Computational investigations on rotational and vibrational spectroscopies of some diatomics in solid environment

Abstract

In this thesis, spectroscopic and structural properties of homogeneous para-hydrogen crystals and diatomic molecules embedded in rare gas crystals are explored by theoretical means. The agreement with experimental signatures indicates the suitability of the chosen quantum and classical methods to accurately describe the various low-temperature physical and chemical problems investigated at the laser laboratory.

A novel explanation of band structures in Raman and IR spectra is given for CO in Ar. Rotational transition rules are found to dictate the spectral broadening of infrared and Raman band structures as a function of temperature. The results show that after photodissociation of matrix-isolated formaldehyde, the formation of intermolecular complex CO-H$_2$ becomes energetically unfavourable in the cage of Ar lattice atoms. The molecular photodissociation products, CO and H$_2$, rotate in lattice sites of varying angular hindrance.

An extensive analysis of long-lived electronic coherences is performed for I$_2$ in Xe. Quantum beats in time-resolved CARS spectra are reproduced with the help of computed potentials. The predicted 2D (time and frequency) representations of the CARS signals and simulated resonance Raman intensity modulations are shown to complement the demonstration of electronic coherence observed as vibronically resolved absorption spectra. The doublet in resonance Raman progression is assigned to molecular iodine residing in two lattice sites of different geometry. The rapid line broadening of the other peak with increasing vibrational quantum number is explained by coupled rotation-translation dynamics in the crystal site that supports reorientations of the molecular axis. The other site fixes the orientation and maintains longer vibrational coherence.

An analytical model for quantum and polarization beat frequencies in the CARS spectra are presented for CN in Xe. The rotational coherences in the ground and electronic states are studied numerically and a predictive control scheme for the detection and emergence of interferences is introduced.

Nonadiabatic molecular alignment is considered as the underlying mechanism in excitation and probing of rotons in para-hydrogen crystals. The relative orientation between the laboratory frame (pump polarization) and the crystal axes is shown to dictate the roton dynamics observed through the pump-probe optical Kerr effect spectroscopy. The numerical results suggest a control over the spatial composition of the rotons by changing the laser beam focus along the surface plane of the crystal.