Interaction of Infrared Carbon Monoxide Laser Radiation with Carbon Monoxide Molecules Chemisorbed on a Palladium Surface: AN Experimental Investigation of Selective Laser-Induced Desorption.
This thesis describes the first experimental investigation dealing with the following problem: does depositing infrared photons into the internal vibrational mode(s) of molecules chemisorbed on a metal surface lead to selective laser -induced desorption?. Overall, the experimental results we obtained allow us to unambiguously reach the following significant conclusion: the direct interaction of a CW infrared laser beam with CO molecules chemisorbed on a palladium surface does not lead to wavelength-selective quantum desorption. Based on the minimum desorption quantum efficiency detectable with our experimental set-up, an upper limit of 3 x 10(' -8) desorbed molecules per absorbed photon was determined for the laser-induced quantum desorption process. The only detectable laser-induced desorption was thermal in nature, coming from the radiative heating of the palladium surface by the laser. Upon suppression of this heating effect, by using an automatic electronic temperature regulation scheme, no desorption signal (pressure increase) was detected in our UHV system when the laser source was turned on. A great deal of effort was dedicated to the design of the surface temperature regulation system and the progress we made in that respect is documented thoroughly. The chemisorption system investigated (CO/Pd) provided, in our opinion, the best "state of the art" opportunity to observe laser-induced desorption from metal surfaces through wavelength-selective processes. Furthermore, we believe that similar results would most likely have been found with many other chemisorption systems, under the same experimental conditions. A comprehensive review of the numerous aspects related to the interaction of infrared radiation with molecule -covered metal surfaces is presented. Various explanations are offered to account for the low efficiency of the process under study. Uppermost amongst them is the direct energy transfer, through electrodynamic interactions, from the adspecies pump mode to the metal surface conduction electrons. The experimental results may help resolve differences in various conflicting theoretical studies which have been published in relation with this subject.
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- Physics: Molecular