EPR in Condensed Gases

Overview of the experimental technique and results.

Since the late eighties the research activity of the laboratory in the field of matrix isolation has been focused on the EPR study of trapped light atoms and radicals. A contribution has been made to the data of matrix shifts of the EPR spectra parameters for hydrogen, H, deuterium, D, nitrogen, N, atoms and methyl, CH₃, and formyl, HCO, radicals and their isotopomers isolated in rare-gas and molecular solids. The deposition from the gas phase on to a cold surface of a quartz finger inserted into the EPR microwave cavity has been utilized to obtain the samples. Two gas flows were provided to the substrate: one of the two was passed through a gas discharge tube where the radicals were formed and another one was set avoiding the gas discharge zone. Shown below is one of the schemes of the major section of the experimental set-up: the microwave cavity of the 3 cm EPR spectrometer with 100 kHz modulation of the magnetic field, the low temperature gas discharge device and the quartz finger.

FIG.1.

Here 1 is the cylindrical TE011-mode cavity, 2 is the bottom of the quartz finger 3, filled with liquid He, and 4 is a waveguide. In order to obtain temperatures above 4.2 K, the bottom can be cooled bt liquid He vapor. An electrodeless RF gas discharge is excited in the glass tube 5 with outlet 6 of 0.6 mm diameter. The matrix gas can be supplied to substrate 2 by glass tube 7 and further by quartz tube 8 inserted into the cavity (channel B). The end of the tube 8 is located close (3 mm) to the bottom 2 which facilitates effective freezing out of the matrix gas. The whole device presented in the figure is cooled externally with liquid nitrogen vapor (LN₂) and its temperature can be varied from 77 to 300 K. A high-frequency (15 MHz) oscillator is used to maintain the discharge. The high-frequency power is fed through a coaxial cable to coil 9 wound over discharge tube 5.

The experimental technique described above made possible to obtain, for the first time, H and D atoms matrix-isolated from the gas phase in solid Ne [1,2]. Thorough experimental study of H and D atoms trapped in various matrices and analysis of both the data obtained in the study and those available from the literature led to the observation and explanation of the isotope effect in the hyperfine constant matrix shifts and EPR linewidths of the atoms in H₂, D₂, N₂, Ne, Ar, Kr and Xe matrices [3-6].

Not only trapped particles but matrices themselves were investigated. Nitrogen atoms isolated in n-H₂ matrix were utilized for the first time to observe and study by EPR technique ortho-para conversion in the normal solid hydrogen stimulated by a paramagnetic impurity [7,8]. In the case of N atoms in N₂ matrix, careful analysis of the EPR experimental data obtained showed that two different types of equilibrium surroundings are possible for a nitrogen atom trapped in a substitutional position, one of which corresponding to an undistorted N2 crystal lattice, and the other to a position in which the axes of nearest neighboring molecules are directed towards the trapped atom, and the centers of gravity of molecules are displaced to the center of the matrix cell. A value of the displacement was assessed.

In order to elucidate factors and processes which limit the concentration of trapped active particles, matrix isolation of nitrogen atoms in solid N₂ was studied under wide range of the flow rates of nitrogen atoms and molecules fed onto the substrate. It was found out [9] that the major factor governing the concentration of impurity atoms was the atom surface diffusion during sample deposition which resulted in the recombination of these active particles.

At present, the laboratory research follows three scientific directions in the field of matrix isolation.

1. Detection and study of free radicals in low-temperature gas-grain reactions of astrophysical interest. Two studies have been done and the results were published [10]. One of them concentrated on the sequence of the H-atom addition gas-grain reactions in solid CO which is acknowledged to be among the fundamental processes responsible for the synthesis of organic molecules in interstellar cloud dust grains. As a result, both formyl (HCO and DCO) and methyl (CH₃ and CD₃) free radicals were detected in a sequence of these reactions. 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. 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 next most challenging problem will be detection, in a laboratory, of intermediate products in producing methanol, CH₃OH, molecules through the low-temperature gas-grain reactions.
   
   
2. Investigation of the ⌠giant■ photoelectron emission from the rare-gas (Ne, Ar, Kr) solids subjected to the action of VUV radiation. The yield of photoelectrons from the solid rare gases exceeds the yield from molecular-gas solids up to orders of magnitude. The photoelectrons are recorded with the use of the electron cyclotron resonance (ECR). During investigation, the attention is paid to factors increasing the emission and to processes of the excitation energy transfer in the bulk of the sample under irradiation and on its surface (see [11-13] and references therein).
   
   
3. EPR study of the quantum rotation of the CH₃-radical and its isotopomers isolated in low temperature matrices. At liquid helium temperatures, before the Boltzmann statistics take over in the classical high temperature realm, the spectral intensities of the radicals are dominated by quantum statistics. These properties can be attributed to quantum effects of inertial rotary motion and its coupling to the nuclear spin of the radical. The theoretical investigation of the above effects and other related quantum effects, as well as recognition of the important physics which lead to them, is not a simple matter. Experimental results obtained in the laboratory (see [14] and references therein) shed light on the problem.

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