Spectroscopy and chemical synthesis of interstellar ice analogues

Nair, Binukumar Gopalakrishnan (2014). Spectroscopy and chemical synthesis of interstellar ice analogues. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f847

Abstract

Molecular synthesis and chemical evolution in the interstellar medium has been studied under laboratory conditions. The method of preparation and energetic processing of interstellar ice analogues on surfaces, spectroscopic principles for monitoring chemistry and morphology of these ice analogues and analysis methodologies are discussed in detail. The modification of a portable, ultrahigh vacuum (UHV) system for electron irradiation, vacuum ultraviolet (VUV) spectroscopy and temperature programmed desorption (TPD) of interstellar ice analogues are described in this thesis. Experimental procedures to grow interstellar ice analogues of pure molecules and mixtures are described. The results from the various experiments discussed in this thesis are classified into four main parts: VUV spectroscopy, electron irradiation of interstellar (IS) ice analogues, simultaneous irradiation and generation of IS ice analogues and temperature programmed desorption of interstellar ice analogues. Temperature dependent vacuum ultraviolet (VUV) photo-absorption spectra of pure molecular ices such as HCONH₂, HCOOH, HCOOCH₃, CH₂CHCH₂OH CH₃COOCH₃, CH₃CH₂COOH, C₆H₆ and O₃ have been measured on the UV1 beamline of the ASTRID Synchrotron at the University of Aarhus in Denmark and UV A1 beamline at NSRRC, Taiwan. These spectra and photo-absorption cross-sections in the condensed phase are also presented. In particular, temperature dependent VUV photoabsorption characteristics of condensed ice films of O₃ are measured for the first time. Electron induced molecular synthesis in pure organic ice films of HCONH₂, HCOOCH₃, CH₃COOH, NO and binary ice mixtures of CH₃OH+NO (1:1) are also reported. Newly identified pathways of molecular synthesis are discussed. Molecular synthesis during simultaneous electron irradiation and physisorption of a binary mixture of CH₃OH+NH₃ (1:1) is studied for the first time. Simultaneous irradiation-deposition has shown very interesting behaviour in terms of efficiency of formation of radical species such as OCN^-, NH₄⁺ etc and biologically important complex organic species such as HCONH₂, HCOOCH₃, CH₃COOH, CH₃OCH₃, CH₂CHCHO etc. Simultaneous irradiation deposition closely simulate the effect of cosmic ray irradiation in a dense molecular cloud and reveal new pathways of formation with higher efficiencies even at lower column densities of reactants. Finally preliminary results of a temperature programmed desorption study of pure NO ice films are also presented along with the future challenges and strategies.