Assignment and Extraction of Dynamics of a Small Molecule with a Complex Vibrational Spectrum:  Thiophosgene

Abstract

The dispersed fluorescence spectrum of the ground electronic state of thiophosgene, SCCl2, is analyzed in a very complex region of vibrational excitation, 7000-9000 cm(-1). The final result is that most of the inferred excited vibrational levels are assigned in terms of approximate constants of the motion. Furthermore, each level is associated with a rung on a ladder of quantum states on the basis of common reduced dimension fundamental motions. The resulting ladders cannot be identified by any experimental means, and it is the interspersing in energy of their rungs that makes the spectrum complex even after the process of level separation into polyads. Van Vleck perturbation theory is used to create polyad constants of the motion and a spectroscopic Hamiltonian from a potential fitted to experimental data. The eigen functions of this spectroscopic Hamiltonian are rewritten as semiclassical wave functions and transformed to a representation that allows us to analyze and assign the spectra with no other work other than to utilize concepts from nonlinear dynamics.

The Fock space method of vibrational analysis

“…Our deduction of important resonances is not surprising for SCCl 2 as in Table 3 of Ref. 9 it is seen that the resonance weights k 156 and k 526 are by far the largest ones of the weights.…”

“…Let us now turn to the case of thiophosgene where previously 9 we had to study wave function densities and phases in three dimensions, i.e., N was 3. In the polyad of SCCl 2 studied 234 out of 288 states turned out to be dominated by the simultaneous effect of the two by far biggest resonances in the Hamiltonian ͑presented in Ref.…”

Dynamical traps lead to the slowing down of intramolecular vibrational energy flow

“…The various parameter values (SI Appendix, Tables S1 and S2) are taken from an earlier work (54) and note that the first three resonances are much larger than the last three resonances. Due to the existence (54) of three conserved quantities (polyads)…”

A refined quartic potential energy surface and large scale vibrational calculations for S0 thiophosgene

“…Thiophosgene is an interesting molecule, that has attracted considerable attention both experimentally as well as theoretically, as it has proved to be a very suitable model system for studying of a large variety of generic spectral and photophysical manifestations in polyatomic molecules [1][2][3][4][5][6][7][8][9][10][11][12]. Thiophosgene has been recognized both as a suitable molecule for studying ''backbone'' intramolecular vibrational energy redistribution (IVR) in the ground electronic state S 0 [3,4,8,9], as well as ''a molecule, tailor-made for studying fundamental concepts of electronic radiationless transitions'' [1,2,[5][6][7].…”

“…This suggests, that characteristic vibrational feature states -well isolated quantum states with assignable quantum number composition -should exist in the whole range of S 0 vibrational excitation and even above the lowest dissociation limits [11]. Many theoretical models, both quantum mechanical and semi-classical, have been designed and applied to the understanding of the extent and mechanisms of vibrational level mixing and vibrational energy redistribution (IVR) in S 0 thiophosgene [4,[9][10][11][12]. In particular, rationalizing the existence of numerous highly excited assignable vibrational states, not strongly affected by vibrational mixing -which contradicts simple statistical theories -is a major theoretical challenge.…”