Laser control of vibrational excitation in carboxyhemoglobin: a quantum wave packet study

A coherent control algorithm is applied to obtain complex-shaped infrared laser pulses for the selective vibrational excitation of carbon monoxide at the active site of carbonmonoxyhemoglobin, modeled by the six-coordinated iron-porphyrin-imidazole-CO complex. The influence of the distal histidine is taken into account by an additional imidazole molecule. Density-functional theory is employed to calculate a multidimensional ground-state potential energy surface, and the vibrational dynamics as well as the laser interaction is described by quantum wave-packet calculations. At each instant in time, the optimal electric field is calculated and used for the subsequent quantum dynamics. The results presented show that the control scheme is applicable to complex systems and that it yields laser pulses with complex time-frequency structures, which, nevertheless, have a clear physical interpretation.     

Ultrafast control of the internuclear distance with parabolic chirped pulses

Recently, control over the bond length of a diatomic molecule with the use of parabolic chirped pulses was predicted on the basis of numerical calculations [Chang; et al. Phys. Rev. A 2010, 82, 063414]. To achieve the required bond elongation, a laser scheme was proposed that implies population inversion and vibrational trapping in a dissociative state. In this work we identify two regimes where the scheme works, called the strong and the weak adiabatic regimes. We define appropriate parameters to identify the thresholds where the different regimes operate. The strong adiabatic regime is characterized by a quasi-static process that requires longer pulses. The molecule is stabilized at a bond distance and at a time directly controlled by the pulse in a time-symmetrical way. In this work we analyze the degree of control over the period and elongation of the bond as a function of the pulse bandwidth. The weak adiabatic regime implies dynamic deformation of the bond, which allows for larger bond stretch and the use of shorter pulses. The dynamics is anharmonic and not time-symmetrical and the final state is a wave packet in the ground potential. We show how the vibrational energy of the wave packet can be controlled by changing the pulse duration.     

A theoretical study on quantum control of photodissociation dynamics by ultrashort chirped laser pulses

By a wavepacket propagation, we demonstrate the possibility of controlling the photodissociation branching ratio between two fragment channels by a single ultrashort linearly chirped laser pulse. It is found that a negatively chirped pulse of a moderate chirp rate completely prohibits the production of one of the photofragment channels. Two characteristics of chirped laser pulses contribute to this remarkable effect: the mechanism of adiabatic rapid passage (ARP) for the population transfer between the ground and excited states and the intrapulse pump‐dump process for determining the branching ratio. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 72: 525–532, 1999     

The influence of intense control laser pulses on homodyne-detected rotational wave packet dynamics in O2 by degenerate four-wave mixing

We illustrate how the preparation and probing of rotational Raman wave packets in O(2) detected by time-dependent degenerate four-wave mixing (TD-DFWM) can be manipulated by an additional time-delayed control pulse. By controlling the time delay of this field, we are able to induce varying amounts of additional Rabi cycling among multiple rotational states within the system. The additional Rabi cycling is manifested as a change in the signal detection from homodyne detected to heterodyne detected, depending on the degree of rotational alignment induced. At the highest laser intensities, Rabi cycling among multiple rotational states cannot account for the almost complete transformation to a heterodyne-detected signal, suggesting a second mechanism involving ionization. The analysis we present for these effects, involving the formation of static alignment by Rabi cycling at moderate laser intensities and possibly ion gratings at the highest intensities, appears to be consistent with the experimental findings and may offer viable explanations for the switching from homodyne to heterodyne detection observed in similar DFWM experiments at high laser field intensities (>10(13) W/cm(2)).     

Coherent control of population transfer in Rydberg atoms via chirped microwave pulses

We present a comprehensive and ab initio nonperturbative investigation of the coherent population transfer among the 3D high-lying Rydberg hydrogen and alkali atomic states via linearly polarized chirped microwave pulses. The time-dependent Schr?dinger equation for the dynamical evolution of Rydberg atoms is accurately and efficiently solved by means of the time-dependent generalized pseudospectral method. For atomic H, the population transfer from n = 35 to 30 with nearly 100% efficiency is achieved by means of the sequential two-photon Deltan = -1 transitions. The calculation fully utilizes all of the available orbital angular momentum l states for a given n, and the interference pattern and population evolution dynamics of individual l states are analyzed in detail. It is shown that the coherent control of the population transfer from the high n to the low n states can be accomplished by the optimization of the chirping parameters and microwave field strength. Similar analysis is performed for the Na atom, where the alkali atomic structure is described by an accurate model potential. We found that while the global population transfer pattern is qualitatively similar, there are significant differences in the dynamical response of atomic H and Na to the chirped microwave fields. Due to the degeneracy of the l states (for a given n) in unperturbed atomic H, the population transfer involves significant coupling and interference among a number of low-lying l states. For the case of Na atoms, however, the population transfer from the n to (n - 1) state is dominated by a single channel, namely, from the |n,l = 0> to the |n - 1,l = 0> state.     

Time-dependent wave packet propagation using quantum hydrodynamics

A new approach for propagating time-dependent quantum wave packets is presented based on the direct numerical solution of the quantum hydrodynamic equations of motion associated with the de Broglie–Bohm formulation of quantum mechanics. A generalized iterative finite difference method (IFDM) is used to solve the resulting set of non-linear coupled equations. The IFDM is 2nd-order accurate in both space and time and exhibits exponential convergence with respect to the iteration count. The stability and computational efficiency of the IFDM is significantly improved by using a “smart” Eulerian grid which has the same computational advantages as a Lagrangian or Arbitrary Lagrangian Eulerian (ALE) grid. The IFDM is generalized to treat higher-dimensional problems and anharmonic potentials. The method is applied to a one-dimensional Gaussian wave packet scattering from an Eckart barrier, a one-dimensional Morse oscillator, and a two-dimensional (2D) model collinear reaction using an anharmonic potential energy surface. The 2D scattering results represent the first successful application of an accurate direct numerical solution of the quantum hydrodynamic equations to an anharmonic potential energy surface.     

Dissociative ionization of methane by chirped pulses of intense laser light

Measurements have been made of optical field-induced ionization and fragmentation of methane molecules at laser intensities in the 10(16) W cm(-2) range using near transform limited pulses of 100 fs duration as well as with chirped pulses whose temporal profiles extend up to 1500 fs. Data is taken both in constant-intensity and constant-energy modes. The temporal profile of the chirped laser pulse is found to affect the morphology of the fragmentation pattern that is measured. Besides, the sign of the chirp also affects the yield of fragments like C2+, H+, and H2+ that originate from methane dications that are formed by optical field-induced double ionization.     

Steering dissociation of Br2 molecules with two femtosecond pulses via wave packet interference

The dissociation dynamics of Br2 molecules induced by two femtosecond pump pulses are studied based on the calculation of time-dependent quantum wave packet. Perpendicular transition from X 1Sigma g+ to A 3Pi 1u+ and 1Pi 1u+ and parallel transition from X 1Sigma g+ to B 3Pi 0u+, involving two product channels Br (2P3/2)+Br (2P3/2) and Br (2P3/2)+Br* (2P1/2), respectively, are taken into account. Two pump pulses create dissociating wave packets interfering with each other. By varying laser parameters, the interference of dissociating wave packets can be controlled, and the dissociation probabilities of Br2 molecules on the three excited states can be changed to different degrees. The branching ratio of Br*/(Br+Br*) is calculated as a function of pulse delay time and phase difference.     

Carbonyl vibrational wave packet circulation in Mn2(CO)10 driven by ultrashort polarized laser pulses

The excitation of the degenerate E(1) carbonyl stretching vibrations in dimanganese decacarbonyl is shown to trigger wave packet circulation in the subspace of these two modes. On the time scale of about 5 ps, intramolecular anharmonic couplings do not cause appreciable disturbance, even under conditions where the two E(1) modes are excited by up to about two vibrational quanta each. The compactness of the circulating wave packet is shown to depend strongly on the excitation conditions, such as pulse duration and field strength. Numerical results for the solution of the seven-dimensional vibrational Schro?dinger equation are obtained for a density functional theory based potential energy surface and using the multi-configuration time-dependent Hartree method.     

Photoionization-induced dynamics of the ammonia cation studied by wave packet calculations using curvilinear coordinates

The determination of the photoelectron spectrum of NH₃ and of the internal conversion dynamics of recently published [A. Viel, W. Eisfeld, S. Neumann, W. Domcke, U. Manthe, J. Chem. Phys. 124 (2006) 214306] is complemented by the investigation of the effect of the vibrational angular momenta couplings on the dynamics. The multi-configurational time-dependent Hartree method is used to propagate a wave packet on the analytical anharmonic six-dimensional three-sheeted potential energy surface for the ground and first excited states of the ammonia cation. Curvilinear coordinates and the associated quasi-exact kinetic energy operator suitable for the multi-configurational time-dependent Hartree scheme are employed. A non-negligible effect of the use of Cartesian normal modes instead of the curvilinear coordinates is observed on the low energy part of the photoelectron spectrum. However, the three different time scales found in the dynamical calculations for the second absorption band are very similar regardless of the use of either normal modes or curvilinear coordinates.     

Assignment of electronic transitions by geometry optimization

Adiabatic excitation energies, excited state geometries, excited state charges, bond orders and dipole moments have been obtained for HCN, CO₂,H₂CO, HFCO, F₂CO, ethylene, trans-butadiene, furan, pyrrole and uracil using the SINDO1 semi-empirical method with configuration interaction. Our results generally agree with those ofab initio calculations and experiment satisfactorily. Geometry optimization is found to mix configurations differing in their allowedness in vertical excitation from the ground state, which in turn helps in the assignment of spectral transitions. TheV excited singlet state of trans-butadiene and various excited states of furan, pyrrole and uracil have been found to be appreciably non-planar. The single and double CC bonds are found to exchange positions due to the lowest triplet and singlet transitions of furan and pyrrole. The first triplet and first singlet transitions of uracil have been found to be of π-π* and π-σ* types respectively in agreement with recent experimental findings. On leave of absence from the Department of Physics, Banaras Hindu University, Varanasi-221005, India     

Closed loop coherent control of electronic transitions in gallium arsenide

A genetic algorithm was used to control the photoluminesce-nce (PL) from GaAs(100). A spatial light modulator (SLM) used feedback from the emission to optimize the spectral phase profile of an ultrashort laser pulse. Most of the experiments were performed using a sine phase function to optimize the integrated PL spectrum over a specified wavelength range, with the amplitude and period of the phase function treated as genetic parameters. An order of magnitude increase in signal was achieved after only one generation, and an optimized waveform, consisting of three equally spaced pulses approximately 0.8 ps apart, was obtained after 15 generations. The effects of fluence, polarization, relative phase of the subpulses, and spectral range of the optimized PL were investigated. In addition, preliminary experiments were performed using the phases of individual pixels of the SLM as genetic variables. The PL spectrum is identified with recombination of electron-hole pairs in the L-valley of the Brillouin zone. Control is achieved by coherent manipulation of plasma electrons. It is proposed that hot electrons excite lattice phonons, which in turn scatter carriers into the L-valley.     

Control of laser induced molecular fragmentation of n-propyl benzene using chirped femtosecond laser pulses

We present the effect of chirping a femtosecond laser pulse on the fragmentation of n-propyl benzene. An enhancement of an order of magnitude for the relative yields of and in the case of negatively chirped pulses and in the case of positively chirped pulses with respect to the transform-limited pulse indicates that in some fragmentation channel, coherence of the laser field plays an important role. For the relative yield of all other heavier fragment ions, resulting from the interaction of the intense laser field with the molecule, there is no such enhancement effect with the sign of chirp, within experimental errors. The importance of the laser phase is further reinforced through a direct comparison of the fragmentation results with the second harmonic of the chirped laser pulse with identical bandwidth.     

Selective excitation of LI2 by chirped laser pulses with all possible interstate radiative couplings

We have numerically explored the feasibility and the mechanism of population transfer to the excited E (1)Σ(g) electronic state of Li(2) from the v=0 level of the ground electronic state X (1)Σ(g) using the A (1)Σ(u) state as an intermediate. In this system, the use of transform limited pulses with a frequency difference greater than the maximum Rabi frequency does not produce population transfer when all possible radiative couplings are taken into account. We have employed two synchronous pulses far detuned from the allowed transition frequencies, mainly with the lower frequency pulse positively chirped, and both pulses coupling the successive pair of states, X-A and A-E. The adiabaticity of the process has been investigated by a generalized Floquet calculation in the basis of 12 field dressed molecular states, and the results have been compared with those obtained from the full solution of time dependent Schro?dinger equation. The conventional representation of the process in terms of three (or four) adiabatic potentials is not valid. It has been found that for cases of almost complete population transfer in full calculations with the conservation of the vibrational quantum number, adiabatic passage is attained with the 12 state Floquet model but not with the six state model. The agreement between the full calculations and the 12 state Floquet calculations is generally good when the transfer is adiabatic. Another characteristic feature of this work is the gaining of control over the vibrational state preparation in the final electronic state by careful tuning of the laser parameters as well as the chirp rate sign. This causes time dependent changes in the adiabatic potentials and nonadiabatic transfers can be made to occur between them.     

Adiabatic squeezing of molecular wave packets by laser pulses

Strong pulse sequences can be used to control the position and width of the molecular wave packet. In this paper we propose a new scheme to maximally compress the wave packet in a quasistatic way by freezing it at a peculiar adiabatic potential shaped by two laser pulses. The dynamic principles of the scheme and the characteristic effect of the different control parameters are presented and analyzed. We use two different molecular models, electronic potentials modeled by harmonic oscillators, with the same force constants, and the Na(2) dimer, to show the typical yield that can be obtained in compressing the initial (minimum width) molecular wave function.     

Nano-scale modification of ionic surfaces induced by electronic transitions

This paper presents a review of recent experimental and theoretical work on surface modification of alkali halides due to electronic excitations caused by electron and photon irradiation. In particular, several examples of free exciton and hot electron–hole pair formation in ionic materials are given. It is demonstrated that evolution of these primary excitations with subsequent defect formation and diffusion from the bulk to the surface, leads to dynamic surface modification and sputtering, often periodically varying with the irradiation dose. In turn, modification of the surface topography could affect and modulate periodically, the diffusion processes driving the defects from the bulk of the material towards its surface. Spectacular examples of such oscillatory yield dependencies, and corresponding surface nano-structure formation and evolution, are shown for electron- and photon-irradiated NaCl, KBr and KCl(0 0 1) surfaces of bulk crystals and thin epitaxial films. Important applications of these findings for quantitative characterisation of insulator surfaces and mass spectrometry are presented.     

Coherent control and time-dependent density functional theory: towards creation of wave packets by ultrashort laser pulses

Explicitly time-dependent density functional theory (TDDFT) is a formally exact theory, which can treat very large systems. However, in practice it is used almost exclusively in the adiabatic approximation and with standard ground state functionals. Therefore, if combined with coherent control theory, it is not clear which control tasks can be achieved reliably, and how this depends on the functionals. In this paper, we continue earlier work in order to establish rules that answer these questions. Specifically, we look at the creation of wave packets by ultrashort laser pulses that contain several excited states. We find that (i) adiabatic TDDFT only works if the system is not driven too far from the ground state, (ii) the permanent dipole moments involved should not differ too much, and (iii) these results are independent of the functional used. Additionally, we find an artifact that produces fluence-dependent excitation energies.     

Dipole-resonance assisted isomerization in the electronic ground state using few-cycle infrared pulses

A computational investigation of HCN → HNC isomerization in the electronic ground state by one- and few-cycle infrared pulses is presented. Starting from a vibrationally pre-excited reagent state, isomerization yields of more than 50% are obtained using single one- to five-cycle pulses. The principal mechanism includes two steps of population transfer by dipole-resonance (DR), and hence, the success of the method is closely linked to the polarity of the system and, in particular, the stepwise change of the dipole moment from reactant to transition state and on to products. The yield drops massively if the diagonal dipole matrix elements are artificially set to zero. In detail, the mechanism includes DR-induced preparation of a delocalized vibrational wavepacket, which traverses the barrier region and is finally trapped in the product well by DR-dominated de-excitation. The excitation and de-excitation steps are triggered by pulse lobes of opposite field direction. As the number of optical cycles is increased, the leading field lobes prepare a vibrational superposition state by off-resonant ladder climbing, which is then subjected to the three steps of the principal isomerization mechanism. DR excitation is more efficient from a preformed vibrational wavepacket than from a molecular eigenstate. The entire process can be loosely described as Tannor-Kosloff-Rice type transfer mechanism on a single potential surface effected by a single pulse, individual field lobes assuming the roles of pump- and dump-pulses. Pre-excitation to a transient wavepacket can be enhanced by applying a separate, comparatively weak few-cycle prepulse, in which the prepulse prepares a vibrational wavepacket. The two-pulse setup corresponds to a double Tannor-Kosloff-Rice control scheme on a single potential surface.     

Nonadiabatic chemical dynamics in an intense laser field: electronic wave packet coupled with classical nuclear motions

Dynamics of molecules in an intense laser field is studied in terms of the quantum electronic wave packet coupled with classical nuclear motions. The equations of motion are derived taking a proper account of molecular interactions with the vector potential of a classical electromagnetic field, along with the nonadiabatic interaction due to the breakdown of the Born-Oppenheimer approximation. With the aid of electronic structure calculations, the present method enables us to track, in an ab initio manner, the dynamics of polyatomic molecules in an intense field. Preliminary calculations are carried out for the vibrational state of LiF and a collision of Li+F under an intense laser pulse, which are limited to the domain of no ionization.     

Phase control of the competition between electronic transitions in a solvated laser dye

Excited state population can be manipulated by resonant chirped laser pulses through pump–dump processes. We investigate these processes in the laser dye LD690 as a function of wavelength by monitoring the saturated absorption of chirped ultrafast pulses. The resulting nonlinear absorption spectrum becomes increasingly complex as the pulse is tuned to shorter wavelengths. However, fluorescence measurements indicate that the excited state population depends weakly on chirp when the pump wavelength is far from the lowest order electronic transition. Using a learning algorithm and closed-loop control, we find nonlinear chirp parameters that optimize features in the transmission spectrum. The results are discussed in terms of competition between excited state absorption and stimulated resonant Raman scattering.