ELSEVIER 3 November 1995 Chemical Physics Letters 245 (1995) 393-399 CHEMICAL PHYSICS LETTERS Femtosecond photoassociation spectroscopy: coherent bond formation Una Marvet, Marcos Dantus Department of Chemistry and Center for Fundamental Materials Research, Michigan State University, East Lansing, M148824-1322, USA Received 19 July 1995; in final form 24 August 1995 Abstract The method femtosecond photoassociation spectroscopy (FPAS) is introduced and we report on its application to the study of the reaction Hg + Hg + hv ~ Hg 2. Our results demonstrate coherent bond formation obtained in the femtosecond time scale from free unbound reactants. The product Hg 2 is formed in the D1 u state and shows a high degree of rotational anisotropy induced by the polarization of the binding femtosecond pulse. 1. Introduction The two most fundamental processes of chemical reactions, bond formation and bond dissociation, have traditionally been studied by techniques which pro- vide detailed information only about the initial and final stages of the reaction, leaving the intermediate steps to inference. These processes are very fast (10-~2-10 -14 s) and only recently have time re- solved methods been able to directly capture the transition state dynamics [1]. Bond-breaking studies have had two critical advantages, the first of which is that the initial impact parameter and molecular orien- tation are well defined. Secondly, the photodissocia- tion event may be triggered by an ultrafast laser pulse, thus providing a 'time zero' for the initiation of the reaction. Using femtosecond techniques one is therefore able to interrogate the transition states of reactions in progress [2,3]. Bond formation, on the other hand, is challenging because collisions occur at random times and with random impact parameters and because the short duration of the laser pulse implies a concomitant loss in the number of atoms or molecules in close proximity. Recently, van der Waals complexes have been introduced to restrict the reagent geometry and to allow the definition of a time zero for bimolecular reactions. These experiments by the groups of Ze- wail, Wittig and others use precursors which undergo a full collision reaction upon liberation of one of the reactants in the complex [4]. Femtosecond probing and control of a bimolecular reaction without precur- sors has been achieved in a pioneering experiment by Zewail and co-workers who introduced time zero of the gas phase collision for the reaction of Xe atoms with 12 to produce XeI [5]. The Xe and 12 have random distributions and are mostly unbound at room temperature. The femtosecond resolution de- fines the configuration space and offers sufficient sensitivity even though the number of atoms in close proximity is relatively small. The photoassociation process, whereby a photon excites a free-to-bound transition, has been applied to the spectroscopic study of excimer and exciplex 0009-2614/95//$09.50 © 1995 Elsevier Science B.V. All fights reserved SSDI 0009-2614(95)01018-1