Heavy snow: IR spectroscopy of isotope mixed crystalline water ice

Abstract

Mid-infrared spectra have been measured for crystalline water ice aerosols of widely varied H/D isotopic composition. Particles with diameters ranging from 10–200 nm were generated via rapid collisional cooling with a cold buffer gas over a range of temperatures from 7–200 K. In near isotopically pure ices, the ν_L band position is slightly red-shifted with increasing temperature whilst in the ν₂ region apparently anomalous shifts in peak maxima are explained by the contribution of a broad 2ν_L band of H₂O and a 3ν_L band of D₂O together with ν₂ intensity that is particularly weak in low temperature crystalline ice. The hydrogen bonded OH (or OD) oscillator bands of near pure H₂O (or D₂O) ices are blue-shifted with temperature, with a gradient very similar to that of the corresponding band in isotope diluted samples, HOD in D₂O (or H₂O). It implies that this observed temperature trend is predominantly due to the intrinsic change in local hydride stretch potential energy, rather than to changes in intermolecular coupling. However, it is also observed that the narrow hydride stretch bands of an isotope diluted sample rapidly develop sub-band structure as the oscillator concentration increases, evidence of strong intermolecular coupling and a high degree of delocalisation. Anomalous blue-shifts in the OD stretch profile as D₂O concentration grows is attributable to Fermi resonance with 2ν₂ of D₂O, in much closer proximity than the corresponding H₂O levels. Theoretical results from a mixed quantum/classical approach are used to validate these findings in the hydride stretching region. Theory qualitatively reproduces the experimental trends as a function of temperature and isotopic variance.