Akhil Mundaplackal, BS (Honours) in Physics, Class of 2024

Supervisor: Professor Soumen Ghosh, School of Arts and Sciences

Thesis Brief

This thesis is a study of positron-molecule interactions, specifically on the resonant positron annihilation beyond the fundamental mode frequencies of benzene and deuterated benzene molecules.

It is known from the literature that positrons bind with fundamental modes of several hydrocarbon molecules and that results in exceptionally large annihilation rates. However, there are many unidentified peaks in the annihilation spectrum, which indicates that there are interactions beyond the fundamental mode resonances. A comprehensive study on symmetry-allowed mode combination is carried out in this thesis by employing concepts from group theory and the theory of symmetry in molecules. A correlation between the combination modes and the measured annihilation suggests a strong possibility of positron coupling with the combination modes. Using group theory calculations, it is shown that unidentified peaks in the infrared spectrum of benzene are due to the combination of fundamental modes, as the infrared peaks matched well with the symmetry-allowed two-mode combinations. The same has also been verified with other molecules such as benzene-d6, belonging to the same symmetry group as benzene. A further analysis related to dipole strength of the combination mode in comparison with that of fundamental modes evidences positron coupling with the combination modes in a way similar to positron coupling with the fundamental modes of a molecule.