Femtosecond wave packet spectroscopy: Coherences, the potential, and structural determination

Recently, we presented a formalism for extracting highly resolved spectral information and the potential of bound isolated systems from coherent ultrafast laser experiments, using I2 as a model system [Gruebele et al., Chem. Phys. Lett. 166, 459 (1990)]. The key to this approach is the formation of coherent wave packets on the potential energy curve (or surface) of interest, and the measurement of their scalar and vector properties. Here we give a full account of the method by analyzing the coherences of the wave packet in the temporal transients of molecules excited by ultrashort laser pulses, either at room temperature, or in a molecular beam. From this, some general considerations for properly treating temporal data can be derived. We also present a direct inversion to the potential and quantum and classical calculations for comparison with the experiments

Optical observation of "band‐to‐band" scattering by time‐resolved phosphorescence line narrowing: Exciton dephasing in a quasi‐one‐dimensional solid

Abstract Unavailable

Spin-quantization and spin-orbit coupling effects on the line shapes of triplet states. II. The "small" exciton problem

This paper presents a detailed study of the effect of spin‐quantization and spin–orbit coupling on the transition energies of triplet state dimers or small excitons. We consider both translationally equivalent (AA) and inequivalent (AB) dimers. For the AA and AB systems, we calculate transition frequency shifts induced by the spin–orbital coupling and by the spin–spin interactions between the plus (+) and minus (−) states of the dimer. As a result of these combined effects the selective coupling between the ± states of the singlet and the ± states of the triplet AA dimer system is not operative in the AB system. Furthermore, the role of the gas‐to‐crystal shifts and the intermolecular spin–spin interactions is to change the observed transition frequencies and hence cause a dispersion in the frequencies of the ± states. The relationship between such a dispersion in the AA and the same AB system is directly related to molecular parameters such as the strength of spin–orbital coupling. These results are applied to three experimental findings obtained for different dimer systems—phenazine, naphthalene, and tetrachlorobenzene dimers isolated in isotopically mixed crystals at T<2 °K. The phenazine results are reported here and the other data on naphthalene and tetrachlorobenzene were obtained from the literature. Agreement between theory and the recent experiments is encouragingly good

Vibronic dephasing of anharmonic molecules. II. Impurity molecules isolated in low-temperature matrices

The quantum‐mechanical theory of vibronic dephasing presented in the first paper of this series is applied to the case of a diatomic impurity dissolved in a solid rare‐gas host. An explicit expression for the pure dephasing rate T_2′^(−1) is derived in terms of microscopic properties of the impurity and host, and the effects of variations in the parameters characterizing these properties are investigated. The expression for T_2′^(−1) is applied specifically to the system Cl_2/Ar in order to relate the results to those of previous classical‐trajectory calculations and of experimental measurements. The significance of anharmonicity in the intramolecular potential curve (of the impurity) is demonstrated

Observation of high-energy vibrational overtones of molecules in solids: Local modes and intramolecular relaxations

In the last few years, the spectra of vibrational overtones (1) (at ~15-20,000 cm^-1) in large molecules have received considerable attention. The focus is on three problems dealing with the origin of relaxation at such high energies, the association of spectral band positions with the local modes (LM) in molecules, and the relevance of these spectra to possible selectivity in laser-induced chemistry

Characterization of vibrational overtones and "local" modes by emission spectroscopy

Abstract unavailable

Macromolecular structural dynamics visualized by pulsed dose control in 4D electron microscopy

Macromolecular conformation dynamics, which span a wide range of time scales, are fundamental to the understanding of properties and functions of their structures. Here, we report direct imaging of structural dynamics of helical macromolecules over the time scales of conformational dynamics (ns to subsecond) by means of four-dimensional (4D) electron microscopy in the single-pulse and stroboscopic modes. With temporally controlled electron dosage, both diffraction and real-space images are obtained without irreversible radiation damage. In this way, the order-disorder transition is revealed for the organic chain polymer. Through a series of equilibrium-temperature and temperature-jump dependencies, it is shown that the metastable structures and entropy of conformations can be mapped in the nonequilibrium region of a “funnel-like” free-energy landscape. The T-jump is introduced through a substrate (a “hot plate” type arrangement) because only the substrate is made to absorb the pulsed energy. These results illustrate the promise of ultrafast 4D imaging for other applications in the study of polymer physics as well as in the visualization of biological phenomena