The Rotational Spectrum of Ar-SiH(4) and Ar-SiD(4).

Microwave spectra of Ar-(28)SiH(4), Ar-(29)SiH(4), Ar-(30)SiH(4), and Ar-(28)SiD(4) were recorded using a pulsed molecular beam Fourier transform microwave spectrometer. The K = 0 and K = 1 components of the J = 3 <-- 2 through the J = 7 <-- 6 transitions were measured and assigned in the 9-24 GHz region. For the primary (28)Si isotopic species, Ar-(28)SiH(4) and Ar-(28)SiD(4), a K = 0, A symmetry, a K = 0, F symmetry, a doubly degenerate K = 1, E symmetry, and an l/K-doubled, K = 1, F symmetry rotational progression are observed at the approximately 1 K rotational temperature of the supersonic expansion. The rotational constants for the K = 0, A state for Ar-(28)SiH(4) and Ar-(28)SiD(4) are B = 1700.40624(9) MHz and 1630.687073(22) MHz and the centrifugal distortion constants are D(J) = 29.089(3) and 20.0198(8) kHz and H(J) = -1.91(3) and -0.851(8) Hz, respectively, where type A expanded uncertainties with a coverage factor, k = 3, are given here and elsewhere. The values of the rotational constants for the K = 0, A, and F states and for the K = 1, E state are in good agreement with the infrared-determined values for Ar-(28)SiH(4). The measured linear Stark effect of the E-state transitions was analyzed to give a dipole moment of 9.24(8) x 10(-32) C. m [0.0277(2) D]. Copyright 1999 Academic Press.

High-Resolution Microwave and Infrared Molecular-Beam Studies of the Conformers of 1,1,2,2-Tetrafluoroethane.

High-resolution microwave and infrared molecular-beam spectra have been measured for 1,1,2,2-tetrafluoroethane (HFC134). For the higher energy, polar, C2 symmetry, gauche conformer, microwave spectra have been recorded for the normal and mono-13C isotopomers and analyzed to determine a C-C bond length of 1.512(4) Å, in good agreement with a recent ab initio value (MP2/6-31G**) of 1.515 Å [S. Papasavva, K. H. Illinger, and J. E. Kenny, J. Phys. Chem. 100, 10100-10110 (1996)]. A tunable microwave-sideband CO2 laser and electric-resonance optothermal spectrometer have been used to measure the infrared spectrum of the nu6, C-C stretch of the gauche conformer near 906 cm-1. Microwave-infrared double resonance and precise ground state combination differences provided by the microwave measurements guide the assignment of the spectrum. The observation of a c-type spectrum definitively establishes that the upper state vibration is of A symmetry in the C2 point group. The spectrum is fit to a Watson asymmetric-top Hamiltonian to a standard deviation of 0.24 MHz. A weak perturbation shifts the line positions for transitions near J = Kc = 20 by as much as 12 MHz. The identity of the perturber is unknown. Pulsed slit-jet diode-laser spectra have been recorded for the nu16 vibration of the anti conformer near 1127 cm-1. An a- and c-type hybrid band is observed, consistent with a Bu symmetry mode. Previous low-resolution studies have attributed the 1127-cm-1 mode to either a Bu or an Au symmetry vibration. A total of 522 nonblended transitions were assigned and fit to determine ground and excited state constants. The ground state constants of A = 5134.952(65) MHz, B = 3148.277(27) MHz, and C = 2067.106(43) MHz are the first experimental determinations of the rotational constants for this conformer. Here, type A standard uncertainties are given in the parentheses. Copyright 1998 Academic Press.

Sub-Doppler Infrared Spectra and Torsion-Rotation Energy Manifold of Methanol in the CH-Stretch Fundamental Region

The infrared spectrum of jet-cooled methanol in the CH-stretch fundamental region has been investigated by two sub-Doppler laser techniques: optothermally detected molecular-beam electric resonance and direct-absorption slit-jet spectroscopy. With the aid of microwave-infrared double resonance and ground state combination differences, 27 subbands in the frequency range 2967 to 3027 cm-1 have been assigned to the nu2 fundamental. Perturbation systems in the K' = 0 E, -1 E, and -2 E symmetry subbands have been analyzed to yield matrix elements of 0.013, 0.041, and 0.75 cm-1, respectively. The A-E torsional tunneling splitting for J = 0 of the nu2 vibration of -3.26 cm-1 is of opposite sign and a factor of three smaller in magnitude than the ground state value of +9.12 cm-1. Copyright 1997 Academic Press. Copyright 1997Academic Press

Infrared Diode-Laser Molecular-Beam Spectrum of the nu2 Band of Chlorine Nitrate at 1293 cm-1

The nu2 band of chlorine nitrate (ClONO2 ) near 1293 cm-1 has been measured in a molecular beam with a diode-laser spectrometer. The low rotational temperature of the molecular beam, approximately 23 K, simplifies the spectrum allowing essentially complete assignment of the 35 Cl and 37 Cl lines. An a /b hybrid band is observed with the a -type transition moment being approximately a factor of 2 larger than the b -type transition moment. An inverted shift of the band origins is found with the 37 Cl band origin blue shifted from the 35 Cl by +0.37 cm-1 . This isotopic shift is attributed to an unidentified anharmonic resonance. Precise spectroscopic constants for the bands of each isotopic species are determined to allow future simulations for modeling atmospheric transmission and for remote sensing applications.