Volume 174, number 6 CHEMICAL PHYSICS LETTERS 23 November 1990 Femtosecond multiphoton dynamics of higher-energy potentials R.M. Bowman, M. Dantus and A.H. Zewail Arthur zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Amos zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Noyes Laboratory ofChemical Physics I, California Institute of zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQ Technology, zyxwvutsrqponmlkjihgfedcbaZYXWVU Pasadena, G4 91125, USA Received 30 August 1990; in final form 26 September 1990 The real-time motion of wave packets prepared coherently in different potential energy surfaces of molecular zyxwvutsrqponmlkjihg iodine is reported. Using multiphoton excitation and depletion techniques, we observe the phase-shifted oscillatory motion of the packet and the different transients characteristic of the bound B O+u( ‘H) and D O*u( ‘Z) state potentials alongwith the repulsive O+g( ‘I,) state potential. The approach helps in extending FTS to higher energy potentials and “dark” states, and illustrates experimental control schemes in a relatively simple system. 1. Intmduction In previous work from this group, femtosecond molecular dynamics on different potentials [ l-5 ] have been probed using single-photon techniques. To access higher energy potentials, we extend the fem- tosecond transition-state (or temporal) spectros- copy, FTS, to the multiphoton domain. Several ex- citation and detection schemes are introduced, including fluorescence depletion. These techniques are demonstrated in experiments performed on mo- lecular iodine. The results show the in-phase and out- of-phase motion of a wave packet in the B 0% ( 3TI ) state and transients characteristic of the bound D O+u(‘E) state and the repulsive O+g(rZ) state. Multiphoton spectroscopy of halogens (for a good review, see ref. [6] ) has been one of the most pro- ductive methods for investigating high-lying elec- tronic states [ 7-131. The use of these methods al- lows the excitation of states otherwise unaccessible zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA due to poor Franck-Condon factors caused by large shifts in the equilibrium internuclear distances. One- photon-forbidden processes, for example g-g elec- tronic transitions, can be probed via two-photon pathways. Highly excited ion-pair states of halogens, that require the generation of VUV pulses, can be reached with multiphoton excitation. Upon inspection of the excited state potential sur- faces [ 141 of I2 and taking into account the Hund’s ’ Contribution No. 8211. case (c) selection rules for one-photon excitations [151, g-u 7 g-g, u*Ll , AQ=O,&l, 0+-o+, o-*0-, 0+-o- , the following states can be accessed by one or more photons of 620 nm light starting in the X O’g(‘C) ground state: B O’u(9) (one photon), O+g( 5) (two photons), D O+u(‘C) -(three photons) and F O+u(‘C) (threephotons).TheB [16],D [7,17-221 and F [ 13,23,24] states are all bound and have been well studied by conventional spectroscopic methods, while little is known about O+g(‘G) [25], a repul- sive electronic state. Both the D [ 111 and F [ 13 ] states have been studied by many multiphoton tech- niques including one-color, three-photon spectros- copy. It is believed that the dissociative O+g(‘2.) state enhances the three-photon transitions to these ion- pair states [ 25 1. Thus, it seems that iodine is a good candidate for multiphoton FTS studies. The three possible cases that can arise from pump- probe processes involving the aforementioned states are shown in fig. 1 and are listed below: pump : probe : X-B, B-+O+g(‘C)+D, F, (a) X+B+O+g( ‘C) , O+g(‘Z)+D, (b) X+B+O+g( ‘E)+D , D-tO+g( ‘C) , (c) 546 0009-2614/90/$ 03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)