THE TORSIONAL SPECTRUM OF CH3SIH3 - THE (UPSILON(6)=3[-1)-BAND

The far-infrared spectrum of gaseous CH3SiH3 has been measured with a Fourier transform spectrometer between 180 and 380 cm(-1) under relati vely large pressure-path length conditions. The absorption path length was 124 m and the pressure was 20 Torr. Measurements were made at roo m temperature with an unapodized resolution of 0.01 cm(-1). A congeste d spectrum due to the overlap of several pure torsional bands has been observed. A total of 576 transitions from the (upsilon(6) = 3 <-- 1) torsional band have been identified, involving 15 different (K, sigma) subbands, where sigma = 0, +1, -1 labels the different torsional subl evels. The upper torsional state of this band (upsilon(12) = 0, upsilo n(6) = 3) is significantly perturbed by the upper level (upsilon(12) = 1, upsilon(6) = 0) in the E(1) vibrational fundamental(upsilon(12) = 1 <-- 0) reported earlier (Moazzen-Ahmadi et al., J. Mel. Spectrosc. 1 37, 166, 1989). The (K = 6, sigma = -1) vibrational subband for Delta K = -1 shows resonant perturbation. The identification of the (P)Q(6)( -1) subband has now been extended in J above the value where the inter acting levels have their minimum separation. Two perturbation-allowed (upsilon(6) = 3 <-- 0) transitions have also been assigned. The measur ements of the (upsilon(6) = 3 <-- 1) band and frequencies from previou sly reported experiments were fitted to within the experimental uncert ainty by an effective Hamiltonian which included 22 parameters for the ground vibrational state, 10 parameters for the upsilon(12) = 1 vibra tional state, and 2 parameters which characterize the interactions bet ween these two states. The global data set contained 2607 frequencies. The form of the effective Hamiltonian is severely constrained by the large number of precision data on various torsional levels in the grou nd vibrational state and the excited vibrational state (upsilon(12) = 1, upsilon(6) = 0). The effective Hamiltonian for a vibrational fundam ental of E(1) symmetry and the interactions between the torsional stac k of this state and that of the ground vibrational state are discussed in detail. (C) 1995 Academic Press, Inc.