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The intramolecular vibrational redistribution (IVR)-assisted ionization-gain stimulated Raman spectroscopy (IGSRS) and the jet-cooled fluorene are described. The fluorene spectra has shown extensive vibrational coupling interactions involving both the aliphatic and aromatic CH-stretching first excited states with nearby background states.

The first excited state (S1) intermolecular potential energy surface for the fluorobenzene-argon van der Waals complex is evaluated. Results are compared to the experimental data available and to those found in previous theoretical papers on ground state potentials for similar complexes.

The first excited state ([S.sub.1]) intermolecular potential energy surface for the p-difluorobenzene-Ar van der Walls complex was evaluated using the coupled-cluster method and the augmented-cc-pVDZ-3321 basis set. The results when compared to the benzene- and fuorobenzene-AR [S.sub.0] intermolecular potential energy surfaces show stronger interaction in the excited state, the dissociation energy being larger and the bond distance shorter.

The ground state intermolecular potential energy surface for the p-difluorobenzene-Ar van der Waals complex is evaluated using the coupled cluster singles and doubles including connected triple excitations [CCSD(T)] model and the augmented correlation consistent polarized valence double-zeta basis set extended with a set of 3s3p2d1f1g midbond functions. The results clearly improve previously available data and show the importance of a good correlation method and basis set when dealing with van der Waals complexes.

Presents an approach toward the dynamically exact calculation of intermolecular states in molecule-large molecule complexes. Employment of an intermolecular Hamiltonian; Use of filter diagonalization techniques to diagonalize the Hamiltonian; Interpretation of the results of the calculation in terms of five internal rotation states.

Splittings of the translation–rotation (TR) eigenstates of the solid light-molecule endofullerenes M@C60 (M = H2, H2O, HF) attributed to the symmetry breaking have been observed in the infrared (IR) and inelastic neutron scattering spectra of these species in the past couple of years. In a recent paper [Felker et al., Phys. Chem. Chem. Phys., 2017, 19, 31274], we established that the electrostatic, quadrupolar interaction between the guest molecule M and the twelve nearest-neighbor C60 cages of the solid is the main source of the symmetry breaking. The splittings of the three-fold degenerate ground states of the endohedral ortho-H2, ortho-H2O and the j = 1 level of HF calculated using this model were found to be in excellent agreement with the experimental results. Utilizing the same electrostatic model, this theoretical study investigates the effects of the symmetry breaking on the excited TR eigenstates of the three species, and how they manifest in their simulated low-temperature (5–6 K) near-IR (NIR) and far-IR (FIR) spectra. The TR eigenstates are calculated variationally for both the major P and minor H crystal orientations. For the H orientation, the calculated splittings of all of the TR levels of these species are less than 0.1 cm−1. For the dominant P orientation, the splittings vary strongly depending on the character of the excitations involved. In all of the species, the splittings of the higher rotationally excited levels are comparable in magnitude to those for the j = 1 levels. For the levels corresponding to purely translational excitations, the calculated splittings are about an order of magnitude smaller than those of the purely rotational eigenstates. Based on the computed TR eigenstates, the low-temperature NIR (for M = H2) and FIR (for M = HF and H2O) spectra are simulated for both the P and H orientations, and also combined as their weighted sum (0.15H + 0.85P). The weighted sum spectra computed for M = H2 and HF match quantitatively the corresponding measured spectra, while for M = H2O, the weighted sum FIR spectrum predicts features that can potentially be observed experimentally. [ABSTRACT FROM AUTHOR]

Examines the interaction between a polarized optical field and the polarizability of a symmetric top in the strong interaction limit. Use of the interaction as a tool in interpreting real measured band contours; Analysis of pendular eigenfunctions and eigenergies; Comparison of theoretical expectations regarding pendular band contours with direct numerical simulations.

Discusses the results of nonlinear Raman spectroscopy on the intermolecular transitions of the perprotonated and perdeuterated isotopomers of benzene-Ar. Bands observed for each species; Performance of band-contour measurements; Comparison of the results to simulated contours.

The results of nonlinear Raman experiments on two intermolecular bands of expansion-cooled naphthalene trimer at 0.03 cm-1 resolution are reported. The polarization and intensity dependences of the observed band contours strongly indicate that the observations pertain to transitions between pendular states that result from the interaction of a strong optical field and the permanent polarizability anisotropy of the trimer. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Mass-selective, ionization-detected stimulated Raman spectroscopies have been applied to a study of low-frequency structure in the ground states of benzene dimer isotopomers. The results reveal two Raman bands below 10 cm-1, as well as structure in the range of 47 to 53 cm-1. Tentative assignments for the bands are proposed. [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]

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. [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]

Theoretical and experimental results pertaining to the excitation of intermolecular vibrations in the Raman and vibronic spectra of aromatic-containing, weakly bound complexes and clusters are reported. The theoretical analysis of intermolecular Raman activity is based on the assumption that the polarizability tensor of a weakly bound species is given by the sum of the polarizability tensors of its constituent monomers. The analysis shows that the van der Waals bending fundamentals in aromatic–rare gas complexes may be expected to be strongly Raman active. More generally, it predicts strong Raman activity for intermolecular vibrations that involve the libration or internal rotation of monomer moieties having appreciable permanent polarizability anisotropies. The vibronic activity of intermolecular vibrations in aromatic-rare gas complexes is analyzed under the assumption that every vibronic band gains its strength from an aromatic-localized transition. It is found that intermolecular vibrational excitations can accompany aromatic-localized vibronic excitations by the usual Franck–Condon mechanism or by a mechanism dependent on the librational amplitude of the aromatic moiety during the course of the pertinent intermolecular vibration. The latter mechanism can impart appreciable intensity to bands that are forbidden by rigid-molecule symmetry selection rules. The applicability of such rules is therefore called into question. Finally, experimental spectra of intermolecular transitions, obtained by mass-selective, ionization-detected stimulated Raman spectroscopies, are reported for benzene–X (X=Ar, –Ar2, N2, HCl, CO2, and –fluorene), fluorobenzene–Ar and –Kr, aniline–Ar, and fluorene–Ar and –Ar2. The results support the conclusions of the theoretical analyses and provide further evidence for the value of Raman methods in characterizing intermolecular vibrational level structures. [ABSTRACT FROM AUTHOR]

Size-selective Raman spectra of carbazole–(Ar)n clusters (n=0–14) obtained between about 1305 and 1325 cm-1 by mass-selective ionization-loss stimulated Raman spectroscopy are reported. The spectra are interpreted with respect to vibrational energy flow and argon evaporation, and are compared with vibronic spectroscopic results on the species. [ABSTRACT FROM AUTHOR]

The characteristics and origin of a new type of rotational coherence effect—asymmetry transients—are considered. Asymmetry transients are of two types, which we call C-type and A-type, respectively. C-type features are spaced by time intervals equal to 1/(4C). A-type features are spaced by time intervals equal to 1/(4A). Both types of transients can provide information on rotational constants that is not available from other types of rotational coherence effects. We show that the C-type transients arise from coherences between rotational eigenstates of the form Jτ and (J+2)τ-2 (in the Jτ notation) and that the A-type ones arise from coherences between eigenstates Jτ and (J+2)τ+2. The fact that such coherences produce asymmetry transients is shown to be a consequence of the limiting behavior of asymmetric top energy levels at high values of J. Experimental results in which asymmetry transients are present are reported for jet-cooled perylene, tryptamine (D conformer), 1-naphthol–water, and fluorene–benzene. [ABSTRACT FROM AUTHOR]

Theoretical and experimental results relating to the picosecond laser implementation of Fourier transform stimulated emission spectroscopy (FT-SES) and Fourier transform hole-burning spectroscopy (FT-HBS) in molecular beams are presented. It is shown that the resolution in the schemes is only limited by the length of the interferogram taken, and not by factors such as the excitation pulsewidths, excitation bandwidths, or the delay between the pump and probe laser pulses. In addition, the factors which determine the homogeneous and Doppler broadening in FT-SES and FT-HBS spectra are examined. It is found that ground-state damping factors determine the homogeneous broadening in FT-SES and excited-state damping factors determine such broadening in FT-HBS. Doppler broadening in both of the methods is shown to be that associated with two-photon resonant vibrational transition frequencies rather than one-photon resonant vibronic transition frequencies. The characteristics of the Fourier transform methods are compared to those of their frequency-domain counterparts. [ABSTRACT FROM AUTHOR]

We present a perturbation theory analysis of four time-resolved fluorescence depletion schemes that are useful, or potentially useful, in rotational coherence spectroscopy. The analysis shows that ground-state rotational constants determine the rotational coherence effects in fully resonant, time-resolved stimulated Raman-induced fluorescence depletion (TRSRFD), excited-state rotational constants determine such effects in time-resolved stimulated emission spectroscopy (TRSES), and both ground- and excited-state constants do so in time-resolved fluorescence depletion (TRFD). An analysis of a variant of the TRSRFD scheme in which the stimulated Raman process is not resonance-enhanced shows that this method gives rise to qualitatively different rotational coherence effects than fully resonant TRSRFD. It is argued that the scheme may, nevertheless, be a viable means of ground-state rotational coherence spectroscopy. Expressions for the calculation of rotational coherence effects in TRFD, TRSRFD, and TRSES traces are also presented. Such expressions are used to show that the magnitudes of rotational coherence transients are similar in all three schemes. Finally, experimental results on molecular iodine are presented to show that, indeed, both ground- and excited-state rotational coherence effects are manifest in TRFD traces. [ABSTRACT FROM AUTHOR]

A perturbation theory analysis of Fourier transform (interferometric) versions of coherent Raman scattering and ionization-detected (or fluorescence-detected) stimulated Raman scattering is presented. The analysis has the principal aim of examining the information content of these spectroscopies as a function of the temporal characteristics of the excitation fields. The general conclusion of the analysis is that the information content of the techniques does not depend on excitation pulsewidths and, in particular, that spectral resolution is not limited by such pulsewidths. This is an important result because it allows for the use of short-pulse laser systems in implementing the methods. It is also shown, however, that the temporal characteristics of the excitation fields do influence the practical implementations of the spectroscopies significantly in that signal-to-noise ratios, Doppler broadening, etc. can be dependent on these characteristics. [ABSTRACT FROM AUTHOR]

Symmetry breaking has been recently observed in the endofullerenes M@C60 (M = H2, HF, H2O), manifesting in the splittings of the three-fold degenerate ground states of the endohedral ortho-H2, ortho-H2O and the j = 1 level of HF. The nature of the interaction causing the symmetry breaking is established in this study. A fragment of the solid C60 is considered, comprised of the central C60 molecule surrounded by twelve nearest-neighbor (NN) C60 molecules. The fullerenes have either P (major) or H (minor) orientational orderings, and are assumed to be rigid with Ih symmetry. Only the central C60 is occupied by the guest molecule M, while the NN fullerenes are all empty. The key proposition of the study is that the electrostatic interactions between the charge densities on the NN C60 molecules and that on M inside the central C60 give rise to the symmetry breaking responsible for the measured level splittings. Using this model, the M@C60 level splittings of interest are calculated variationally and using perturbation theory, for both the P and H orientations. Those obtained for the dominant P orientation are in excellent agreement with the experimental results, with respect to the splitting magnitudes and patterns, for all three M@C60 systems considered, pointing strongly to the quadrupolar M–NN interactions as the main cause of the symmetry breaking. The level splittings calculated for the H orientation are about 30 times smaller than the ones in the P orientation. [ABSTRACT FROM AUTHOR]