title: Ultrafast molecular dynamics in amorphous media. A spectroscopic study of molecular motions, vitreous vibrations, and saline structure
Creators: name: Farrell, Andrew J.
Publisher: name: University of Glasgow
Year: 2022

description: This thesis presents three studies conducted over the course of 3 years within the chemical photonics research group of the University of Glasgow. Molecular dynamics and structure are intertwined; one can infer the microscopic structure by the molecular dynamics and vice versa. This thesis is primarily a spectroscopic study of carefully chosen inter- and intra-molecular vibrations in condensed phase media that are highly sensitive to microscopic structure and closely linked to their thermodynamic properties. The two major techniques used throughout are optical Kerr-effect spectroscopy, and Fourier transform infrared spectroscopy. Optical Kerr-effect spectroscopy (OKE), to pick one of the technique’s many names, has been a mainstay in the study of intermolecular interactions for many decades. Despite this there is no standard approach to the interpretation of OKE spectra. The research within this thesis seeks to address this issue by converging upon a general blueprint for OKE line-shape interpretation through robust experimentation and iterative line-shape fitting attempts. As is the case within most OKE studies, the line-shape fitting procedure used throughout is self-consistent. What makes this work worthwhile and useful to the field is the apparent robustness of the final fitting procedure across a very broad range of experimental data. This includes alterations in firstly the molecular size of alkanes and cycloalkanes, secondly the temperature of liquid propane, and finally molecular anisotropy through studying liquid methane and six-carbon ring molecules from cyclohexane to benzene—5 sets of experiments; 24 OKE spectra in total. Though not perfect, the method employed provides (at the very least) a justified starting point for the interpretation of more complex liquids. Indeed, this analytical framework guides subsequent work here. One of the most notorious THz spectral anomalies is the so-called ‘boson peak’. It has been reported in Raman and neutron scattering spectra for wide range of amorphous condensed phases, manifesting as an excess in the vibrational density of states, typically occurring in the 1 THz region. Little is known of the nature of this vibrational mode other than the fact that it has a transverse character. The boson peak is very difficult to spot due to sharing its characteristic frequency with various other oscillatory dynamics, esp. librations. This thesis aims to observe an entirely isolated boson peak through the selection of near-isotropically polarisable glass-forming molecular liquids with vastly simplified intermolecular spectra. With librations rendered OKE-invisible, the remaining sub-THz dynamics result from intermolecular collisions, i.e., washed-out phonon-like modes. This work will compare and contrast the nature of these localised vibrations present within various glass-forming liquids with the behaviour expected of the boson peak. Results show that the remaining collision-induced band may be consistent with transverse acoustic vibrations, however there are additional low-frequency modes visible in some glass forming liquids whose origin is less clear and warrants further investigation. Furthermore, this highlights the importance of these two modes being distinguished from each other, as boson peaks have not always been assigned in a consistent manner. Lastly, this thesis takes aim at the putative liquid-liquid transition in water. Its proposed existence is a product of an explanation of the peculiar thermodynamics of water. Frustratingly, such a liquid-liquid transition is believed to occur at an experimentally inaccessible region of the H2O phase diagram, where ice nucleation will occur rapidly and spontaneously. This work utilises aqueous salt solutions that suppress the onset of crystallisation and enable the probing of amorphous solvent H2O from ambient to cryogenic temperatures. H2O structure and dynamics are studied using OKE and FTIR spectroscopies, while using various dopants to probe local electrostatic environments. Spectra of magnesium perchlorate and lithium chloride solutions are probed and analysed in this chapter, the latter of which is often touted as an experimental proxy for pure H2O dynamics. Their spectra are far from dull, but exhibit temperature dependent phenomena that are explainable outwith the liquid-liquid transition concept. In concentrated lithium chloride solution, the evidence suggests the occurrence of nanophase segregation on cooling that is driven by reduced solubility. This results in an expulsion of solutes into a rich brine phase. Magnesium perchlorate solution at the eutectic composition meanwhile appears to only vitrify on cooling, suggesting that this solution may only be analogous to pure water at pressures too high to access the liquid-liquid transition.