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Photothermal lensing effects are observed in the early expansion stages (X/D=1–7) of a supersonic free jet and narrow P and R branch rotational lines have been recorded at 100 MHz resolution for the ν2 +5ν3 band of acetylene. Rotational Raman loss spectra were also recorded and, from the collision-broadened linewidths of these, it is estimated that about 100 collisions occur in the beam transit from pump to probe positions. This is sufficient to produce rotational and some vibrational relaxation in forming the lens. The photothermal intensities are consistent with a Boltzmann rotational distribution in the ground state and a rotational temperature of 70 K is deduced for 100 PSI expansions of 20% C2 H2 in helium. Charac- teristics of the photothermal signal in the jet are described and the time and spatial dependence of the signal is used to measure a flow velocity 625 m/s for the jet. This photothermal method may prove useful as a diagnostic probe of beam properties and as a spectroscopic measure of weak one- and two-photon transitions of molecules cooled in jets. [ABSTRACT FROM AUTHOR]

Abstract: For symmetric-top molecules, the normal Δk =0, Δl =0 and Δk =±1, Δl =±1 selection rules for parallel and perpendicular bands, respectively, do not allow the determination of the K-dependent rotational constants, C 0 (or A 0), , and . However, we show here that several different combinations of allowed and apparently unperturbed rovibrational infrared transitions can give access to those constants. A necessary ingredient for the application of this technique is a band with selection rules Δk =±1 (or Δk =0), Δl = 2, such as an overtone or difference band, and appropriate other bands. Bands with selection rules Δk =±2, Δl = 1 are also useful but are seldom found. As a general rule, more than one vibrational transition is needed. Examples are given for boron trifluoride (BF3), sulfur trioxide (SO3), and cyclopropane (C3H6) for which there are microwave measurements that provide a check on the derived constants. The technique is also extended to a D2d molecule, allene, even though we have no measurements to use as an example. Examples are also given for the determination of dark states from difference bands, and/or hot bands, and also whole forbidden bands that arise from mixing with distant energy levels. [Copyright &y& Elsevier]

Abstract: Infrared spectra of the small strained cage molecule [1.1.1]propellane have been obtained at high resolution (0.0015cm−1) and the J, K, and l rovibrational structure has been resolved for the first time. We recently used these spectra to obtain combination-differences to deduce ground state parameters for propellane; over 4100 differences from five fundamental and four combination bands were used in the fitting process. The combination-difference approach eliminated potential errors caused by localized perturbations in the upper states and gave well-determined ground state parameters. In the current work, these ground state constants were fixed when fitting the upper state parameters for the ν 12 (e′) perpendicular and parallel bands. Over 4000 infrared transitions were fitted for each band, with J, K values ranging up to 71, 51 and 92, 90, respectively. While the transition wavenumbers for both bands can be fit nicely using separate analyses for each band, the strong intensity perturbations observed in the weaker ν 12 band indicated that Coriolis coupling between the two modes was significant and should be included. Due to correlations with other parameters, the Coriolis coupling parameter for the ν 15 and ν 12 interaction is poorly determined by a transition wavenumber fit alone but, fortunately, the intensity perturbations gave useful added constraints on . By combining the wavenumber fit with a fit of experimental intensities, a value of −0.42 was obtained, quite close to the value of −0.44 predicted by Gaussian ab initio density functional calculations using a cc-pVTZ basis. This intensity fit also yielded a (∂μ z /∂Q 15)/(∂μ x /∂Q 12a ) dipole derivative ratio of 36.5, in reasonable agreement with a value of 29.2 predicted by the ab initio calculations. This ratio is unusually high due to large charge movement as the novel central C axial–C axial bond is displaced along the symmetry axis of the molecule for the ν 15 mode. [Copyright &y& Elsevier]

Abstract: The infrared spectrum of [1.1.1]propellane has been recorded at high resolution (0.002cm−1) with individual rovibrational lines resolved for the first time. This initial report presents the ground state constants for this molecule determined from the analysis of five of the eight infrared-allowed fundamentals ν 9(e′), ν 10(e′), ν 12(e′), , as well as of several combination bands. In nearly all cases it was found that the upper states of the transitions exhibit some degree of perturbation but, by use of the combination difference method, the assigned frequencies provided over 4000 consistent ground state difference values. Analysis of these gave for the parameters of the ground state the following values, in cm−1: B 0 =0.28755833(14), D J =1.1313(5)×10−7, D JK =−1.2633(7)×10−7, H J =0.72(4)×10−13, H JK =−2.24(13)×10−13, and H KJ =2.25(15)×10−13, where the numbers in parentheses indicate twice the uncertainties in the last quoted digit(s) of the parameters. Gaussian ab initio calculations, especially with the computed anharmonic corrections to some of the spectroscopic parameters, assisted in the assignments of the bands and also provided information on the electron distribution in the bridge-head carbon–carbon bond. [Copyright &y& Elsevier]

Abstract: High resolution (0.001cm−1) coherent anti-Stokes Raman spectroscopy (CARS) was used to directly examine the ν 1 symmetric stretching mode of the planar symmetric D 3h molecules 10BF3 and 11BF3. Simulations of the spectra were done using ν 1 rovibrational parameters deduced from published infrared hot-band and difference-band studies and the close similarity to the observed CARS spectra confirms the validity of the infrared constants. No significant perturbations by Fermi resonance or Coriolis interactions with nearby states are observed, in marked contrast to the case of sulfur trioxide, a similar D 3h molecule recently studied. In the harmonic approximation, the 10BF3 and 11BF3 ν 1 Q-branches would be identical since the isotopic substitution is at the center of mass but, interestingly, the ν 1 stretching frequency for 11BF3 is found to be 0.198cm−1 higher than for the lighter 10BF3 isotopomer. This counterintuitive result is reproduced almost exactly (0.200cm−1) by ab initio calculations (B3LYP/cc-pVTZ) that included evaluation of cubic and quartic force constants and x ij anharmonicity constants. The ab initio computations also predict to within 1% the ΔB, ΔC changes in the rotational constants in going from the ground state to the v 1 =1 vibrational level. The results illustrate nicely the complementary interplay of modern infrared, Raman, and ab initio methods in obtaining and analyzing rovibrational spectra. [Copyright &y& Elsevier]

Several new infrared absorption bands for 32S16O3 have been measured and analyzed. The principal bands observed were ν1+ν2 (at 1561 cm-1), ν1+ν4 (at 1594 cm-1), ν3+ν4 (at 1918 cm-1), and 3ν3 (at 4136 cm-1). Except for 3ν3, these bands are very complicated because of (a) the Coriolis coupling between ν2 and ν4, (b) the Fermi resonance between ν1 and 2ν4, (c) the Fermi resonance between ν1 and 2ν2, (d) ordinary l-type resonance that couples levels that differ by 2 in both the k and l quantum numbers, and (e) the vibrational l-type resonance between the A1′ and A2′ levels of ν3+ν4. The unraveling of the complex pattern of these bands was facilitated by a systematic approach to the understanding of the various interactions. Fortunately, previous work on the fundamentals permitted good estimates of many constants necessary to begin the assignments and the fit of the measurements. In addition, the use of hot band transitions accompanying the ν3 band was an essential aid in fitting the ν3+ν4 transitions since these could be directly observed for only one of four interacting states. From the hot band analysis we find that the A1′ vibrational level is 3.50 cm-1 above the A2′ level, i.e., r34=1.75236(7) cm-1. In the case of the 3ν3 band, the spectral analysis is straightforward and a weak Δk=±2, Δl3=±2 interaction between the l3=1 and l3=3 substates locates the latter A1′ and A2′ “ghost” states 22.55(4) cm-1 higher than the infrared accessible l3=1 E′ state. [Copyright &y& Elsevier]