Librational motion of CO in solid Ar: Raman and IR spectra and quantum simulations
Johan Lindgren1, Adriana Olbert-Majkut2, Mika Pettersson1, and Toni Kiljunen1
1Department of Chemistry, Nanoscience Center, P.O. Box 35, FI-40014 University of Jyväskylä, Finland
2Faculty of Chemistry, Wroclaw University, F. Joliot-Curie 14, 50-383 Wroclaw, Poland
Received April 20, 2012
Rovibrational Raman spectra of CO molecules isolated in solid Ar were measured for the 9–30 K temperature
range and compared to past and present IR spectra. The fundamental band appears as a triplet-split structure,
where the center peak shows completely different response to temperature in the Raman and IR spectroscopies. The peak is sharp and stable in Raman but reversibly broadens beyond recognition in IR upon annealing. The red-shifted, intense line of the triplet is found thermally inert similarly in both spectroscopies. The third line is the weakest, and as concentration dependent, it is assigned to a dimer as before. The CO–H2O impurity complex is identified as a side band. We employ crystal field and quantum chemical modeling to interpret the disparity between the spectroscopies. The stable and broadening lines are given assignments to double- and singlesubstitution sites, respectively. Thermal excitations are not effective in the former case of angularly tightconfined, deep potential well. In the single-substitutional case, the librational level structure shows up in discriminating between the Raman and IR selection rules. An effectively ΔJ = 0 totally symmetric transition is found for Raman that is uncoupled from lattice phonons and corresponding broadening mechanisms. The low-temperature limit necessitates the use of a fixed lattice approach while the warmer end of the range is best described with an adiabatic, pseudorotating lattice approach.
PACS: 33.20.Ea Infrared spectra ; PACS: 33.20.Fb Raman and Rayleigh spectra (including optical scattering); PACS: 33.20.Vq Vibration-rotation analysis; PACS: 34.20.Gj Intermolecular and atom-molecule potentials and forces; PACS: 31.15.xv Molecular dynamics and other numerical methods.
Ключові слова: matrix isolation spectroscopy, solid argon, CO molecule, rovibrational analysis, hindered rotation, libration-phonon interaction, crystal field theory, ab initio calculation.