Structure and dynamics of electronically excited molecular systems

Abstract

Structure and dynamics of electronic excited states of molecular systems is of contemporary interest to understand a rich variety of processes ranging from molecular spectroscopy to reaction dynamics. Developments over the past decades have established that coupling of electronic and nuclear motion is a generic feature in polyatomic molecular systems which causes a breakdown of the Born-Oppenheimer approximation. Such a coupling leads to conical intersections in non-linear and glancing degeneracy in linear polyatomic systems. While the former case is quite general and has wide variances depending on the nature of the coupling on the symmetry ground, the latter one is specific to the linear systems having axial symmetry. The Jahn-Teller (JT) systems form a subclass in the former category and the latter category constitutes the Renner-Teller (RT) systems. These systems are characterized by high point group symmetry and orbital degeneracy. The latter splits upon distortion along suitable symmetry reducing nuclear displacement. The JT or the RT split component states may undergo symmetry allowed coupling among another JT or RT split component states or with some non-degenerate states. These are termed as the pseudo-JT and pseudo-RT coupling, respectively.We have studied a variety of photo-induced molecular processes with the aid of ab initio quantum chemistry and first principles quantum dynamics methods. We will document here some of the important developments in the field focusing on the work done on the JT and RT systems primarily in our own laboratory.

Photodetachment band of the fluorenyl anion: a theoretical rationalization

“…The primary objective of the present study is to investigate the experimental photodetachment band of the fluorenyl anion 42 using the FC_Class algorithm within the adiabatic Hessian (AH) approach [58][59][60][61][62] and Poisson spectra simulations 63 using vibronic coupling theory (VCT). 57,[64][65][66][67][68][69] The current study of adiabatic and non-adiabatic nuclear dynamics [70][71][72][73] of the electronic states of the fluorenyl radical using VCT is based on the estimation of the vertical detachment energies (VDEs) along dimensionless normal coordinates (DNCs); later, those energy points are fitted with the analytic forms of the constructed molecule Hamiltonian to estimate the intrastate and interstate vibronic coupling parameters. The positions of the energy minima of the seam of conical intersections (CIs) 64,65,74,75 and the energy minimum of the electronic states are then computed using the vibronic coupling parameters, harmonic frequencies of the vibrational modes and VDEs at the optimized ground state of the fluorenyl anion.…”

A theoretical study of vibronic coupling in the photoelectron spectra of Al6N−

“…For this, we have employed the vibronic coupling theory approach, [10][11][12][13][14][15][16] which has been very effective for various photodetachment processes. [17][18][19][20][21][22][23][24][25][26] In this approach, we obtained the optimized geometry followed by ab initio single-point energy calculations (described in the Theory and computational details section) for the first seven low-lying electronic states of Al 6 N À .…”

Understanding of the Photodetachment Spectrum of Anionic Mixed Carbon–Boron Cluster C3B5– Following Adiabatic and Nonadiabatic Quantum Chemistry Approaches

“…In the present paper, we attempt to advance the understanding of the photodetachment spectrum for C 3 B 5 – following a combination of quantum dynamics methods. It is noted that the structural confirmations of the bare boron clusters and their structural cousins using nuclear dynamics study were first initiated by Mahapatra and co-workers. − We request the interested reader to go to the recent review of Mahapatra in order to access more details about nuclear dynamics. The success of the method has been remarkable and was employed to unravel intriguing features in the photodetachment spectra of many molecules.…”

“…Later, these energies are used to extract vibronic Hamiltonian parameters on a diabatic electronic basis through a fitting procedure. Vibronic couplings between different electronic states are determined by implementing the multimode vibronic coupling approach. − The locations of the CIs between different electronic states can be found from the vibronic coupling parameters. The dynamic observables described in this work are computed with the use of both time-independent (TI) and time-dependent (TD) methods on the electronic states of our concern.…”