ISSN:
1089-7690
Quelle:
AIP Digital Archive
Thema:
Physik
,
Chemie und Pharmazie
Notizen:
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.
Materialart:
Digitale Medien
URL:
http://dx.doi.org/10.1063/1.472814
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