Electronic state-resolved evaporation dynamics at the gas-molten metal interface

The gas-liquid interface represents a fascinating albeit challenging environment essential to understanding a wide range of atmospheric, chemical, and biological processes. This talk focuses on a relatively simple model system: quantum state resolved evaporation of metal atoms from the gas-molten metal interface. In particular, we combine hot molten metal crucibles in high vacuum (10^-8 Torr) with high sensitivity laser induced fluorescence (LIF) obtained from atoms in an Nd:YAG pumped-frequency tripled dye laser/photomultiplier (PMT) detection system. Specifically, we probe both ground and spin-orbit excited Ga(²P_1/2 and ²P_3/2) atoms evaporating from molten Ga metal as a function of temperature (600-1100K), for which the signal intensities and spin-orbit ratios permit quantitative assessment of i) the thermodynamics and ii) equilibrium vs. non-equilibrium nature of the atomic evaporation event, respectively. The temperature dependence of the Ga signals permits rigorous extraction of vaporization enthalpies (ΔH_vap) for such ultralow vapor pressure molten metal systems. Conversely from time reversal symmetry and detailed balance considerations, the equilibrium vs. non-equilibrium spin-orbit quantum state distributions report on the adsorption/desorption dynamics, e.g., whether an incident Ga (²P_1/2 or ²P_3/2), atom approaching the molten metal “sticks” (i.e., adsorbs and thermalizes) or “bounces” (i.e., inelastically scatters) from the gas-liquid interface.