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

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

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

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

Photodissociation of partially solvated molecules in beams by the picosecond-jet technique: Hydrogen bond breakage

In this communication we wish to report the measurement of photodissociation rates of jet-cooled solute-solvent complexes in various stages of solvation. The observation of fluorescence decays as a function of excess vibrational energy for isoquinoline (IQ), IQ-(methanol)n, and IQ-(water)n complexes reveals threshold behavior for the decays rates of 1:1 solute-solvent complexes. The thresholds at ~3 kcal/mol provide new information on the breakage of excited state hydrogen bonds

Direct Observation of Nonchaotic Multilevel Vibrational Energy Flow in Isolated Polyatomic Molecules

With picosecond spectroscopy and molecular beams it is shown that nonchaotic multilevel vibrational energy flow is present in large polyatomic molecules. This Letter reports on this novel observation and its probing of the fundamental process of energy redistribution in molecules

Sobre la ocurrencia del cretáceo superior marino en Coihaique , Provincia de Aisén

Using the CCSD­(T) model, we evaluated the intermolecular potential energy surfaces of the He–, Ne–, and Ar–phosgene complexes. We considered a representative number of intermolecular geometries for which we calculated the corresponding interaction energies with the augmented (He complex) and double augmented (Ne and Ar complexes) correlation-consistent polarized valence triple-ζ basis sets extended with a set of 3s3p2d1f1g midbond functions. These basis sets were selected after systematic basis set studies carried out at geometries close to those of the surface minima. The He–, Ne–, and Ar–phosgene surfaces were found to have absolute minima of −72.1, −140.4, and −326.6 cm^–1 at distances between the rare-gas atom and the phosgene center of mass of 3.184, 3.254, and 3.516 Å, respectively. The potentials were further used in the evaluation of rovibrational states and the rotational constants of the complexes, providing valuable results for future experimental investigations. Comparing our results to those previously available for other phosgene complexes, we suggest that the results for Cl₂–phosgene should be revised