High-resolution spectroscopy of matrix-isolated ReO_4^− molecules

The temperature-dependent linewidth of the saturable infrared active nu(3) vibrational mode (10.8 microm) of ReO(4)(-) molecules in pure-KI host crystals has been measured between 2 and 600 K. Below 20 K the absorption line is inhomogeneously broadened and has a 0.016-cm(-1) width (FWHM). Above 30 K the transition is homogeneously broadened by elastic acoustic phonon-scattering processes. The low-temperature inhomogeneous width of ReO(4)(-) molecules in KI:RbI alloys is 2 orders of magnitude larger than for the pure KI crystal, and the absorption is coincident with the 10.8-microm P(42) CO(2) laser line at all temperatures.