Analysis Of Laser Desorption By Secondary Ion Mass Spectrometry

Analysis of laser desorption by secondaryion mass spectrometryAlan R. BurnsSandia National LaboratoriesAlbuquerque, New Mexico 87185AbstractFor the first time, laser- induced thermal desorption is examined with secondary ionmass spectrometry (SIMS). The desorption of small molecules on a polycrystalline tungstensurface by high- intensity (up to 176 MW /cm2), 1064 -nm pulses of 8 -10 nsec duration isanalyzed by monitoring relative changes in the SIMS intensities. Despite the very highsurface temperatures (3000 K) obtained by the absorption of intense laser radiation, theobserved changes are much smaller than predicted by the equilibrium model of thermaldesorption.IntroductionDespite the evident theoretical interest in the interaction of laser radiation withsurfaces,1-3 there has been very little definitive experimental work on the actualsurface physics and chemistry of laser desorption. Among the many questions that need tobe answered are: (1) How efficient is the desorption process? (2) Is the equilibrium modelof thermal desorption applicable in describing the kinetics of laser desorption? (3) Howdoes the extremely rapid heating rate due to the absorption of laser radiation effect therates and pathways of energy exchange between the surface and the adsorbed molecules? Inorder to begin to answer these questions, a sensitive surface analysis technique like SIMSis used to evaluate the effects of laser radiation on the coverages of adsorbed atoms andmolecules. These effects are then compared with those calculated from the theories oflaser surface heating and thermal desorption.SIMSSIMS has a surface sensitivity of less than 10 -4 monolayer4.5 due to the moderatelyefficient production of charged secondary ions, coupled with the high collection efficiencyand high detector gain of mass spectrometers. When a very low primary ion beam current isused (< 50 nA /cm2), the presence of specific molecular species on a metal surface can becontinuously monitored for several hours without significantly changing the surfacecomposition.6 Rabelais and co- workers7'8 have clearly demonstrated that valuablesurface structural and chemical information can be obtained from molecular SIMS as long asthe beam energy is low enough (< 600 eV) to preclude ion -beam -induced recombination anddissociation. The use of a low- energy ion beam also assures a negligible yield of secondaryions that originate from layers deeper that about 10 A below the surface,9 thus makingthe spectra indicative of surface composition. Since the process of laser- induced thermaldesorption is extremely rapid (nsec time scale), it would be virtually impossible tomonitor in real -time the actual dynamics of desorption. Thus most of the data presentedhere takes the form of complete SIMS spectra taken prior to laser radiation, followed bycomplete spectra taken afterwards. The net changes in surface coverage are then analyzed.Laser heatingThe elevated surface temperature of a metal substrate due to the absorption of laserenergy can be estimated by the method discussed by Rear. 10 For opaque solids liketungsten, 1064 -nm light penetrates only a few hundred 11 so Nd:YAG laser heatingcan be approximated as a heat source at the surface of a semi -infinite solid: