MOLECULAR-BEAM DIODE-LASER SPECTRUM OF THE $\nu_{9}$ BAND OF $N_{2}O_{4}$ AT $1757 cm^{-1}$
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Date
1993
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Publisher
Ohio State University
Abstract
The sub-Doppler, jet-cooled beam (25\% $NO_{2}$ in Ar) spectrum of the $\nu^{9}$ band of $N_{2}O_{4}$ has been measured with a diode-laser spectrometer and a slit nozzle. The spectra were calibrated against BOMEM FTS spectra of $CH_{2}O$ and isotopic water lines. Comparing calculated spectra against observed spectra leads to an estimate of the beam temperature of 25-35 K. Unlike the a-type $\nu_{11}$ band at $1263 cm^{-1}$, whose analysis was reported at this meeting last year, the $v^{9}$ band is a b-type band with selection rules $\Delta K = \pm 1$ and $\Delta J = 0, \pm 1$. The molecule is moderately asymmetric with $K = 0.37$, and in the P- and R-branches high $J (<12)$ and low $K_{a} (>6)$ lines form distinctive clumps which, with the help of spin statistics ($K_{a}$ odd missing: $K_{c}$ even lines twice the weight of K$c$ odd lines) makes line assignment intuitive. Transitions with J as high as 50 and $K_{a}$ up to 23 have been assigned. Unlike the band, which is badly perturbed, only one line of this band is perturbed despite a four-fold increase in the density of states in this region, and it was possible to assign most of the lines: in all, about 600 lines were included in the band fitting. The standard deviation of this fitting was $0.00024 cm^{-1}$ and unlike the perturbed lower-frequency band, it was necessary to use only quartic distortion constants in the fitting. Combination differences obtained from this hand were combined with those from the $\nu^{9}$ band. In all 690 combination differences were fit with $\sigma= 0.00032 cm^{-1}$ to obtain an improved set of ground state constants. A refined structure for the molecule has been obtained.
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Author Institution: Instituto de Estructura de la Materia; Molecular Physics Division NIST