CALORIMETRIC STUDIES OF TUNNELING PHENOMENA

Calorimetric investigations have been performed for the molecules and ions that show tunneling in solids and adsorbed monolayers. A combination of heat capacity measurement and neutron scattering experiment yields valuable information about the energy scheme and the kinetic behavior such as spin conversion. Finding of a phase transition of those solids also gives us helpful information to understand the whole nature of tunneling phenomena. For ammonium ions diluted in a KBr crystal and for methane molecules adsorbed on the surface of graphite, the energy scheme of the rotational ground state was investigated. The rate of conversion between different spin species was determined for ammonium ions in KBr, methane molecules on graphite and solid 4-methylpyridine N-oxide. New phase transitions were found in 4-methylpyridine N-oxide, 1,3,5-tribromo-2,4,6-trimethylbenzene and 2,6-dimethylpyridine. A hydrogen-bonded crystal Rb3 H(SeO4)2 has an excess contribution in heat capacity below 30 K, which may come from the proton tunneling.