W. Jakubetz et al., STATE-SELECTIVE EXCITATION OF MOLECULES BY MEANS OF OPTIMIZED ULTRASHORT INFRARED-LASER PULSES, Journal of physical chemistry, 97(48), 1993, pp. 12609-12619
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.