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

Optimal control theory is used to design ultrashort (subpicosecond) infrared laser pulses inducing state-selective vibrational excitation processes. For a Thiele-Wilson model Hamiltonian with parameters adapted for the HDO molecule, complete control of vibrational excitation by such pulses is demonstrated, including the selective excitation of OH or OD local vibrations (mode or bond selectivity). It is also shown how the constraint of fluence minimization can be used to achieve the additional objective of keeping the laser intensity as low as possible. We find that the approach works well from a computational point of view, and no numerical difficulties are encountered in a conjugate-gradient implementation of the pulse optimization. The spectral composition of the resulting minimum-fluence pulses follows a simple pattern. These pulses can be understood as superpositions of few components, each one inducing resonant transition between two levels in a series forming a ladder from the initial state to the target state. Thus in this ultrafast regime stepwise excitation by overlapping, phase-adjusted subpulses of low photonicity is seen to be more efficient than mechanisms related to or derived from direct multiphoton excitation. Fluence minimization is found to be an essential prerequisite for keeping the laser intensities below the range where molecular ionization and dissociation become nonnegligible processes, but even so the intensity requirements are formidable and limit the application of this technique to moderate degrees of vibrational excitation. The suitability of this approach as a tool in mode- or bond-selective chemistry is discussed.