Laser probing of nascent gaseous product species formed in chemical reactions on surfaces

A review of research conducted at the Naval Research Laboratory illustrates the unique advantages of using lasers to probe the newly formed gaseous products from chemical reactions on surfaces. Examples to be discussed are taken from the areas of catalytic reactions, silicon etching, and surface photochemistry. UHV experiments on the catalytic oxidation of H2, D2, and NH3 by 02 and of H2 by NO2 used the laser-induced fluorescence technique to measure the apparent desorption energies of OH radicals and the energy accommodation of the desorbing OH, OD, and NO reaction products. The observed different degrees of rotational cooling in the different products are discussed. In multiphoton ionization mass spectrometry (MPI/MS) experiments, spectroscopies of three different MPI processes for the SiF2 radical were studied and two of these were applied to the study of silicon etching by XeF2, F2, and NF3. The only observed gaseous products were SiF4 and the SiF2 radical and their apparent production energies correlated with the ease of surface fluorine atom production. In the area of surface photochemistry, an excimer laser was used to photolyze iodobenzene, trimethylgallium, and trimethylindium adsorbed on a cold surface. The time-of-flight spectra of the fragments or reaction products were measured by using a mass spectrometer equipped with MPI or electron ionization (El).