FROM LASER CONTROL OF VIBRATIONALLY MEDIATED PHOTODISSOCIATION TO PHOTODESORPTION - MODEL SIMULATIONS OF BREAKING METAL-LIGAND BONDS IN ORGANOMETALLIC MOLECULES, CLUSTERS, AND ADSORBATES AT SURFACES

Three specific model systems, HCo(CO)(4), Na . NH3, and NO/Pt(111), ar e used to extend the strategy of vibrationally mediated photodissociat ions of organometallics, via small clusters of metal atoms and small m olecules, to photodesorption of small molecules from metal surfaces. A ll systems and strategies are similar with respect to breaking metal-l igand bonds by means of infrared IR and visible or ultraviolet UV phot ons. Specific properties of the systems call, however, for different i mplementations of the overall tools. In the case of HCo(CO)(4), tradit ional continuous wave (CW) IR + UV 2-photon excitations enhance the ra tes of HCo bond homolysis. A detailed analysis discovers three effects which result from Franck-Condon transitions in the domains of vibrati onally excited wave functions: (i) ultrafast (approximate to 20 fs) bo nd rupture starting from the steeply repulsive wall of the potential e nergy surface of the excited singlet state; (ii) efficient fast (appro ximate to 200 fs) predissociation via tunneling through neighboring po tential barriers; and (iii) decreasing contributions from indirect dis sociations via slow (approximate to 46 ps) intersystem crossing induce d by spin-orbit coupling. In the case of Na . NH2, we suggest a vibrat ionally mediated pump-and-dump scheme, similar to the strategy of Tann er, Rice, and Kosloff, with proper control of the delay (ca. 70 fs) be tween ultrashort (ca. 30 fs) pump-and-dump laser pulses. Ultimately, t his strategy shifts specific lobes of the vibrationally excited wave p ackets into a steeply repulsive wall of the potential energy surface o f the electronic ground state, with subsequent fast (ca. 100 fs) ruptu res of the Na-NH3 bond, similar to effect (i) for HCo(CO)(4). Finally, we show that a similar, vibrationally mediated pump-and-dump scheme m ay also support photodesorption of NO from Pt(111), with an intrinsic relaxation step for the electronically excited system NO/Pt(111) inste ad of active pump-and-dump control for Na . NH3. All strategies are si mulated by fast Fourier transform propagations of representative wave packets on two potential energy surfaces. (C) 1996 John Wiley & Sons, Inc. (A)ll strategies are simulated by fast Fourier transform propagat ions of representative wave packets on two potential energy surfaces. (C) 1996 John Wiley & Sons, Inc.