High-resolution Fourier transform infrared and cw-diode laser cavity ringdown spectroscopy of the ν2+2ν3 band of methane near 7510 cm-1 in slit jet expansions and at room temperature

The nu(2)+2nu(3) combination band of (CH4)-C-12 near 7510 cm(-1) was studied with the recently introduced technique of cavity ring-down spectroscopy employing a cw-diode laser in a pulsed supersonic slit jet expansion and with Doppler-limited Fourier-transform infrared spectroscopy at room temperature. nu(2)+2nu(3) is the strongest absorption band in the high-wave-number region of the N=2.5 icosad of methane. First assignments of the combination band are provided. The vibrational origin of nu(2)+2nu(3) at 7510.3378+/-0.0010 cm(-1), the integrated band strength G=(1.3+/-0.2)x10(-4) pm(2) and the vibrational transition moment parallel tomu(nu)parallel to=(1.0+/-0.1)x10(-3) D have been determined. The values represent benchmarks to test effective vibrational Hamiltonians and ab initio calculations for methane. Although an isolated band analysis was possible at low J-values, the influence of strong perturbations becomes evident at higher rotational excitation. The F-1-component of nu(2)+2nu(3) interacting by a strong Coriolis resonance with the IR-active F-2-component appears to be a dominant perturber. (C) 2002 American Institute of Physics.