Depth-profiling and diffusion measurements in ice films using infrared laser resonant desorption.

A new infrared laser resonant desorption (LRD) technique has been developed that permits depth-profiling and diffusion measurements in ice. This LRD technique utilizes an Er:YAG rotary Q-switched laser with an output wavelength of lambda = 2.94 microm and a pulse duration of approximately 100 ns. The Er:YAG laser light resonantly excites O-H stretching vibrations in the H2O molecules that form the ice. This laser resonant heating induces H2O desorption at the ice surface. Control experiments were conducted on pure and isotopically mixed laminated ice films to determine the optimum experimental parameters for the LRD depth-profiling and diffusion measurements. Depending on laser energy, the measured desorption depth was either less than, comparable to, or larger than the optical penetration depth of approximately 0.8 microm at lambda = 2.94 microm. LRD studies were used to analyze H2 18O/H2 16O stacked multilayers and laminate sandwich structures. These measurements revealed that the LRD technique can depth-profile into ice films with submicrometer spatial resolution and high sensitivity. Two types of experiments employing LRD depth-profiling were demonstrated to monitor diffusion in ice. HCl hydrate diffusion in ice was measured versus time after depositing ice/HCl/ice sandwich structures. Na diffusion into ice was studied after adsorbing Na using a continuous Na source for a given exposure time at the diffusion temperature.