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Intermolecular kinetic-energy operators are derived (in the rigid monomer approximation) for solute–solvent[sub n] clusters of the type B–A[sub n], where B is a molecule and A is either an atom or a molecule. The operators are obtained for a body-fixed frame embedded in the B moiety and parallel to the principal axes of that species. They are expressed in terms of intermolecular coordinates that represent the projection along the body-fixed axes of position vectors pointing from the center of mass of B to the centers of mass of the A species. The results are particularly useful for calculations on clusters in which A–B interactions dominate over A–A interactions in the intermolecular potential energy surface and/or there is minimal interaction between subsets of the A moieties. This utility is demonstrated in variational calculations of intermolecular states in the (1|1) isomer of anthracene–He[sub 2]. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

In this paper we describe the results of picosecond fluorescence polarization (sub-Doppler) experiments designed to determine the role of purely rotational coherence in two jet-cooled molecules: trans-stilbene and anthracene. Observations of the manifestations of purely rotational coherence in t-stilbene are reported. The relationship of purely rotational coherence to molecular parameters (excited state rotational constants and transition dipole directions) is confirmed by comparison of our measurements with the results of the theory described in paper I [P. M. Felker and A. H. Zewail, J. Chem. Phys. 86, 2460 (1987)]. The sum of rotational constants B’ and C’ of the t-stilbene S1 electronic state is determined with a precision of better than 1 part in 700 (B’+C’=0.5132±0.0007 GHz). The influence of molecular beam expansion conditions and fluorescence detection conditions on our measurements is also investigated and compared with the theroretical findings of paper I. Also measurements of time-resolved and polarization-analyzed fluorescence as a function of excess vibrational energy in the S1 electronic states of both t-stilbene and anthracene are reported. We are able to distinguish the contribution of purely rotational coherence from the contributions of purely vibrational (or rovibrational) coherence to the evolution of fluorescence from the vibrationally excited molecule. The results are first analyzed on the basis of a model in which strict separability of vibrational and rotational motion is assumed. This provides a test of the extent of coupling of these motions and its influence on intramolecular vibrational energy redistribution (IVR). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]