Library

Notes: Six internal rotation/vibration bands of ArH2O are observed in a slit supersonic expansion via direct absorption of a tunable diode laser in the v2 bend region of H2O. The spectra obtained for the ortho H2O manifold are well represented by a pseudodiatomic model with nearly free internal rotation of the H2O subunit. By way of contrast, the para bands show significant mixing between the internal rotor and stretch states, indicative of strong angular-radial coupling in the intermolecular potential. The spectra for the para Ar–H2O species can be deperturbed based on a three state Coriolis plus angular-radial coupling model which includes microwave, far-ir and near-ir data. The results indicate a redshift of (approximate)0.58 cm−1 upon bend excitation of the H2O subunit, and in general rather modest changes in the excited state intermolecular potential from the ground state potential. No indication of predissociation broadening is found, and the instrument-limited linewidths place a lower limit on the vibrational lifetime in the excited state of τ≥7.2(6) ns. © 1997 American Institute of Physics.

Notes: Absolute state-to-state cross sections are reported for rotationally inelastic scattering in crossed jets of HF with He, Ne, and Ar at mean center-of-mass collision energies of 480, 390, and 350 cm−1, respectively. HF seeded in Ar diluent gas is cooled into the J=0 ground rotational state in a pulsed supersonic expansion, followed by single collision rotational excitation with rare gas atoms from a second pulsed supersonic jet. The column-integrated densities of HF in both the initial and final scattering states are probed in the jet intersection region via direct absorption of light from a narrow bandwidth (0.0001 cm−1), continuously tunable, color center laser. Total inelastic cross sections for collisional loss out of J=0 and collisional excitation into J〉0 states are determined in absolute units from the dependence of infrared absorption signals on collider gas concentration. Full close coupling scattering calculations are performed on several ab initio and empirical potential energy surfaces for each of the three HF+rare gas systems. Agreement for He+HF and Ar+HF integral cross sections is remarkably good, but significant discrepancies are noted for the less accurately determined Ne+HF surface. Photoelastic polarization modulation of the IR laser is used to probe for rotational alignment in the scattered HF flux; the measurements set an upper polarizance limit for collisionally populated J=1 HF molecules [probed on P(1)] of |P|〈2%. High resolution IR laser Dopplerimetry reveals velocity structure in the collisionally excited J=1 Doppler profiles, which is in excellent qualitative agreement with theoretical predictions of rainbow features in the J=1←0 state-to-state differential cross section. © 1997 American Institute of Physics.

Notes: Three high-resolution rovibrational bands of ortho H2 with both para and ortho H2O are observed in a slit supersonic expansion, based on direct absorption of a tunable diode laser in the ν2 bend region of H2O near 1600 cm−1. Complexes containing para H2O are responsible for a Σ←Σ type band associated with intramolecular bending excitation of H2O, while complexes containing ortho H2O exhibit two bands associated with (i) the intramolecular HOH bend (Π←Π) and (ii) an inter+intramolecular combination band (Σ←Π) corresponding to simultaneous HOH bend plus internal rotor excitation. From high-resolution line broadening studies, each upper state has a different vibrational predissociation lifetime; for bend excited para H2O complexes it is 5.1(14) nsec, while for the bend excited state and bend+internal rotor combination state of ortho H2O, it is 2.53(14) and 1.85(33) nsec, respectively. Analysis of the spectra supplemented by 2D quantum calculations indicate large amplitude, slightly hindered internal rotation of the H2O subunit in the complex. Nevertheless, the internal rotor splittings yield potential parameters that suggest ortho H2–H2O is best described with the H2 predominantly pointing towards the O atom in a H2O proton acceptor geometry. © 1999 American Institute of Physics.

Notes: The theory developed by Zare [Ber. Bunsenges. Phys. Chem. 86, 422 (1982)] for using electric-dipole-allowed photoexcitation with linearly polarized light to align linear and symmetric top molecules via parallel transitions is extended to include perpendicular transitions, as well as the alignment of asymmetric tops via a-, b-, or c-type transitions. Analytical expressions for the spatial distribution of a symmetric top figure axis following a parallel or perpendicular transition are presented. A prescription is developed for determining the spatial distribution of each principal axis of an asymmetric or symmetric top following parallel or perpendicular type transitions. The degree of alignment obtainable via photoexcitation for symmetric and asymmetric tops is discussed, with the somewhat surprising result that all three principal axes of an asymmetric top can be highly aligned via photoexcitation. A simple computer program for calculating the degree of alignment of each principal axis of a symmetric or asymmetric top following an a-, b-, or c-type transition is described and made available. © 1997 American Institute of Physics.

Notes: Ar2CO2 is studied using direct absorption infrared spectroscopy. The van der Waals molecules are formed when a mixture of CO2 and Ar gases is expanded in a supersonic slit jet. To probe the clusters, the ν3 asymmetric stretch of the CO2 monomer is then monitored in absorption. Sixty-one trimer transitions are assigned and fit to a Watson asymmetric top Hamiltonian. Rotational constants for the upper and lower vibrational states permit determination of vibrationally averaged molecular structures, which indicate that the Ar atoms lie in the plane that bisects CO2 and is perpendicular to the CO2 intramolecular axis. These geometries are consistent with an equivalent "T-shaped'' ArCO2 geometry for each Ar atom. Vibrational origins for the ν3 CO2 asymmetric stretch frequency in ArnCO2 are found to shift approximately linearly for zero, one, and two Ar atoms. Calculations using pair potentials are used to extrapolate these red shifts out to the bulk phase and to compare the results to experimental matrix data. Finally, the slight nonlinearity in the red shift between ArCO2 dimer and Ar2CO2 trimers is interpreted in the context of three-body forces. © 1996 American Institute of Physics.

Notes: High resolution infrared spectra of a previously unidentified noncyclic isomer of (CO2)3 have been obtained via direct absorption of a 4.3 μm diode laser in a slit jet supersonic expansion. Two vibrational bands (labeled νI and νIII) are observed, corresponding to the two most infrared active linear combinations of the three constituent CO2 monomer asymmetric stretches: νI is redshifted −5.85 cm−1 from the monomer vibrational origin and is predominately a c-type band of an asymmetric top, while νIII is blueshifted +3.58 cm−1 and is predominately an a-type band. Transitions with Ka+Kc=odd (even) in the ground (excited) state are explicitly absent from the spectra due to the zero nuclear spin of CO2; this rigorously establishes that the noncyclic isomer has a C2 symmetry axis. The vibrational shifts and relative intensities of the bands are interpreted via a resonant dipole interaction model between the high-frequency stretches of the CO2 monomers. Rotational constants are determined by fits of transition frequencies to an asymmetric top Hamiltonian. These results are used to determine vibrationally averaged structural parameters for the complex, which is found to be stacked asymmetric but with C2 symmetry about the b inertial axis. The structural parameters are then used to test several trial CO2–CO2 interaction potentials. © 1996 American Institute of Physics.

Notes: Nascent quantum states of CO2 subliming from CO2 thin films at rates of 1 to 103 monolayers (ML) per second are probed via direct infrared absorption of the ν3 asymmetric stretch with a frequency ramped diode laser. The high spectral resolution (Δν≈15 MHz) of the diode laser and the use of polarization modulation techniques permit individual rotational, vibrational, translational, and even MJ degrees of freedom of the subliming flux to be studied with quantum state resolution. Measured rotational and ν2 bend vibrational distributions indicate that the molecules sublime from the surface in a Boltzmann distribution characterized by the thin film temperature Ts. Similarly, the velocity distributions parallel to the surface are well described by a Maxwell velocity distribution at Ts, as determined by high resolution Doppler analysis of the individual rovibrational line shapes. The MJ distribution of subliming rotational states is probed via polarization modulation methods; no alignment is detected within experimental sensitivity. This places an upper limit on the anisotropy in the rotational distribution of |n⊥/n(parallel)−1|〈0.02, where n⊥/n(parallel) is the ratio of molecules with J perpendicular vs parallel to the surface normal. By virtue of the direct absorption technique, the absolute sublimation rates from the surface can be obtained from the measured column integrated densities. Via detailed balance, these fluxes are compared with equilibrium vapor pressure measurements to retrieve the absolute sticking coefficients S for gas phase CO2 impinging on a solid phase CO2 thin film. For sublimation rates 〈103 ML/s, the data indicate S=1.0±0.2, irrespective of quantum state, rotational alignment, and tangential velocity component. For sublimation rates (approximately-greater-than)103 ML/s, the onset of a mild supersonic expansion is observed, with post-desorption collisions cooling the rotational temperature by as much as 15 K below Ts. Modeling of the gas–surface interaction using realistic CO2–CO2 pair potentials demonstrates that the gas–surface potential is relatively "soft'' and highly corrugated, which promotes efficient translational and rotational energy transfer to the surface. The scattering analysis also suggests that nonequilibrium quantum state distributions in the subliming flux are not expected for translational and rotational energies less than or comparable to the binding energy of CO2 to the surface. © 1996 American Institute of Physics.

Notes: The effects of finite-switching rates of media on measured coercivities are studied using two methods. The first starts from expressions for the switching times of magnetic media and goes on to derive simple expressions for the coercivities of media subjected to various applied field wave forms. The second uses a well-developed numerical micromagnetic model to investigate both the switching times and coercivities. The variations of coercivity with frequency derived using the two methods show similar results. © 2001 American Institute of Physics.

Notes: A model of properties of colloidal dispersions of fine paramagnetic particles has been investigated. The model has been used to investigate the effects of large magnetic fields on the short-range order within the dispersions. The onset of significant short-range order at a well-defined critical field has been found, in agreement with previous experimental work.