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Double resonance spectra have been obtained for nitrogen and benzene molecules in the low density region of a molecular beam by use of stimulated Raman pumping followed by ionization probing. For N2 the photoionization step was via a 2+2 resonantly enhanced multiphoton ionization process using the a1Πg(v=2)←X1Σg(v=1) two-photon resonance. The detection limit is 103–104 times lower than for the direct optical measurement of a stimulated Raman signal. For benzene, a more efficient 1+1 ionization step permitted measurements at state densities less than 1010 cm-3 and the Raman step yielded linewidths of 30 MHz for the Q-branch lines of the weak Raman active ν16 fundamental. © 1996 American Institute of Physics. [ABSTRACT FROM AUTHOR]

High resolution coherent anti-Stokes Raman spectroscopy has been used to study the symmetric CH stretching mode of methyl radical formed by UV laser photolysis of CH3I cooled in a free jet expansion. Spectra obtained under near-nascent conditions (∼3 to ∼6 collisions) show that most of the CH3 product is formed in the ground vibrational state, with little excitation seen in the ν2 out-of-plane bending coordinate [v2=1/v2=0, population ratio 0.27(10)]. This is in accord with recent theoretical calculations favoring slow, adiabatic CH3 relaxation from a pyramidal to planar structure as the C–I bond breaks. Extensive N,K rotational structure is resolved and the distributions obtained lend support to those deduced for nascent CH3 by Chandler and co-workers from modeling of unresolved resonance enhanced multiphoton ionization spectra. The results are consistent with conservation during dissociation of CH3 ortho-para nuclear spin forms and of K spinning angular momentum about the symmetry axis. CH3 product tumbling motion is slightly greater (1–2 units of angular momentum) than predicted theoretically. Rapid collisional excitation of higher rotational levels is seen and the rich spectrum observed after ∼50 collisions has been analyzed to give improved or new vibrational-rotational parameters for 1000 and 1100 states. The 1100 band origin is determined to be 2996.21(4) cm-1, yielding -8.23(5) cm-1 for the x12 anharmonicity constant. [ABSTRACT FROM AUTHOR]

Coherent anti-Stokes Raman spectroscopy (CARS) was used to measure the vibrational–rotational Q-branch structure of the ν1 symmetric stretch of methyl radicals produced by 266 nm laser photolysis of methyl iodide. Spectra were recorded in both flow cells and free jet expansions at instrumental resolutions ranging from 0.25 to 0.005 cm-1. Due to the high recoil velocity of the CH3 fragment, Doppler and collisional broadening of the transitions is appreciable. Even at the highest resolution such broadening of the transitions leads to interference effects among the closely spaced Raman transitions that influence both the line positions and intensities in the observed CARS spectra. The molecular parameters (cm-1) obtained from the analysis are ν1=3004.42(4), αB1=0.0851(8), αC1=0.0475(7), DN1-DN0=-0.000 046(8), DNK1-DNK0=0.000 083(20), and, with assumptions, DK1-DK0=-0.000 039. These results and infrared data in the literature yield a CH bond length of 1.08378(5) Å for the (1000) state and, with some assumptions, an equilibrium bond length Re of 1.076 Å for this prototypic case of sp2 bonding. [ABSTRACT FROM AUTHOR]

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]