SPECTROSCOPIC EFFECTS IN CH₄/H₂O ICES

Abstract

Recent observations of CH 4 in different astrophysical objects encourage laboratory research on methane/water ice mixtures. An IR spectroscopy laboratory investigation is presented on these systems. Co-deposited samples are formed by vapor deposition of CH 4 and H 2 O on a cold substrate, in a wide range of stoichiometries, from very diluted mixtures to CH 4 /H 2 O = 2.5 values. Samples are prepared at 14 K and at 40 K, and their temperature behavior is studied when they are warmed up to 60 K. The spectroscopic analysis is centered on the methane features, and also on the water dangling bonds (DBs) that appear in the spectra of the mixtures. The IR forbidden ν 1 band shows up in the spectrum (3.44 μm), indicating some form of distorted methane. The combination bands ν 3 + ν 4 and ν 1 + ν 4 are seen at 2.32 and 2.38 μm, and the ν 2 + ν 3 band weakly at 2.21 μm. Whereas ν 3 is not shifted in spectra of mixed samples, the wavenumber peak of ν 4 and its combination bands vary in a 6 cm −1 range, providing a possible estimation for the relative methane concentration in the sample. Bands in the spectra of mixtures are always broader than their counterparts in pure CH 4 ice. The intensity of ν 4 appears to increase in mixed samples with respect to the pure solid. Raising the temperature of the ices up to 60 K liberates part of the methane, but a fraction is retained with a maximum value of ∼7% ± 2%. This limit may provide information on the temperature properties of astrophysical objects. The different spectral characteristics of water DBs with increasing methane proportion in mixed samples can also furnish information to estimate the stoichiometry of the mixture.

Interaction of CH4 and H2O in ice mixtures

“…The n 1 band of methane is a strong feature in Raman spectra, either of the gas phase 31 or from noble gas matrices, 32 and has recently been reported in infrared spectra of CH 4 ice mixtures by Hodyss et al 20 and Ga´lvez et al 19 The observation of this breathing vibration in infrared spectra implies a breakdown of the symmetry of the system, and consequently the formation of a distorted form of CH 4 . The adsorption of CH 4 molecules on water ice surfaces, or the presence of very small methane crystallites distorted from their cubic structure through interactions with the amorphous water environment, could cause such an effect.…”

“…For methane the interaction with water seems weaker, and CH 4 -dist, as we may call this distorted structure, is mainly manifested by the appearance of the forbidden n 1 band. Small shifts or broadenings are also observed on some of the other bands, 19 but they do not allow an independent and simultaneous identification of both methane structures.…”

“…This fraction probably corresponds basically to CH 4 -dist in the inner recesses of the bulk ASW, keeping the same value that it had for the lowest CH 4 /H 2 O proportions. In a recent work, 19 the amount of methane retained in co-deposited samples heated at 60 K was estimated to have a saturation value of B10 AE 2% irrespective of the initial CH 4 excess concentration. In the former investigations on CO 2 /H 2 O and CO 2 /CH 3 OH mixtures mentioned above, 25,[33][34][35] some CO 2 was also retained above its sublimation temperature, B90 K.…”

“…Laboratory experiments conducted on CH 4 /H 2 O samples reported changes in peak position, profiles and relative strength of IR bands with temperature and methane concentration. [15][16][17][18][19][20] There are also several studies on the morphology of water ices through CH 4 adsorption experiments. [21][22][23] We present in this article an investigation on methane/water ice mixtures in the 14-60 K temperature range.…”

OPTICAL CONSTANTS AND BAND STRENGTHS OF CH4:C2H6 ICES IN THE NEAR- AND MID-INFRARED

“…Also Quirico & Schmitt (1997) and Gálvez et al (2009) found that CH 4 peaks shift to higher frequencies when highly diluted in N 2 or in H 2 O, respectively. On the other hand, for CH 4 :H 2 O ratios with large methane amount, the opposite behavior was reported, leading to a shift to longer wavelengths (see Figure 3 in Gálvez et al 2009). In the CH 4 :C 2 H 6 mixtures investigated in this work, CH 4 peak positions move to lower frequencies for all dilutions.…”