STATE-SELECTIVE EXCITATION OF MOLECULES BY MEANS OF OPTIMIZED ULTRASHORT INFRARED-LASER PULSES

Optimal control theory is used to design ultrashort (subpicosecond) in frared laser pulses inducing state-selective vibrational excitation pr ocesses. For a Thiele-Wilson model Hamiltonian with parameters adapted for the HDO molecule, complete control of vibrational excitation by s uch pulses is demonstrated, including the selective excitation of OH o r OD local vibrations (mode or bond selectivity). It is also shown how the constraint of fluence minimization can be used to achieve the add itional objective of keeping the laser intensity as low as possible. W e find that the approach works well from a computational point of view , and no numerical difficulties are encountered in a conjugate-gradien t implementation of the pulse optimization. The spectral composition o f the resulting minimum-fluence pulses follows a simple pattern. These pulses can be understood as superpositions of few components, each on e inducing resonant transition between two levels in a series forming a ladder from the initial state to the target state. Thus in this ultr afast regime stepwise excitation by overlapping, phase-adjusted subpul ses of low photonicity is seen to be more efficient than mechanisms re lated to or derived from direct multiphoton excitation. Fluence minimi zation is found to be an essential prerequisite for keeping the laser intensities below the range where molecular ionization and dissociatio n become nonnegligible processes, but even so the intensity requiremen ts are formidable and limit the application of this technique to moder ate degrees of vibrational excitation. The suitability of this approac h as a tool in mode- or bond-selective chemistry is discussed.