Coriolis analysis of several high-resolution infrared bands of bicyclo[111]pentane-d₀ and -d₁

High-resolution infrared absorption spectra have been analyzed for two bicyclo[111]pentane isotopologues, C₅H₈ (-d₀) and C₅H₇D (-d₁), where in the latter the D-atom replaces a hydrogen on the C₃ symmetry axis such that the molecular symmetry is reduced from D₃ₕ to C₃ᵥ. Two (a"₂) parallel bands, v₁₇ and v₁₈, of bicyclopentane-d₀ were studied and the former was found to be profoundly affected by Coriolis coupling with the nearby (e') perpendicular band, v₁₁. Weaker coupling was observed between the v₁₈ band and the nearby v₁₃(e') band, for which fewer transitions could be assigned. For bicyclopentane-d₁, the v₅ parallel band was also studied along with the nearby v₁₅(e') band to which it is coupled through a similar type of Coriolis resonance. For both isotopologues, quantum calculations (B3LYP/cc-pVTZ) done at the anharmonic level were very helpful in unraveling the complexities caused by the Coriolis interactions, provided that care is taken in identifying the effect of any Coriolis resonances on the theoretical values of aB and q rovibrational parameters. The ground state B₀ constants were found to be 0.2399412(2) and 0.2267506(11) cm⁻¹ for the -d₀ and -d₁ isotopologues. The difference yields an Rₛ substitution value of 2.0309(2) Å for the position of the axial H atom relative to the -d₀ center of mass, a result in good accord with a corresponding Ra value of 2.044(6) Å from electron diffraction data. For both isotopologues, the theoretical results from the quantum calculations are in good agreement with all corresponding values determined from the spectra

COHERENT ANTI-STOKES RAMAN SPECTROSCOPY OF GASES

Author Institution: Naval Research LaboratoryCoherent anti-Stokes Raman spectroscopy (CARS) is a new and exciting method for optically probing matter. If two input laser beams (one of which is tunable) are focused in a medium (gas, liquid or solid), a coherent, laser-like beam at the anti-Stokes frequency is generated. Conversion efficiencies can become quite high (as much as 1% of the tunable laser output) as the difference in frequency between the two exciting lasers approaches a Raman resonance in the material. A trace of the CARS output, with respect to the frequency difference in the two laser beams constitutes a CARS spectrum. The laser system used in these experiments consists of a double $TEM_{00}$-Nd:YAG laser operating at 532 nm with $0.03 cm^{-1}$ line width and 5 MW output in 18 ns pulse width at up to 10 Hz. A portion of this laser is split off to pump a tunable dye laser and amplifier. A telescope-grating-tuning-element combination yield a dye laser line width of $\sim 0.3$ $cm^{-1}$. An additional etalon may be used to reduce the line width to $\sim$ $cm^{-1}$. Because narrow line widths in laser may be achieved with only modest sacrifices in output power, measuring high resolution CARS Spectra of gases (0.03-$0.03 cm^{-1}$) at low pressures ($< 1$ Torr) are possible. In this presentation we show the CARS spectra of a number of different gases (e.g., $D_{2}$, $CH_{4}$, $NH_{3}$, various hydrocarbons, etc.). Some of these materials have been probed at the center of electrical discharge and/or in flames, where the anti-Stokes and coherent properties, together with the high intensity of CARS output, have enabled one to easily record Raman spectra under conditions very unfavorable for measurement by spontaneous Raman scattering. Vibrational and rotational temperatures of the species within the discharge or flame have been measured. For example, a vibrational temperature of $1050^\circ K$ and a rotational temperature of $400^\circ$ have been determined for $D_{2}$ (at $\sim50$ Torr) at the center of a dc electrical discharge