Proton-impact ionization cross sections of adenine measured at 0.5 and 2.0 MeV by electron spectroscopy

Abstract

Double-differential ionization cross sections (DDCSs) of vapor-phase adenine molecules (${\mathrm{C}}_{5}{\mathrm{H}}_{5}{\mathrm{N}}_{5}$) by 0.5- and 2.0-MeV proton impact have been measured by the electron spectroscopy method. Electrons ejected from adenine were analyzed by a 45${}^{\ensuremath{\circ}}$ parallel-plate electrostatic spectrometer over an energy range of 1.0--1000 eV at emission angles from 15${}^{\ensuremath{\circ}}$ to 165${}^{\ensuremath{\circ}}$. Single-differential cross sections (SDCSs) and total ionization cross sections (TICSs) were also deduced. It was found from the Platzman plot, defined as SDCSs divided by the classical Rutherford knock-on cross sections per target electron, that the SDCSs at higher electron energies are proportional to the total number of valence electrons (50) of adenine, while those at low-energy electrons are highly enhanced due to dipole and higher-order interactions. The present results of TICS are in fairly good agreement with recent classical trajectory Monte Carlo calculations, and moreover, a simple analytical formula gives nearly equivalent cross sections in magnitude at the incident proton energies investigated.