearance and disappear-ance of NeBr2 in the B state, ��=16, along with the best-fit curve, �=74 ps. �c� Appearance of Br2, B state, ��=15 product from vibrationalpredissociation of the NeBr2, B state, ��=16, along with the best-fit curve,�=84 ps.

054311-4 Cabrera et al. J. Chem. Phys. 123, 054311 �2005�

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this work, the procedure employed will be described in somedetail. Product appearance data, such as the bottom curve ofFig. 5, are fitted to the function,

Pf�ti� = S0 + A�−�

ti

CC�t��1 − exp�− �ti − t�/���dt , �1�

where Pf is the fitting function, S0 is the base line of the data,A is an amplitude scaling parameter, � is the lifetime to beextracted, and CC�t� is the laser cross-correlation curve, as-sumed to be Gaussian, as discussed above, and obtained byfitting to Br2 double-resonance data. Equation �1� is leastsquares fitted to the data with a manual parameter searchusing the least-squares criterion for goodness of fit. Thefunction for fitting the double-resonance delay curves issimilar,

DRfit�ti� = S0 + A�−�

ti

CC�t��exp�− �ti − t�/���dt . �2�

Two types of error analysis were performed on the life-time data. First, for each data set the error bars for � weredetermined by changing � from the best-fit value and reiter-ating the other parameters. When a value for � was foundthat did not produce a satisfactory fit, that value was assignedas one of the error bars. To test the validity of this procedure,a statistical analysis was performed for NeBr2 ��=16. Sixindependent data sets for product state appearance were in-dividually fitted. This procedure resulted in a lifetime of82±8 ps, in agreement with both the absolute value and withthe error bars from the more visual analysis described above.The error bars on product appearance data are generally nar-rower than those on the double-resonance data. Product ap-pearance data collected before the probe-laser pulse deter-mine the base line and the data collected a long time after theprobe-laser pulse determines the intensity-scaling factor. Thedata that represent the change between these limiting inten-

sities determine the value of �. In contrast, double-resonancedata start and end on the base line, so both the intensity-scaling factor and � must be extracted from the intermediatetime data and the two parameters are correlated.

For NeBr2 ��=16 a study was also performed to measurethe isotope dependence of the product appearance lifetime.Enough data sets were recorded to use the statistical erroranalysis. The values obtained are Ne79,79Br2, �=82±8 ps;Ne79,81Br2, �=92±8 ps; and Ne81,81Br2, �=89±8 ps. Thedifferences between these values are at the edges of the errorbars, just large enough to be tantalizing.

Data similar to that illustrated above were obtained forNe79Br2, 16��29. Table I gives the lifetimes from bothdouble resonance and product appearance curves for all lev-els that were studied and compares the values to lifetimespreviously extracted from linewidth data20,21 and theory.3 Alldata were fitted to single exponential functions as describedabove. This is a good assumption for the longer-lived states,while for the shorter-lived states there is not sufficient timeresolution to distinguish between single and multiple expo-nential dynamics. For several higher levels, spectral conges-tion made it impossible to measure double-resonance appear-ance and decay due to the relative weakness of these signals.For ��=26 and 27, the double-resonance signal was not ob-served even though there was no significant interferencefrom spectral congestion.

Limits on the lifetimes can be estimated from the inten-sity of the NeBr2 double-resonance signal. As the lifetimegoes down, the peak intensity of the double-resonance signalgets weaker. We performed model calculations to estimatethe magnitude of this effect. Taking the double-resonanceintensity for �=100 ps as the standard, the calculated peakintensity drops to 50% for �=20 ps, and 20% for �=5 ps.Unfortunately, as shown in Fig. 4, the double-resonance sig-nals are not particularly strong even for transitions to states

TABLE I. Vibrational predissociation lifetimes for Ne79Br2, B state.

�

Appearance ofBr2 product

� �ps�

Disappearanceof Ne79Br2

� �ps�

Linewidthmeasurements

� �ps� Theorya � �ps� Br2 �cm−1�

16 84±8 74±8 95±9b 71 108.118 53±7 47±8 99.619 50±10 41±8 95.320 40±9 28±5 35±2b 26 90.921 25±6 21±6 86.522 17±3 19±7 7±3c 82.023 14±4 d 77.524 15±4 16±6 6±1c 72.925 10±3 d 5±1c 68.226 15±3 �10 2.2±0.5c 63.527 10±3 �10 1.7±0.3c 58.827e 19±328e 22±529e 15±4

aFrom Ref. 3.bFrom Ref. 20.cFrom Ref. 21.dNeBr2 double resonance was not observed due to spectral congestion.eRefers to probing of the ��=−2 NeBr2 dissociation channel.

054311-5 Spectra of NeBr2 J. Chem. Phys. 123, 054311 �2005�

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with ��20 ps, probably due to small Franck-Condon factorsfor the van der Waals modes. For ��=26 and 27, we expectedto see double resonance, but did not. We believe that this isdue to the short NeBr2 excited-state lifetime for these levelsand we assign a lifetime of less than 10 ps in each case.Although we also did not observe any double-resonancetransitions for ��=25, 28, and 29, we did not assign shortlifetimes in these cases because the transitions may havebeen obscured by spectral congestion.

IV. DISCUSSION

New data are reported for the dissociation rates of theNeBr2 B electronic state vibrational levels ranging from ��=16, which is expected to decay via direct vibrational pre-dissociation; through the range of 22����27, for which thedynamics is expected to be dominated by intramolecular vi-brational relaxation; to ��=28 and 29 for which the ��=−1 is a closed dissociation channel. For most of these lev-els we are able to measure both the decay of the NeBr2

excited-state population produced by the pump laser and theappearance of the free Br2 products. However, we did notdetect any new, long-lived IVR intermediate states.

For ��=16 and 20, for which the dynamics are expectedto follow the direct vibrational predissociation mechanism,the new data are in good agreement with previous experi-mental results from linewidth measurements and with theo-retical calculations. For these two levels both the new dataand the previous linewidth data yield lifetimes that are some-what longer than the calculated lifetime. This indicates thatthe potential-energy surface employed for the calculationslightly overestimates the coupling between the Br–Brstretch and the van der Waals modes in the B state.

The dynamics for 22����27 are more intriguing. Thenew measurements yield significantly longer product stateappearance times, with 10 ps���20 ps, than those inferredfrom the previous linewidth measurements. The linewidthmeasurements follow the energy-gap law, and predict thatthe lifetimes decrease to about 2 ps over this range. For thesevibrational levels the theory predicts multiexponential dy-namics and non-Lorentzian line shapes.3 The details of thesecalculations are expected to be quite sensitive to thepotential-energy surface since they depend on near reso-nances between the IVR active states. These IVR activestates involve orbiting resonances in the ��=−1 manifoldthat are nearly isoergic with the zero-order bright state.Double resonance on the NeBr2 B state levels initially popu-lated by the pump laser was studied for ��=22 and 24. Ineach of these two cases the rates are slow enough that ourexperiment can provide reliable results. The decay of thedouble resonance yields dynamics with the same time con-stant as the product appearance data within experimental un-certainty. The values obtained are about 2.5 times those es-timated from linewidths.20

Although the lifetimes extracted from the ��=24 data areconsiderably longer than the prediction of the energy-gaplaw, they are still less than the pump-probe autocorrelationwidth, and so we were especially careful with the data analy-sis. Figure 6�a� shows double resonance via the free Br2, B

state, ��=24. As described above these data were used toextract the t=0 point and the laser autocorrelation widthwhich is 28 ps in this case. Figure 6�b� shows the NeBr2, Bstate, ��=24 double-resonance data, the best fit, 16 ps, andthe best fits that are generated using � values of 10 and 22 ps.Figure 6�c� presents a similar analysis of the ��=23 productappearance data, yielding �=15±4 ps. This analysis showsthat our error bar estimates are quite conservative. It is clearfrom this analysis that the actual time constant for the data isconsiderably longer than the 6-ps value previously obtainedfrom linewidth data.

The data for ��=26 are particularly tantalizing. For thislevel, we did not observe NeBr2 double resonance eventhough the region of the spectrum where we expected to seeit was not obscured by congestion.22 As discussed above, thisleads us to assign a lifetime of less than 10 ps to this level.However, the Br2 product that results from this excitationyields a � value of 15 ps. The theoretical line-shape calcula-tion for this level3 predicts two resonances, one of which isnarrow enough to be consistent with a 15-ps lifetime. Howwould such an intermediate resonance show up in the probespectrum? Our hypothesis is that it would look like a rovi-brationally excited resonance above the ��=−1 dissociationthreshold, as recently reported for HeICl.9 We might thenexpect the B→E spectrum of this resonance to be a slightlyperturbed resonance of the free Br2 ��=25 level. However,no such resonances were observed for this, or any other levelthat we studied. The probe spectra in this regime are some-what congested due to the large number of �� channel prod-ucts that are produced. Thus a careful analysis of high signal-

FIG. 6. Three time-delay scans for excitation to NeBr2, B state, ��=24. �a�Free Br2, B state, ��=24 double resonance and laser autocorrelation curve,28-ps FWHM. �b� Appearance and disappearance of NeBr2, B state, ��=24, along with the best-fit curve, �=16 ps, calculated for the data and thetwo curves generated by the ±6-ps error bars. �c� Appearance of Br2 productin B, ��=23 from vibrational predissociation of the NeBr2 B state, ��=24,along with the best-fit curve, �=15 ps, and the curves generated by the±4-ps error bars.

054311-6 Cabrera et al. J. Chem. Phys. 123, 054311 �2005�

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to-noise data may eventually reveal such a resonance,especially if these measurements are aided by calculations ofthe predicted signal.

The ��=−1 channel is only slightly open for ��=27,severely limiting the number of open product rotationalstates for initial J=0.10 Thus, any rotational excitation of theinitial state may have a significant effect on the expecteddynamics. For this reason, the ��=−1 data observed for ��=27 most likely give higher weight to excited rotationalstates than do the ��=−2 data. It is thus interesting thatslightly different excited-state lifetimes result from theanalysis of the ��=−1, �=10 ps, and ��=−2, �=19 ps,product state data. As illustrated in Fig. 7, these differencesare greater than the experimental uncertainty. The ��=−2 peak clearly rises more slowly than does the ��=−1peak. The fact that different lifetimes are measured for thesetwo channels means either that they arise from different dis-tributions of initial states, probably different sets of rota-tional levels, or that at least one intermediate state standsbetween the initially excited states and the product levels.We suspect that both possible effects play a role. The inter-play between IVR and direct VP has been discussed for sucha case, and indicates that the rates should be the same for thetwo channels.4 However, there are no theoretical predictionsfor how rotational excitation will affect the dynamics ofthese levels.

For ��=28, the ��=−1 channel is completely closed and��=−2 is the first open channel. The lifetime for productappearance for this channel is 22 ps, longer than that of theprevious six levels, and an order of magnitude longer thanthe value that was extracted from line-shape data. Since thedynamics of these levels is expected to be dominated by

IVR, this implies that there is an intermediate resonance be-tween the initially excited level and the products. To ourknowledge, there have been no detailed theoretical studies ofthe excitation line shapes and dissociation dynamics forNeBr2 levels for which ��=−1 dissociation is closed, exceptfor much higher energy levels near the Br2 dissociationlimit.5

Although the data presented here are consistent with theexisting theoretical predictions that NeBr2 undergoes IVReven in the ��=−1 dissociation regime, we have not ob-served the long-lived intermediate resonances that wouldhave provided direct evidence for IVR. It may be that higherspectral and time resolution would resolve probe transitionsin addition to those we have observed to date. Based on thedata presented above, the ��=26 level appears to be the bestcandidate for a direct observation of IVR resonances in the��=−1 regime. It will also be very interesting to extend thestudy of the ��=−2 regime.

Another important result of the present study is the ob-servation of double resonance for the NeBr2 molecule fromthe X state, through the B state to the E state. As shown inFig. 4, these double-resonance signals show no significantinhomogeneous widths, and are thus probably dominated by0-0 transitions in the van der Waals modes. The Ne–Br2

bond energy for �=2 of the E state is found to be 82 cm−1.The fact that NeBr2 B-E spectra have now been observedindicates that theoretical calculations can be performed topredict the spectra for the elusive intermediate resonances.

V. SUMMARY AND CONCLUSIONS

Real time pump-probe spectra are reported for NeBr2 Bstate vibrational levels with 16����29. For ��=16 and 20,for which direct vibrational predissociation is expected todominate the dynamics, the results are consistent with previ-ous linewidth measurements. The lifetimes are slightlylonger than those predicted by theory, so the theoretical po-tential slightly overestimates the coupling between the bro-mine stretch and the van der Waals modes. This is importantsince the details of calculations in the IVR regime are verysensitive to the potential. For levels 22����29, the ob-served lifetimes are significantly longer than those from pre-vious linewidth measurements. The dependence of lifetimeon vibrational level becomes nonmonotonic, and the life-times measured by double resonance for ��=26 and 27 areshorter than those measured by product appearance. Thissupports the IVR model. However, we did not detect the IVRintermediate resonance itself. The observation ofNeBr2 B-E probe transitions indicates that such resonancesmay eventually be observed, perhaps with the help of addi-tional theoretical calculations. The bond energy of Ne to theBr2 , E state, �=2 level, is found to be 82 cm−1.

ACKNOWLEDGMENTS

The authors would like to acknowledge discussions andaccess to unpublished results from Nadine Halberstadt, Tho-mas Stephenson, Octavio Roncero, Alberto García-Vela,Adolfo Bastida, Ramon Hernández-Lamoneda, and RichardLoomis. This work was funded by the National Science

FIG. 7. Three time-delay scans for NeBr2, B state, ��=27, �a� Free Br2 , Bstate, ��=27 double resonance and laser autocorrelation curve, 28-psFWHM. �b� Appearance of Br2 product in the B state, ��=26 from vibra-tional predissociation of the NeBr2 B state, ��=27, along with the best-fitcurve, �=10 ps. �c� Appearance of Br2 product in the B state, ��=25 fromvibrational predissociation of the NeBr2 B state, ��=27, along with the best-fit curve, �=19 ps.

054311-7 Spectra of NeBr2 J. Chem. Phys. 123, 054311 �2005�

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Foundation, Award No. CHE-0213149. The UC Mexus-Conacyt program funded visits between Cuernavaca �Profes-sor Hernández� and Irvine to discuss the results that are pre-sented in this paper. One of the authors �C.R.B.�acknowledges support from the Hewlett-Mellon Faculty De-velopment Fund and the Herbert H. and Grace A. Dow Trust-ee’s Professorship of Albion College. Another author�B.C.O.� received additional financial support from the Al-bion College Foundation for Undergraduate Research, Schol-arship and Creative Activity.

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