Detection of free radicals in low-temperature gas-grain reactions of astrophysical interest

Ioffe Physico-Technical Institute, 26 Politekhnicheskaya, St. Petersburg 194021, Russia

Corresponding author: R. A. Zhitnikov, zhitnikv.mares@pop.ioffe.rssi.ru

Received: 15 October 2001
Accepted: 20 February 2002

Abstract

For laboratory detection and investigation of free radicals which often appear in chemical reactions of astrophysical interest either as important intermediate or as final products, we pioneered the use of the electron paramagnetic resonance (EPR) technique, which is very effective in observing these substances. This has allowed both formyl (HCO and DCO) and methyl (CH₃ and CD₃) free radicals to be detected in a sequence of low-temperature gas-grain reactions of H and D atom addition in solid CO. The solid samples subjected to the EPR study were obtained by simultaneous independent deposition of CO molecules and either H or D atoms on the substrate cooled to liquid helium temperatures. The sequence of the H-atom addition gas-grain reactions in solid CO is acknowledged to be among the fundamental processes responsible for the synthesis of organic molecules in interstellar cloud dust grains. The present observation of the above free radicals confirms that the solid-state gas-grain sequence of the reactions is efficient at low temperatures; it also suggests that there should be considerable concentrations of formyl and methyl radicals in the ISM. Another study concentrated on the formation of ethyl free radicals (C₂H₅) in a low-temperature gas-grain reaction of H-atom abstraction from a C₂H₆ molecule by free H-atom in solid CH₄. These experiments were carried out by deposition onto a substrate, cooled by liquid helium, of a flow of CH₄ molecules containing a small amount of impurities such as ethane molecules (C₂H₆), free H-atoms and CH₃ radicals formed in a discharge in a pure gaseous methane. EPR spectra of CH₃ radicals, H-atoms, and C₂H₅ radicals matrix-isolated in solid CH₄ were detected. The relative concentrations of the radicals were found to depend on the experimental conditions. The abstraction reaction, C₂H₆ + H C₂H₅ + H₂, took place in CH₄-ice.