A.P. Penner

Effect of line broadening mechanisms on resonance Raman excitation profiles: A possible method for measuring short lifetimes

Abstract The structure of a Raman excitation profile is shown to depend on the nature of the mechanism responsible for the lineshape in the absorption spectra. In principle, this makes it possible to separate line broadening corresponding to the lifetime of the excited state from solvent-induced broadening.

Exactly solvable models for vibronic coupling in molecular spectroscopy. III. The pseudo Jahn–Teller effect

A model system consisting of two electronic manifolds coupled through a nontotally symmetric mode of vibration is solved exactly and self‐consistently by the method described in Paper I [J. Chem. Phys. 65, 2071 (1976)]. As in I, the model is defined in terms of harmonic diabatic potentials, but the restriction to harmonic adiabatic potentials, applied in I, is lifted here. As a result, the adiabatic coupling operator which has the same analytical form as in I, can assume a much wider range of values. It leads to adiabatic potentials which in general are anharmonic and may exhibit a double minimum. The coupling is taken to be an odd function of the vibrational coordinate so that it describes the (pseudo‐) Jahn–Teller effect. Absorption and emission spectra are calculated for selected combinations of four spectroscopic parameters: (1) the electronic energy gap; (2) the diabatic harmonic frequency difference; (3) a linear adiabatic coupling parameter; and (4) a nonlinear (quasiquadratic) adiabatic coupling p...

Radiationless decay of vibronically coupled electronic states

Radiationless transitions from an optically prepared state to the ground state are studied on a model consisting of three electronic states and two harmonic modes of vibration. The effect of the upper excited state on the nonradiative decay properties of the lower excited state is investigated for systems in which these states are coupled through the same non‐totally‐symmetric mode that couples the lower excited state to the ground state. If only this mode is considered, the model is exactly solvable and allows one to test the assumption that the initially prepared state is an adiabatic Born–Oppenheimer state. This assumption is found to be accurate unless the zeroth‐order adiabatic vibrancy state from which the transition originates is very close to, e.g., within one vibrational quantum of, a zeroth‐order state of the upper excited state manifold. Strong nonadiabatic mixing occurs when a vibrationally excited level of the lower excited state is in resonance with a level of the upper state. In general, th...

Comparison of two linear approximations to vibronic coupling

Abstract Two commonly used linear approximation of vibronic coupling in molecules are investigated. It is shown that one leads to physically unaccepted potentials if the coupling is no extremely weak while the other leads to well-behaved, but anharmonic, adiabatic potentials.

Resonance Raman scattering in molecules with double minimum potentials

Abstract Resonance Raman excitation profiles and depolarization dispersion curves are calculated for model systems exhibiting pseudo Jahn-Teller and pseudo Renner-Teller effects.

Anharmonic potentials and vibronic energy levels for closely coupled molecular electronic states

Abstract New anharmonic adiabatic potentials are derived to represent pairs of closely coupled electronic states subject to a pseudo Jahn—Teller or pseudo Renner—Teller effect. These potentials are combined with harmonic diabatic potentials to construct a model that yields exact vibronic energy levels for arbitrary coupling strength.

A simple model of rotational effects in thermal decomposition

A simple analytical model of rotational effects in thermal decomposition is developed. The model describes the non-equilibrium population distribution of the reactant, as a function of the average vibrational energy lost per deactivating collision. The corresponding non-equilibrium rate constant is evaluated, and the results are compared to two previous models.

Nature of an optically prepared state undergoing radiationless decay

Abstract The assumption that the initially prepared state in a molecule undergoing internal conversion is an adiabatic Born-Oppenheimer state is tested on an exactly solvable three-state model.

On the strong coupling approximation in two-state molecular spectroscopy

Abstract Two different but complementary perturbation solutions to a two-state strong vibronic coupling model are presented.

Some properties of the mean first-passage time for thermal decomposition

We relate the mean first-passage time for thermal decomposition to the steady-state and equilibrium rate constants using a general separable transition probability model. These relations are used to assess the relative merits of various approximate rate constants.