Computational Chemistry and Molecular Modeling
Atomic, Molecular and Optical Physics
Wiley InterScience Backfile Collection 1832-2000
Chemistry and Pharmacology
Three specific model systems, HCo(CO)4, Na · NH3, and NO/Pt(111), are used to extend the strategy of vibrationally mediated photodissociations of organometallics, via small clusters of metal atoms and small molecules, to photodesorption of small molecules from metal surfaces. All systems and strategies are similar with respect to breaking metal-ligand bonds by means of infrared IR and visible or ultraviolet UV photons. Specific properties of the systems call, however, for different implementations of the overall tools. In the case of HCo(CO)4, traditional continuous wave (CW) IR + UV 2-photon excitations enhance the rates of HCo bond homolysis. A detailed analysis discovers three effects which result from Franck-Condon transitions in the domains of vibrationally excited wave functions: (i) ultrafast (≈ 20 fs) bond rupture starting from the steeply repulsive wall of the potential energy surface of the excited singlet state; (ii) efficient fast (≈ 200 fs) predissociation via tunneling through neighboring potential barriers; and (iii) decreasing contributions from indirect dissociations via slow (≈ 46 ps) intersystem crossing induced by spin-orbit coupling. In the case of Na · NH3, we suggest a vibrationally mediated pump-and-dump scheme, similar to the strategy of Tannor, Rice, and Kosloff, with proper control of the delay (ca. 70 fs) between ultrashort (ca. 30 fs) pump-and-dump laser pulses. Ultimately, this strategy shifts specific lobes of the vibrationally excited wave packets into a steeply repulsive wall of the potential energy surface of the electronic ground state, with subsequent fast (ca. 100 fs) ruptures of the NA(SINGLEBOND)NH3 bond, similar to effect (i) for HCo(CO)4. Finally, we show that a similar, vibrationally mediated pump-and-dump scheme may also support photodesorption of NO from Pt(111), with an intrinsic relaxation step for the electronically excited system NO/Pt(111) instead of active pump-and-dump control for Na · NH3. All strategies are simulated by fast Fourier transform propagations of representative wave packets on two potential energy surfaces. © 1996 John Wiley & Sons, Inc.
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