Abstract

Previous work by Herek, Materny, and Zewail has demonstrated that the timing between two ultrafast laser pulses can be used to control the branching ratio of Na* to Na production in the photodissociation of NaI [1]. In their approach, a transform limited pump pulse excites the ground ionic electronic state of NaI to the electronically excited covalent state and then a second transform limited pulse intercepts the evolving covalent wave packet and further excites to a higher lying state on which it dissociates into Na^*+I. They showed that the branching ratio of Na to Na^* production can be controlled by changing the timing between the two pulses. In this work we show that the Na to Na^* production ratio can also be controlled at fixed pump-probe delay by varying the linear chirp of the pump laser pulse while keeping its spectrum and intensity versus time fixed. What is being controlled is the degree of localization or focusing of the wave packet at the time of its interception by the pump pulse, which carries a localized wave packet with greater probability away from the state decaying to Na up to the state decaying to Na^*. It has been demonstrated both theoretically and experimentally that chirped ultrashort laser pulses can be used to focus nuclear wave packets of evolving quantum systems [2]. In our control scheme, the first control laser pulse ω_c is tailored to focus the vibrational wave packet at a particular time and internuclear distance such that a second ultrafast light pulse ω_p will intercept the wave packet and further excite it to the higher lying Na^*+I state. We demonstrate both theoretically and experimentally that the degree of wave packet focusing caused by the control pulse can affect the Na^* to Na branching ratio, and that this focusing and subsequent increase in the Na^* to Na branching ratio is due to the linear chirp of the control pulse ω_c.

© 1996 Optical Society of America