G. K. Paramonov

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.

Controlled surface photochemistry: Bond- and isotope-selective photodesorption of neutrals by adsorbate vibrational preparation with infrared laser pulses

The possibility of controlling surface photochemistry by the selective vibrational preparation of adsorbates with infrared (ir) laser pulses is investigated theoretically. In particular, the selective ir plus ultraviolet (uv) light-induced desorption of different isotopomeric neutral adsorbates from metal surfaces is studied with the help of nuclear density matrix theory. As a concrete example the system NH3/ND3/Cu(111) is chosen. In a first step of the “vibrationally mediated chemistry” advocated here, based on computed two-mode dipole functions and model potentials, optimal infrared laser pulses are designed to selectively excite the umbrella mode ν2 of either adsorbed NH3 or ND3. In a second step, an uv/visible photon enforces an electronic transition, leading, after ultrafast quenching, to desorption induced by electronic transitions (DIET). It is argued that despite strong dissipation, the proper vibrational preparation not only increases desorption yields substantially, but also allows for an almost...

Design of substituted semibullvalenes suitable for control of the cope rearrangement by two ps IR laser pulses

Abstract A systematic experimental search for substituted semibullvalenes yields systems which have nearly thermoneutral reactants and products ( ΔH 0 ≈ 0.5–1 kJ mol −1 ) separated by moderately low barrier ( ΔE 0 ǂ ≈ 10–30 kJ mol −1 ), e.g. 2,6-dicyanoethylmethylsemibullvalene DEM-SBV (= 7 c ⇌ 7 c ′). Theoretical model simulations suggest that their Cope rearrangement may be controlled by two infrared picosecond (IR ps) laser pulses: The first pump pulse excites the reactants to the transition state, represented by a delocalized wave packet with energy above the potential barrier. The second dump pulse stabilizes the products.

INFRARED-LASER DRIVEN VIBRATIONAL EXCITATION OF RELAXING ADSORBATES : QUANTUM DYNAMICAL ASPECTS

As a first step to the active manipulation of adsorbates by external, time-dependent electromagnetic fields, the infrared-laser driven selective excitation of molecular vibrations of adsorbates at metal surfaces is investigated here in the framework of time-dependent open-system density matrix theory. Special emphasis is given to the inclusion of vibrational damping, caused by the coupling of the adsorbate vibrations to possibly electronic substrate degrees of freedom. For the example system NH3/Cu, a non-Markovian, two-mode open-system Liouville–von Neumann model for the vibrational relaxation of an excited adsorbate is proposed. After studying the field-free decay of excited adsorbates, it is shown that even in rapidly relaxing environments optimal IR laser pulses in the picosecond domain can be designed which lead to temporarily high populations of selected target states of adsorbates at metal surfaces.

Ultrafast laser control of vibrational dynamics for a two-dimensional model of HONO2 in the ground electronic state: separation of conformers, control of the bond length, selective preparation of the discrete and the continuum states

Abstract Selective excitation of the vibrational bound and the continuum states, controlled by subpicosecond infrared (IR) laser pulses, is simulated within the Schrodinger wave function formalism for a two-dimensional model of the HONO2 molecule in the ground electronic state. State-selective excitation of the OH bond is achieved by single optimal laser pulses, with the probability being 97% for the bound states and more than 91% for the resonances. Stable, long-living continuum states are prepared with more than 96% probability by two optimal laser pulses, with the expectation energy of the molecule being well above the dissociation threshold of the ON single bond, and its life-time being at least 100 ps. The length of the ON single bond can be controlled selectively: stretching and contraction by about 45% of its equilibrium length are demonstrated. Laser separation of spatial conformers of HONO2 in inhomogeneous conditions occurring on an anisotropic surface or created by a direct current (DC) electric field is analysed. The relative yields of target conformers may be very high, ranging from 10 to 108, and the absolute yields of up to 40% and more are calculated.

Selective vibronic excitation and bond breaking by picosecond UV and IR laser pulses: application to a two-dimensional model of HONO2

Abstract A new method to control a bond-selective dissociation is presented: first, an optimal UV laser pulse accomplishes vibrationally state-selective complete population transfer from the ground to the excited electronic state and, second, an optimal IR laser pulse induces an efficient bond-selective multiphoton dissociation. The method is demonstrated by means of computer simulation within the Schrodinger wavefunction formalism for a two-dimensional model of HONO 2 wherein the OH and ON single-bond stretches are treated explicitly. Selective breaking of the ON single bond is achieved on a picosecond timescale, with the dissociation probability being almost 100%. The proposed control scheme is much less demanding of the field strength of the dissociating IR laser as compared to that required for breaking the ON single bond solely in the ground electronic state.

Control of breaking strong versus weak bonds of BaFCH3 by femtosecond IR + VIS laser pulses: theory and experiment

Intense (≈80 GW cm−2) ultrashort (≈100 fs) infrared (IR) laser pulses may be employed for excitation of a high frequency (≈3500 cm−1) local mode vibration in a molecule. Subsequently, an intense (16–256 GW cm−2), ultrashort visible (VIS) laser pulse yields electronic excitation with near adiabatic transfer of the vibrational energy, which has been accumulated by the IR pulse. The net result of these sequential IR + VIS laser pulses may be the breaking of a strong molecular bond close to the pre-excited one. In contrast, exclusive excitation by just a visible laser pulse breaks a competing weak bond. The effects of IR + VIS laser pulse control may be considered as an extension of vibrationally mediated chemistry, from ns pulses or continuous wave (cw) excitations to sub-ps laser pulses, and from direct vibrational pre-excitation of the bond to be broken to a neighboring bond, thus exploiting intramolecular vibrational redistribution (IVR) from the pre-excited local mode to the bond to be broken in the electronic excited state. The mechanism is demonstrated by quantum simulations for the model system BaFCH3, where BaF-, FC- and CH3 play the roles of the weak and strong bonds to be broken, and the vibrationally pre-excited CH3 stretch. The theoretical predictions are confirmed experimentally. Various extensions of the control by IR + VIS laser pulses include the control of the branching ratio of weak versus strong bond breaking, as well as isotopomer selectivity depending on the vibrational pre-excitations.