I. IR Photodissociation Spectroscopy of Large van der Waals Clusters. II. UV Laser-Induced Photochemistry of Fe(CO)₅ on Single Crystal Surfaces

Chapter 2 is a summary of vibrational predissociation spectra of weakly bound clusters containing either CF3I or CF3Br. Spectra indicative of ArCF3I, KrCF3I, (CF3I)2, ArCF3Br and (CF3Br)2 clusters are presented with comparison to the matrix-isolation spectra of CF3I and CF3Br. The infrared photodissociation technique can be applied to the study of weakly bonded clusters which do not absorb in the infrared by attaching an IR chromophore; this work is presented in Chapter 3. Vibrational predissociation spectra of large clusters of Ar, Kr, N2 and CH4 containing a single CH3F or C2H4 chromophore are obtained as a function of cluster size. The IR spectra distinguish three regimes of cluster size in the CH3F case. The dissociation profiles yield information on the chromophore environment and the lifetime of the excited vibration. The clusters are gas-phase analogies to the matrix-isolation technique, although differences between the two are consistent with the presumed icosahedral cluster geometry, and "matrix" and "surface" sites are distinguishable. Chapter 4 presents the application of a simple two-level-with-decay model to the photodesorption of weakly bound adsorbates on crystal surfaces. Feasibility estimates are reported and establish that vibrational predissociation of the excited adsorbes can be induced in the infrared, where energy quenching mechanisms on surfaces can be quite fast. The effect of these quenching mechanisms on the lineshapes present the opportunity to probe the processes with the use of the photodesorption technique. Extraction of the phenomenological rate constants is described. Also reported are attempts to observe photodesorption induced by low-power lasers. The photodecomposition of Fe(CO)5 on three single crystal surfaces, sapphire (Al2O3), Si(100) and Ag(110), using UV irradiation is described in Chapter 5. The quenching of excited electronic states of adsorbates is expected to be quite different on the surfaces, yet dissociation of the Fe(CO)5 is observed in all cases. The results allow determination of the dissociation mechanism and also imply that the Fe(CO)5 dissociation rate is ultra-fast (< 10-13). Extensions of this initial investigation are discussed.