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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: 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.