Direct measurement of the crystal field splitting of isolated J=1 impurities in solid parahydrogen

The crystal field splitting of single J = 1 impurity orthohydrogen molecules in an otherwise pure J = 0 parahydrogen crystal has been studied experimentally and theoretically over many years. We have measured this quantity directly from the fine structure of the Q(3)(0) (v = 3<--0, J = 0<--0) overtone transition induced by J = 1 hydrogen, in the near infrared using a Ti:sapphire laser. From our observed eight pairs of split spectral lines, we have determined the value Delta/k = [E(M = +/- 1) - E(M = 0)]/k = 0.0102 +/- 0.0003 K including its sign. A straightforward numerical calculation based on ab initio calculations of the H-2-H-2 potential reproted by Mulder, van der Avoird, and Wormer gives Delta/k = 0.0116 K which is close to the experimental value. This suggests that the static and dynamic effects of phonons on the splitting may have been overestimated in the past.