A model and detailed calculations are presented to describe the flow of energy in a shocked solid consisting of large organic molecules. The shock excites the bulk phonons, which rapidly achieve a state of phonon equilibrium characterized by a phonon quasitemperature. The excess energy subsequently flows into the molecular vibrations, which are characterized by a vibrational quasitemperature. The multiphonon up pumping process occurs because of anharmonic coupling terms in the solid state potential surface. Of central importance are the lowest energy molecular vibrations, or ‘‘doorway’’ modes, through which mechanical energy enters and leaves the molecules. Using realistic experimental parameters, it is found that the quasitemperature increase of the internal molecular vibrations and equilibration between the phonons and vibrations is achieved on the time scale of a few tens of picoseconds. A new mechanism is presented for the generation of ‘‘hot spots’’ at defects. Defects are postulated to have somewhat greater anharmonic coupling, causing the vibrational temperature in defects to briefly overshoot the bulk. The influence of the higher defect vibrational temperature on chemical reactivity is calculated. It is shown that even small increases in defect anharmonic coupling have profound effects on the probability of shock induced chemistry. The anharmonic defect model predicts a size effect. The defect enhanced chemical reaction probability is reduced as the particle size is reduced.

Shocked molecular solids: Vibrational up pumping, defect hot spot formation, and the onset of chemistry

One‐ and two‐color time‐domain probing of the resonant dichroic response of iodine in n‐hexane following femtosecond B‐X excitation at 580 nm is described. The detected signals contain both ground and excited state vibrational coherence contributions to the third‐order polarization. The dichroic response can be separated into positive and negative amplitude contributions: B‐X absorption and stimulated emission are positive but absorption from the B‐state can yield either positive or negative signals depending on the direction of the transition moment. Wave packet motion on both the ground and excited states of iodine is studied with a frequency tunable femtosecond probe. It is shown that the positive signals can be interpreted as B‐X dichroic response using the classical Franck principle. The classical Franck principle also provides information about the potential probed in absorption from the B state. From the probe wavelength dependent delay in the signal appearance, it is concluded that the absorptive signal for blue probe wavelengths arises from a repulsive state reached by solvent‐induced predissociation of the B state. Dephasing of B state vibrational coherence results from this solvent‐induced predissociation of iodine. We discuss the evolving reaction in terms of possible dissociative potential energy curves a1g(3Π) and a’0g+(3Σ−). The time evolution of the bluest probe dichroism signals is representative of continuing atom separation; the experiments have not yet probed large enough internuclear separations to evidence a buildup of dissociated product or momentum reversal, i.e., caging.

http://www.cchem.berkeley.edu/grfgrp/Papers/1993%20Scherer%20JChemPhys99153.pdf

Femtosecond wave packet and chemical reaction dynamics of iodine in solution: Tunable probe study of motion along the reaction coordinate

The well-known benzophenone intersystem crossing from S(1)(n,pi*) to T(1)(n,pi*) states, for which direct transition is forbidden by El-Sayed rules, is reinvestigated by subpicosecond time-resolved absorption spectroscopy and effective data analysis for various excitation wavelengths and solvents. Multivariate curve resolution alternating least-squares analysis is used to perform bilinear decomposition of the time-resolved spectra into pure spectra of overlapping transient species and their associated time-dependent concentrations. The results suggest the implication of an intermediate (IS) in the relaxation process of the S(1) state. Therefore, a two step kinetic model, S(1) --> IS --> T(1), is successfully implemented as an additional constraint in the soft-modeling algorithm. Although this intermediate, which has a spectrum similar to the one of T(1)(n,pi*) state, could be artificially induced by vibrational relaxation, it is tentatively assigned to a hot T(1)(n,pi*) triplet state. Two characteristic times are reported for the transition S(1) --> IS and IS --> T(1), approximately 6.5 ps and approximately 10 ps respectively, without any influence of the solvent. Moreover, an excitation wavelength effect is discovered suggesting the participation of unrelaxed singlet states in the overall process. To go further discussing the spectroscopic relevancy of IS and to rationalize the expected involvement of the T(2)(pi,pi*) state, we also investigate 4-methoxybenzophenone. For this neighboring molecule, triplet energy level is tunable through solvent polarity and a clear correlation is established between the intermediate resolved by multivariate data analysis and the presence of a T(2)(pi,pi*) above the T(1)(n,pi*) triplet. It is therefore proposed that the benzophenone intermediate species is a T(1)(n,pi*) high vibrational level in interaction with T(2)(pi,pi*) state.

The Benzophenone S1(n,π*) → T1(n,π*) States Intersystem Crossing Reinvestigated by Ultrafast Absorption Spectroscopy and Multivariate Curve Resolution

Degenerate four wave mixing (DFWM) is applied as a diagnostic to study OH and NH radicals in flames. DFWM is a coherent technique which offers the advantages of a highly collimated signal beam permitting efficient rejection of interfering radiation and requiring minimal optical access. Rotational temperatures have been determined from the DFWM spectra and are in close agreement with the temperatures measured using coherent anti-Stokes Raman scattering of nitrogen.

Degenerate four-wave mixing diagnostics on OH and NH radicals in flames

Diagrammatic perturbation theory combined with a spherical tensor treatment allows the degenerate four‐wave mixing (DFWM) signal resulting from an isotropic molecular sample to be decomposed into a sum of three multipole moments in the weak‐field (no saturation) limit. The zeroth moment gives the relative internal‐state population contribution, the first moment the orientation contribution, and the second moment the alignment contribution to the DFWM spectra. This treatment makes explicit how the magnitude of the DFWM signal depends on the polarizations of the other three beams and the collisional relaxation caused by the environment. A general expression is derived for the DFWM signal for an arbitrary geometric configuration of the beams (arbitrary phase matching geometry). Under the assumption that the rates of collisional relaxation of the population, the orientation, and the alignment are the same, simple analytic expressions are found for the most commonly used experimental configurations, which shou...

/pdf/reduction-of-degenerate-four-wave-mixing-spectra-to-relative-4pr7d2qaub.pdf

Reduction of degenerate four-wave mixing spectra to relative populations II. Strong-field limit

Picosecond transient grating experiments in sodium and iodine vapors, involving the 3S→3P and X→B transitions, respectively, are discussed in detail. Population gratings in sodium demonstrate that the technique can be used to measurevelocity distributions in the gas phase. It is shown that the time dependent transient grating signal is related to the Fourier transform of the velocity distribution. Similar experiments on iodine illustrate the effect of state changing collisions on the grating signal. Theoretical calculations for a model in which the change of state is caused by a single collision are given. Close agreement with the data is observed for the situation in which the collision takes the initial velocity into a random velocity distribution. From this model a collision cross section is determined. The results demonstrate that information on collision dynamics can be obtained from grating experiments. In addition, the sodium experiments are used to illustrate a new type of time domain high resolution spectroscopy. When the grating excitation pulses have perpendicular polarizations, a polarizationgrating, rather than the usual population grating, is formed. Diffraction from the sodiumpolarizationgrating shows larger time dependent oscillations in the diffraction efficiency. These oscillations yield the ground state and excited state hyperfine frequencies (1.77 GHz and 189 MHz, respectively). A detailed theoretical description of the origin of the oscillations is presented. The results suggest that polarizationgratingspectroscopy can have applications in other areas, such as molecular rotational dynamics.

Gas phase dynamics and spectroscopy probed with picosecond transient grating experiments

The magnetically active isotope of oxygen 17O has been used to probe the changes in the electron charge and spin density distributions in oxygen valence orbitals which occur when benzophenone is excited to its lowest triplet state. The data obtained include the optically detected magnetic resonance (ODMR) and electron‐nuclear double resonance spectra at both zero and high magnetic fields. New methods of analysis of zero‐field ODMR spectra, appropriate when the second‐order hyperfine splitting exceeds the quadrupole coupling, are described. This analysis yields the principal values of the electron fine‐structure (D), oxygen hyperfine (A), and oxygen quadrupole (Q) tensors, and the orientation of their principal axes with respect to the molecular frame. It is found, consistent with expectations for an nπ* state, that the direction of the largest component of Q is different from that of the ground state. It is also found, by two independent methods, that the principal transverse axes of A and Q do not confor...

Optically detected magnetic resonance studies of photoexcited 17O‐benzophenone. Orbital rotation in the lowest triplet state

Picosecond transient grating experiments in sodium vapor: velocity and polarization effects

A new experimental method for the direct observation of electronic excitation–phonon coupling is presented. The CARS signal from the 69 cm−1 optical phonon mode of naphthalene crystals is resonantly enhanced by coupling to the electronic states of dilute pentacene impurities. The resonantly enhanced signal decays over three times faster than the nonresonant decay in the same crystal. This is explained in terms of impurity perturbed domains.

Impurity perturbed domains: Resonant enhancement of bulk mode CARS by coupling to the electronic states of dilute impurities

Detection of deuterium quadrupole splittings by odendor in the excited triplet state of benzophenone

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