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

Abstract

We present high resolution (∼0.01 cm−1) infrared absorption spectra of the ν4 band of methane doped parahydrogen (CH4/pH2) solids produced by two different techniques: gas condensation in an enclosed cell at T≈8 K, and rapid vapor deposition onto a T≈2 K substrate in vacuum. The spectrum of the rapid vapor deposited solid contains a novel progression of single peaks with ≈5 cm−1 spacing, superimposed over the known spectrum of CH4 molecules trapped in sites of D3h symmetry in hexagonal close-packed (hcp) solid pH2. New theoretical calculations of the rovibrational transitions of a tetrahedral molecule in an external field of Oh symmetry permit the assignment of this new progression to CH4 molecules trapped in crystalline face centered cubic (fcc) regions of the pH2 solid. Annealing of the rapid vapor deposited samples to T≈5 K decreases the intensities of the CH4/pH2(fcc) absorptions, and results in intensity changes for parallel and perpendicularly polarized CH4/pH2(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 pH2 hosts.