Experimental Studies of Amorphous and Polycrystalline Ice Films Using FT-RAIRS

Abstract

A systematic investigation of amorphous and crystalline vapor deposited ice layers with thickness ranging from less than 100 nm to more than 5 μm has been performed using Fourier transform (FT) reflection−absorption infrared spectroscopy (RAIRS). Al and Au surfaces were used for the vapor deposition and very similar results were obtained on both. The spectra were recorded both with polarized and nonpolarized radiation and simulated with a simple Fresnel reflection model and empirical optical indices from the literature. Optical effects peculiar to this technique like surface suppression or enhancement of vibrational modes, saturation of intense absorptions, and IR interferences, are found to distort the spectra to a greater or lesser extent over the whole thickness range investigated. The diverse spectral band shapes and intensities are globally well reproduced with the mentioned Fresnel model. Some noteworthy discrepancies are, however, observed in the most intense peaks of the polarized spectra, which are affected by larger distortions. Whenever possible, the present measurements have been compared with published spectra recorded under similar conditions and a good accordance has been found. This comparison and the spectral simulations can reconcile seeming discrepancies in the previous literature data.

OPTICAL CONSTANTS OF NH3AND NH3:N2AMORPHOUS ICES IN THE NEAR-INFRARED AND MID-INFRARED REGIONS

“…The apparatus used has been previously described by Maté et al (2003) and Herrero et al (2010), and only a brief account will be given here. The experimental arrangement consists of a high vacuum chamber with background pressures in the 10 −8 mbar range coupled with a closed-cycle helium cryostat.…”

“…The ice film thickness was measured by monitoring the interference pattern of a He-Ne laser beam incident on the growing ice film at θ 1 = 4.0 ± 0.2 • from the surface normal, as described previously (Maté et al 2003). The reflected light was detected by a photodiode and converted to a digital signal.…”

SPECTROSCOPIC EFFECTS IN CH4/H2O ICES

“…With this configuration the pressure in the QMS chamber can be maintained below 2 × 10 −6 mbar for all the experimental conditions of the present work. The absolute deposition pressure was determined from the growth rate of solid films of the pure species on the substrate (Maté et al 2003). Film thickness values, needed for the calculation of the growth rate, were derived by simulating our transmission spectra with optical constants from the literature.…”

Trapping and adsorption of CO2 in amorphous ice: A FTIR study

“…Our experimental set-up has been described in detail before (Carrasco et al, 2002;Maté et al, 2003;Gálvez et al, 2007). In brief, it consists of a high vacuum cylindrical chamber, evacuated by a turbomolecular pump.…”

“…The background pressure in the chamber is usually in the 10 -8 mbar range. The chamber is coupled to a Fourier transform infrared spectrometer (Bruker Vertex70) through a (Gálvez et al 2007, Maté et al 2008, is an improvement of our original configuration (Carrasco et al 2002, Maté et al 2003, intended to establish a pressure difference of up to two orders of magnitude between the deposition chamber and the QMS chamber. This modification allows the use of the QMS to control the vapor composition even for relatively high deposition pressures like some of those selected in the present work (P > 5x10 -5 mbar).…”