Electron-Induced Chemistry in the Condensed Phase

Jan Hendrik Bredehöft

doi:10.3390/atoms7010033

Abstract

Electron–molecule interactions have been studied for a long time. Most of these studies have in the past been limited to the gas phase. In the condensed-phase processes that have recently attracted attention from academia as well as industry, a theoretical understanding is mostly based on electron–molecule interaction data from these gas phase experiments. When transferring this knowledge to condensed-phase problems, where number densities are much higher and multi-body interactions are common, care must be taken to critically interpret data, in the light of this chemical environment. The paper presented here highlights three typical challenges, namely the shift of ionization energies, the difference in absolute cross-sections and branching ratios, and the occurrence of multi-body processes that can stabilize otherwise unstable intermediates. Examples from recent research in astrochemistry, where radiation driven chemistry is imminently important are used to illustrate these challenges.

Highlights

In many physical processes that involve ionizing radiation, chemical reactions take place, sometimes unbeknownst to the experimenter
Values for most chemical compounds can be found in the literature or in databases such as the NIST Chemistry Webbook [7]
The most commonly used one is the Binary Encounter-Bethe (BEB) model [8,9], which takes a number of molecular parameters as input, all of which are quite accessible by quantum chemical calculations

Summary

Introduction

In many physical processes that involve ionizing radiation, chemical reactions take place, sometimes unbeknownst to the experimenter. Depending on the exact energy of the electron, its interaction with a molecule can trigger three main principal processes, which produce reactive species that can go on to form bigger and more complex chemical structures: Above the ionization threshold, the impinging electron will knock out one other electron from the molecule in a process called electron impaction ionization (EI) The first one is neutral dissociation (ND), where inelastic scattering of the impinging electron leads to excitation of the targeted molecule and its subsequent breakup into smaller uncharged fragments This process is believed to follow a similar energy dependence as EI, albeit with lower total cross-sections and an onset energy that is lower than the ionization threshold. I will present a few recent examples of astrochemical research from my lab that highlight the typical challenges faced by experimenters, when trying to apply gas-phase data to condensed-phase problems

The Ionization Potential

Possible reaction pathways to formamide formamideasas well expected

Prediction of Possible Reaction Routes

Conclusions