VIS-NIR overtone bands of snow: Photoacoustic spectroscopy

摘要

Photoacoustic (PA) spectrum of natural dry snow was obtained in the wavelength range of 200-1100 nm, using an indigenously developed PA Spectrometer (Kapil, J.C, Joshi, S.K., Rai. A.K., 2003. Insitu Photoacoustic Investigations of some optically transparent samples like ice and snow. Review of Scientific Instruments 74 (7), 3536-3543), working in a temperature range of room temperature to —40 ℃. Fundamental frequencies (ν_1, ν_2, ν_3) as well as overtone frequencies of O-H vibrations in a snow crystal were identified and the corresponding combinational frequencies were assigned in the UV-VIS— NIR region of electromagnetic spectrum. The PA spectrum of snow thus obtained was compared with the PA spectra of distilled water and hexagonal ice (ice-Ih), in the same wavelength region. Bathochromic shifts (942→974→983 nm) in the third overtone frequency (3ν_1) of fundamental O-H vibrations in the H_2O molecule were observed when the phase changes as vapor→liquid→solid. These shifts have been explained in the context of increased contents of hydrogen bonding (skeleton of H-bonds) in the denser phases (liquid and solid) as compared to the rare phase (vapor). The effect of temperature on the creation or breakage of H-bonds in snow crystals has been demonstrated by the relative shifting of the absorption maxima. For monitoring the effect of H-bonding leading to the intricacy in the crystallography of snow, vibrational absorption bands were analyzed from the relative shifting associated to the absorption maxima of snow and snow-melt water at two different wavelengths (535 and 826 nm) near the phase transitions. Also, our investigation reveals that ΔE_j (change in vibrational energy) are symmetrically aligned about the Δ?-axes (change in vibrational quantum number) at phase transitions.