ALBERT

Notes: We report the measurement of two new perpendicular (D2O)3 torsional bands by terahertz laser vibration–rotation–tunneling (VRT) spectroscopy of a planar pulsed supersonic expansion. The first (28.0 cm−1) band corresponds to the k=±2l←0 transition, and is the lowest frequency vibrational spectrum observed for a water cluster. The second (81.8 cm−1) band originates in the first excited torsional state, and has been assigned as k=3u←±1l. An effective three-dimensional Hamiltonian is derived to describe the rotational structure of each torsional state. Degenerate torsional levels with k=±1 and k=±2 exhibit a Coriolis splitting linear in K implying the presence of vibrational angular momentum, and a second-order splitting from off-diagonal coupling between degenerate sublevels with +|k| and −|k|. With this effective Hamiltonian we fit a total of 554 rovibrational transitions in five different bands connecting the lowest nine torsional states, with a rms residual of 1.36 MHz. The data set comprises the two new VRT bands together with the 41.1 cm−1 parallel band, the 89.6 cm−1 parallel band, and the 98.1 cm−1 perpendicular band. This analysis provides a comprehensive characterization of the torsional energy levels in (D2O)3 up to 100 cm−1 above the zero-point energy, and confirms the torsional assignments for all five (D2O)3 VRT bands observed to date. Moreover, it vindicates the adiabatic separation of the trimer torsional and hydrogen bond stretch/bend vibrational modes which underlies the torsional model. © 1999 American Institute of Physics.

Notes: We report the measurement of two new (H2O)3 bands by terahertz laser vibration–rotation–tunneling (VRT) spectroscopy. Both bands have been assigned to torsional ("pseudorotational") transitions and are highly perturbed by Coriolis interactions. The 42.9 cm−1 band corresponds to the k=±2←±1 transition while the 65.6 cm−1 band corresponds to the k=±2←0 transition. A model Hamiltonian is derived which allowed a global fit of 361 VRT transitions of these two new bands and the previously reported torsional band at 87.1 cm−1. Each of the bifurcation tunneling components is accurately described. This global fit represents a complete description of the VRT transitions of (H2O)3 up to 150 cm−1, and complements our similar treatment of the (D2O)3 torsional dynamics. © 1999 American Institute of Physics.

Notes: 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. © 1995 American Institute of Physics.

Notes: We report the design and performance of a pulsed slit valve for generation of supersonically cooled species in a long path length planar expansion. Utilizing three commercial solenoids driven synchronously by an economic power transistor circuit, the valve produces pulses adjustable in width from 500 to 1000 μs with a repetition rate up to 80 Hz. The pulsed valve can be operated continuously for 12 h, and operation over one month is typical before major maintenance is required. The path length × density product attained by this pulsed source is sufficient for observing large cluster species, such as the water hexamer, on our far-infrared spectrometer. With the addition of a detachable sample oven, it can also be heated up to 230 °C to inject nonvolatile molecules into planar supersonic expansions. © 1996 American Institute of Physics.

Notes: We have measured and interpreted a microwave spectrum of the HeAr+ ion in which all of the observed energy levels lie within 8 cm−1 of the lowest dissociation limit, He(1S)+Ar+(2P3/2). We use an ion beam technique in which the HeAr+ ions are formed by electron impact, accelerated to kilovolt potentials, and mass-analyzed. After passage through an appropriate section of waveguide, the ions enter an electric field lens in which state-selective fragmentation occurs; the Ar+ ions produced in the lens are separated from all other ions by means of an electrostatic analyser and detected with an electron multiplier. Microwave transitions induced in the waveguide section result in population transfer which produces detected changes in the electric field-induced Ar+ fragment current. Many transitions have also been observed by a microwave–microwave double resonance technique. We have observed 68 lines spanning the frequency range 6–170 GHz; no immediately recognizable pattern is apparent. We have measured the Zeeman splitting produced by a small axial magnetic field for almost every line, which enables us to determine the values of the total angular momentum J involved in each transition, and also effective g factors for the two levels involved. We are therefore able to construct a purely experimental pattern of 37 levels lying within 8 cm−1 of the dissociation limit. The data are treated first by means of a conventional effective Hamiltonian in a case (c) basis, which allows electronic and vibrational quantum numbers to be assigned to most of the levels; the assignments are approximate, however, because very strong rotational-electronic coupling undermines the Born–Oppenheimer approximation.A more complete theoretical treatment is then presented, using the coupled-channel method in a case (e) representation to calculate the energy levels without making the Born–Oppenheimer approximation. The microwave transition frequencies and g-factors are fitted, together with earlier ultraviolet spectra, to provide a new interaction potential (designated MAL1) for He interacting with Ar+(2P3/2 and 2P1/2). The MAL1 potential is substantially more accurate than previous potentials, especially in the long-range region and for the A1 2Π3/2 state, which had not been observed before. An important new feature of the MAL1 potential is that the long-range C6 coefficient is strongly anisotropic, so that the different electronic curves have substantially different C6 coefficients. © 1995 American Institute of Physics.