Gamma-ray spectra of methane in the positron-electron annihilation process

TL;DR

This study investigates how bound electrons influence gamma-ray spectra in methane during positron-electron annihilation, using ab initio quantum methods to identify the dominant electrons involved.

Contribution

It demonstrates that the 2a1 electrons of methane predominantly contribute to the annihilation gamma-ray spectra, highlighting the role of positrophilic electrons.

Findings

2a1 electrons dominate the gamma-ray spectra

C-H bonds are polarized with partial charges

Negative electrostatic potential facilitates positrophilic electrons

Abstract

Bound electron contribution to the Doppler-shift of gamma-ray spectra in the positron-electron annihilation process of molecular methane has been studied in gas phase. Two accurate ab initio quantum mechanical schemes, i.e. the delocalized molecular orbital (MO) and the localized natural bond orbital (NBO) schemes, are applied to study the multi-centred methane molecule. The present ab initio calculations of methane indicate that the C-H bonds are polarized with the partial negative charge of -0.36 a.u. on the carbon atom and the partial positive charge of +0.09 a.u. on each of the hydrogen atoms. The positively charged hydrogen atoms produce repulsive Coulomb potentials to a positron. Both the MO and NBO schemes further reveal that the 2a1 electrons of methane, that is, the 2a1 electron component of the C-H bonds rather than the whole C-H bonds of methane, predominates the positron-electron annihilation gamma-ray spectra of the molecule. Electrons of a molecule which are dominant the positron-electron annihilation processes are called positrophilic electrons in the present study. It is further shown that the negative electrostatic potential (ESP) of methane facilitates with the density of the positrophilic 2a1 electrons of methane. Other valence electrons (e.g. 1t2) in the C-H bonds play a minor spectator role in the annihilation process of methane.