Friday, January 9, 2009
Laser Induced Desorption of Amorphous Solid Water
MARC U*STAR Fellow
Laser Induced Desorption of Amorphous Solid Water Amorphous solid water (ASW) is a metastable form of ice. It is the most abundant phase of water in interstellar clouds.1,2 Despite its ubiquity, microphysical processes such as adsorption, desorption, diffusion, and reaction kinetics are not well known or published data is conflicting.3 CO2 has been used as a guest molecule to probe the porosity of ASW using temperature programmed desorption (TPD) and Fourier transform infrared (FTIR) techniques.4 Samples were prepared by depositing vapor on an MgO crystal attached to a liquid nitrogen cryostat under ultrahigh vacuum (~ 10-10 torr). Experiments of depositing 13CO2 on top of ASW, depositing 13CO2 underneath ASW, and codepositing 13CO2 and ASW reveal that 13CO2 diffuses into ASW grown at 90 K. At 106 K, 13CO2 sublimates from the surface of the ASW layer. Guest molecules remain trapped in the ASW upon annealing. At 165 K, trapped 13CO2 molecules are abruptly emitted from the pores of the ASW, which indicates the phase transition of H2O from amorphous to crystalline. Recent experiments employ the use of a pulsed Nd:YAG laser for desorption as opposed to annealing by heating the MgO substrate. Infrared light is used to excite the O-H stretching vibration in H2O to cause desorption.5 IR laser radiation of 2930 nm (~ 3400 cm-1) is generated by Raman6 shifting 1064 nm laser light from the Nd:YAG using a 1.1 m D2 Raman cell at 900 psi. The IR light is irradiated through a CaF2 window into the ultrahigh vacuum chamber on the substrate. Detection of desorbed molecules using a pulsed time-of-flight mass spectrometer has proven invaluable for attaining temporal spectra giving way to desorption velocities.