On the nature of highly vibrationally excited states of thiophosgene#

Abstract

In this work an analysis of the highly vibrationally excited states of thiophosgene (SCCl2) is made in order to gain insights into some of the experimental observations and spectral features. The states analyzed herein lie in a spectrally complex region where strong mode mixings are expected due to the overlap of several strong anharmonic Fermi resonances. Two recent techniques, a semiclassical angle space representation of the eigenstates and the parametric variation of the eigenvalues (levelvelocities) are used to identify eigenstate sequences exhibiting common localization characteristics. Preliminary results on the influence of highly excited out-of-plane bending modes on the nature of the eigenstates suggest a possible bifurcation in the system. I. INTRODUCIONUnderstanding the nature of the highly excited molecular eigenstates is equivalent to deciphering the mechanism of intramolecular vibrational energy redistribution (IVR) occuring in the molecule [1]. However, the assignment of eigenstates is far from simple. The existence of and interplay of several strong anharmonic resonances result in complicated spectral patterns and highly mixed states. Nevertheless, eigenstates of molecular systems rarely exhibit the extreme scenario of being completely ergodic or completely regular [2]. A generic situation, even at fairly high energies, is the coexistence of several classes of eigenstates with differing degree of mixing being interspersed among each other [3,4]. More interestingly, the interspersed states can show several sequences associated with certain identifiable common localization characteristics. In other words, although a full set of quantum numbers do not exist for labeling such states, there are reasons to believe that a sufficient number of approximate quantum numbers do exist to understand, and hence assign, the eigenstates. Since the approximate quantum numbers arise out of the local dynamics due to specific resonances, relevant at the energies of interest, the assignment is inherently dynamical in nature. In a nutshell, several decades of work have shown that dynamical assignments can be done reliably only if the structure of the corresponding classical phase space is well understood [5].This work is concerned with the dynamical assignment of the eigenstates of thiophosgene SCCl 2 . The Hamiltonian of interest is a highly accurate spectroscopic Hamiltonian obtained [6] by Gruebele and Sibert via a canonical Van Vleck perturbation analysis of the experimentally derived normal mode potential surface [7]. The Hamiltonian can be expressed as H = H 0 + V res withThe zeroth-order anharmonic Hamiltonian is H 0 , whose eigenstates are the feature states and V res accounts for the important anharmonic resonances responsible for the IVR dynamics. In the rest of the paper the various resonances will be refered to by simply indicating the modes. For instance, the first resonance term in V res above will be refered to as the 526-resonance. The hermitian adjoints of the operators are denoted by c.c., and the op...

More Protected Vibrational States at the Dissociation Limit of SCCl2

“…The semiclassical analyses of the transition region all indicate that several sequences of states “collide” at about 300 THz, leading to complex vibrational wave functions that can nonetheless be visualized. , This complexity is evidently resolved again (at least for the protected states) above 500 THz. The semiclassical analyses are consistent with our observations: the highest density of missing assignments (Supporting Information, Table S1) and the largest systematic perturbations of the energies (Figure ) occur in the range from 350 to 450 THz.…”

“…Instead, LIF and SEP experiments revealed that SCCl 2 exhibits several completely regular vibrational progressions (illustrated in color in Figure B) up to at least 90 THz (∼2700 cm –1 ) above its first dissociation limit (at. ∼597 THz or ∼20,000 cm –1 ). , The progressions can be classified by polyad quantum numbers. − We say that such states are protected from IVR and thus call them “protected states”.…”

Scaling Perspective on Intramolecular Vibrational Energy Flow: Analogies, Insights, and Challenges

“…The recent review [196] by Farantos, Schinke, Guo, and Joyeux gives an up to date account of the recent developments. For example, dynamical assignments of highly excited states eigenstates in several systems clearly indicate the existence of sequences of eigenstates sharing common localization characteristics [197,198]. Note that states in such sequences are neither regular nor ergodic, and certainly not measure zero.…”

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.…”