Crystal-field spectroscopy of UCl4 by neutron inelastic scattering

The crystal-field level scheme of uranium tetrachloride has been the subject of a number of investigations by both optical and magnetic measurements. Agreement does not exist. To attempt to resolve this problem we have performed experiments with the pulsed neutron source at ANL which allows electronic spectroscopy to be extended to approx.2000 cm-1. Initially using incident energies of 400 cm-1 we examined the GAMMA4 (ground state) to GAMMA5 (excited state) level with the sample at 10K. A very strong line is seen at 92 +- 1 cm-1 which is clearly the GAMMA4 → GAMMA5' transition. This is in good agreement with a previous neutron study but in disagreement with the 110 to 120 cm-1 assignment by other techniques. On warming to 50K the peak position shifts to lower energy by approx.4%, broadens, and decreases dramatically in magnitude to approx.1/3 of its value at 10K. Boltzmann statistics would predict a decrease of only 20% on warming from 10 to 50K. We assume from this that a strong coupling occurs between the crystal-field and vibronic states. We have also used an incident energy of 2000 cm-1 to search for the higher GAMMA5' state. This was found at 1125 +- 3 cm-1, in good agreement with previous work. On warming the sample this GAMMA4 → GAMMA5 transition strength rapidly diminishes. Since the phonon spectrum does not extend beyond approx.300 cm-1, this behavior is most unusual. Because of this strong relaxational process, we have not been able to determine the positions of the other crystal-field levels. 6 refs