JET-COOLED INFRARED SPECTRA AND INVESTIGATION OF THE TORSION-ROTATION ENERGY LEVELS OF METHANOL IN THE C-H STRETCH REGION

Abstract

Jet-cooled absorption spectra of the C-H stretching bands of methanol have been recorded between 2965 and $3027 cm^{-1}$ on two different molecular beam machines. In both cases, a color-center laser pumped by $\sim$1.6 W from a krypton ion laser at 647 nm was used to generate 8-9 mW of tunable IR radiation. The first spectrum between $2977-3027 cm^{-1}$ was recorded in direct absorption with a 2 $\times$ 0.01 cm slit nozzle. The molecular rotational temperature was -17 K, and the linewidth was -75 MHz due to residual Doppler spread. Beseline subtraction was employed to suppress laser intensity fluctuations. Frequency calibration was accomplished by simultaneously recording absorption lines of ethlene and the transmission fringes of a vacuum-spaced 150 MHz-marker etalon. The second spectrum between $2965-3024 cm^{-1}$ was recorded on a molecular-beam optothermal spectrometer. A mixture of a few percent methonol in He carrier was introduced to the beam chamber through a 60-$\mu$m pinhole nozzle with $\sim$ latm backing pressure, giving a rotational temperature of $\sim$10 K. The beam was focused onto a He-cooled bolometer using quadrupole focusing in the flight chamber between the skimmer and the detector. In a few pathological regions, $H_{2}O$ absorption completely obscured the methanol spectrum and spoiled control of the laser scan. Calibration traces were recorded to minimize errors due to gaps and drifts. On a given day the calibration interferometer drift was typically less than $\sim$10 MHz, defining the overall precision. Separations of close lines in a single scan should have precision better than 1 MHz. The two spectra were combined for analysis, with the straight absorption spectrum giving particularly nice and easily recognizable spectral patterns. Line assignments were initially made by MW-IR double resonance, pattern recognition, and fits of individual subbands to polynomials in $m (m = J + l, J$, and $-J$ for $R -, Q -$, and $P$ -branch transitions, respectively). They were checked with stringent combination loop tests. About 20 subbands have been confirmed up to $K^{\prime}_{\max} = 3 $ for $A$ and $E$ torsional symmetries respectively, all for the $v_{2}A^{\prime}$ asymmetric C-H stretching fundamental. (The vibrational identification is based on literature information.) For several subbands, perturbations were identified along with associated satellite components. Torsion -$K$-rotation energies were deduced for the excited vibrational state by adding the experimental IR subband origins to calculated ground state energies. The $A / E$ torsional energy pattern is found to be reversed compared to the ground state, as seen for other methanol fundamentals and other molecules. For the systems with identified perturbations, off-diagonal interaction matrix elements have been determined, and attempts to fit the observed excited-state energy pattern are in progress.