The nonlinear nonperturbative response of atoms in intense laser fields has been extensively studied both experimentally and theoretically in the past twenty years leading to new unexpected effects such as Above Threshold Ionization, ATI, high order frequency generation etc. and these are documented in recent book The similar studies of molecules is a new chapter in the pursuit of laser control and manipulation of molecules. The nonlinear nonperturbative response of molecules to intense (I<1015 W/cm2 ) and ultrashort (V10 fs) laser pulses [2] is expected to yield new effects due to the extra degrees of freedom nuclear motion as compared to atoms [3], such as creation of Laser Induced Molecular Potentials, LIMP' s, Charge Resonance Enhanced Ionization, CREI [4] and molecular High Order Harmonic Generation [5]. These nonlinear nonperturbative in effects were seen in experiments [6] and were predicted and confirmed by high-level numerical simulations of appropriate time-dependent Schrodinger equations [3-5,7], TDSE's, of molecules in laser fields. Our recent supercomputer simulations of H2+ molecule dynamics in intense laser fields, [7-9] based on TDSE, also allowed us to propose two new molecular imaging techniques: a) LCEI, Laser Coulomb Explosion Imaging [8] and b) LPEI, Laser Photoelectron Imaging [9]. The first is based on the analysis of the kinetic energy of molecular fragments after Coulomb Explosion, CE, whereas the latter imaging uses the shape of ATI electron peaks, produced by an intense laser pulse. We describe summarily in the present communication these two imaging methods which were developed using high level supercomputer simulations
This paper is devoted to the dynamics of attosecond pulses created during the high order harmonic generation process. In this goal we study Ti:sapphir laser pulses propagating in a H2+ gas. The dynamics
and propagation of the incident pulse is obtained by solving the macroscopic Maxwell equations. The
molecular gas reaction on the electric field, the polarization, is derived from TDSE's following the model
presented in [9], [10]. We are especially interested in this work, in the attosecond pulse dynamics and the
intensity of the first harmonics dependently of the propagation length inside the gas, on the attosecond
pulse generation and propagation and the energy of return graphs in function of the driver phase.
A theoretical study is presented suggesting possible photoelectron spectroscopy measurement of electron dynamics
on molecules on "attosecond" time scales. In particular, we study analytically the photoionization of
a coherent superposition of molecular electron states by an ultrashort, attosecond X-ray pulse. We show that
the broad photoelectron spectrum inherent in attosecond pulse ionization contains detailed information about
the time-dependent electron wave packet. Such ultrashort pulse photoionization leads to an asymmetry of the
photoelectron momentum distribution and this seems to measure the momentum asymmetry of the initial coherently
prepared electron bound state. We show further that universal molecular Coulomb interference effect occur in ultrashort pulse photoionization and other processes with fast momentum transfers. Thus "attosecond"
photoelectron spectra and their inherent asymmetry allow to monitor and in principle to control electron motion
on the attosecond time scale.
We consider a d-dimensional conductor (a superlattice) within the independent-electron one-band approach
taking into account slow relaxation processes. Its nonperturbative response to time-periodic electric fields
is studied in the nearly coherent regimes: a) dynamic (short-time) and b) kinetic (long-time). We provide a
classification and analysis of field-induced dynamic localization and response through the dc/ac current and mean
square displacement of electrons. We demonstrate that the overall localization increases in passing from the
periodic regime through the commensurate to the incommensurate one (governed by the relation of field period
and Bloch frequency) both in the dynamic and kinetic cases. Simultaneously, exceptional localization (for some
particular values of field parameters or symmetries) typically retains its order in the small relaxation rate, but on
the background of increasing overall localization becomes less pronounced, both in dynamic and kinetic regimes.
In the dynamic regime exceptional localization is manifested through diffusion and dc response, in the kinetic - through diffusion and ac response. The commensurate case with long-range-overlap qualitatively resembles the periodic one; within nearest-neighbor approximation the commensurate regime becomes qualitatively analogous to the incommensurate one. We also obtain explicit analytic solutions for the dynamic and kinetic harmonic generation spectra. We demonstrate that in strong and smooth fields the harmonic spectrum has a plateau-like shape. In the kinetic regime all the harmonics vanish under "exceptional" localization, whereas in the dynamic regime there is no such effect. In a strong dc field there is an effect of weak ac harmonics amplification. Coherent control of induced localization and harmonic generation is discussed.
The alignment dynamics of HCN in intense laser fields are studied numerically, using time-dependent quantum mechanics. The alignment, with respect to the laser field polarization vector, is measured from the angular distribution of the molecule using the half-angle (theta) 1/2. It is shown that it can be achieved on a sub-picosecond timescale. The role of the vibration of the two molecular bonds is investigated, as well as the effect of the permanent and field-induced dipole moments on alignment.
Numerical solutions of the time-dependent Schroedinger equation for the molecular ion H2+ in intense laser fields, I approximately equals 1014 W/cm2 and long wavelengths, (lambda) equals 1064 nm lead to the conclusion that electron motion can be controlled at these high intensities as a function of laser phase difference for two color photoionization, either in (omega) + 3(omega) or (omega) + 2(omega) combinations. A new phenomenon, charge resonance enhanced ionization, CREI, has been discovered and can be rationalized in terms of a tunneling model. Finally, the ionized electron is rescattered by the molecular ion core at long laser wavelengths, thus suggesting the use of such rescattering to study laser- induced electron diffraction, LIED, in order to 'clock' time-dependent nuclear dynamics.
We use penalty methods derived from Augmented Lagrangians coupled with unitary exponential operator methods to solve the optimal control problem for molecular time-dependent Schodinger equations involving laser pulse excitations. A stable numerical algorithm is presented which propagates directly from initial states to given final states. Results are reported for an analytically solvable model for the complete inversion of a three-state system.
A coupled equations approach based on a general artificial channel method is used to demonstrate two color laser coherent control of photopredissociation and photoionization in Cl2. Photopredissociation angular distributions illustrate the suppression of the corresponding transition in selective rotational states.
We examine two consequences of the unique behavior of molecular ions in intense laser fields. First, laser-induced crossings can be created with femtosecond laser pulses resulting in trapping of the molecular ion in high-field induced potential wells. We demonstrate this by numerical solutions of the time-dependent Schroedinger equation for H2+. Second we show results of numerical calculation with vibrationally excited H2+ of harmonic generation (HG), which can be enhanced by the trapping mechanism. We show next that in molecular ions, two plateaus appear in the harmonic generation spectrum, one of a molecular nature and the other atom like. Finally, we present an example of phase control of HG which can be explained in terms of a classical model.
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