SUBLIMATION DYNAMICS OF CO2 THIN-FILMS - A HIGH-RESOLUTION DIODE-LASER STUDY OF QUANTUM STATE-RESOLVED STICKING COEFFICIENTS

Nascent quantum states of CO2 subliming from CO2 thin films at rates o f 1 to 10(3) monolayers (ML) per second are probed via direct infrared absorption of the nu(3) asymmetric stretch with a frequency ramped di ode laser. The high spectral resolution (Delta nu approximate to 15 MH z) of the diode laser and the use of polarization modulation technique s permit individual rotational, vibrational, translational, and even M (J) degrees of freedom of the subliming flux to be studied with quantu m state resolution. Measured rotational and nu(2) bend vibrational dis tributions indicate that the molecules sublime from the surface in a B oltzmann distribution characterized by the thin him temperature T-s. S imilarly, the velocity distributions parallel to the;surface are well described by a Maxwell velocity distribution at T-s, 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 to)/n(parallel to) --1\<0.02, where n(perpendicular to)/n(parallel to) is the ratio of mo lecules with J perpendicular vs parallel to the surface normal. By vir tue of the direct absorption technique, the. absolute sublimation rate s from the surface can be obtained from the measured column integrated densities. Via detailed balance, these fluxes are compared with equil ibrium vapor pressure measurements to retrieve the absolute sticking c oefficients'S for gas phase CO2 impinging on a solid phase CO2 thin fi lm. For sublimation rates <10(3) ML/s, the data indicate S=1.0+/-0.2, irrespective of quantum state, rotational alignment, and tangential ve locity component. For sublimation rates >10(3) ML/s; the onset of a mi ld supersonic expansion is observed, with post-desorption collisions c ooling the rotational temperature by as much as 15 K below T-s. Modeli ng of the gas-surface interaction using realistic CO2-CO2 pair potenti als demonstrates that the gas-surface potential is relatively ''soft'' and highly corrugated, which promotes efficient translational and rot ational energy transfer to the surface. The scattering analysis also s uggests that nonequilibrium quantum state distributions in the sublimi ng flux are not expected for translational and rotational energies les s than or comparable to the binding energy of CO2 to the surface. (C) 1996 American Institute of Physics.