Theory of fluorescence excitation spectra using anharmonic-coriolis coupling in S1 and internal conversion to S. I. General formalism

Abstract

A treatment of one-or two-photon fluorescence excitation spectra is described using the vibration-rotation coupling of zeroth order states in the excited electronic state and nonadiabatic coupling to the ground state. Using perturbation theory, experimental harmonic frequencies, an anharmonic force field, and various theoretical Coriolis coupling constants, a quasistationary molecular eigenstate in an excited electronic state S, is first calculated. The S, eigenstate is then coupled via the nonadiabatic nuclear kinetic energy operator (internal conversion) to rovibronic states in the ground state manifold, the latter states approximated in a simple manner. A search algorithm is used to select the S, dark states and the Sc states. Both the perturbation theory coefficient and the Franck-Condon factors are employed in the evaluation function used in the search. The results are applied in part II to the channel three problem in benzene.

Isotopomer-Selective Overtone Spectroscopy of Jet-Cooled Benzene by Ionization Detected IR + UV Double Resonance: TheN= 2 CH Chromophore Absorption of12C6H6and13C12C5H6near 6000 cm-1

“…Since then, this unusual dynamic behavior has been studied in detail using experimental techniques such as “chemical timing” (O 2 -pressure dependence of emission spectra), rotationally resolved high-resolution spectroscopy, or time-resolved photoionization experiments. , The high-resolution studies revealed a strong rotational dependence of the nonradiative decay rate . These findings may be explained by a mechanism where very fast IVR occurs from bright rovibronic states to S 1 background states induced by rotationally dependent Coriolis and anharmonic couplings. , These S 1 background states may then undergo rapid internal conversion to S 0 , possibly aided by a conical intersection of the S 1 and S 0 potential energy surfaces. ,,− After internal conversion, isoenergetic S 0 states are expected to have low ionization cross-sections due to small Franck−Condon factors to the ionization continuum. , It is not yet clear to which extent these dynamic processes reduce the ionization efficiency in the present scheme, because different S 1 states than previously studied are accessed by the IR + UV double resonance absorption. It is clear, however, that under the present experimental conditions ionization can compete successfully with the fast intramolecular relaxation, because photoionization is observed.…”

“…7 These findings may be explained by a mechanism where very fast IVR occurs from bright rovibronic states to S 1 background states induced by rotationally dependent Coriolis and anharmonic couplings. 7,82 These S 1 background states may then undergo rapid internal conversion to S 0 , possibly aided by a conical intersection of the S 1 and S 0 potential energy surfaces. 7,9,[82][83][84] After internal conversion, isoenergetic S 0 states are expected to have low ionization cross-sections due to small Franck-Condon factors to the ionization continuum.…”

“…7,82 These S 1 background states may then undergo rapid internal conversion to S 0 , possibly aided by a conical intersection of the S 1 and S 0 potential energy surfaces. 7,9,[82][83][84] After internal conversion, isoenergetic S 0 states are expected to have low ionization cross-sections due to small Franck-Condon factors to the ionization continuum. Benzene ionized by the IR + UV scheme used here has a characteristic mass spectrum, which is dominated by the parent ion and some weaker fragment ion peaks ranging from C x)2-6 H y)0-5 to C. Obviously, some fragmentation occurs after ionization, because the UV wavelength and intensity chosen are not suitable for efficient resonantly enhanced or nonresonant multiphoton ionization of neutral C atoms or other fragments.…”

Two-Photon Spectroscopy of Jet-Cooled Benzonitrile: New Results for a Charge Transfer Molecule

“…Helman and Marcus [40,41] explained the experimentally found [39,42,43] rotationally selective intramolecular process near the onset of the "channel three" as a combined Coriolis and anharmonic coupling to vibrational background states within the S 1 state. Helman and Marcus [40,41] explained the experimentally found [39,42,43] rotationally selective intramolecular process near the onset of the "channel three" as a combined Coriolis and anharmonic coupling to vibrational background states within the S 1 state.…”

Spectroscopic study on deuterated benzenes. III. Vibronic structure and dynamics in the S1 state

“…From the theoretical analysis on intensity distribution of the observed rotational transitions, the parallel Coriolis interaction was shown to be superior for low J levels, while the perpendicular Coriolis interaction also becomes active at high J levels. 12,31,33,34 Consequently, the main IVR channel in jet-cooled benzene is considered to be the parallel Coriolis interaction around the out-of-plane c axis. This interaction leads to mixing between E 1u vibronic levels in the S 1 states followed by enhancement of IC, which causes drastic shortening of fluorescence lifetimes.…”