Infrared spectra and structure of methane – noble gas mixed crystals: the influence of temperature and methane concentration on the v₃ vibration band of methane

Abstract

The infrared spectra of CH,, at low concentrations in the noble gases, were obtained at temperatures ranging from 4 "K up to the respective boiling points of argon, krypton, or xenon. It is concluded from this study that methane occupies substitutional cavities in these lattices. Furthermore, it is established that the potential hindering the rotation of the molecules perturbs the higher rotational levels to a greater extent than the low lying levels.In a second series of experiments, the methane concentration was varied from 0.06 to 55 % for krypton; narrower ranges were used for argon and xenon. The experimental results clearly demonstrate that the influence of the CH, concentration is different for the three systems and that the fine structure does not result from molecular aggregation.The spectrum of 0.06% CD, in solid krypton shows fine structure which confirms the rotational origin of the four components observed for CH,. The CD, spectra previously published were complicated with additional lines that probably arose from the nonhomogeneity of the samples.Les spectres infrarouges du CH, en faibles concentrations dans les gaz nobles furent enregistres a des temperatures allant de 4 "K jusqu'aux points d'ebullition de I'argon, du krypton ou du xenon. Cette etude nous a permis de conclure que le methane occupe des cavites substitutionnelles dans ces reseaux.On a de plus constate que le potentiel s'opposant a la rotation des molecules se manifeste a un plus haut degrC sur les niveaux de rotation eleves que sur les premiers niveaux.La concentration de methane fut ensuite variCe de 0.06 a 55 % pour le krypton; des domaines moins Ctendus furent utilists pour I'argon ou le xenon. Les rCsultats exptrimentaux obtenus dimontrent clairement que I'effet de la concentration en CH, est different pour les trois systemes et que la structure fine n'est pas causee par I'agglomCration des molCcules.Le spectre du CD, en solution a 0.06% dans le krypton montre une structure fine qui confirme la nature rotationnelle des quatre composantes observees pour le CH,. Les spectres de CD, publies antirieurement Ctaient compliquCs par des raies additionnelles probablement attribuables a I'inhomogtnCitC des Cchantillons.

Infrared Spectra and Structures of Solid CH4 and CD4 in Phases I and II

“…Table 7 illustrates the similarity between the spectrum of neat CH, crystals and that of CH, isolated in a krypton matrix. The spacings between the various lines are very similar to the spacings found in noble gas matrices (20 the v, region (Fig. 2) the band at 987.5 belongs to type B molecules whereas that at 992.3 arises from type A molecules; on the high frequency side, a shoulder is assigned to v, of CHD, molecules present as an impurity whereas on the low frequency side the shoulder is due to 13CD, in natural abundance.…”

“…structure. The resemblance with krypton is particularly striking since both crystals have cavities of very similar sizes; this was previously discussed by one of us (18,20,21). Since quantized molecular rotation of methane was shown to exist in the crystals of the noble gases we will try to assign the sharp lines left unexplained in the spectra of the neat crystals to vibration-rotation transitions of the type B molecules.…”

“…The same general pattern was found for CD, isolated in noble gas matrices except for the greater number of components corresponding to the Boltzmann distribution amongst the more closely spaced energy levels of CD, which has a greater moment of inertia. It must be remembered that although the fine structure is well resolved at 8 OK, it broadens very rapidly with temperature and that around 30 OK only the rotational envelope is preserved (20). The spectra of the neat crystals were recorded at 9 OK for CH, and 23 O K for CD,; therefore from the noble gas matrices results it is obvious that the rotational features, if present, will be easier to detect in CH, than in CD,.…”

“…The i.r. (18)(19)(20) and Raman (21) spectra of both molecules in noble gas matrices have also been extensively studied.…”

Theoretical study of the ν3 and ν4 Raman bands of methane in rare gas matrices

“…for the symmetry of the total Hamiltonian [3], elaborated the rotational and the low-lying vibration-rotation states in the truncated rotational space J < 10, using the WRF's as the basis functions. Now we construct the wavefunctions of the rotational and vibrationrotation eigenstates utilizing the GRF's given in the preceding section, and thus make the assignment for each state under the group O,O, instead of 0 0 .…”