Mikhail V. Korolkov

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 Schrodinger 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 H2O 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.

Theory of ultrafast laser control of isomerization reactions in an environment: Picosecond cope rearrangement of substituted semibullvalenes

An efficient approach to control isomerization reactions by ultrashort infrared laser pulses in the presence of a thermal environment is developed and demonstrated by means of model simulations within the reduced density matrix formalism beyond a Markov‐type approximation for a picosecond Cope rearrangement of 2,6‐dicyanoethyl‐methylsemibullvalene coupled to a quasi‐resonant environment. The population transfer from the reactant state via the delocalized transition state to the product state is accomplished by two picosecond infrared laser pulses with a probability up to 80% despite the rather strong coupling to the environment, which reduces the lifetime of the transition state into the femtosecond time domain. Simulations, carried out for helium (4 K), nitrogen (77.2 K) and room (300 K) temperatures, show that low temperatures are preferable for state‐selective laser control of isomerization reactions.

State‐selective control for vibrational excitation and dissociation of diatomic molecules with shaped ultrashort infrared laser pulses

Ultrafast state‐selective dynamics of diatomic molecules in the electronic ground state under the control of infrared picosecond and femtosecond shaped laser pulses is investigated for the discrete vibrational bound states and for the dissociative continuum states. Quantum dynamics in a classical laser field is simulated for a one‐dimensional nonrotating dissociative Morse oscillator, representing the local OH bond in the H2O and HOD molecules. Computer simulations are based on two approaches — exact treatment by the time‐dependent Schrodinger equation and approximate treatment by integro‐differential equations for the probability amplitudes of the bound states only. Combination of these two approaches is useful to reveal mechanisms underlying selective excitation of the continuum states and above‐threshold dissociation in a single electronic state and for designing optimal laser fields to control selective preparation of the high‐lying bound states and the continuum states. Optimal laser fields can be de...

Vibrationally state-selective photoassociation by infrared sub-picosecond laserr pulses: model simulations for O + H → OH(ν)

Abstract The quantum dynamics of a photoassociation reaction in the electronic ground state controlled by an infrared picosecond laser pulse is investigated. The association reaction O + H → OH(ν) is simulated by representative wavepackets. The OH molecule to be formed is modeled as a non-rotating Morse oscillator. It is shown that the initial free continuum state of O + H can be transferred selectively into a specified vibrational bound state by interaction with an infrared laser pulse. Optimal design of the laser control field leads to high association probability with very high vibrational state selectivity.

A reflection principle for the control of molecular photodissociation in solids: model simulation for F2 in Ar

Laser pulse induced photodissociation of molecules in rare gas solids is investigated by representative quantum wavepackets or classical trajectories which are directed towards, or away from cage exits, yielding dominant photodissociation into different neighbouring cages. The directionality is determined by a sequence of reflections inside the relief provided by the slopes of the potential energy surface of the excited system, which in turn depend on the initial preparation of the matrix isolated system, e.g. by laser pulses with different frequencies or by vibrational pre-excitation of the cage atoms. This reflection principle is demonstrated for a simple, two-dimensional model of F2 in Ar.

Subpicosecond spin-flip induced by the photodissociation dynamics of ClF in an Ar matrix

Ultrafast spin-flip is used to monitor the subpicosecond intersystem crossing dynamics from the 1Π to the 3Π state following photodissociation of ClF isolated in an Ar matrix by means of pump–probe spectroscopy. After photoexcitation of the 1Π state analysis of the populations of triplet states shows that about 50 percent of the spin-flip occurs during the first bond stretch which takes about 250 fs. The early time dynamics of the Cl–F bond in an Ar matrix is investigated theoretically by selecting representative singlet and triplet excited states from a diatomics-in-molecules Hamiltonian. In a one-dimensional model, wave-packet simulations for the first excursion are performed which give a lower limit of about 60 fs for the spin-flip process. The ultrafast spin flip is supported by the caging of the wave packet by the neighboring Ar atoms. Already before collision of the F and Ar atoms the rather large energy gap between the 1Π and 3Π states in the Franck–Condon region is reduced rapidly to near degeneracy. As a consequence the spin–orbit interaction becomes dominant, inducing more than 40% admixture of the triplet character in the 1Π state. Subsequent kinetic energy transfer from ClF to Ar, not yet included in the model, should slow down the Cl and F atoms on their way back toward shorter bond distances, implying stabilization of the wave packet in the 3Π state, where it is monitored by the probe laser pulse.

Coherent spin control of matrix isolated molecules by IR¿UV laser pulses: Quantum simulations for ClF in Ar

Two coherent sequential IR+UV laser pulses may be used to generate two time-dependent nuclear wave functions in electronic excited triplet and singlet states via single (UV) and two photon (IR+UV) excitation pathways, exploiting spin-orbit coupling and vibrational pre-excitation, respectively. These wave functions evolve from different Franck-Condon domains until they overlap in a domain of bond stretching with efficient intersystem crossing. Here, the coherence of the laser pulses is turned into optimal interferences of the wave packets, yielding the total wave packet at the target place, time, and with dominant target spin. The time resolution of spin control is few femtoseconds. The mechanism is demonstrated by means of quantum model simulations for ClF in an Ar matrix.

Spin-orbit induced association under ultrafast laser pulse control

Abstract The possibility of spin–orbit induced association reactions controlled by optimized femtosecond laser pulses is demonstrated for the example of Br + ( 3 P )+ H ( 2 S )→ HBr + ( X 2 Π/ A 2 Σ + ) association. The nuclear wavepacket dynamics is simulated on the basis of ab initio data for HBr+. To achieve permanent, vibrationally state–selective association, both a Feshbach resonance between collision and vibronic energies and optimal timing between spin–orbit induced and laser pulse assisted transitions are important. The novel method can be effective even when direct photoassociation is forbidden.

Infrared picosecond laser control of acceleration of neutral atoms: model simulations for the collision pair O + H

Abstract The quantum dynamics of an atomic collision pair interacting with the electric field of an infrared sub-picosecond laser pulse is investigated by means of propagation of representative wavepackets. Depending on the optimal choice of the laser pulse, two competing types of scattering events are encountered. First, for continuum → bound transitions, effecitve (85%) vibrationally state-selective photoassociation reactions O + H → OH( ν ) are induced by stimulated emission [Chem. Phys. Lett. 260 (1996) 604]. Second, for non-resonant cases, laser-controlled acceleration of the colliding atoms can be achieved. Laser field optimization allows one to design the energy distribution of the scattered atoms

Spin–orbit induced predissociation dynamics of HCl+ and HBr+ ions: temporal and spectral representations

Abstract The spin–orbit induced predissociation dynamics of HCl + and HBr + ions have been investigated by numerical solution of four coupled time-dependent Schrodinger equations based on ab initio potential energy data. For HCl + exponential decay dominates for all vibrational levels in the electronic excited 2 Σ + state. For HBr + pronounced nonexponential decay occurs due to significant multichannel competition. The comparison of temporal and spectral representations of the dynamics reveals that the derivation of dynamic information from experimental frequency domain spectra may be difficult in the case of multichannel competition.