High resolution Raman spectroscopy in the α and β crystalline phases of N2

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Abstract
The line shape of the Eg libron at 32 cm−1 and of the two components (Ag and Tg) of the vibron at 2330 cm−1 of solid α‐N2 at normal pressure were measured as a function of temperature between 5 and 35.6 K by means of high‐resolution Raman spectroscopy with a limiting spectral resolution of 0.006 cm−1. A suitable deconvolution procedure provides the respective weights of Lorentzian and Gaussian contributions to the true phonon line shapes. The measured residual bandwidth of the Eg libron at low temperature is noticeably narrower than that previously quoted in the literature (0.16 cm−1 instead of 0.8 cm−1). At high temperature it is shown that the libron relaxation mechanisms are mainly governed by four‐phonon processes. The temperature dependence of the vibron dephasing mechanisms are discussed in terms of contributions arising from depopulation, elastic and inelastic quartic processes as well as from disorder. The influence of the low energy optical phonons (ω≂32 cm−1) in the vibron relaxation mechanisms is emphasized. It is shown that between 25 K and the α–β transition temperature at Tc=35.6 K, the temperature dependent vibron linewidths are driven by inelastic processes. Finally some results on the frequency and the bandwidth of the ν1 mode in the β phase are reported.