Portal de Periódicos da CAPES

Vibrational relaxation in molecular crystals. High-resolution Raman band profiles of some naphthalene and naphthalene-d8 phonons at low temperature

1984; Elsevier BV; Volume: 86; Issue: 1-2 Linguagem: Inglês

10.1016/0301-0104(84)85160-5

1873-4421

P. Ranson, R. Ouillon, S. Califano,

Solid-state spectroscopy and crystallography

The accurate determination of the shape of phonon Raman bands in crystals is of great importance for the determination of the phonon lifetimes and for the study of their relaxation mechanisms. Several molecular crystals show at low temperatures the occurrence of long-living phonons with Raman bandwidths which are much narrower than the spectral slitwidth of standard Raman instruments. Information on their relaxation processes is thus normally extracted only from time domain experiments using picosecond CARS techniques. In order to extend these measurements to the frequency domain high resolution is needed. For this purpose we have built a high-resolution Raman instrument coupling together a Raman spectrometer and an interferometer. The tandem instrument has a limiting resolution of 0.02 cm−1, largely sufficient to measure the bandwidths of long-living phonons in molecular crystals. With this instrument we have measured the profile of the six Raman-active phonons of naphthalene-h8 at 57.5, 69, 511, 514, 766 and 1385 cm−1 and of three corresponding phonons of naphthalene-d8 at 54, 64 and 493 cm−1 at 5 K. The observed bandwidths are well fitted by lorentzian curves. Comparison with available recent picosecond CARS data is used to prove the competitivity of high-resolution Raman with time-delayed spectroscopy. Bandwidths measured with the two techniques show differences of a few thousands of cm−1 for most of the bands studied. Only in the case of the two lowest phonons of naphthalene-h8 larger differences are observed. These are interpreted in terms of optical crystal quality. The observed band profiles are interpreted in terms of population decay mechanisms based on anharmonic calculations of three-phonon decay processes. Pure dephasing processes are shown to give vanishing contributions to the low-temperature widths.