High resolution neutron spectrometer LAM-80ET and rotational tunnelling in 4-methylpyridine N-oxide

A high energy-resolution neutron spectrometer with large mica mirrors, LAM-80ET, has been constructed at National Laboratory for High Energy Physics in Japan (KEK). The highest resolution achieved with this spectrometer is 1.2 μeV. In order to examine the capability of the spectrometer, the rotational tunnelling of the methyl group in 4-methyl pyridine N-oxide (N-oxy γ-picoline) has been measured. Using the (004) reflection of the mica mirrors, we have clearly observed four peaks at ε=60, 115, 190 and 274 μeV. Measurements of the Q-dependence of these tunnelling peaks show that the tunnelling potential may be three-fold. It is also shown that the line width of tunnelling modes and the spin conversion at T > 5 K are derived by the libron-phonon process with EL = 30 - 40 K. (author)