Sublimation Dynamics of CO2 Thin Films: A High Resolution Diode Laser Study of Quantum State Resolved Sticking Coefficients

Published

July 1, 1996

Author(s)

M J. Weida, J M. Sperhac, David Nesbitt

Abstract

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 V₃ asymmetric stretch with a frequency ramped diode laser. The high resolution (Δ}v=15 MHz) of the diode laser spectrometer 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 V₂ bend vibrational distributions indicate that the molecules sublime from the surface in a Boltzmann distribution characterized by the thin film temperature, T_g. Similarly, the velocity distributions are well described by a Maxwell velocity distribution at T_g, as determined by high resolution Doppler analysis of the individual rovibrational line shapes. The M_j 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(perpendicular)/n(parallel)-1<0.02, where n(perpendicular)/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 format he 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 onset of a mild supersonic expansion is observed, with post-desorption collisions cooling the rotational temperature by as much as 15 K below T_s. Modeling of the gas-surface interaction using the realistic CO₂-CO₂ pair potential of Murthy et al. [Mol. Phys. 50,531 (1983)] 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 would not be expected for translational and rotational energies less than or comparable to the binding energy of CO₂ to the surface.

Citation

Journal of Chemical Physics

Volume

105

Issue

No. 2

Pub Type

Journals

Keywords

alignment, CO₂, high resolution, infrared, quantum state resolved, sticking coefficient, sublimation, surface, thin films

Citation

Weida, M. , Sperhac, J. and Nesbitt, D. (1996), Sublimation Dynamics of CO₂ Thin Films: A High Resolution Diode Laser Study of Quantum State Resolved Sticking Coefficients, Journal of Chemical Physics (Accessed April 18, 2024)