CONTROLLED DISSOCIATION OF I-2 VIA OPTICAL-TRANSITIONS BETWEEN THE X-ELECTRONIC AND B-ELECTRONIC STATES

Recent progress in both theoretical and experimental methods has led to the widespread belief that control of chemical reactions with ultrashort laser pulses is in principle achievable. In this article, the I2 molecule, which has been the subject of a great deal of femtosecond work, is studied theoretically as a prospect for experimental control. An unconstrained optimization of the electric field is performed, with the objective of dissociation on the B state. Krotov's optimization theory is used for this unconstrained optimization, which reduces the CPU time usage by a factor of four. The field which results from the optimization yields a probability of 99% for B state dissociation. The Husimi transform of the optimal field indicates a simple underlying structure, in contrast with many previous fields obtained by optimal control theory: the optimal field closely resembles a sequence of three Gaussian pulses with the same central frequency. Analysis of the optimal field indicates it operates by a quasi-cw excitation directly into the B continuum. A constrained optimization was then performed: the electric field was restricted onto the frequency interval from 0 to 19752 cm-1 to avoid the quasi-continuous-wave excitation mechanism. The optimal solution of this problem uses a three-photon pump-dump-pump (PDP) mechanism. The optimal field was approximated by sequences of three and four Gaussian-shaped laser pulses; the optimal and the two approximating fields gave 34%, 6% and 11% for the dissociation probability, respectively. The fields from the constrained optimization have fwhm on the order of 30-50 fs and intensities in the range of 10(10) W/cm2 and should be experimentally producible with available technology.