High resolution infrared absorption spectra of methane molecules isolated in solid parahydrogen matrices

摘要

We present high resolution (approx 0.01 cm~(-1)) infrared absorption spectra of the v_4 band of methane doped parahydrogen (CH_4/pH_2) solids produced by two different techniques: gas condensation in an enclosed cell at T approx= 8 K, and rapid vapor deposition onto a T approx= 2 K substrate in vacuum. The spectrum of the rapid vapor deposited solid contains a novel progression of single peaks with approx= 5 cm~(-1) spacing, superimposed over the known spectrum of CH_4 molecules trapped in sites of D_(3h) symmetry in hexagonal close-packed (hcp) solid pH_2. New theoretical calculations of the rovibrational transitions of a tetrahedral molecule in an external field of O_h symmetry permit the assignment of this new progression to CH_4 molecules trapped in crystalline face centered cubic (fcc) regions of the pH_2 solid. Annealing of the rapid vapor deposited samples to T approx= 5 K decreases the intensities of the CH_4/pH_2(fcc) absorptions, and results in intensity changes for parallel and perpendicularly polarized CH_4/pH_2(hcp) transitions. We discuss these phenomena, and the narrow (0.01-0.04 cm~(-1) full width at half-maximum) absorption linewidths, in terms of the microscopic structure of the pH_2 hosts.