Hindered rotation and site structure of methane trapped in rare gas solids

High resolution infrared absorption spectra have been recorded for the ν4 vibrational mode of CH4 trapped in rare gas solids. In argon, krypton, and xenon hosts two dominant sites are observed which we call site R and site A,B. For both sites we believe that the CH4 molecule replaces one host atom. The fine structure of site R at a temperature of 5 K is that predicted for a tetrahedral molecule rotating in an octahedral field; singlet peaks for P(1), Q(1), R(0), and a doublet for the R(1) transitions. The barrier for this rotation is not clearly defined. Site A,B shows site symmetry splitting and orientational ordering suggestive of C3v symmetry. Rotational structure is also associated with site A,B; it is not clear whether the hindered rotation for this site takes place about only the C3 axis or about axes perpendicular to C3 as well. From annealing studies and the effects of deposition temperature we conclude that site R is for CH4 molecules trapped in the normal cubic-close-packed environment while site A,B is for CH4 trapped in hexagonal-close-packed pockets induced by stacking faults formed during deposition. Temporal changes of peak intensities while the matrices stand at 5 K are indicative of a slow relaxation of nuclear spin from a triplet state (J=1) to a quintet state (J=0). For site A,B the triplet to quintet spin relaxation is considerably faster than for site R; for example, for a matrix of CH4 in krypton (1/10 000) at 5 K the spin relaxation half-lives are 87 minutes and 26 minutes for sites R and A,B, respectively.