Non-Markovian near-infrared Q branch of HCl diluted in liquid Ar

Abstract

By using a non-Markovian spectral theory based in the Kubo cumulant expansion technique, we have qualitatively studied the infrared Q branch observed in the fundamental absorption band of HCl diluted in liquid Ar. The statistical parameters of the anisotropic interaction present in this spectral theory were calculated by means of molecular dynamics techniques, and found that the values of the anisotropic correlation times are significantly greater (by a factor of two) than those previously obtained by fitting procedures or microscopic cell models. This fact is decisive for the observation in the theoretical spectral band of a central Q resonance which is absent in the abundant previous researches carried out with the usual theories based in Kubo cumulant expansion techniques. Although the theory used in this work only allows a qualitative study of the Q branch, we can employ it to study the unknown characteristics of the Q resonance which are difficult to obtain with the quantum simulation techniques recently developed. For example, in this study we have found that the Q branch is basically a non-Markovian (or memory) effect produced by the spectral line interferences, where the PR interferential profile basically determines the Q branch spectral shape. Furthermore, we have found that the Q resonance is principally generated by the first rotational states of the first two vibrational levels, those more affected by the action of the dissolvent.

Comparing thermal wave function methods for multi-configuration time-dependent Hartree simulations

“…However, these approaches contain approximations that are difficult to control and are invalid for certain systems, for example, non-Markovian baths. 7,8 An alternative approach is to propagate the wave function of the system and a sufficiently large model bath by solving a system-bath Schrödinger equation. [9][10][11][12] While such calculations are rather costly, they converge to the exact solution as the number of basis functions is increased.…”

Two-Dimensional Infrared Spectroscopy From the Gas to Liquid Phase: Density Dependent J-Scrambling, Vibrational Relaxation, and the Onset of Liquid Character

“…This growing intensity in the nominally forbidden gas phase Q-branch region is phenomenologically analogous to the triplet structure previously observed in the numerous studies of the absorption spectra of hydrogen halides in nonpolar liquids. 2,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] 2DIR studies of these diatomics in these dense nonpolar fluids can also establish the free rotor character in these diatomic probe systems and their state point dependence. MD simulations and calculations have been carried out to understand the origins of these absorption spectra and what they reveal about solvation in the liquid and high-pressure environments.…”

“…analysis. 2,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] A characteristic phenomenological feature of these IR spectra is the observation of a broad and overlapping "triplet" structure for vibrational bands of diatomics in these liquids and high pressure fluids. The two feature or shoulders on the red and blue of the vibrational absorption band are attributed to unresolved P (DJ = -1) and R (DJ = +1) gas-phase-like rovibrational branches.…”

“…Explanations offered for these diatomics in liquid and dense fluid vibrational bandshapes span the range from dipole-induced-dipole effects, 36 J-mixing due to anisotropic interaction potential, 37 van der Waals complex formation or long lived anisotropic spatial correlations in the fluid, 2,38,39,48 "modified rotor" or hindered rotor descriptions, 42,43,50 to P and R spectral interference effects. 49 Some analyses have speculated that these complex lineshapes are due to both mixed free rotor character and modified rotor character resulting from slowly varying anisotropic solute-solvent potentials and the role of barriers on the lower lying rotational levels. 41,[45][46][47] The obvious experimental feature that limits more detailed interpretation of the high fluid density rovibrational absorption spectra is the lack of resolved rotational features.…”

“…6 There is an extensive literature on the observation and molecular-level interpretation of the vibrational spectra of small molecules, especially IR absorption spectra of hydrogen halides in high pressure and simple non-polar liquids, that closely relates to this 2DIR analysis. 2,[34][35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50] A characteristic phenomenological feature of these IR spectra is the observation of a broad and overlapping "triplet" structure for vibrational bands of diatomics in these liquids and high pressure fluids. The two feature or shoulders on the red and blue of the vibrational absorption band are attributed to unresolved P (DJ = -1) and R (DJ = +1) gas-phase-like rovibrational branches.…”