Inhomogeneous broadening of infrared and Raman spectral bands of amorphous and polycrystalline thin films

Abstract

Infrared and Raman spectroscopy are used to examine effects of solid structure on vibrational line shapes in thin solid films of CO 2 and N 2 0 prepared by vapor deposition. A new method is described for forming films consisting of weakly-interacting microcrystals by deposition of the solute in an argon matrix followed by complete evaporation of the argon. The infrared absorption bands of these films exhibit inhomogeneous spectral broadening in the range between the transverse and longitudinal frequencies of the bulk crystal. The origin of this broadening is shown to be the dependence of vibrational frequencies on size and shape of the microcrystals due to long-range dipole interactions. In contrast, the spectra of polycrystalline films formed by direct vapor deposition are the same as for single crystals because the long-range dipole interactions effectively couple vibrational motions across the grain boundaries. An analysis shows that inhomogenously broadened spectra are obtained whenever the separation between the individual crystal grains is comparable to or greater than their dimensions.

RADIATION DAMAGE AND ASSOCIATED PHASE CHANGE EFFECT ON PHOTODESORPTION RATES FROM ICES—LYα STUDIES OF THE SURFACE BEHAVIOR OF CO2(ICE)

“…As shown in Figure 5, the strong ν 3 mode of 13 CO 2 (ice) displays evidence for the inhomogeneous broadening of this molecular mode with the appearance of LO (2314 cm −1 ) and TO (2280 cm −1 ) phonon modes from the lattice vibrations of the ice (Ovchinnikov & Wight 1993). The lattice-related LO spectral band consists of a shoulder on the high wavenumber side of the TO mode.…”

“…This absorption band contains inhomogeneously broadened LO (longitudinal) and TO (transverse) lattice phonon modes, which are related to the ice crystal structure (Ovchinnikov & Wight 1993). The ν 3 line shape changes slightly during irradiation, consistent with the disruption of the ice structure, and is therefore unsatisfactory for quantitative kinetic measurements.…”

“…The absorbance in this shoulder is related to the 13 CO 2 crystallite size, crystalline shape, and crystalline order in the ice, as well as to changes due to CO 2 depletion. When the crystallite size is made small compared to the wavelength of the IR light, the absorption is intensified near the LO lattice mode (Ovchinnikov & Wight 1993). Increasing the lattice damage in the crystallites is expected to cause a decrease of the relative intensity of the absorption in the region near the LO mode since vibrational coupling between CO 2 molecules will diminish in the disordered ice (Ovchinnikov & Wight 1993).…”

“…When the crystallite size is made small compared to the wavelength of the IR light, the absorption is intensified near the LO lattice mode (Ovchinnikov & Wight 1993). Increasing the lattice damage in the crystallites is expected to cause a decrease of the relative intensity of the absorption in the region near the LO mode since vibrational coupling between CO 2 molecules will diminish in the disordered ice (Ovchinnikov & Wight 1993). Figure 5 shows spectral changes in the ν 3 region where spectra are shown during Lyα irradiation at 75 K. It is seen that 13 CO 2 photodepletion leads to losses in intensity in both the LO and TO spectral regions.…”

Surface science investigations of the role of CO 2 in astrophysical ices

“…Infrared studies of crystalline CO 2 ices generally show a broad, multipeak, feature between 2340 and 2380 cm −1 [27,[64][65][66][67][68]. In particular, Ovchinnikov & Wight [27] observed the transverse optical (TO) and orthogonal, or longitudinal optical (LO), modes of the CO 2 υ 3 stretching mode, split as a consequence of the crystalline nature of the CO 2 ice, to be at 2344 and 2381 cm −1 , respectively. However, this does not explain the observation of the band at 2382 cm −1 in amorphous ices.…”