Vibrational line broadening in the solid system N2-Kr: a molecular dynamics study

The vibrational band shapes and the related parameters of N2N2 in Kr have been calculated by molecular dynamics simulations as a function of the nitrogen concentration. Most of the simulations have been applied to the solid hcp phase at 5 GPa and 296 K. The calculated spectra have been obtained by full analysis of the relaxation function. Due to the limited size of the system, the particles remain near the same lattice point throughout a simulation run (no N2-KrN2-Kr exchange). Upon dilution, the vibrational frequency of nitrogen in krypton shows a red shift. The full width at half maximum is extremely composition dependent, with a maximum value of 3.5 cm−1 at equal mole fractions. In addition, for the 50 and 75 mol % systems, a few special configurations with ordered distributions have been simulated. On the basis of these results together with earlier experimental data it is suggested that, in the real solid system, the nitrogen and krypton particles exchange places rapidly so that in time, each N2N2 molecule vibrates with all possible frequencies. To make an estimation of the exchange rate, several simulations have been performed during which the particles exchange randomly at various rates. The calculated widths depend strongly on the exchange rate. By comparison of the calculated and the experimental width of the spectra, an estimation of the exchange rate in the real system is made