More Protected Vibrational States at the Dissociation Limit of SCCl₂

Abstract

Local vibrational coupling models predict that intramolecular vibrational energy redistribution (IVR) is not completely statistical even at the dissociation limit of polyatomic molecules. Thus states protected from IVR and from rapid dissociation form regular progressions and can be assigned vibrational quantum numbers. We previously observed such regular progressions of states in vibrational spectra of the molecule SCCl2, but a discrepancy in the density of such states remained between theory and experiment. Here we show that the gap can be closed by observing and assigning additional vibrational transitions above the dissociation limit of SCCl2, and by carefully analyzing the theoretically expected density of protected states. The newly observed transitions originate from recently assigned and more highly excited vibrational levels in the B̃ electronic state, connecting to the X̃ ground state by different Franck-Condon factors. Based on our analysis of Franck-Condon activity, we conclude that theory and experiment agree within measurement uncertainty. Consistency between theory and experiment implies that even more protected states should be observed for larger molecules, leading to nonstatistical dissociation reactions.

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].…”

“…The vibrational spectroscopy of thiophosgene has also been studied in considerable detail, although even at the present time some vibrational fundamentals have not been directly observed. As a result of an extensive research effort, many ground electronic state frequencies have been measured up to extremely high vibrational excitation energies, of $23,000 cm À1 , using low resolution IR [13][14][15][16] and high resolution synchrotron IR spectroscopy [17], laser induced fluorescence and stimulated emission pumping from both excited electronic states S 1 and S 2 [4,8,11,19,20]. This range of observed vibrational frequencies extends up to and even exceeds the first two dissociation limits ($19,900 cm À1 to CS + Cl 2 , and $22,200 cm À1 to Cl + SCCl).…”

“…This range of observed vibrational frequencies extends up to and even exceeds the first two dissociation limits ($19,900 cm À1 to CS + Cl 2 , and $22,200 cm À1 to Cl + SCCl). A surprisingly large number of the spectroscopically observed highly excited vibrational levels (several hundred), including many levels above dissociation limit, have been successfully observed and assigned using a simple nonresonant effective Hamiltonian expression by Gruebele and collaborators with only 20 adjustable parameters [4,11]. 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].…”

“…A surprisingly large number of the spectroscopically observed highly excited vibrational levels (several hundred), including many levels above dissociation limit, have been successfully observed and assigned using a simple nonresonant effective Hamiltonian expression by Gruebele and collaborators with only 20 adjustable parameters [4,11]. 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].…”

On readout of vibrational qubits using quantum beats

“…As the simplest model system for the readout, we include two computational states in our calculations to study readout from one vibrational qubit, and four states for readout from the vibrational two-qubit system. The energy spectrum and transition dipole moments of our model are taken from the experiment 53,54 and are given in Table I. The intensity of fluorescence from the readout state is linearly proportional to its population.…”