Methyl group rotation of p-chlorotoluene in cyclohexane: A nuclear spin conversion study via UV-hole burning

We performed hole burning experiments in the UV with p-chlorotoluene as a probe in cyclohexane. The 4 K absorption spectrum shows a rich structure of sites. In all sites which we investigated hole burning was possible, but only in some of them the central hole was accompanied by sharp antiholes. In these cases hole burning is exclusively based on a light-induced change of the total nuclear spin of the methyl group. The hole-antihole splitting is strongly site-dependent. In one of the sites we measured the nuclear spin conversion relaxation of the methyl group. It is governed by a Raman process.