Energy dependence of electronic relaxation processes in polyatomic molecules

The Stark level‐crossing spectra of S0 D2CO are analyzed for evidence of energy level correlations and quantum ergodicity. The analysis for short and long range level correlations gives an apparent energy level spacing distribution which lies between the Poisson and GOE limits. However, the Stark level‐crossing method diminishes any existing correlations. The true level spacing distribution must be closer to the Gaussian orthogonal ensemble (GOE) than to the Poisson limit. Complete distributions of S1−S0 coupling matrix elements and S0 dissociation rates are reported and subjected to statistical tests for ergodicity. The distribution of S1−S0 coupling matrix elements indicates that the dynamics of intramolecular vibrational redistribution of energy (IVR) is very nearly quantum ergodic. There is strong coupling and free flow of energy among vibrational degrees of freedom in this molecule above its dissociation threshold. The average S0 decay rate can be accounted for by RRKM theory with tunneling correctio...

Eigenstate‐resolved unimolecular reaction dynamics: Ergodic character of S0 formaldehyde at the dissociation threshold

Time- and frequency-resolved fluorescence of dye molecules with femtosecond resolution gives new information on the intramolecular and intermolecular dynamics of large molecules in solution. We analyze data that were obtained recently by the upconversion technique together with new results: Access to vibrational coherence effects and to vibrational redistribution, thermalization, and cooling is illustrated for fluorescent dyes. The ultrafast fluorescence decay that accompanys internal conversion of a triphenylmethane dye is also analyzed.

Subpicosecond fluorescence dynamics of dye molecules

The electronic relaxation of biacetyl in the vapor phase

We have observed quantum beats in the reversible intersystem crossing of simple α‐dicarbonyls, and we have analyzed them to obtain information concerning the density of interacting states and the average intramolecular coupling energy. We have analyzed most of the biacetyl quantum beats using a method based on the properties of random matrices which is described elsewhere and we present the results here. We also present an analysis based on perturbation theory using the Fourier transforms of the quantum beats which is appropriate for understanding the methylglyoxal quantum beats. The density of vibrationally hot triplet states which interact with excited singlet states that are connected to the ground electronic state via optical excitation (∼22 000 cm−1) is found to increase with the amount of vibrational excitation in the accessible singlet state at roughly the same rate as the overall density of triplet vibrational stress increases. We find satisfactory agreement between the density obtained from the quantum beats and that calculated using well known analytical formulas and direct state counting. The density of interacting states increases with the rotational quantum number of the initially excited singlet rovibronic state. The average value of the magnitude of the spin–orbit interaction is 1–10 MHz independent of the amount of vibrational–rotational excitation present. Radiationless transitions in these highly excited molecules are evidently not subject to any overriding selection rules other than spatial symmetry, and conservation of total energy, total angular momentum, and nuclear spin. The biacetyl quantum beats are collisionally quenched with helium at the same rate as the overall fluorescence. The cross section for this process is roughly 300±200 A2.

J.M. Delory

Papers

Time interferences and nonexponential decays of quasi-isolated molecules