Volume 208, number 3,4 CHEMICAL PHYSICS LEITERS 1 I June 1993 Time-dependent laser-induced molecular formation from repulsive surfaces Theodoros Mercouris, Ioannis D. Petsalakis and Cleanthes A. Nicolaides zyxwvutsrqponmlkjihgfedcbaZ Theoretical and PhysicalChemistry Institute, NationalHellenicResearchFoundation, 48 Vasileos Constantinou Avenue, I16 35 Athens, Greece Received 27 November 1992; in final form 6 April 1993 We have computed time-dependent probabilities of formation of the HeH* bound excited states, at their equilibrium distance of about R,,= 1.5 au, upon realistic femtosecond laser excitation during the slow collision of He with H on the ground repulsive surface. The computation involved the time integration of ten states, represented by configuration interaction wavefunctions and coupled through the full Hamiltonian. The effect of radiative and radiationless damping was incorporated in terms of complex energies computed from first principles. We tind that the excitationrate goes througha maximumas a functionof the field strength Experiments on this or on similar molecular formation problems seem possible, provided efficient techniques for pre- paring the initial state are applied. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1. Introduction The emergence of the technology of short laser pulses in the range lo- lz-lO-ls s, has created a spec- trnm of opportunities for studying the time evolu- tion of energy transfer processes in atoms, molecules and solids, such as ionization, dissociation, predis- sociation or internal conversion. These phenomena involve transitions which start from a bound state and end in another bound state (intramolecular rearrangement) or in the free particle continuum. Experimental work by Dantus et al. [l] has also probed a transition state of an excited potential en- ergy surface. From the theoretical point of view, in order to understand them quantitatively, one must solve the time-dependent Schriidinger equation us- ing accurate wavefunctions and a realistic represen- tation of the spectrum. In this Letter, we present the results of a quantum- mechanical study of a time-dependent transition which starts from a repulsive surface and zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHG ends on a bound excited state, driven by a femtosecond laser pulse. In particular, we have considered the reaction H(ls)+He(ls’) z (HeH)* (bound Rydberg states) (1) and have computed its time dependence using reli- able molecular data obtained from first principles. We find that, for slow collisions, reaction ( 1) indeed takes place at R = 1.5 au, for a laser pulse of 1= 3030 8, and duration 40 fs, suggesting the possibility of ex- perimental realization of this and of similar laser-in- duced molecular formation (LIMF) processes and of novel photochemical reaction paths for nonreac- tive species #I. Reaction (1) was first treated by Hutchinson and George [ 31 (for XeF), within a time-independent resonance discrete-continuous mixing formalism, where the weak field coupling is assumed to induce a single photon absorption to the first excited state while a second photon induces a vibrational transi- tion within it. The present time-dependent theory aims at an experimentally verifiable prediction and involves the coupling of many excited Rydberg states represented by correlated adiabatic wavefunctions. *’ Analogous to ( 1 ), where He is replaced by an electron, is the f-bound excitation mechanism recently proposed [2] for the laser-induced formation of extraordinary bound excited states, starting from an electronic continuum which is modified by the presence of a resonance whose lifetime is of the order of 1 zyxwvutsrqpo O- ” s. 0009-2614/93/$ 06.00 0 1993 Elsevier Science Publishers B.V. AU rights reserved. 197