Velocity Map Imaging at molecular ice surfaces*

Photodesorption from ice-coated grains in interstellar space is believed to be an important source of molecular gases in cold interstellar space.  Photoexcitation also leads to formation of complex molecules which are released on warming up of the grain. We study photodesorption induced by pulsed UV-VUV irradiation of a thick film of molecules frozen onto the repeller plate of a velocity map imaging [1] setup.  The ejected plume is probed by laser ionization taking place 1-3 mm above the surface and the ions formed are projected onto a two-dimensional detector using VMI conditions.  Imaging allows us to distinguish intact atoms and molecules directly ejected at the surface from atom or molecular fragments created by the laser ionization process.  The later process creates unique patterns at the detector which allow us to identify the parent species probed by the detection process. Many unexpected processes are revealed including surface enhanced two-photon absorption and intact ejection of highly excited molecules from a range of simple molecular ices including O₂, CO₂, H₂O, and  (doped) rare gas solids.  

[1].ATJB Eppink, DH Parker, Review of Scientific Instruments 68 , 3477-3484, (1997)