Theory of ultrafast laser control for state-selective dynamics of diatomic molecules in the ground electronic state: vibrational excitation, dissociation, spatial squeezing and association

Abstract

An overview of the current state of the art in the laser control of molecular dynamics is presented with a special emphasis on the ultrafast vibrationally state-selective processes controlled by short and shaped infrared laser pulses. Ultrafast state-selective vibrational dynamics and dissociation of isolated diatomic molecules in the electronic ground state under the control of intense and shaped infrared laser pulses of picosecond and femtosecond duration is investigated within the Schrödinger wavefunction formalism. The laser driven dissipative dynamics is investigated within the reduced density matrix formalism beyond and within a Markov-type approximation for the ultrafast state-selective excitation of diatomic molecules, which are coupled to an unobserved quasi-resonant thermal environment. Quantum dynamics in a classical electric field is simulated for a one-dimensional Morse oscillator, representing the local OH bond of the H 2O and HOD molecules in the electronic ground state. Flexible tools of optimal laser control are developed and demonstrated on a picosecond timescale, which enable to localize the population with a very high probability at any prescribed vibrational level of OH, including those close to the dissociation threshold, without substantial dissociation. Comparative analysis of the Markovian and non-Markovian dissipative quantum dynamics reveals that the Markov approximation results in a pronounced decrease of a predicted probability for ultrafast selective preparation of very high vibrational bound states. The laser-controlled dissociation from selectively prepared high vibrational bound states is investigated for a wide range of the laser carrier frequencies, revealing the role of the phase of the dissociating laser pulse. In the limiting case of small laser frequencies, for half-cycle pulses, a spatial squeezing of highly excited molecules is discovered. It is demonstrated that the optimally controlled dissociation may be very efficient, and the dissociation probability may approach the maximal value. Quantum dynamics of vibrationally state-selective association of a diatomic molecule in the electronic ground state controlled by shaped sub-picosecond infrared laser pulse is investigated by means of representative wavepackets. It is shown, in particular, that a colliding pair of O and H atoms can be transferred selectively into a prespecified vibrational bound state of OH(ν). Optimal design of the laser field controlling this process results in a high association probability with a very high vibrational state-selectivity.