Peter M. Felker

Purely rotational coherence effect and time‐resolved sub‐Doppler spectroscopy of large molecules. I. Theoretical

In this and the accompanying paper we present a theoretical treatment and experimental study, respectively, of the phenomenon termed purely rotational coherence. This phenomenon has been demonstrated to be useful as a time domain means by which to obtain high resolution spectroscopic information on excited state rotational levels of large molecules [Felker et al., J. Phys. Chem. 90, 724 (1986); Baskin et al., J. Chem. Phys. 84, 4708 (1986)]. Here, the manifestations in temporally resolved, polarization-analyzed fluorescence of coherently prepared rotational levels in samples of isolated symmetric and asymmetric top molecules are considered. These manifestations, for reasonably large molecules at rotational temperatures characteristic of jet-cooled samples, take the form of polarization-dependent transients and recurrences with temporal widths of the order of tens of picoseconds or less. The transients, which arise from the thermal averaging of many single molecule coherences, are examined with respect to their dependences on molecular parameters (rotational constants, transition dipole directions) and experimental parameters (polarization directions and temperature). A physical picture of rotational coherence as a reflection of the time-dependent orientation of molecules in the sample is developed. And, the influence of rotational coherence in experiments designed to probe intramolecular energy flow is discussed. In the accompanying paper, we present experimental results for jet-cooled t-stilbene and anthracene. For t-stilbene we determine rotational constants for vibrational levels in the S1 electronic state (from the recurrences) and we monitor the trends in rotational coherence vs vibrational coherence as the total energy in the molecule increases.

Jet spectroscopy of anthracene and deuterated anthracenes

Fluorescence excitation and SVL fluorescence spectra of jet-cooled h10-, 9d1-, 9,10d2-, and d10-anthracene are reported. Ground state vibrational assignments are presented for all these species and are compared with literature values. In addition, assignments for the first excited singlet state of h10-anthracene are made using SVL spectra and rotational band contours as guides. The work presented herein serves as an essential reference for other work from this research group concerning the dynamics of excited anthracene (see accompanying papers), and completes the spectroscopy of the polyacene series.

Picosecond excitation and TRANS-CIS isomerization of stilbene in a supersonic jet: Dynamics and spectra

New details of the dynamics and spectroscopy of trans-stilbene are revealed by picosecond excitation of jet-cooled and collisin-free molecules. The fluorescence spectra clearly show the low-frequency torsional modes of the ground state. The time-resolved fluorescence following S_1 excitation exhibit exponential decay times ranging from 2.7 ns to ≈190 ps for excitation wavelengths varying from 3102 A (0 excess energy) to 2850 A (2851 cm^(−1) excess energy). The results indicate that an energy threshold for isomerization occurs at ≈1200 cm^(−1). The redistribution of vibrational energy from the optically excited modes to the ethylene bond torsional modes leading to isomerization is discussed.

Quantum beats and dephasing in isolated large molecules cooled by supersonic jet expansion and excited by picosecond pulses: Anthracene

The modulation depth and lifetime were measured for quantum beats observed in the fluorescence decay of anthracene. (AIP)

Dynamics of intramolecular vibrational‐energy redistribution (IVR). II. Excess energy dependence

The results of picosecond‐resolved measurements of intramolecular vibrational‐energy redistribution (IVR) in jet‐cooled anthracene at different excess energies are presented. From these results, the nature of IVR as a function of vibrational energy, the relevant time scales for the process, and the details of pertinent vibrational couplings are determined.

Picosecond dynamics and photoisomerization of stilbene in supersonic beams. II. Reaction rates and potential energy surface

Using picosecond excitation in a supersonic jet, we present a full account of our earlier report on the dynamics of state-selective photoisomerization of t-stilbene. Collisionless isomerization in this case indicates the twisting of the molecule about the ethylene bond away from the trans configuration Central to this reaction is the question of vibrational energy redistribution or IVR. From direct (single vibronic level) time-resolved measurements, relative fluorescence quantum yields from relaxed and unrelaxed states, and a thorough vibrational analysis from excitation and dispersed fluorescence spectra (previous paper), the following conclusions are reached: (i) The IVR yield is state selective being more extensive from combination modes than from fundamental modes of similar energy. The IVR yield becomes very significant above [approximately-equal-to]900–1000 cm^−1. The rate is much faster than the reaction at all energies studies. (ii) The barrier to isomerization is observed at 3.3±0.2 kcal/mol (1100–1200 cm^−1). The radiative lifetimes, measured from the 0° level fluorescence decays, are 2.7±0.1 ns (h12) and 2.5±0.1 ns (d12). (iii) The observed isomerization rates in the isolated molecule are approximately an order of magnitude less than the calculated RRKM rates and observed solution phase rates. (iv) The apparent non-RRKM behavior in the isolated behavior is explained by considering the nature of IVR and by adopting a diabatic representation of the reactive surface (i.e., an allowed surface) using a Landau–Zener–Stueckelberg model. (v) Finally, we compare t-stilbene with other related isolated molecules and to solution phase t-stilbene results in order to assess the role of mode mixing and the nature of the reactive surface.

Dynamics of intramolecular vibrational‐energy redistribution (IVR). I. Coherence effects

In this series of papers, theoretical and experimental results concerning the dynamical manifestations of intramolecular vibrational-energy redistribution (IVR) in temporally resolved fluorescence are presented. In this paper (I) we present a general treatment of IVR and coherence effects in multilevel vibrational systems. Specifically, the concern is with the derivation of the characteristics of the beat-modulated fluorescence decays which arise from vibrational coupling among N levels within a molecule. Relations connecting quantum beat frequencies, phases, and modulation depths to coupling parameters are presented. Likely sources of deviation of experimental results from theoretical predictions are considered. And, finally, the direct link between IVR and time-resolved fluorescence experiments is discussed with emphasis placed on the physical interpretation of vibrational quantum beats and the nature of IVR as a function of vibrational energy in a molecule.

Picosecond dynamics and photoisomerization of stilbene in supersonic beams. I. Spectra and mode assignments

In this and the following paper, we present a full account of our earlier report [Syage et al., Chem. Phys. Lett. 88, 268 (1982)] on the spectra and picosecond dynamics of stilbene isomerization in supersonic jets. The jet-cooled excitation and dispersed fluorescence spectra of t-stilbene-h12 and -d12 are reported and assigned for the Bu 0 0 wavelengths for h12 and d12 (in excitation) are 3101.4 and 3092.5 A, respectively. Previously unidentified low frequency modes (as low as 20 cm^−1 in S0 for -h12) have been observed and tentatively assigned as out-of-plane modes of au symmetry in C2h. This indicates that t-stilbene has a propeller-like geometry involving phenyl rotation (i.e., C2 symmetry). A Franck–Condon analysis of the low frequency modes and particularly the ag, nu25 in-plane symmetric bend mode indicates that a large geometry change takes place upon electronic excitation possibly due to a delocalization of double bond character from the Ce–Ce bond to Ce–[cursive phi] bond. The geometry change of the in-plane Ce–Ce–[cursive phi] between S1 and S0 was determined from the Franck–Condon and a normal mode analysis to be 1.3°±0.3°. The rms amplitude of this bend motion for the symmetric nu25 bend mode (for one quanta in S0) is | ^2|^1/2=1.0±0.2°. Most ag modes involving benzene-type vibrations (other than C–H stretch modes) have been assigned. Dispersed fluorescence spectra exhibited a broad background indicative of IVR which increased rapidly with S1 vibrational energy. The spectra were completely diffuse above 1200 cm^−1 which is consistent with the barrier for isomerization being at about 1100–1200 cm^−1. The excitation spectra show a rapid decline in intensity at higher energies due to the process of isomerization which competes with radiative decay. However, sharp (albeit weak) structure could still be discerned at energies well in excess of 2000 cm^−1. In the accompanying paper, we present results on the dynamics of isomerization and its dependence on mode mixing and the nature of the reactive surface (adiabatic vs diabatic).

Dynamics of intramolecular vibrational‐energy redistribution (IVR). III. Role of molecular rotations

Experimental results on jet‐cooled anthracene pertaining to the role of rotations in IVR processes are presented. For theoretical comparison, we consider the effects of molecular rotational level structure on the beat‐modulated decays that arise as manifestations of IVR. It is shown theoretically for anharmonic coupling that small differences in rotational constants between coupled vibrational states give rise to decays, the beat envelopes of which decay faster than the unmodulated portions of the decays. These envelope decay rates are shown to be rotational temperature dependent. The experimental results reveal behavior entirely consistent with the theoretical expectations. The results also show that although rotational effects are present in experimental decays, they are not so marked as to wash out the manifestations of vibrational coherence.

Raman‐vibronic double‐resonance spectroscopy of benzene dimer isotopomers

1. B.F. Henson, G.V. Hartland, V.A. Venturo, R.A. Hertz, and P.M. Felker, Chem. Phys. Lett.176,91 (1991). 2. B.F. Henson, G.V. Hartland, V.A. Venturo, and P.M. Felker -- to be submitted.