Fig. 1-9 Electron spin resonance (ESR) spectrum of ortho- and parahydrogen anion radicals produced and trapped in a gamma-ray irradiated solid parahydrogen at 4.2 K Solid parahydrogen is a unique quantum system which offers very high resolution-ESR spectroscopy due to the lack of hyperfine interaction (two opposite nuclear spins cancel each other). This allows us to interpret the ESR spectrum shown above as not due to the e-H2+ pair, HO2, H3, nor H2+ but solely due to the H2- anion.

How do chemical reactions proceed at very low temperatures for example, on the dust particles in outer space? The energy necessary for chemical reactions may be available from UV- or cosmic-rays. Atomic motion may be determined not by thermal energy dynamics but by quantum mechanical tunneling effects. Recently we found that, on irradiating with gamma-rays of solid parahydrogen (p-H2; two nuclear spins in the molecule are opposite to each other), the electron generated in the ionization event was captured by the p-H2 molecule. Hydrogen molecular anions thus formed are in the para (25%) and ortho (75%) states and disappear gradually in about a day by the tunneling mechanism at 4.2 K. This mechanism of chemical reaction which has long been unknown, is very important to understand, for instance, for the formation of protein precursors in the space.

Reference T. Kumada et al., Experimental Observation of H2- Anions by High-Resolution ESR spectroscopy in gamma-Ray Irradiated Quantum Solid Parahydrogen at 4.2 K, Chem. Phys. Lett., 251, 219 (1996).

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