Astron. Astrophys. 333, 1025-1033 (1998)

3. Results

The mid-infrared spectrum of solid methane shows four active bands, two fundamentals at 1302 cm^-1 (7.68 µm, C-H deformation mode) and 3010 cm^-1 (3.32 µm, C-H stretching mode), and two combination modes, the at 4203 cm^-1 (2.38 µm) and the at 4303 cm^-1 (2.32 µm). In this section we present a detailed study of the two fundamental modes in pure methane and ice mixtures, as deposited and after ion irradiation. All the spectra were first divided, in intensity scale, by a fitted continuum. Then they were converted to an optical depth scale, and peak positions and FWHM were taken. The intensity between sampled points were interpolated by cubic splines. The error in the peak position and the FWHM are estimated, from the noise level in the continuum, to be typically about 0.5 cm^-1, anyway always less than the sampling (1 cm^-1).

3.1. The 1302 cm^-1 C-H deformation mode

Fig. 1a shows the profile of the mode for pure CH₄ at T = 12.5 K, in optical depth scale and normalized to unit integrated area, at the end of the deposition.

Fig. 1. a The profile of the band at 1302 cm-1 in pure CH4 at T = 12.5 K (full line) and its resolution in two components (dotted lines). The band has been normalized to unit integrated area. b The profile of the band at 3010 cm-1 in pure CH4 at T = 12.5 K (full line) and its resolution in components (dotted lines). The main peak is reproduced by the superposition of the second and third functions listed in Table 3. The band has been normalized to unit integrated area.

It can be resolved in two components, that have been fitted with pseudo-voigt functions. A pseudo-voigt function is simply the weighted mean of a gaussian and a lorentzian with the same peak position and FWHM. A third parameter, , gives the weight of the gaussian, and, consequently, gives the weight of the lorentzian. The parameters were derived from a combined fit of all the spectra taken at various thicknesses of the sample, during its deposition. The parameters of the two fitting functions are shown in Table 1. The choice of the fitting functions has been just one of convenience to better reproduce the profile. The fitted profiles are shown dotted in Fig. 1a. While the fit is rather good, the main component in the experimental spectrum is slightly asymmetric, suggesting that the real profile may be more complex, possibly with more unresolved components.

Table 1. The parameters of the pseudo-voigt functions that best fit the profile of the 1302 cm^-1 band in pure CH₄ as deposited at T = 12.5 K.

The thickness of the pure CH₄ sample was measured during deposition from the interference fringes of a He-Ne laser beam reflected by the interfaces film-vacuum and film-substrate. Since the substrate is tilted of with respect to the IR beam, the thickness traversed by the latter is approximately times the measured thickness of the sample. We could therefore calculate the column density of CH₄ molecules in the path of the IR beam, using a density of solid CH₄ of 0.52 g cm^-3 (Landolt-Börnstein 1971). Fig. 2a shows the points corresponding to the integrated optical depth of the band at various stages of the deposition, and thus various thicknesses, of the sample, plotted against the column densities of CH₄. The slope of the straight line through the origin fitted to the data yields a value of 6.4 10^-18 cm mol^-1 for the integrated absorbance. This value is somewhat smaller than that of 7.3 10^-18 cm mol^-1 derived by Boogert et al. (1997) based on data previously published by Hudgins et al. (1993).

Fig. 2. The points represent individual measures of the integrated optical depth of the CH4 fundamental bands at 1302 cm-1 (inset a) and 3010 cm-1 (inset b) versus the column density of the sample, as deposited at T = 12.5 K. Superimposed is the line through the origin that best fits the points.

The value of the integrated absorbance can not, at present, be measured when CH₄ is codeposited with other species, because we are not able to obtain an accurate estimate of the relative amount of each species in the mixture. In fact the assumption that the species deposit in the same ratio as they are prepared in the mixing chamber might in some case be incorrect. Thus in the following we will assume that the integrated absorbance of CH₄ bands does not change in mixtures.

Fig. 3 shows the band profile in different ice mixtures containing CH₄ for different irradiation doses. Fig. 4 shows the band profile as produced after ion irradiation of ice mixtures containing CH₃ OH and H₂ O at different ratios. Table 2 lists peak positions and widths of the band in all the samples examined, at various irradiation doses, before warming up. The peak position of the C-H deformation mode of CH₄ was found to range between 1299.3 to 1304.0 cm^-1 in the examined ice mixtures. The gas phase position of the origin of the band lies at 1305.5 cm^-1, so that the band appears always more or less redshifted, indicating an attractive interaction with the solid matrix. The highest redshift was observed for CH₄ codeposited with NH₃, even higher than that observed for pure CH₄ at low temperatures, while the lowest was observed for CH₄ produced irradiating CH₃ OH or mixtures of CH₃ OH and water with energetic ions. The full width at half maximum, at low temperatures, ranges from 6.7 cm^-1 in pure CH₄ as deposited to 13.8 cm^-1 in H₂ O+NH₃ +CH₄ as deposited. It can be seen that, exception made for pure CH₄, the general trend is that this band shifts, if any, to slightly higher wavenumbers and gets narrower for higher irradiation doses. It can also be seen that, in all samples examined, if CH₄ is produced by ion irradiation of ice mixtures containing CH₃ OH, the band tends to peak at higher wavenumbers. Where comparable, our results agree closely with those published by Boogert et al. (1997).

Fig. 3. Variations of the shape and position of the band at 1302 cm-1 of CH4 codeposited in various ice mixtures upon increasing ion irradiation doses.The initial ice mixture is indicated in top left corner of each box, along with the temperature of the sample. The dose, in units of eV/16 a. m. u., is indicated beside each plotted band. The position of the origin of the band in gaseous CH4 (1305.5 cm-1) is indicated by the dashed line, the position of the peak in pure solid CH4 as deposited (1301.5 cm-1) is indicated by the dash-dotted line.

Fig. 4. Variations of the shape and position of the band at 1302 cm-1 of CH4 produced by ion irradiation in various ice mixtures upon increasing doses.The initial ice mixture is indicated in top left corner of each box, along with the temperature of the sample. The dose, in units of eV/16 a. m. u., is indicated beside each plotted band. The position of the origin of the band in gaseous CH4 (1305.5 cm-1) is indicated by the dashed line, the position of the peak in pure solid CH4 as deposited (1301.5 cm-1) is indicated by the dash-dotted line.

Table 2. Position (cm^-1) and FWHM (cm^-1) of the 1302 cm^-1 band of CH₄ in various ice mixtures at different doses of ion irradiation, all at low T (10-16 K). The dose is expressed in eV/16 a.m.u

3.2. The 3010 cm^-1 C-H stretching mode

Fig. 1b shows the profile of the mode for pure CH₄ at T = 12.5 K, in optical depth scale and normalized to unit integrated area, at the end of the deposition. It shows a considerable structure, and can be fitted with a superposition of five pseudo-voigt functions with the parameters given in Table 3. As before, the choice of such functions was simply one of convenience.

Table 3. The parameters of the pseudo-voigt functions that best fit the profile of the 3010 cm^-1 band in pure CH₄ as deposited at T = 12.5 K.

The fitted profiles are shown dotted in Fig. 1b. Clearly, the superposition of the second and third function reproduces the profile of the main peak, while the others seem to represent real fine structure. The first one is very weak and might be simply noise, even though it is consistently present in all spectra taken at different thicknesses of the sample, as the parameters were derived from a combined fit of all these spectra. Moreover, such structures are expected to appear in CH₄ due to the breaking of symmetry of the higher vibrational level involved in the transition by the crystal electric field and to the presence of different nonequivalent sites (see, for example, Bohn et al., 1994). Again, to measure the integrated absorbance we used all the spectra taken at different thicknesses of the sample, during deposition. The slope of the straight line through the origin fitted to the data yields a value of 9.5 10^-18 cm mol^-1. The data and the fitted line are shown in Fig. 2b. Using the data from the paper of Hudgins et al. (1993), assuming for pure CH₄ ice a density of 0.52 g cm^-3 (Landolt-Börnstein 1971), as done by Boogert et al. (1997) for the band at 1302 cm^-1, one derives a value of 11.0 10^-18 cm mol^-1, which is again somewhat larger than our measurement.

Fig. 5 shows the band profile in different ice mixtures containing CH₄ for different irradiation doses. Fig. 6 shows the band profile as produced after ion irradiation of CH₃ OH. It is evident that the signal to noise ratio in this band is generally much worse, especially when water is present in the ice mixture, since this band lies on top of a very strong and broad water band that has to be subtracted first. Table 4 lists peak positions and widths of the band in all samples examined, at various irradiation doses, before warming up. The peak position of the C-H stretching mode of CH₄ was found to range between 3007.7 to 3011.7 cm^-1 in the examined ice mixtures. The gas phase position of the origin of the band lies at 3020.3 cm^-1, so that the band appears again more or less redshifted. The highest redshift was observed for CH₄ codeposited with NH₃, just as for the C-H deformation band, while the lowest was observed for CH₄ codeposited with N₂. The full width at half maximum, at low temperatures, ranges from 5.2 cm^-1 in pure CH₄ as deposited to 14.7 cm^-1 in H₂ O+NH₃ +CH₄ as deposited, again just as for the other fundamental band.

Fig. 5. The profiles of the band at 3010 cm-1 of CH4 codeposited in various irradiated and unirradiated ice mixtures. The position of the origin of the band in gaseous CH4 (3020.3 cm-1) is indicated by the dashed line, the position of the peak in pure CH4 ice as deposited (3009.2 cm-1) is indicated by the dash-dotted line.

Fig. 6. The spectrum of the band at 3010 cm-1 of CH4 produced by ion irradiation of CH3 OH. The position of the origin of the band in gaseous CH4 (3020.3 cm-1) is indicated by the dashed line, the position of the peak in pure CH4 ice as deposited (3009.2 cm-1) is indicated by the dash-dotted line.

Table 4. Position (cm^-1) and FWHM (cm^-1) of the 3010 cm^-1 band of CH₄ in various ice mixtures at different doses of ion irradiation, all at low T (10-16 K). The dose is expressed in eV/16 a.m.u

3.3. Warm-up effects

Figs. 7 and 8 show the evolution of the band profiles of the C-H deformation and the C-H stretching upon warming up the irradiated and unirradiated samples. Tables 5 and 6 list measured peak positions and FWHMs. Generally, the band at 1302 cm^-1 changes much less than that at 3010 cm^-1 with temperature. The band at 1302 cm^-1 always gets slightly narrower at first to get wider again at higher temperatures only in some of the samples. The peak position of this band changes very little, except for pure CH₄ and H₂ O+CH₄ (1:1) as deposited, for which phase changes suggested by abrupt changes in the profile of the bands can be clearly seen respectively between 12.5 and 16 K (see CH₄ in Figs. 7 and 8) and between 27 K and 47 K (see H₂ O+CH₄ (1:1) in Fig. 7). The band at 3010 cm^-1 is, on the other hand, extremely sensitive to the temperature in all the samples in which it was detected. Its width always increases greatly upon warm-up, particularly in pure CH₄, for which it changes from 5.2 to about 20 cm^-1. Again, variations in the peak position are much smaller. The strong variations in width may result from a competition between annealing, that tends to reduce the number of different nonequivalent sites that a CH₄ molecule can occupy, and another mechanism, that might be hindered rotation (Jones et al., 1986; Nelander, 1985), that tends to increase the width of the bands. However, it is clear that the band at 3010 cm^-1 is much more sensitive to temperature changes than that at 1302 cm^-1. Since both bands arise from transitions from the fundamental level to a vibrational level of the same species, if their widths were due to unresolved hindered (almost free) rotational structure, one would expect them to be very similar at all temperatures, and especially to have similar widths in energy scale. Since this is not the case, not even for pure CH₄, either hindered rotation is not the sole cause of broadening, or the molecule rotation must be rather differently hindered in the two different excited vibrational states. Our measuments of the peak positions of the bands at 1302 cm^-1 and 3010 cm^-1 of pure CH₄ as deposited at various temperatures, upon comparison with those by Hudgins et al. (1993) at 10, 20 and 30 K, are very close but systematically shifted to slightly lower wavenumbers with respect to them, by about 0.5 cm^-1.

Fig. 7. Variations of the shape and position of the CH4 band at 1302 cm-1 in irradiated and unirradiated ice mixtures upon warm-up. The initial mixture is indicated in top left corner of each box, along with the dose, in units of eV/16 a. m. u.. The temperature is indicated beside each plotted band. The position of the origin of the band in gaseous CH4 (1305.5 cm-1) is indicated by the dashed line, the position of the peak in pure CH4 ice as deposited (1301.5 cm-1) is indicated by the dash-dotted line.

Fig. 8. Variations of the shape and position of the CH4 band at 3010 cm-1 in various ice mixtures upon warm-up. The initial mixture is indicated in top left corner of each box, along with the dose, in units of eV/16 a. m. u.. The temperature, in K, is indicated beside each plotted band. The position of the origin of the band in gaseous CH4 (3020.3 cm-1) is indicated by the dashed line, the position of the peak in pure CH4 ice as deposited (3009.2 cm-1) is indicated by the dash-dotted line.

Table 5. Position (cm^-1) and FWHM (cm^-1) of the 1302 cm^-1 band of CH₄ in various irradiated and unirradiated ice mixtures upon warm-up.

Table 6. Position (cm^-1) and FWHM (cm^-1) of the 3010 cm^-1 band of CH₄ in various irradiated and unirradiated ice mixtures upon warm-up.

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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