Joseph W. Nibler

Resonant CARS spectra of NO2

Resonant CARS spectra are reported for nitrogen dioxide gas for frequency shifts of 1200–3400 cm−1 from a 532 nm ω1 pump source. The spectra change dramatically with slight changes in ω1 frequency. Much vibrational–rotational structure is observed but the analysis is complicated by the contributions of more than one resonant process. Various possible resonances are considered and absorption spectra and intensity measurements are used to assess the importance of some of these.

A new direct infrared laser absorption method for state-to-state rotational energy transfer in crossed supersonic jets: Experimental results and quantum scattering analysis for Ar+CH4

A new method for measuring state‐to‐state rotational energy transfer in crossed supersonic beams is described. The method is based on direct absorption of tunable, high‐resolution infrared laser light by target molecules collisionally excited into final rotational states. The direct IR absorption approach offers high sensitivity, full quantum state resolution, a Doppler probe of final velocity components, and is applicable to any target molecule that absorbs in the near IR. Preliminary results are presented for Ar+CH4 scattering in crossed supersonic beams at a mean center‐of‐mass collision energy of 41 meV. Because of the high spectral resolution, the method can readily distinguish rotational fine structure states of A, F, and E symmetry in the tetrahedral group, as well as the much more energetically separated final j states. The results are compared with full quantum close‐coupling calculations on two different Ar+CH4 potential energy surfaces. The state‐to‐state scattering results provide a sensitive ...

High resolution rotational and ν3 coherent Raman spectra of C2H6

Coherent anti‐Stokes Raman spectroscopy has been used to study C2H6 in the rotational and ν3 CC stretching regions. Pure rotational transitions for torsionally excited molecules are seen for the first time and direct analysis of the peak maxima gives B values of 0.663 06 and 0.660 37 cm−1 for the υ4=0 and 1 torsional levels. The change in B is combined with ab initio calculations to show that the primary response of the molecule to the increased torsional amplitude is a CC extension of 0.0031 A, plus a slight increase of 0.10 degrees in the CCH angle. For the ν3 vibrational Q branch, a complex mixture of four band systems is seen, a consequence of Fermi resonance interactions causing the predicted four torsional sublevels of this state to separate. Spectra of jet‐cooled samples aid in the assignment of the spectrum and the four torsional components are determined as 994.973 (E3s), 994.878 (E3d), 994.864 (A1s), and 993.791 cm−1 (A3d). The A and B rotational constants decrease by 0.003 27 and 0.006 21 cm−1,...

Combined Electron-Diffraction and Spectroscopic Determination of the Structure of Spiropentane, C5H8

Gas-phase electron-diffraction (GED) data have been combined with recent spectroscopic rotational constants to determine the rα0 structural parameters for spiropentane, C5H8. The structure has D2d symmetry, and the results yield values of 1.105(2) Å for the CH bond length, 1.557(3) Å for the distal CC bond length, and a smaller value of 1.482(1) Å for the four lateral CC bonds that connect to the central carbon atom. The HCH angle is 113.7(13)°, and the HCH flap angle, defined as the angle of the HCH bisector and the distal CC bond, is 150.2(16)°. Corresponding rg values are 1.122(2) Å, 1.560(3) Å, 1.485(1) Å, 115.1(13)°, and 148.9 (16)°. The results are in good accord with values from density functional calculations (B3LYP/cc-pVTZ) and resolve some questions about the structure reported in an earlier GED study, in particular about the HCH angle and anomalous rotational constants calculated for the structure.