Laser control product branching ratios

Prof. S. A. Rice has pointed to another experimental verification of the Tannor-Rice-Kosloff scheme, carried out by Prof. G. R. Fleming. I would like to ask Prof. Fleming whether he could explain to us his experiment, that is, how are the two pump and control laser pulses used to control the branching ratio of competing chemical products ... 

Control over the product branching ratio in the photodissociation of Na2 into Na(3s) + Na(3p), and Na(3s) + Na(3d) is demonstrated using a two-photon incoherent interference control scenario. Ordinary pulsed nanosecond lasers are used and the Na2 is at thermal equilibrium in a heat pipe. Results show a depletion in the Na(3d) product of at least 25% and a concomitant increase in the Na(3p) yield as the relative frequency of the two lasers is scanned. 

The coherence properties of laser radiation provide an opportunity to exert some external control over intramolecular dynamics. Control over photofragmentation product branching ratios has been achieved in both time and frequency domain experiments. 

The conceptually simplest approach towards controlling systems by laser field is by teaching the field [188. 191. 192 and 193]. Typically, tire field is experimentally prepared as, for example, a sum of Gaussian pulses with variable height and positions. Each experiment gives an outcome which can be quantified. Consider, for example, an A + BC reaction where the possible products are AB + C and AC + B if the AB + C product is preferred one would seek to optimize the branching ratio... 

Prof. G. Gerber and A. H. Zewail have presented to us three fascinating experiments on femtosecond laser control of the branching ratio of competing product channels ... 

Following early pioneering work on the theory of coherent control, Rabitz and co-workers demonstrated experimentally that photoionization branching ratios can be controlled in a series of ketones (Levis et ai, 2001). For example, in the photoionization of acetone ((CH3)2CO), the CH3C0 ion, which was almost below the detection limits for an untailored pulse, became a substantial product when the laser pulse was tailored by the learning algorithm. This is particularly... 

Experiments with HCl (Park et al. 1991) have confirmed the predictions of coherent-control theory, particularly the sinusoidal dependence of the ionization rate on the relative phases of the two exciting lasers, as well as the dependence of the degree of sinusoidal modulation of the ionization cinrent on the relative laser field intensities. This technique was also used in experiments on controlfing the product ratio in the photodissociation of HI (Zhu et al. 1995) and the branching ratio in the photodissociation of Na2 (Shnitman et al. 1996). 

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