Theoretical Study on Molecules of Interstellar Interest. I. Radical Cation of Noncompact Polycyclic Aromatic Hydrocarbons

Abstract

Polycyclic aromatic hydrocarbons (PAHs), in particular, their radical cation (PAH(+)), have long been postulated to be the important molecular species in connection with the spectroscopic observations in the interstellar medium. Motivated by numerous important observations by stellar as well as laboratory spectroscopists, we undertook detailed quantum mechanical studies of the structure and dynamics of electronically excited PAH(+) in an attempt to establish possible synergism with the recorded data. In this paper, we focus on the quantum chemistry and dynamics of the doublet ground (X̃) and low-lying excited (Ã, B̃, and C̃) electronic states of the radical cation of tetracene, pentacene, and hexacene molecule. This study is aimed to unravel photostability, spectroscopy, and time-dependent dynamics of their excited electronic states. In order to proceed with the theoretical investigations, we construct suitable multistate and multimode Hamiltonians for these systems with the aid of extensive ab initio calculations of their electronic energy surfaces. The diabatic coupling surfaces are derived from the calculated adiabatic electronic energies. First principles nuclear dynamics calculations are then carried out employing the constructed Hamiltonians and with the aid of time-independent and time-dependent quantum mechanical methods. The theoretical results obtained in this study are found to be in good accord with those recorded in experiments. The lifetime of excited electronic states is estimated from their time-dependent dynamics and compared with the available data.

Dissociation of polycyclic aromatic hydrocarbons at high energy: MD/DFTB simulations versus collision experiments

“…It seems to validate the general assertion that internal conversion is very effective and that the excited electronic states reached by excitation either by collision or by photon absorption rapidly decay non adiabatically to the ground-state. The non radiative lifetime for PAH radical cations in excited electronic states is indeed expected to be very short (less than several tens of fs [38,39]) because of numerous conical intersections, where non adiabatic decay prevails [20,50]. It also validates the energy distribution model in which this assumption is made.…”

Dissociation of polycyclic aromatic hydrocarbons: molecular dynamics studies

“…This is probably reasonable assuming the general statement that internal conversion is very effective and that the excited electronic states reached by excitation either by collision or by photon absorption rapidly decay non-adiabatically to the ground state. The non-radiative lifetime for PAH radical cations in excited electronic states is indeed expected to be very short (less than several tens of fs [50,51]) because of numerous conical intersections, where non-adiabatic decay prevails [52,53]. However, the presence of conical intersections may lead to overexcitation in specific modes, which could orient further fragmentation.…”

Rationalization of photo-detachment spectra of the indenyl anion (C9H7−) from the perspective of vibronic coupling theory

“…7,8 The identification of particular PAHs in the circumstellar envelopes of carbon rich asymptotic giant branch (AGB) stars is still questionable, 9 but it is believed that PAHs are carriers of the various unidentified and mysterious diffuse interstellar bands (DIBs) in infra-red and visible wavelengths. [10][11][12][13][14][15][16][17] In general, the study of molecular evolution of PAHs is exclusively focused on possible expansion of the cyclo-C 6 ring(s) in the parent benzene moiety in a linear [18][19][20][21][22][23][24][25][26][27] and compact [28][29][30][31][32][33][34] fashion. Naphthalene and phenanthrene are the simplest prototypical examples of linear and compact PAHs formed due to cyclo-C 6 ring extension of benzene.…”