Possible identification of the dihydrogen radical anion in irradiated solid hydrogen

It has recently been shown that when solid para-hydrogen is exposed to ionising radiation at 4.2 K a narrow, 1:1 doublet, centered on g = 2.007 with a splitting of 12.4 G grows in the ESR spectrum. This was discussed in terms of a zero-field splitting of a triplet state comprising (H2+·−et−) pairs. The aim of the present study is to suggest that these features are the central doublet for H2−· radical anions. If this is correct, the real g-value becomes 2.0033, and the 1H hyperfine splitting, estimated from the 12.4 G splitting, is 203 G for two equivalent protons. The outer, MI = ± 1 features were not detected. This is not surprising since there should be a large anisotropic component for the outer features, which should greatly reduce their intensities relative to the very narrow central components. (These have now been detected by Professor T. Miyazaki and co-workers.)