Showing papers by "Richard J. Saykally published in 1995"

Determination of the intermolecular potential energy surface for (HCl)2 from vibration–rotation–tunneling spectra

Abstract: An accurate and detailed semiempirical intermolecular potential energy surface for (HCl)2 has been determined by a direct nonlinear least‐squares fit to 33 microwave, far‐infrared and near‐infrared spectroscopic quantities using the analytical potential model of Bunker et al. [J. Mol. Spectrosc. 146, 200 (1991)] and a rigorous four‐dimensional dynamical method (described in the accompanying paper). The global minimum (De=−692 cm−1) is located near the hydrogen‐bonded L‐shaped geometry (R=3.746 A, θ1=9°, θ2=89.8°, and φ=180°). The marked influence of anisotropic repulsive forces is evidenced in the radial dependence of the donor–acceptor interchange tunneling pathway. The minimum energy pathway in this low barrier (48 cm−1) process involves a contraction of 0.1 A in the center of mass distance (R) at the C2h symmetry barrier position. The new surface is much more accurate than either the ab initio formulation of Bunker et al. or a previous semiempirical surface [J. Chem. Phys. 78, 6841 (1983)].

Vibration--rotation--tunneling dynamics calculations for the four-dimensional (HCl)2 system: a test of approximate models.

Abstract: Several commonly used approximate methods for the calculation of vibration–rotation–tunneling spectra for (HCl)2 are described. These range from one‐dimensional models to an exact coupled four‐dimensional treatment of the intermolecular dynamics. Two different potential surfaces were employed—an ab initio and our ES1 experimental surface (determined by imbedding the four‐dimensional calculation outlined here in a least‐squares loop to fit the experimental data, which is described in the accompanying paper [J. Chem. Phys. 103, 933 (1995)]. The most important conclusion deduced from this work is that the validity of the various approximate models is extremely system specific. All of the approximate methods addressed in this paper were found to be sensitive to the approximate separability of the radial and angular degrees of freedom, wherein exists the primary difference between the two potentials. Of particular importance, the commonly used reversed adiabatic angular approximation was found to be very sensitive to the choice for fixed R; an improper choice would lead to results very much different from the fully coupled results and perhaps to false conclusions concerning the intermolecular potential energy surface.

Infrared laser spectroscopy of uracil in a pulsed slit jet

Abstract: A new high‐temperature pulsed slit jet source has enabled the first observation of a rotationally resolved vibrational spectrum of a nucleotide base. The spectrum, centered at 1703.888 cm−1, has been assigned to the fundamental out‐of‐phase mixed carbonyl stretching vibration of the diketo tautomer of uracil, clarifying an ambiguous assignment from low‐resolution studies.

Cavity ringdown laser absorption spectroscopy and time‐of‐flight mass spectroscopy of jet cooled platinum silicides

Abstract: The cavity ringdown technique has been employed for the spectroscopic characterization of the AuSi molecule, which is generated in a pulsed supersonic laser vaporization plasma reactor. Fifteen rovibronic bands have been measured between 340 nm–390 nm, 8 of which have been analyzed to yield molecular properties for the X and D 2Sigma states of AuSi. This assignment is in disagreement with previous emission studies of AuSi, which had assigned the ground electronic state as a 2Pi state. A time-of-flight mass spectrometer simultaneously monitors species produced in the molecular beam and has provided evidence for facile formation of polyatomic gold silicides. Comparison of AuSi with our recent results for CuSi and AgSi indicates regular bonding trends for the three coinage metal silicide diatoms.

A search for the C3 carbon cluster in the Interstellar Medium

Abstract: We have searched for rovibrational emission and absorption transitions arising from the 63 cm(exp -1) nu(sub 2) (pi(sub u)) bending vibration of the C3 carbon cluster in the interstellar medium using the Betz/Boreiko heterodyne receiver on board the Kuiper Airborne Observatory. The Q(4) line at 1896.707 GHz was searched for in the IRc2 Orion/M42 and the W3 sources, and the R(2) transition at 1968.594 GHz was searched for in Sgr B2. No emission lines were observed in any source. However, a weak absorption was detected in Sgr B2 with a LSR velocity of 63.7+/-0.6 km s(exp -1) and a FWHM linewidth of 7.9+/-0.8 km s(exp -1). This absorption is tentatively identified as the R(2) transition of the C(sub 3) bending mode.