Alfons Weber

Ro‐vibronic species, overall allowed species, and nuclear spin statistical weights for symmetric top molecules belonging to the Dnd and Dnh (n≤6) point‐groups

Nuclear spin statistical weight factors have been calculated for the rotational energy levels of symmetric top molecules belonging to the Dnd (n=2,3,4,5,6) and Dnh (n=3,4,5,6) point‐groups. The ro‐vibronic species for the levels have been calculated on the basis of Hougen’s classification scheme, using the point‐group of the molecule. The Longuet–Higgins permutation–inversion‐group was used to determine the overall allowed species permitted by the Pauli exclusion principle and the ramifications of inversion symmetry. The ro‐vibronic species and the statistical weights are given in a set of easy to use tables. They are presented explicitly for all possible vibrational states of a totally symmetric electronic state. The weights are given in terms of the coefficients mα which occur in the expression ΓS=ΣmαΓαG for the reduction of the nuclear spin representation ΓS with respect to the irreducible representations ΓαG of the point‐group G. General formulas for the characters of the nuclear spin representation a...

Raman scattering in La1−xSrxMnO3 single crystals (x = 0, 0.1, 0.2, 0.3)

Abstract We report polarized Raman spectra of La1−xSrxMnO3 crystals (x = 0, 0.1, 0.2 and 0.3) in the temperature range from 5 K to 300 K. The small distortion of the nearly cubic lattice of doped La1−xSrxMnO3 single crystals results in a structured phonon Raman spectrum which is composed of two parts with different contributions depending on the value of doping. The first part is assigned to the distorted non-cubic perovskite lattice and follows the selection rules for a tetragonal structure. This suggests the possible presence of a tetragonal structure in doped La1−xSrxMnO3 single crystals. The second part is mainly due to the density of vibrational states and is attributed to the second-order Raman scattering. The frequency of the A1g-like mode was found to be sensitive to doping but the observed shift is opposite to that expected from the difference in the atomic weight of Sr and La. This mode exhibits also an anomalously large shift of ∼20 cm−1 in the temperature range from 5 K to 300 K. A possible explanation of this behaviour is suggested.

Diagnostics of “colossal” magnetoresistance manganite films by Raman spectroscopy

Polarized Raman scattering by phonons is used to characterize thin films prepared by laser ablation of La1−xCaxMnO3 targets. It was found that, in the temperature range from 6 to 300 K, phonon spectra of La0.7Ca0.3MnO3 films exhibit observable differences from those in bulk materials (microcrystalline ceramics and single crystals). A significant difference was found in the spectra of “as-grown” films compared to those annealed in oxygen at 800 °C. The observed Raman peaks and their linewidths exhibit an irregular temperature dependence near Tc. A correlation of Raman data with magnetization of the sample was also found.

Amplitude-modulated heterodyne polarization spectroscopy

Amplitude-modulated heterodyne polarization spectroscopy was applied for detecting spectral features with small transition probabilities. The intrinsic sensitivity of Doppler-free polarization spectroscopy can be enhanced toward the shot-noise limit, using a frequency-offset saturating beam together with high-frequency chopping. Heterodyne detection of weak transitions with absorption in the 10−6 range was achieved. Examples are given of spin-forbidden transitions in Na2 and of two-photon taransitions in Na2 and Na.

Rovibronic species, overall allowed species, and nuclear spin statistical weights for symmetric top molecules. II. Point groups Cnv and Cnh (n≤6)

The method described in I [A. Weber, J. Chem. Phys. 73, 1952 (1980); 74, 4754 (1981)] has been used to derive the rovibronic species, overall allowed species, and the nuclear spin statistical weights for symmetric top molecules belonging to the point groups Cnv or Cnh with n⩽6. Rules are presented by means of which the rovibronic species of Cnv and Cnh molecules are obtained from the results given in I. The overall species and the nuclear spin statistical weights are given in a new set of tables.

Analysis of the ν8 band of methylene fluoride

Abstract The high resolution infrared absorption spectrum of the ν 8 band of methylene fluoride has been measured on a BOMEM DA 3.002 Fourier transform spectrometer in the region 1400–1465 cm −1 . More than 900 transitions have been assigned in this A -type band centered at 1435.6355 cm −1 . The data have been combined with the upper state microwave measurements in a weighted least-squares fit to obtain molecular constants for the upper state resulting in an overall standard deviation of 0.0004 cm −1 .

Nuclear spin statistical weights for the trimers and tetramers of C2H2, C2HD, and CO2

Nuclear spin statistical weights have been calculated for the rotational levels of C2H2 trimers and tetramers for various geometries having D3h,C3h,D4h,C4h, and D2d symmetries as well as for the several possible planar trimer and tetramer forms of C2HD to assist in the interpretation of high resolution rotation–vibration spectra of these complexes. Results are also given for the trimers and tetramers of CO2. The weights obtained for the 12C2H2 trimer with C3h symmetry as well as the weights for the trimers of 12C2HD are directly applicable to the trimers of HF and H12CN, respectively. Reference is made to earlier calculations of the nuclear spin statistical weights for monomer molecules, the results of which may sometimes be transferred to van der Waals and hydrogen‐bonded complexes. A discussion is presented on the role of the separable degeneracies occuring in the classification of energy levels of molecules belonging to one of the Cnh or other point groups with complex characters.

Raman scattering from single crystal YBa2Cu3O7-δ in a magnetic field

A magneto-Raman study of a YBa2Cu3O7-δ single crystal(Tc = 92 K) was carried out at a resolution of 2 cm-1 over the temperature range 5–125 K. At temperatures belowTc we observed a slight narrowing of the 340 cm-1 Raman mode in a magnetic field of 5.15 T directed parallel to thec-axisof the crystal. The observed magneto-temperature dependences of the Raman continuum intensity measured in the low-frequency region, at ca. 40 cm-1, indicate contributions of electronic excitations arising from a pair-breaking process. Within the experimental uncertainty, the Raman intensity of the high-frequency continuum was observed to be independent of the magnetic field strength.