Rotational level involvement in the T1→S intersystem crossing transition in thiophosgene

Abstract

We propose and develop theoretically a general mechanism for the involvement of rotational motion into the nonradiative transitions that occur in an isolated polyatomic molecule. The treatment is based on the different rotational constants and different (asymmetric top-symmetric top) molecular structures in the two combining electronic states. We focus our attention on the T(1)-->S(0) intersystem crossing (ISC) transition in thiophosgene and show how the rotational mechanism could lead to a considerable enhancement in the effective level density for the process. Inserting the rotational mechanism into our recently developed technique and algorithm for combined spin-orbit coupling+intramolecular vibrational redistribution analysis, we have carried out large-scale calculations that have led to a better understanding of the ISC (T(1)-->S(0)) in thiophosgene.

High-resolution synchrotron infrared spectroscopy of thiophosgene: The ν2 and ν4 fundamental bands near 500 cm−1

“…The m 2 (symmetric C-Cl stretch) and m 4 (out-of-plane bend) fundamental bands in the 500 cm À1 region have been analyzed for the two most abundant isotopomers, 35 Cl 2 12 C 32 S and 35 Cl 37 Cl 12 C 32 S. For each of these four bands, some 600-1700 assigned transitions were fitted to obtain accurate upper state rotational and centrifugal distortion parameters. Ground state parameters were fixed at the microwave values of Carpenter et al [12], which proved to be entirely satisfactory.…”

“…Thanks in part to its rich electronic spectrum in the visible region, the thiophosgene molecule is a favorite model system for studies of photophysics, vibrational dynamics, and intersystem interactions [1][2][3]. Many hundreds of vibrational levels in the ground and excited electronic states have been experimentally observed, allowing the development of detailed anharmonic force fields including all six vibrational modes [4][5][6].…”

The role of rotational relaxation in the intersystem crossing between a triplet and a singlet electronic state

“…In recent years, we have performed a series of theoretical studies on the ISC process in thiophosgene14–16. Initially, we performed a model calculation of the ISC rate constant K ISC , using a modified Fermi golden rule expression14.…”

“…Initially, we performed a model calculation of the ISC rate constant K ISC , using a modified Fermi golden rule expression14. The explicit calculation of spin–orbit (SO) coupling matrix elements between the initial T 1 level $|s\rangle$ and isoenergetic, highly vibrationally excited S 0 levels $|l_{k}\rangle$ was based on an analysis of the vibrational structure of the ground electronic state8, in terms of symmetrized vibrational coordinates16. In that work, a decay time of about τ ISC ≈ 40 ps was obtained, which was in general (qualitative) agreement with the earlier calculations5 of τ ISC ∼ 20 ps.…”

“…However, the detailed ISC mechanism remained unclear, as only a minor part of the available isoenergetic { $|l_{k}\rangle$ } level manifold was coupled perceptibly to $|s\rangle$ via direct SO coupling matrix elements. In our subsequent work17, we carried out a combined ISC + intramolecular vibrational redistribution (IVR) study, where we combined the two approaches developed in our previous work15, 16. For this purpose, an IVR analysis was performed on each state $|l_{k}\rangle$ that was directly SO coupled to $|s\rangle$ to implicate additional levels $|l_{k}^\prime\rangle$ that were coupled to $|l_{k}\rangle$ by anharmonic interactions from higher order terms in the S 0 vibrational potential.…”