J. Phys. Chem. 1995, 99, 1768-1775
1768
Intramolecular Relaxation of Photoelectron Spectroscopy
S1
Benzene Studied with Picosecond Photoionization and
Jonathan M. Smith,?Xu Zhang, and J. L. Knee* Department of Chemistry, Wesleyan University, Middletown, Connecticut 06459 Received: August 30, 1994; In Final Form: October 17, 1994@
Picosecond pump-probe multiphoton ionization and zero electron kinetic energy (ZEKE) photoelectron spectroscopy are applied to study five vibronic bands in the S1 state of benzene. The goal is to measure the nonradiative decay processes in the channel three region including both overall electronic state relaxation which leads to a loss of S I population and intramolecular vibrational redistribution (IVR) which is due to vibronic coupling within the S1 state. The lowest energy band studied, 6112 at 2367 cm-’, is below the channel three region while the other four bands, 6113, 7l, 7111, and 6114, are considered to be near or above the channel three threshold. The measurements of overall electronic state decay agree with the previous determinations by Sumitani et al. The picosecond ZEKE experiments reveal that the 7l and 7l1’ bands show little or no evidence for IVR on the time scale of the electronic state decay (’100 ps) while the 6’13 band apparently undergoes rapid IVR with a time constant of -35 ps. The results are compared with some of the recent studies on benzene, including the picosecond fluorescence results of Sumitani et al., the chemical timing experiments of Longfellow et al., and the rotationally resolved spectral and dynamics studies of Riedle et al.
14112,the experiment revealed that at low J only the K = 0 lines were observed whereas at high J only the K = J lines Nonradiative decay in the fiist excited state of benzene has were observed. The line width of the observed transitions varied been studied extensively over the years with a wide array of considerably, but some were as narrow as 2 MHz.12 Decay e ~ p e r i m e n t a l l -and ~ ~ t h e o r e t i ~ a l * ~approaches -~~ applied to the measurements were made on some of the narrower bands, and problem. These studies are far too numerous to summarize here it was shown that they decayed with a single exponential and so the reader is referred to several review article^^^,^^ and recent a time constant consistent with the observed widths. This papers for the proper perspective. confirmed the homogeneous nature of the transitions. Many Benzene has been known to exhibit a rather abrupt increase lines in the band, however, were not observed at all, and there in nonradiative processes when vibrational levels in excess of absence is attributed to fast nonradiative decay which is highly 3000 cm-’ are excited. This phenomenon was first reported rotational state dependent. Similar experiments were performed by Callomon et aL,6 who inferred the rapid dynamics from on the one-photon 6113 band,13 but in this case jet-cooled broadened room temperature absorption spectra. Since that time molecules were studied to sufficiently reduce the Doppler width. many experiments have been performed which have revealed The results on this band also showed rotational state dependent much of the details concerning the rates and nature of the decay, but with somewhat different J , K dependence of the dynamic processes. Quantum yield and excited state lifetime nonradiative rates. These impressive rotationally resolved m e a ~ u r e m e n t s ~showed ~ ~ , ~ ~that - ~while ~ ~ ~ the ~ quantum yield results make it clear that interpretation of non-rotationallywas quite low in this region the “channel three” behavior was resolved results should include consideration of the averaging not as dramatic as indicated by the linewidth measurements, over the behavior of the individual rotational states which for both the one-photon and two-photon transitions. The contribute to the observed signal. For instance, if one uses a discrepancy between line width data and nonradiative decay fast, broad pulse to excite an entire vibronic band, as in our times can be resolved by invoking IVR as an initial fast process experiment, then the observed results will be the sum of the which can occur, sequentially or in parallel with the electronic contributions from a number of rotational states, with perhaps state decay. Much of the more recent work has focused on widely different decay behavior. The rotationally resolved determining the details of these possible pathways.11-14,17-24,28,29 results yield specific lifetimes for the longer-lived lines but only Most studies have focused on the vibrational aspect of the give an upper limit for the features which are broadened into problem, but by far the most detailed and informative experithe background. Therefore, it is difficult to directly compare ments have been the rotationally resolved spectroscopy of Riedle the rotationally averaged picosecond results to the observed et. aZ.11-13 Using both one-photon and two-photon specrotationally resolved information; however, one might expect troscopies, they have accurately measured the line widths of that the time-resolved results would be the sum of a large single rotational transitions for a number of bands in the channel number of exponential decays. three region. What is observed is highly J- and K-dependent In this work we apply picosecond time-resolved multiphoton line widths, including the observation that many lines are not ionization (MPI) and zero electron kinetic energy (ZEKE) observed at all due to rapid nonradiative decay which is photoelectron experiments to try and determine the involvement rotational state dependent. In the case of the two-photon band, of IVR in the nonradiative decay of a number of bands in the channel three region. In this context the most relevant previous Present address: Department of Chemistry, Yale University, New studies are those of Riedle et al.11-13 mentioned above, Sumatani Haven, CT 065 11. et aZ.,18-21and Longfellow et ~ 1 Sumatani . ~ ~ et a1.18-21per‘Abstract published in Advance ACS Abstracts, January 15, 1995. Introduction
0022-365419512099-1768$09.0010
0 1995 American Chemical Society
Intramolecular Relaxation of SI Benzene formed picosecond time-resolved fluorescence experiments on a number of bands in the channel three region in and perdeuteriobenzeneZ1and also made relative quantum yield measurements. These measurement were performed primarily on room temperature samples, but jet-cooled studies were performed on some of the stronger bands. They measured fast nonradiative decay in the channel three region with many instances of biexponential decay observed. The lifetimes were generally subnanosecond and did not show any pronounced mode specific variation. In particular, the 611n and 711n series did not exhibit widely different behavior in their total fluorescence lifetimes. They concluded that the previously measured line broadening of the 6l1" bands relative to the 711n series is due to rapid IVR in the former, which is not present in the latter. To explain the observed biexponential decays, they invoked another coupled electronic state and presented a kinetic model which agrees with their experimental data. The work of Longfellow et ~ 1 is perhaps . ~ ~to date the most direct attempt to measure IVR for the vibronic bands in the channel three region. Chemical timing experiments were performed using room temperature benzene and 0 2 as a quencher. This technique is described fully in ref 32 with additional applications reported on p-difluorobenzene and p-fluorotoluene. The results of this work indicate that IVR is fast (5 -200 ps) throughout the channel three region, including the 6l l2band which is below the channel three region but was still measured to have an IVR lifetime of 200 ps. Furthermore, the results indicate no qualitative difference between the behavior of the 6l1" series and the 7'1" series, which is contrary to much of the reported data of other experiments.
Experimental Section The dynamics of vibronic states of benzene in the channel three region were investigated with picosecond pump-probe experiments with two types of detection. In one set of experiments a picosecond pulse prepares the vibronic state of interest which is subsequently probed by photoionization with a probe pulse and detection of the total ion signal. In the second set of experiments the probe pulse is tuned to a specific ion resonance, and the ZEKE photoelectron signal of that resonance is detected as a function of the pump-probe delay time. In the case of total photoionization detection the probe is tuned sufficiently to the blue (well beyond the vertical ionization energy) such that all vibrational states in S1 should be effectively probed in this experiment.14 Therefore, both the initial state and any SI vibrations subsequently populated by IVR should be detected, and thus any decay would reflect a total loss of SI population, Le., electronic state nonradiative decay. In the case of ZEKE probing the initially pumped state is expected to have well-defined transitions to particular ion states, and specifically in our case we have focused on probing the Av = 0 transitions. Thus, for instance, if the 7; transition is pumped, then the 7:' transition would be probed by proper tuning of the ZEKE probe laser. In this case nonradiative decay to other vibrations in S1 (IVR) would lead to a loss of ZEKE signal at the 7:' probe wavelength. We have demonstrated ZEKE probing of IVR in a number of molecular system^.^^-^^ Of course, electronic nonradiative decay would also lead to a loss of this ZEKE signal so it is important to compare the total ion signal decay (above) to the decay of the ZEKE signal. In this way we can try to assess the role of IVR on the total electronic state decay in the channel three region of benzene. As a further check, the ZEKE spectrum is scanned at early and late time, which can help reveal overall changes in the photoelectron spectrum as a function of time.
J. Phys. Chem., Vol. 99, No. 6,1995 1769 Many of the experimental details for such studies have been reported p r e v i ~ u s l y ~ - ~ ~a brief description will be given so~only here. The laser is an amplified two-color picosecond system. A frequency-doubled CW mode-locked Nd:YAG (Antares, Coherent Inc.) synchronously pumps two dye lasers at 76 MHz. The low-energy dye laser pulses are amplified in separate threestage linear dye amplifiers which are pumped by the second harmonic of a Q-switched regenerative amplifier (Continuum Inc., RGA-67) operating at 30 Hz. Each amplified dye laser pulse is 0.5-2 mJ per pulse with a width of 7-20 ps depending on the degree of amplifier saturation. For these experiments (as described below) the time resolution was usually sacrificed for higher dye laser power, but generally this was not the limiting factor in the measured decays. For picosecond studies of the S1 state of benzene the generation of the pump wavelength is quite difficult. Our approach was to use sum frequency generation of the doubled dye laser (-630 nm) with the 1.064 p m output of the regenerative amplifier. The regenerative amplifier (regen) pulses are -70 ps in width but have significantly higher pulse energy ('30 mJ residual, 1.064 pm available). The pulses from the regen were directed to a delay line and then recombined with the UV (doubled dye laser) and mixed in an angle tuned KD*P crystal. The overall power of the pump is estimated at '10 pJ, which produced adequate signal levels and at times had to be attenuated. The pulse-topulse amplitude of the pump was poor and limited the ultimate signal to noise. As mentioned above, the probe for the ZEKE experiments was nominally a Av = 0 transition to the cation, which is approximately 36 470 cm-' (i.e., IP(S1) = 74 55638 086 cm-'). The exact probe energy would depend upon the vibrational frequency shifts between the S1 state and the cation which could be obtained or estimated from previous photoelectron experiments. The 36470 cm-' cannot be obtained by directly doubling the dye laser since that energy lies to the blue of the tuning range. To obtain this wavelength, the probe dye laser was mixed with part of the 532 nm pulse from the regen, in a KDP crystal. This allowed the probe tuning range to extend to approximately 36 700 cm-'. This mixing process was quite efficient, and we estimate the W power to be > 100 pJ/pulse. As mentioned above, to measure the purely electronic state decay, it is necessary to detect the total ion signal with a probe sufficiently to the blue to effectively probe all the vibrations potentially populated by IVR in SI. To ensure this, and for experimental simplicity, the total ion probe used was the 266 nm fourth harmonic of the regenerative amplifier. This also simplified the experimental setup but at the cost of a broader response function since this UV pulse is estimated to have a width of -60 ps. A number of total ion transients were also obtained with the dye laser probe which resulted in a narrower response function, but the results were consistent with the 266 nm probe. The experiments were conducted in a two-chamber, differentially pumped molecular beam apparatus. Benzene was seeded in He at a vapor pressure of approximately 40 TOIT, expanded in a pulsed valve (General Valve, Inc.), and skimmed. The second chamber contains the laser interaction region and ZEKE photoelectron spectrometer, which has been described in detail p r e v i ~ u s l y . ~ For ~ - ~MPI ~ experiments the detection system is configured as a standard Wiley and M ~ C l a r e nTOF ~~ mass spectrometer with a 40 cm drift tube and the detector biased to detect ions. For ZEKE experiments the interaction region is field-free when pump and probe laser excitation occur, but approximately 1 ps later a negative pulse is applied to the lower grid to field ionize any high-lying Rydberg states remaining and repel the resulting electrons to the detector which
Smith et al.
1770 J. Phys. Chem., Vol. 99, No. 6, I995 is biased to detect electrons. Since high resolution in the ZEKE spectra was not necessary, the extraction pulse used was large, -35 V/cm, to enhance the overall collection efficiency. It has been shown that such extraction pulses result in a ZEKE resolution of 5- 10 cm-1.38 For each experiment care was taken to properly tune the pump wavelength to reproducibly excite the proper resonances. To do this the MPI signal was monitored, and the pump was scanned in the region of interest until the intended resonance was optimized. The probe laser was then introduced, and the two-laser signal (MPI or ZEKE) was optimized by adjusting the overlap of the beams. The pump and probe beams were made parallel and closely spaced and then overlapped at a gentle focus of a 1 m lens. The pump beam traversed a stepper motor controlled optical delay line which utilized a corner cube reflector. The delay line was carefully optimized such that the beam movement was a minimum as the delay line was scanned. For transient decays the signal, MPI or ZEKE, was monitored with a boxcar averager and computer as a function of the delay line position. In another experiment the ZEKE spectrum could be scanned at different fixed positions of the delay line. Thus, time-gated spectra of the S1 to ion transition could be obtained.
TABLE 1: Summary of Measured Lifetimes for Five SI Bands in Benzene; Fluorescence Data of Sumatani et aLzo Are Included for Comparison fluorescence lifetimeb (ps)
energy
measured
SIband
(cm-')
lifetime" (ps)
tl
6Il2 71 6113 7111 6l l4
2367 3080 3290 4003 4213
'1000 > 1000 285c 115 70
47000 4000 250 130 99
tz
18000 2900
The measured lifetimes are the average of measurements made in this work including both MPI and ZEJSE results. Results reproduced from ref 20, including two lifetime components as indicated. Other bands, not measured here, were also reported in this work. With MPI measurements a fast component of 35 ps was also measured for the 6113 band.
Results Measurements were conducted on a total of five vibronic bands of benzene in the channel three region, specifically 6112 (2367 cm-l), 7' (3080 cm-I), 6113 (3290 cm-'), 7111 (4003 cm-'), and 6114 (4213 cm-'). The following section is a summary of the measurements for each of these individual bands. The lifetimes of the individual decays were obtained by a nonlinear least-squares fit of the data, including deconvolution of an assumed Gaussian response function. The actual system response could not be measured so the Gaussian response function width was varied to obtain the best fit, particularly to the rising edge of the data. For most of the data the observed decays were well in excess of the response function, and therefore it was not a significant factor. For the 6114band where the decay is fast and the 266 nm probe led to a broad response function, the uncertainty in the reported lifetimes is greater. For the 6113band the data were fit in some cases to a biexponential decay, again with deconvolution of a system response function. Due to experimental limitations long delay scans could not be measured so that any of these long components have a large degree of uncertainty associated with them. In general, for all the data, it is often possible that a small amount of long component is present and not accounted for. However, the data always contain a prezero signal which is incorporated in the fit, forcing this to be the actual baseline in the fit. Therefore, any significant long-time signal (> 10%) should be noticed and would force consideration of a long component to the decay. A summary of the measured lifetimes is given in Table 1. 6112. This band is considered to be below the channel three threshold and will therefore help establish a baseline for what signals are observed in the absence of any rapid electronic state decay. Figure 1 shows the ZEKE spectrum in the region of the 6 : l F cation band,39which is a Av = -1 transition in V I . This probe region is accessed by using the frequency-doubled probe dye laser. To access the Av = 0 region, the dye laser was mixed with the 532 nm output of the regenerative amplifier. The features in the spectrum in Figure 1 are assigned as 6l1 1+ (&3/2) by analogy to the structure observed by Krause and Neussep and Linder et aL41 for the 6l+ (&3/2) transition. Also shown in Figure 1 is the time-dependent ZEKE signal obtained by gating on the strong resonance in the spectrum and scanning the pump-probe delay line. Over the time scale of this rather
I
0
100
200
300
400
500
El
Pump-Probe Delay Time (ps)
Figure 1. (top) ZEKE spectrum obtained by pumping the 6112 band at 2367 cm-' and probing in the Av = - 1 (in V I )region of the spectrum. The two larger peaks are assigned to the 6:12 transitions to the f3/2 components of the Jahn-Teller split ezg V 6 m ~ d e .(bottom) ~ ~ . ~ Timedependent behavior of the ZEKE signal when gating on the largest feature in the top panel.
narrow scan (our delay line is limited to -1.5 ns) it is clear that no dynamics are occuning. Again, this ZEKE probing is state-specific to the S1 resonance and so should show a decay for both IVR and electronic nonradiative decay. This result is in contrast to an IVR lifetime of 200 ps reported by Longfellow et ~ 1for. this~ band. ~ 7l. Figure 2 shows the ZEKE spectrum obtained from pumping the 7l band and scanning in the region of A v = 0. The 7l+ resonance is clearly observed and is sharp. The time evolution of the 7l band is obtained by gating on the ZEKE 7l+ signal and scanning the pump-probe delay time. As shown in Figure 2, the signal rises promptly and then remains flat, indicating no dynamics, IVR, or electronic nonradiative decay on this 800 ps time scale. ZEKE spectra obtained at late time delay show the same sharp spectrum of the 7I+ resonance. 6*13. The 6113 band is accepted to be in the channel three region and has been measured to have a low quantum yield
J. Phys. Chem., Vol. 99, No. 6, 1995 1771
Intramolecular Relaxation of S1 Benzene
r,=30+10 ps
b)
I , ,
1 I I I , I I I I 1
29002950
I
3050
3000
I
/
,
I
3100
I I I I I I 1 l I I I L
3150
3200325
Excess Ion Energy (cm-')
pa
+,=275+35
ps
3
z?
iij
200
Yw
N
I
I
l
200
*
l
l
.
I
400
1
*
600
1
# I 600
Pump-Probe Delay Time (ps)
Figure 2. (top) ZEKE spectrum obtained by pumping the SI7l band and scanning the probe in the Av = 0 region of the cation. The observed
peak is assigned to the 7;' transition. (bottom) Time dependence of the ZEKE signal when gating on the 7;' probe transition.
I
Nhn
6'13 Band
3000
3100
3200
1
3300
3400
Excess Ion Energy (cm-') Figure 3. ZEKE spectrum obtained from pumping the 6Il3 band in
SI and scanning the probe in the Av = 0 region of the cation. This spectrum was obtained at zero pump-probe delay and is quite weak but reproducible. No assignments are offered for these broad peaks.
and commensurate short excited state lifetime.18-21 In our experiments this band gave a strong MPI signal, including two laser-enhanced signals from both dye laser and 266 nm probing. We performed extensive ZEKE experiments on this band by probing both the 6: 1:' and 6: 1:' transitions. The results were elusive with no sharp well-defined ZEKE transitions found at any delay times. Figure 3 shows the diffuse ZEKE spectrum which we were able to record and which we found to be reproducible. After numerous attempts, with frequent checks to confirm that we could quickly obtain the ZEKE spectrum of the 6l l2and 7l band, we became convinced that the broad ZEKE spectrum we measured was the spectrum of bands populated by rapid IVR from the 6113 band. The broad ZEKE structure appears similar to bands in other molecules (fluorene33 and a ~ e n a p h t h e n e ~where ~) the broadening was shown to be conclusively due to IVR. Ideally, one would like to see a sharp ZEKE spectrum at early time, which would then quickly evolve
400
600
Pump-Probe Delay Time (ps) Figure 4. Measurements of MPI signal as a function of pump-probe delay obtained when pumping the 6113 band and probing at three different energies. Decay a is obtained with 266 nm probing (37 594 cm-l) and has a noticeably slower rise due to broad 266 nm pulse width. Traces b and c have probe wavelengths of 36 575 and 35 562 cm-l, respectively. into the broadened spectrum measured; however, all attempts to measure a sharp spectrum at early time failed. There was, however, a consistent trend in both the MPI and ZEKE transient decays. When probing with MPI in the region of the Av = 0 transition, 6:1:+, a predominantly singleexponential decay, was measured with a lifetime of 285 ps (the average of many measurements). When probing in the Av = -1 (in VI)region, a biexponential decay was measured with a fast component of 35 ps and a slow component of -285 ps. We explain this observation as follows. For Av = 0 probing all vibrational levels should have strong transitions because the Franck-Condon factors are all large for Av = 0. This would include the transition from the originally pumped level, 6:1?, as well as transitions from any levels populated by IVR. On the other hand, when probing in the Av = -1 v1 region, the strength of the probe transition will depend on the FranckCondon factors to this low-energy region. For total ion detection this will in fact be the sum of the Franck-Condon factors from the ionization threshold to the probe energy. Transitions from the initially prepared state, 6113, to this lower energy can be made by the change of just one quantum, the 6:1:+ for instance ( i e . , Av = -1 in V I ) . The low-frequency combination and overtone bands populated by IVR may (depending on their particular constitution) have to change several quantum numbers to access the lower probe region. Thus, the Franck-Condon overlap from the initially prepared state to the lower ionic states is expected to be larger than that for the redistributed states. In this scenario then the overall ionization cross section (for probing to the red) is expected to decrease as IVR proceeds, thus explaining why the biexponential decay is observed in the total ion signal. With the dye laser probe above the Av = 0 region, including the much bluer 266 nm probe, only singleexponential decays are measured for this band. This analysis would then suggest an IVR rate of -35 ps and an overall electronic state decay rate of 285 ps. 7111. This band at 4003 cm-' is well into the channel three region and is known to have a low quantum yield and fast decay. The ZEKE results obtained for this band were quite good, and a number of experiments were performed. Pumping the 7111
1772 J. Phys. Chem., Vol. 99, No. 6,1995
and conclude that the electronic state nonradiative decay from this band is quite fast. No conclusion can directly be made about IVR for this band.
7'1' Band
t = O ps
w
Discussion
t=50 ps
3800
4000
3900
4100
Excess Ion Energy (cm-')
lypl h. 0
200
Smith et al.
~=107+10pa
400
600
Pump-Probe Delay Time (ps) Figure 5. (top) ZEKE spectra obtained by pumping the 7ll band and probing the ZEKE spectrum in the Av = 0 region at t = 0 and t = 50 ps. The ZEKE spectrum is sharp at early and late probe time. (bottom) Comparison of ZEKE probing of the 7tl;' transition to probing with MPI (probe = 36 528 cm-'), which measures the total ion signal. Both transients decay with a time constant of -115 ps. band the ZEKE spectrum was obtained both in the Av = 0 region and the Av = -1 in V I . Figure 5 shows these ZEKE spectra obtained at early and late times. Unlike the 6113 band these spectra are sharp and exhibit the expected transitions, 7:1:+ and 7 i l y , respectively, for the initially prepared states. The spectra remain unchanged at late time except that they are weaker. The transient decay of the 7ll level are obtained in two ways. First, the 7:l:' (and 7:lY) ZEKE signal is measured as a function of pump-probe delay. This is shown in Figure 5 . Fitting this and several other transients for this band results in an average decay lifetime of 115 & 10 ps. This ZEKE measurement is sensitive to both IVR and electronic state decay and may contain contributions from both. Second, the 7111 band is probed by total ionization detection using 266 nm in one experiment and the probe dye laser in another. Both probes yielded a similar lifetime of -1 15 ps. These measurements should be insensitive to IVR decay. The quantitative agreement between total ion and ZEKE detection decay times requires that the IVR decay time be significantly longer than the time scale of these experiments. 6114. This is the highest energy band measured at 4213 cm-'. We were unsuccessful at recording a ZEKE spectrum of this band, which could be due to the fact that the band is inherently quite weak or possibly because the IVR leads to a broadened ZEKE spectrum even at early time, which could not be effectively probed. We were successful at probing the total ionization signal using the 266 nm probe. The result was a decay of 70 ps which was close to the system response function using this probe wavelength. The fitting routine should, however, reliably deconvolute this lifetime from the system response. At any rate we can use this lifetime as an upper limit
The above data show what we believe are very clear differences between the behavior of the 711n and 6l1" series. The time-resolved ZEKE measurements demonstrate that for the 7l and 7l1' bands IVR does not occur to any significant extent on the time scale of the electronic state nonradiative decay. This is in stark contrast to the results of Longfellow et ~ 1 but. is ~consistent ~ with the original line width studies of Callomon et uL6 Sumitani et ul.18-20supported this interpretation in their work, but their data could not conclusively demonstrate the role of IVR in the overall observed decays. Even though the 7ll band showed no evidence of rapid IVR, it still undergoes a fast nonradiative relaxation. Our measured rate for this decay, using both MPI and ZEKE probing, is very close to the value measured by Sumitani et ul.19-20(130 ps) using fluorescence decay measurements. The 6l l3band is less conclusive, as outlined above, but our interpretation is that IVR is rapid for this band, and the manifestation of it could be the -35 ps fast component measured with MPI using a redder probe. At any rate, the lack of any sharp early time measured ZEKE resonances suggests that the IVR rate is comparable to our system response function. Although this is a negative result, it is supported by the fact that the 6112, 7l, and 7111 bands all gave rise to sharp ZEKE resonances with good signal to noise. We can only conclude that we could not make the ZEKE measurements of the 6113 band because of rapid IVR. The ZEKE spectrum did show broad structure which we attribute to the signal from the redistributed states but which could not be assigned. Our data for the overall electronic state decay are consistent with the measured time decays of Sumatani et ul.,18-21except that we did not see any obvious biexponential decays except for the 6113 band. The biexponential decay which we do find for the 6113 band arises presumably from a different source (IVR), and in fact our slow component (285 ps) agrees very well with their observed fast component (250 ps). The majority of the data of Sumitani et u1.18-21are for room temperature samples, so in fact the good agreement is between our jet-cooled results and their room temperature results. For their beam experiments the resolution was limited to -500 ps, and for the 6l l3 and 7l1 bands they just report a fast component decay of
