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

We present high resolution (similar to 0.01 cm(-1)) infrared absorption spe ctra of the nu(4) band of methane doped parahydrogen (CH4/pH(2)) solids pro duced by two different techniques: gas condensation in an enclosed cell at T approximate to 8 K, and rapid vapor deposition onto a T approximate to 2 K substrate in vacuum. The spectrum of the rapid vapor deposited solid cont ains a novel progression of single peaks with approximate to 5 cm(-1) spaci ng, superimposed over the known spectrum of CH4 molecules trapped in sites of D-3h symmetry in hexagonal close-packed (hcp) solid pH(2). New theoretic al calculations of the rovibrational transitions of a tetrahedral molecule in an external field of O-h symmetry permit the assignment of this new prog ression to CH4 molecules trapped in crystalline face centered cubic (fcc) r egions of the pH(2) solid. Annealing of the rapid vapor deposited samples t o T approximate to 5 K decreases the intensities of the CH4/pH(2)(fcc) abso rptions, and results in intensity changes for parallel and perpendicularly polarized CH4/pH(2)(hcp) transitions. We discuss these phenomena, and the n arrow (0.01-0.04 cm(-1) full width at half-maximum) absorption linewidths, in terms of the microscopic structure of the pH(2) hosts. (C) 1999 American Institute of Physics. [S0021-9606(99)01533-0].