P. Ranson

Temperature dependence of the bandwidths and frequencies of some anthracene phonons. High-resolution Raman measurements

Using a coupled interferometer—spectrometer with a resolution of 0.02 cm−1 we have measured the Raman band profiles of the four low-frequency anthracene phonons ω1(ag), ω2(ag), ω6(bg) and ω7(bg) in the temperature range 2–70 K. These phonons possess very narrow bandwidth at low temperature which are convinently measured under high resolution. In particular the two lowest-frequency phonons ω1(ag) and ω6(bg) have a bandwidth at 2 K of 0.045 cm−1. The other two phonons ω7(bg) and ω2(ag) have bandwidths at 2 K of 0.165 and 0.4 cm−1, respectively. A detailed analysis of the bandwidth variation with temperature was made in terms of three-phonon decay processes. The exrerimental variation of the bandwidth with temperature was correctly reproduced assuming a single down-and up-process. The following results were obtained: ω1(ag): 49.45 cm−1 = 2×24.72 cm−1, 49.45 cm−1 = 98.45 cm−1 −49.0 cm−1; ω6(bg): 57.50 cm−1 = 2×28.75 cm−1, 57.50 cm−1 = 108.50 cm−1 −51.0 cm−1; ω7(bg): 71.20 cm−1 = 2×35.6 cm−1, 71.20 cm−1 = 120.20 cm−1 −49.0 cm−1: ω2(ag): 82.40 cm−1 = 57.50 cm−1 +24.9 cm−1, 82.40 cm−1 = 138.4 cm−1 −56 cm−1. The efficiency of the down- and up-processes is discussed in terms of the two-phonon density of states. The bandwidths at 2 K follows very closely the variation of the two-phonon sum density of states, whereas the relative importance of the up-processes follows well the two-phonon difference density of states. The anharmonic frequency shifts are corrected for the thermal expansion of the crystal using the Gruneisen single-phonon parameters and the thermal expansion coefficients given in the literature. This permits an estimation of the variation of the anharmonic shifts in the temperature range studied.

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

Abstract 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- h 8 at 57.5, 69, 511, 514, 766 and 1385 cm −1 and of three corresponding phonons of naphthalene- d 8 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- h 8 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.

A Raman scattering study of the phase transitions in SrTiO3 and in the mixed system (Sr1-xCax)TiO3 at ambient pressure from T = 300 K down to 8 K

The room-temperature Raman spectra of Sr1-xCaxTiO3 (SCT) for x = 0.06 (SCT06), 0.12 (SCT12) and 0.30 (SCT30) are shown to contain first-order Raman lines corresponding to Eg and B1g modes, which appear below 105 K in pure SrTiO3. The Eg line shows slight asymmetry, indicating the orthorhombic structure of SCT for x≥0.06. The low-temperature Raman spectra of SCT06 and SCT12 contain Raman-active modes of symmetry B2g and Eg, predicted by group theory but not observed in SrTiO3 or SCT0.7 (x = 0.007) by previous workers. The polar hard modes TO2 (175 cm-1) and TO4 (550 cm-1) are present even at room temperature for SCT06 and SCT12 due to Ca2+-centred ferroelectric microregions. The intensity of these modes starts increasing below 55 and 80 K for SCT06 and SCT12 respectively, which are nearly 2 Tc, where Tc is the temperature of the smeared dielectric peak. The presence of such an extended precursor region is akin to relaxor behaviour. For SCT30, two new lines at 79 and 128 cm-1 appear below the paraelectric to antiferroelectric phase transition temperature of 230 K. It is shown that Raman lines characteristic of both ferroelectric and antiferroelectric phases are present in SCT12 below 100 K, confirming the frustration model for the highly smeared dielectric response for this composition.

High resolution Raman spectra of the Fermi diad in crystalline CO2 at 6 K

High resolution Raman spectra of the Fermi diad ω1:2ω2 in crystalline CO2 at 6 K were obtained by means of a tandem interferometer‐spectrometer instrument. For the first time the two components of the factor group splitting of the Ω− hybrid level were perfectly resolved. The bandshape of the two overlapping components of the Ω+ state were obtained by means of polarization measurements. The very different residual bandwidths of these crystal modes are discussed in terms of decay processes involving the two‐phonon continuum and the isotopic impurity levels.

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

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 ...

Relaxation processes in molecular crystals. Temperature dependence of the bandwidth of Raman active lattice phonons of crystalline CO2

We have measured the bandwidth of the three Raman active lattice phonons of crystalline CO2 as a function of temperature in the range 6–130 K. For bandwidths narrower than 1 cm−1 we used a high resolution tandem interferometer–spectrometer instrument and for broader bands a standard Raman instrument. We found that our measurements, made at constant pressure, differ considerably from those obtained previously by other authors at constant volume. These differences are explained as due to the deconvolution procedure utilized for the constant volume measurements. The evolution with temperature of the phonon bandwidth shows that fourth‐order processes play a significant role in the phonon relaxation.

Low-temperature Raman spectra of nitromethane single crystals. Lattice dynamics and Davydov splittings

By means of Raman scattering experiment performed on single crystals of solid nitromethane in various polarization geometries at normal pressure and low temperatures, the Davydov splitting of the vibrational levels in the solid phase of hydrogenated CH3NO2 and perdeuterated CD3NO2 nitromethane is reported for the first time. A re-examination of the lattice vibrations in their various symmetry species is also given. These experimental data are discussed on the basis of calculations using harmonic models of stretching, bending, and torsional motions of an isolated molecule and using density functional theory and second-order Moller–Plesset perturbation theory. Then, following Sorescu et al. [J. Phys. Chem. B 104, 8406 (2000).], the intermolecular interactions (Buckingham 6-exp form plus charge–charge Coulombic interactions) were added to the intramolecular potential. The frequencies of the q≈0 phonons or vibrons of different symmetry species were calculated paying a special attention to the symmetric and an...

High resolution Raman study of phonon and vibron bandwidths in isotopically pure and natural benzene crystal

The frequency and the bandwidth of the 12 Raman active lattice modes and of more than 45 internal vibrons of natural and 12C isotopically pure benzene have been measured under high resolution with the aid of a tandem interferometer–spectrometer system or alternatively of a two meter focal length grating Sopra spectrometer. For five lattice phonons and for several vibrons the measurements were extended to a sufficient temperature range to study the relaxation processes in terms of anharmonic phonon–phonon and phonon–vibron coupling processes. The effect of the isotopic impurities on the phonon lifetime has been shown to be very important for some vibrons, in particular, for those resonant with the impurity level. The bandwidth evolution with temperature has been interpreted in terms of down- and up- three- and four-phonon as well as of dephasing processes.

Anharmonicity of zone-centre optical phonons: Raman spectra of the isomorphous ?-quartz, berlinite and gallium phosphate in the temperature range 8-300 K

The temperature dependence of the optical-phonon wavenumbers and bandwidths in the isomorphous α-quartz, berlinite (AlPO4) and gallium phosphate (GaPO4) was investigated by Raman scattering experiments in the temperature range 8–300 K. Combining these data with those from high-pressure Raman scattering experiments (up to 20 GPa), we estimated the implicit (thermal volume expansion) and explicit (pure self-anharmonic) temperature contribution to the temperature wavenumber shifts observed in isobaric experiments. In the framework of the Gruneisen approximation, the mode-Gruneisen parameters of the A1 modes in GaPO4 are given. As expected, these results are similar to those previously reported for quartz and berlinite. In the temperature range 8–300 K, which is less than the α–β temperature of transition (Tα → β ≈ 850 K) of quartz and berlinite, the phonon population decay processes appear to be the main cause of phonon damping in the three materials. Copyright

Fermi resonance states in the natural isotopically mixed CS2 crystal

High‐resolution Raman spectroscopy (HRRS) is used to analyze the anharmonic interactions between the modes ν1 and ν2 in the CS2 crystal and its natural isotopic aggregates. Statistics of aggregates (mainly formed with molecules containing 34S and 33S isotopes) are calculated. Using the exciton theory formalism, a unified description of the Fermi resonance going from the molecule to the crystal is provided and used to calculate the various eigenmodes of aggregates. HRRS allows an accurate analysis, in the ν1 frequency domain, of the complex band structure due to the various configurations of isotopic aggregates (monomers, dimers, trimers,...). In the crystal, the weak Fermi resonance interaction (ν1, 2ν2 ) leads to the appearance of bound states ν+ and ν−. HRRS allows one to clearly separate the two Raman active Davydov components (Ag and B3g ) in both the ν− and ν+ bands. Their thermal evolution is also given and compared to other incoherent as well as coherent Raman data.