J. Phys. Chem. 1993,97, 8718-8726
8718
Hot-Band Spectroscopy of Ground-State Levels of Perylene following Predissociation of van der Waals Complexes Stacey A. Wittmeyert and Michael R. Topp' Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323 Received: December 29, I992
Experiments involving laser-induced hot bands have interrogated the energies and populations of ground-state vibrational levels of perylene. In the bare molecule, many ground-state levels persist into the 10-7-106-s regime without significant relaxation. Even in the perylene/Arl van der Waals complex, a ground-state quantum of 353 cm-l persists for >120 ns. On the other hand, residual collisions in the expanding free jet cause significant relaxation of energy deposited in low-energy (G) modes of perylene. Hot bands of free perylene are also generated following vibrational predissociation of van der Waals complexes. The argon, methane, and nitrogen complexes all show efficient hot-band population and show distribution of energy into low-energy, out-of-plane modes of the product aromatic molecule at higher energies. The COz complex dissociated at 11270 cm-1 in SIand the ethylene complex at 11320 cm-l. Corresponding ground-state binding energies are S1235 and 1 1160 cm-l. In these two cases, hot-band excitation spectroscopy at x2-cm-l resolution provided the means to detect the onset of hot bands of nascent free perylene, as the pump laser scanned through a congested region of the spectrum.
1. Introduction Molecular clusters prepared under high-vacuum conditions offer valuable opportunities to test theories of intermolecular forces. They also allow fundamental studies of the interactions and dynamics that, on a larger scale, are characteristic of condensed phases. Contemporary studies in this area range from van der Waals complexes having as few as three atoms to large clusters involving dozens of atoms or molecules. For complexes involving aromatic molecules, it is of great interest to make accurate measurementsof binding energies and internal potential energy barriers and to study the mechanisms of predissociation events. Calculations using semiempirical pair-potential techniques'" have successfully predicted binding energies to within 10-2096 of experimental values, for small clusters (i.e., 12:l) of Ar and occasionally CH, around aromatic hydrocarbons. Such agreement results from the parametrization of thermodynamic data for atoms and molecules adsorbed onto extended surfaces, such as graphite, and molecular Almost ail measurements of binding energies reported so far have involved raregas atoms, which have low binding energies, and can be examined in relatively uncongested regions of the spectrum. Extensions of computational approaches to molecular complexes have been less successful both in binding energy and structure calculations. It is especially important to consider complexes involving unsaturated molecules, since the electrostatic components of the intermolecular potential energy are considerable and may determine the structures. The spectra of many double-aromatic complexes arecomplicated by the effects of electronicallyinduced structural change, induced largely by changes in the electrostatic interaction. Other cases exhibit simple spectra, such as the complexes of perylene with benzene and naphthalene.10 The structure of perylenelbenzene was recently found by Felker and co-workers" to correspond closely to a parallel sandwich arrangement ( C d , in both the SOand S1states. Similarly, despite the large quadrupole moment possessed by ethylene,lZ the uncomplicated absorption spectrum of perylene/(ethylene) (observed by hole-burning ~pectroscopy)~3J~ suggests a dominant dispersive component and a simple center-of-mass structure for the complex. Also, the large red shift of 162 cm-* for ethylene is consistent with the above cases. On the other hand, the C02 t Present address: Dupont-Marshall Laboratories, Philadelphia, PA.
0022-3654/93/2097-87 18$04.00/0
molecule has a disproportionately large quadrupole moment for its size.15 so thevibronicspectrumof perylene/(COz)1 is a sequence of low-frequency mode progressions. Here, the red shift is anomalously low, at 33 cm-l. Because of the sensitive balance between dispersive and electrostatic interactions, structure computations are quite uncertain. Experimental determinations of both the structure and binding energy are needed to support refinement of theoretical procedures. Direct experiments to measure the binding energies of largemolecule aggregates have largely employed fluorescence spectroscopy. These approaches can in principle provide a dynamical model of the predissociation event as a function of the excess energy. For example, a restricted RRKM model was used by Zewail and co-workers16to analyze the predissociation dynamics of dimeric species involving single-ring aromatic molecules. On the other hand, recent experiments involving picosecond timedomain fluorescence studies of larger molecules have shown a wide variation in the predissociation dynamics of simple argon and methane complexes of different aromatic species.l7J8 This limits the applicability of RRKM models restricted to intermolecular modes.19 Also, as binding energiesincrease into the region >1000 cm-l, direct excitation of bare-molecule transitions in the same region as the less abundant complexes becomes unavoidable. In such a regime, timeresolved fluorescence measurementsrelying on spectral resolution of the component species become difficult to apply. For example, the binding energies of alkane complexes of a large molecule such as perylene lie in the range 1500-2500 cm-l. Despite red shifts of 200-400 cm-I, the appropriate resonances coincide with a congested region of the bare-molecule absorption spectrum. Another approach to measure binding energies is to focus on the products of vibrational predissociation via double-resonance spectroscopy. This is a well-developed technique for rare-gas atom complexes of single-ring aromatic molecules, where a stateby-state analysis of the product species can readily be carried out.20-z2 This kind of approach is also desirable for complexes involving large molecules. However, such experiments must in general interrogate higher-energy, congested regions of the vibronic spectra, where fluorescence or ionization probes of the excited state could yield ambiguous results. When a jet-cooled molecule is excited, fluorescence will populate nonzero vibrational levels of the ground state, according 0 1993 American Chemical Society
Hot-Band Spectroscopy of Ground-State Perylene to the Franck-Condon profile. Subsequent irradiation can interrogate these levels via "hot-band" spectroscopy, as Hopkins et al.23 demonstrated in their study of the ground-statevibrational dynamics of alkylbenzenes. Detection of the ground-state level populations with a high-power, narrow-band laser requires only a few percent predissociation in competition with fluorescence dccay. Experiments todetect laser-inducedhot bands complement high-resolution dispersed emission spectroscopy, since both techniques can examine the properties of vibrational levels in both SOand SI. Hot-band techniques have the advantage of laser-limited spectral resolution and sensitivity to dynamics in the electronic ground state. In this respect, they complement stimulated emission pumping and related multiphoton experiment~.2~Js Hot-band studies of directly excited, uncomplexed aromatic molecules are the counterpart of hole-buming (absorptiondepletion) studies.13J6J7 In the latter, the redistribution of population during an excitation-fluorescence cycle is probed via the reduction of preexisting absorptions. Hot-band experiments, on the other hand, probe the newly populated levels directly. In addition, theuse of a pump-dump sequenceto populatevibrational levels of the ground state can selectively amplify weaker hotband transitions. Knight and co-workers2* used this kind of approach to study the effect of collisions on the vibrational relaxation of ground-state p-difluorobenzene. These principles can be applied to the predissociation of molecular aggregates. After vibrational predissociation near the threshold of an S1-excitedvan der Waals complex, the population of the product aromatic molecules is distributed over low-lying vibrational levels of SI. Subsequent fluorescence (*IO ns) then populates ground-state levels of the aromatic, creating a distribution that can be probed after a suitable delay via "hot" bands in the fluorescence excitation spectrum. Since the hot-band transitions are narrow, this approach offers the opportunity to detect specifically those molecules that fluoresced following predissociation. If significant amounts of uncomplexed species are coincidentally excited, their fluorescence spectra will be broadened by vibrational coupling, and they will usually not lead to narrow hot-band spectra. In this paper, we demonstrate the application of hot-band spectroscopy to perylene and the predissociation of some of its complexes. 2. Experimental Section
The supersonicjet apparatus has been described elsewhere. A pulsed solenoid valve operated at 8-10 Hz, with helium carrier gas at =20 psig and an orifice diameter of 0.75 mm, to produce an expanding free jet. Perylene was entrained in the helium flow via a small oven maintained at =200 OC. The oven consisted of a few turns of heating tape around the pulsed valve and the last centimeter or so of 6-mm steel tube used to suspend the valve insidethe vacuumchamber. A chromel+nstantan thermocouple monitored the temperature. The perylene sample was held in place by a plug of glass wool. Sample gases were introduced by bubbling through liquid paraffin in a side arm, controlled by a needle valve. A Molectron UV-14pulsed N2laser pumped two separatedye lasers, each tuned by a grazing-incidencegrating. One pulse (1) was directed into the vacuum chamber after an optical path of a few feet. The second (2) passed through an optical fiber delay of 120 ns before entering the chamber, exactly counterprop agating with the first beam. The spot size at the jet was -51 mm. Two detectors were used: a reference photomultiplier used at low gain detected fluorescence excited by the pump pulse, while the signal phototube was gated to avoid crosstalk from the pump channel. Most of the experiments reported here involved two photons, in which fluorescence excited by pulse 1 populates ground-state
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The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 0719
i
Delay
W
I
Figure 1. Energy diagram illustratingthe generationof hot-bandsignals using two laser pulses separated by about 120 ns.
A
2A
1
T
-.
120 ns
Figure2. Energy diagramillustratingthe generationof hot-bandsignals using three laser pulses. The second and third pulses are derived from splitting a single pulse (Le., 90%to 2A and 104% to 2B), sending pulse 2A to the jet delayed after pulse 1 by a few ns and pulse 2B by 120 ns.
levels, after which the delayed pulse 2 reexcites fluorescence through hot-band transitions (see Figure 1). An alternative approach uses three pulses. As Figure 2 shows, the excited state is generated by an initial pulse as in the two-pulse case. In the present experiments, 90% of the intensity of pulse 2 is diverted (2A) to be 5 ns behind pulse 1. Therefore, a significant fraction of the& population isstimulateddownward (i.e., before significant fluorescence occurs) to levels of the ground state as the second laser is tuned. The ground-state population is then sampled by the remainder of pulse 2 (pulse 2B), which has passed through the 20-m optical fiber. Notably, those molecules that have been stimulated downward by pulse 2A and that have not undergone further relaxation can be reexcited to SIby the delayed portion of the same pulse (i.e., pulse 2B). This selectively amplifies those hot-band transitions that also appear in the emission spectrum, providing a straightforward means of measuring the fluorescence spectrum with laser-bandwidth-limited resolution and high sensitivity.
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3. Results aod Discussion
6
3.1. Hot Bands of Bare Peryleoe. 3.2.1. and A:, Excitation. Following vibronic excitation, fluorescence carries a molecule to a distribution of levels in the electronic ground state determined by the emission Franck-Condon profile. A sequence of fluorescence spectra of perylene obtained following excitation via the 353; sequence (n = 0-3) is shown in Figure 3. Table I lists the relevant Franck-Condon factors, both in absorption and emission. Subsequent reexcitation of the system can detect both the loss of absorption resulting from the depletion of the zero-point level (hole buming) and new excitation resonancesdue to the fluorescentpopulation of nonzerovibrational levels (hot bands).
Wittmeyer and Topp
TABLE II: Limiting Flporescence Excitation and Hot-Band
dV=
_-3
-2
-1
0
1
2
3
1
.d
I
1057
I1
t s
-
s
4
1
w
wavenumber ofprow
no. excitation
1057 705 353 0
0.01 0.05 0.20 0.41
"8 I $ 1
1
Y
1
705
I
353
705
1057
0.006 0.036 0.152 0.252
0.005 0.039 0.132 0.228
0.005 0.031 0.130 0.213
0.046 0.012 o.oO06
0.040 0.010 0 . m
0.033
Depletion
I
I
initial excitation wavenumberb 0
-353 -705 -1057
n
-
0.007 0.046 0.169 0.299
Hot Bands 0.027 0.002 0.m5
0.005
o.oooo5
Numbers refer to the sum of the Franck-Condon factors for the constituent transitions at a particular probe wavenumber. Calculations used cq 1. * Relative to the 0; transition. a
for 353: of 0.01. The remaining signal at the 0; position from these levels alone should then be 0.299/0.41,or 73% of the signal observed without saturation. This is consistent with the results of hole-burning e~periments.~~ We can summarize this in an equation as follows:
1050
700
350
0
-350 -700 -1050 -1400
Relative Wavenumber Figure 3. Sequence of dispersed fluorescence spectra for uncomplexed pcrylene, following excitation into the sequence of bands (A 3 353 an-').This illustrates the evolution of the Franck-Condon profile and the differentdistributionsof ground-statelevelspopulated by fluormxnce. The 'S" in the top trace indicates a scattered light contribution, which overwhelms the weak 3-0 (6u = -3) band.
e3
TABLE I: Franck-Condon PrONes for Perylene Levels (353Cm-I Sequence).
3 4 1 2 0 -2 -1 bv= -3 X.b 434.6 428.1 421.7 415.5 409.5 430.7 398.1 392.5 398.1 403.7 409.5 415.5 421.7 428.1 434.6 441.4
0 353 705 1057
0.01
0.05 0.11
0.20 0.23 0.18
0.41 0.10 0.006 0.01
0.20 0.23 0.18 0.11
0.05 0.11 0.15 0.16
0.01 0.03 0.05 0.08
0.001 0.004 0.01 0.02
a Table I shows Franck-Condon profiles for absorption and emission spectra involving the sequence of levels 353, or 353,. The absorption data denote the spectra from the ground-statelevels 353,, whereas the emission data refer to the spectra from the excited state levels 353'". The numerical values are taken from a harmonic-oscillator fit to the experimentalspectra but also include a factor of 0.67 to allow for other vibronic transitions not shown here.
Following excitation of the 0; resonance, the absorption spectrum originally 100% from the zero-point level is replaced by one corresponding to population partly in other levels. If the absorption is saturated by a pulse shorter than the fluorescence decay time, yet rotational coherence effects can be neglected, at the end of the pulse there should be a 5050 population split between the zero-point levels of S1and SO.Of the 50%in SI, 41% will subsequentlyfluoresceto the zero-point level of SO.It happens that the 20% of this excited population that fluorto 3531 can still absorb photons resonant with 0;. (Le., 415.5 nm for perylene), because there is a very small vlbrational energy shift between SIand SO.Therefore, this Av = 0 transition also can contribute to the signal, although the Franck-Condon factor of the 1 1 transition is now 0.10,compared with 0.41 for 0 0 (see Table I). In addition, 5% of the excited moleculm will fluoresce to 3532, which has an absorption Franck-Condon factor at 415.5 nm of 0.006. The last level in this sequence that can contribute a significant resonant signal is 3533, which receives 1% of the fluorescent population and has a Franck-Condon factor
-
-
Here, thef are Franck-Condon factors, the subscript i denotes the vibrational level in SIirradiated by the saturation pulse, k is the intermediate level in SOreached by fluorescence, and 6 is the vibrational quantum number change in the band probed. The quantities a and @ represent the residual populations in the radiatively coupled ground (a)and excited (8) states at the end of the laser pulse; commonly, a @ 0.5. For S < 0 (i.e., for hot bands),fa = 0. Table I1 summarizes the results of calculations using eq 1, for vibronic bands in the series 353:; for (r = fl = 0.5. Thesecorrespond toboth hot bands and saturated absorption bands, showing that depletion intensities of more than 35% can be observed, purely from the emission Franck-Condon profde, and independently of vibrational coupling. Table I1 also predicts that hot-band intensities can exceed 10% of the unsaturated 0; transition. Both predictions have been verified experimentally. Figure 4 shows a sequence of hot-band spectra obtained following excitation of perylene into a sequence of levels 353r. These spectra all contain prominent hot-band features due to 'A: and ,A ': at -353 and -705 cm-l, respectively. The main subsidiary features in the hot-band sequence of Figure 4 are due to the 'G" mode (see next section). This results in resonances displaced =97 cm-' (Gi) and -195 cm-l (G:) to higher energies than the main hot bands. For example, the prominent band at -257 cm-l corresponds to AF'G;, while that at -155 cm-1 corresponds to AG :' ;. According to the dispersed emission spectra of Figure 3, several different ground-state quantum levelsof the 3 5 3 4 " ' progression are populated. For example, fluorescence following excitation of S1via 4 (+353 cm-l) populates comparably the u = 1-3 levels of the ground state, via the Au = &2 transitions, respectively. AU of these have significant Franck-Condon factors for a Av = -1 hot-band transition at 421.7nm, according to Table I. Similarly, excitation produces levels 1,3, and 4,whereas 4 (1057cm-l) excitation results in roughly equal amounts of the ground-state levels 1, 2,4,and 5, and very little 0 and 3. The presence of a single hot band at -353 cm-l (421.7nm) in each trace of Figure 4 shows that all of the A ' : transitions fall within the operational bandwidth of -2 cm-l. Similar observations are possible for the 'A : bands, seen at -705 cm-l, near 428 nm. This proves that this *A"-band sequence is highly harmonic in both the ground and excited states and that the frequency is effectivelythe same in the ground and excited states.
--
4
The Journal of Physical Chemistry, Vol. 97, No. 34, 1993 8721
Hot-Band Spectroscopy of Ground-State Perylene
1
1057
f
..--'---J-I 0
-100 -200 -300 -400
-500
-600
-700
Relative Wavenumber Fioprr 4. Sequence of two-pulse hot-band spectra for bare perylene, following a pump pulse into the same levels of SIas for the fluorescence spectra of Figure 3. Despitethcsignihnt differencesin level populations, the hot-band spectra are very similar due to the harmonicity of the A mode.
These simple spectra also show little or no vibrational coupling in either So or S1 for this sequence of levels. Unlike the SIcase, where the observation 'window" is equivalent to the fluorescence decay time (i.e., -10 ns), for ground-state species the present experiments sample the vibrational level population after 120ns. This serves to emphasize the weakness of vibrational coupling of the sequence of "A" modes of ground-state perylene. We have used thrwpulse sequences in some cam to clarify assignments of hot-band transitions. In the normal fluorescence spectrum, the transition 353: has a Franck-Condon factor of 0.20 such that, following 0; excitation, 20% of the population terminates in level 3531. On the other hand, using a saturation flux from pulse 2A (DUMP), this amount can be increased to 50%. representing an amplification of at least a factor of 2. Weaker hot bands, such as 550: or 353;, can be amplified by more than a factor of 10. Although laser power limitations in the present experiments did not permit full amplification, in some cases large differences can be seen between spectra taken with, and without, the DUMP pulse. For example, Figure 5 compares two such runs for 0; excitation of bare perylene. As the data show, use of the DUMP pulse brings many weaker resonances above the experimental noise. It is significantthat ground-statevibrational excitations in the regime 800-900 cm-l (e&, 'R", 'A C") persist after 120 ns. Since the intensities reflect the FranckCondonfactors, significantlylonger relaxationtimes are indicated. Thisemphasizesthat many levels of thislarge moleculeare weakly coupled. At high laser intensity,thegreatest amplificationresults for the weakest bands,consistent with predictions. This procedure also allows more confident assignment of vibronic transitions and of the ground-state energies. These results, obtained at
