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 lowtemperature 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 gasgrain 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.

The effect of formation in a neon cryocrystal of atomic-type 2pJ3s 3P2 centres obtained as a mu11 of He gas discharge products being trapped in a growing neon cryocrystal is discovered. The unstable excited 2p53s3Pz states in the neon cryocrystal were detected by ESR. This effect is interpreted as a new phenomenon: quasi-resonance transfer of excilakion energy from the metastable He Z3S1 atom trapped in a growing neon cryocrjstal to the exciton energy band of the neon crystal followed by the exciton self-happing into the 2ps3p state and subsequent decay, ending in the 2p53s3Pz state recorded by ESR in our experiment. A temperature audy showed that Lhe rate constant A of the process of excitation energy transfer from the He Z3St atom to the neon cryocrystal exciton band follows the Arrhenius-like law A -A0 exp(-ElkT); the 'activation energy' E of the process nuns out to be O.OOlO(5) eV.

The methods are described for producing unstable paramagnetic excited states in rare gas cryocrystals Ne, Ar, Kr, and Xe through the trapping, in the cryocrystals growing from the gas phase, the products of the gas discharge taking place in the same or other rare gas. The paper presents a technique and results of an observation and investigation of excited states in rare gas cryocrystals with electron paramagnetic resonance (EPR). The discovered unstable paramagnetic centers are interpreted as being local metastable excited np5(n+1)s atomic-type states in rare gas cryocrystals which are subject to the action of the anisotropic electric field resulted from the crystal surroundings distorted by the center. An account is given of the mechanisms for formation of observed paramagnetic excited states in cryocrystals which arise owing to the excitation energy of the metastable P23 atoms of Ne, Ar, Kr, Xe and He 23S1 and 21S0 atoms that form in the discharge in an appropriate gas and trap in the growing cryocrystal.

No abstract

Local metastable excited states are found in Ar and Kr cryocrystals when He gas-discharge products are trapped in the growing cryocrystals. These states are detected by ESR and are interpreted as being local metastable excited np5(n+1)s3P2 atomic-type states in Ar and Kr cryocrystals. The study showed that the yield of the 3P2 excitations in the above process decreases with increasing temperature. Analysis of the results allows the following explanation of the observed effect to be given. Metastable excited He atoms from the He gas discharge are trapped in the growing Ar or Kr cryocrystals and transfer their excitation energy to the cryocrystal to form, in the process of internal ionization, a RG+ ion and a free electron in the conduction band, whereupon the fast (10-12 s) self-trapping reaction of a hole follows: RG+RG to RG2+. Thereafter either the dissociative recombination reaction RG2++e to RG2** to RG+RG*(3P2) or recombination RG2++e to RG+RG to produce ground-state atoms could take place. The former is likely at lower temperatures, and the latter at higher temperatures when the vibrational relaxation rate of the RG+ molecular ion increases and the mobility of free electrons in the conduction band decreases. This is the reason for the observed temperature dependence.