Thomas R. Dyke

Partially deuterated water dimers: Microwave spectra and structure

Radio frequency and microwave spectra for various isotopically substituted water dimers have been studied by molecular beam electric resonance spectroscopy. Resolved radio frequency hyperfine transitions have provided information about the tunneling–rotational levels of water dimer. The microwave spectra have been analyzed with a rigid rotor model to give the following structural information: Roo=2.976 A (+0.000, −0.030 A), ϑd=−51(10)°, ϑa=57(10)° and χa=6(20)°. The effects of large amplitude vibrational motion have been estimated and the equilibrium geometry should lie within the above limits. The experimental data is also consistent with χa and φd equal to zero for the equilibrium geometry. The water dimer structure, therefore, has a symmetry plane, a trans configuration, and a linear hydrogen bond within quoted error limits.

The molecular beam spectrum and the structure of the hydrogen fluoride dimer

The molecular beam spectrum of the hydrogen fluoride dimer has been investigated in detail. Microwave and radiofrequency transitions of (HF)2, HFDF, and (DF)2 are reported for both the Ka =0 and Ka =1 rotational levels. The results provide information on the structure of the dimers and on the nature of the intermolecular potential energy surface. The average F–F distance is shown to be approximately 2.78 A but with a probably significantly shorter equilibrium distance. The nonhydrogen bonded hydrogen atom is bent 63°±6° from the F–F axis. In addition, analysis of the perpendicular electric dipole moment indicates a nonlinear hydrogen bond. The large centrifugal distortion effects and the unusual quantum mechanical tunneling provide a crude model for the potential surface associated with the hydrogen bond; the effective bond stretching force constant is (1.36±0.03)×104 dyn cm−1.

Group theoretical classification of the tunneling–rotational energy levels of water dimer

A permutation‐inversion group theoretical classification scheme for the tunneling–rotational levels of the water dimer molecule is given. Electric dipole selection rules and nuclear spin statistics are discussed. Application of these results to the microwave spectrum of water dimer, observed by molecular beam techniques, is also presented.

Molecular-beam infrared absorption studies of rare gas-ocs complexes

Abstract Resolved vibration-rotation spectra for Ne·OCS, Ar·OCS and Kr·OCS in the ν 3 region of the OCS monomer have been obtained by absorption spectroscopy of pulsed molecular beams. Rotational constants for the upper and lower vibrational states and the band origins were determined, and effective structures for these molecules were calculated.

Molecular beam electric deflection study of ammonia polymers

The molecular beam electric deflection behavior of (NH3)n, n=1 to 6, has been determined. The ammonia dimer is found to be polar and presumably has a single hydrogen‐bond structure. The higher polymers are nonpolar, compatible with cyclic, hydrogen‐bonded ring structures.

Rotational spectra and structure of the ammonia–water complex

Microwave and radio frequency spectra for NH3⋅H2O and deuterated analogs have been observed by molecular beam electric resonance spectroscopy. Rotational constant, Stark effect, and nitrogen quadrupole coupling interaction data were obtained. This complex is found to have a linear, hydrogen bonded structure with water as the proton donor. The NH3 monomer symmetry axis was found to have a vibrationally averaged displacement of 23.1° from the N...O axis. No evidence for transfer of a proton from water to the ammonia was observed.

Water dimer tunneling states with K=0

Tunneling–rotational transitions of water dimer with K=0 have been observed and assigned in the radio frequency and microwave region of the spectrum. Rotational constants and electric dipole moments were obtained from these spectra. The rotational constants show surprisingly large variations with tunneling state for (H2O)2, but not for (D2O)2, indicating that the former species may be following behavior characteristic of a low‐barrier tunneling case. A tunneling splitting of 19 526.73 MHz has been observed for water dimer and 1172.23 MHz for the completely deuterated species. The nuclear hyperfine structure of (H2O)2 radio frequency transitions has been assigned and was quite useful for determining the symmetries of the observed states. The nuclear spin–spin coupling constants have been interpreted in terms of the tunneling state of observation and of the water dimer structure.

The structure of H2S⋅HF and the stereochemistry of the hydrogen bond

The radiofrequency and microwave spectra of H2S⋅HF and deuterium substituted analogs have been studied by molecular beam electric resonance spectroscopy. Rotational constant data, electric dipole moments, and nuclear hyperfine interactions were obtained from the spectra. The structure determined from this data has H2S as the proton acceptor. The hydrogen bond is linear to within ±10°. The S⋅⋅⋅F distance is 3.249 A. The plane of the H2S is oriented perpendicular to the hydrogen bond axis within conservative error limits of ±10°. The right angle structure of H2S⋅HF compared to the tetrahedral orientation of the proton acceptors in (H2O)2, (HF)2, and HF⋅HCl shows a striking analogy to the geometries of first and second row hydrides. This result is discussed in terms of traditional chemical concepts.

Infrared absorption and microwave–infrared double resonance studies of Ne⋅OCS molecular beams

Infrared absorption spectra for molecular beams of Ne⋅OCS have been observed with a diode laser for the vibrational transition near 2062 cm−1 correlating with the monomer ν3 mode. The linewidths were ∼150 MHz (FWHM), giving rotationally resolved spectra and allowing the upper and lower vibrational state A, B, and C rotational constants to be determined along with the frequency of the band origin. No broadening in excess of that expected from Doppler effects and laser linewidth was observed, setting a lower limit of 10−9 s on the lifetime of the upper state. Rotational transitions for the vibrational ground state were observed by microwave–infrared double resonance experiments. The ∼150 kHz linewidths in these experiments increased the precision of the rotational constants and permitted the quartic centrifugal distortion constants for the ground state to be determined. The effective structure of the Ne⋅OCS complex was calculated from the rotational constant data. The vibrational frequency and structural re...

Wave packet interferometry and quantum state reconstruction by acousto-optic phase modulation.

Studies of wave packet dynamics often involve phase-selective measurements of coherent optical signals generated from sequences of ultrashort laser pulses. In wave packet interferometry (WPI), the separation between the temporal envelopes of the pulses must be precisely monitored or maintained. Here we introduce a new (and easy to implement) experimental scheme for phase-selective measurements that combines acousto-optic phase modulation with ultrashort laser excitation to produce an intensity-modulated fluorescence signal. Synchronous detection, with respect to an appropriately constructed reference, allows the signal to be simultaneously measured at two phases differing by 90 degrees. Our method effectively decouples the relative temporal phase from the pulse envelopes of a collinear train of optical pulse pairs. We thus achieve a robust and high signal-to-noise scheme for WPI applications, such as quantum state reconstruction and electronic spectroscopy. The validity of the method is demonstrated, and state reconstruction is performed, on a model quantum system--atomic Rb vapor. Moreover, we show that our measurements recover the correct separation between the absorptive and dispersive contributions to the system susceptibility.