Molecules In Intense Laser Fields Research Articles

Sequential nonadiabatic excitation of large molecules and ions driven by strong laser fields

Electronic processes leading to dissociative ionization of polyatomic molecules in strong laser fields are investigated experimentally, theoretically, and numerically. Using time-of-flight ion mass spectroscopy, we study the dependence of fragmentation on laser intensity for a series of related molecules and report regular trends in this dependence on the size, symmetry, and electronic structure of a molecule. Based on these data, we develop a model of dissociative ionization of polyatomic molecules in intense laser fields. The model is built on three elements: (i) nonadiabatic population transfer from the ground electronic state to the excited-state manifold via a doorway (charge-transfer) transition; (ii) exponential enhancement of this transition by collective dynamic polarization of all electrons, and (iii) sequential energy deposition in both neutral molecules and resulting molecular ions. The sequential nonadiabatic excitation is accelerated by a counterintuitive increase of a large molecule's polarizability following its ionization. The generic theory of sequential nonadiabatic excitation forms a basis for quantitative description of various nonlinear processes in polyatomic molecules and ions in strong laser fields.

Non-Hermitian Quantum Mechanics for High-Order Harmonic Generation Spectra

High-order harmonic generation spectra of atoms and molecules in intense laser fields show a wide plateau of odd harmonics, with photon energies high up to the soft X-ray regime. The mechanism leading to the absence of other frequencies, so-called hyper-Raman lines or sidebands, is still unclear. By the use of non-Hermitian quantum mechanics for the many-electron atomic/molecular Hamiltonian system, we derive closed-form expressions for the high-order harmonic generation spectra. The analysis of these expressions provides the (already known) conditions for which the hyper-Raman lines are missing and also the conditions for which they are observable. The new information obtained here is proof that in order to observe sidebands it is not sufficient to populate two different Floquet states. When the duration of the laser pulse is sufficiently long, the two resonance Floquet states should have similar widths (inverse lifetimes). For short pulses, the condition for observable sidebands is more complicated, and...

Description of molecular dynamics in intense laser fields by the time-dependent adiabatic state approach: application to simultaneous two-bond dissociation of CO2 and its control.

We theoretically investigated the dynamics of structural deformations of CO(2) and its cations in near-infrared intense laser fields (approximately 10(15) W cm(-2)) by using the time-dependent adiabatic state approach. To obtain "field-following" adiabatic potentials for nuclear dynamics, the electronic Hamiltonian including the interaction with the instantaneous laser electric field is diagonalized by the multiconfiguration self-consistent-field molecular orbital method. In the CO(2) and CO(2+) stages, ionization occurs before the field intensity becomes high enough to deform the molecule. In the CO(2)(2+) stage, simultaneous symmetric two-bond stretching occurs as well as one-bond stretching. Two-bond stretching is induced by an intense field in the lowest time-dependent adiabatic state |1> of CO(2)(2+), and this two-bond stretching is followed by the occurrence of a large-amplitude bending motion mainly in the second-lowest adiabatic state |2> nonadiabatically created at large internuclear distances by the field from |1>. It is concluded that the experimentally observed stretched and bent structure of CO(2)(3+) just before Coulomb explosions originates from the structural deformation of CO(2)(2+). We also show in this report that the concept of "optical-cycle-averaged potential" is useful for designing schemes to control molecular (reaction) dynamics, such as dissociation dynamics of CO(2), in intense fields. The present approach is simple but has wide applicability for analysis and prediction of electronic and nuclear dynamics of polyatomic molecules in intense laser fields.

Alignment of CS2 in intense nanosecond laser fields probed by pulsed gas electron diffraction

A pulsed gas electron diffraction apparatus was developed and applied to investigate an alignment process of molecules in intense laser fields. A two-dimensional (2D) electron diffraction pattern of jet-cooled CS2 in intense nanosecond laser fields (1064 nm, ∼0.64 TW/cm2, 10 ns) was measured using short-pulsed 25 keV electron beam packets (∼7 ns) generated by irradiating a tantalum photocathode with the 4th harmonics of pulsed YAG laser light. The observed anisotropic 2D diffraction pattern was analyzed quantitatively by taking into account the spatio-temporal distributions of the laser pulses, the electron beam packets, and the molecular beam through a numerical simulation of the observed diffraction pattern. The anisotropy of the spatial distribution of molecular axes of CS2 in the laser polarization direction is accounted for by the effect of the intense laser fields. Considering the spatio-temporal averaging effect, the temporal pulse width of an electron packet required for real-time probing of the alignment process of molecules in intense nanosecond laser fields is discussed. A numerical simulation of temporal and spatial profiles of an electron packet is also performed to examine conditions for generating sub-picosecond ultrashort electron pulse for real-time probing of ultrafast molecular dynamics by the pulsed gas electron diffraction method.

Interference effects in above-threshold ionization

We discuss four recent photoionization experiments with atoms and molecules in intense laser fields. So far, photoionization under these conditions was considered to be amenable to classical arguments. In contrast with this, the effects described here are due to quantum interference and, accordingly, have to be described by a quantum-mechanical theory.

Intense laser induced field ionization of C2H2, C2H4, and C2H6

Using HOMO Field Ionization Model, the tunneling probabilities and the theoretical threshold intensities of the field ionizations of acetylene, ethylene, and ethane in intense laser field are calculated. C2H2, C2H4, and C2H6 were irradiated by 800 nm, 100 fs laser pulses with the intensity range of 1013–1014 W/cm2. A TOF-mass spectrometer was coupled to the laser system and used to experimentally investigate the field ionization of these molecules. The experimental ionization threshold intensities are obtained. The calculating results of the three molecules agree well with the experimental results, indicating that HOMO Field Ionization Model is valid for the ionization of polyatomic molecules in intense laser field.

Interference Effects in Above-Threshold Ionization

We discuss four recent photoionization experiments with atoms and molecules in intense laser fields. So far, photoionization under these conditions was considered to be amenable to classical arguments. In contrast to this, the effects described here are due to quantum interference and, accordingly, have to be described with a quantum mechanical theory.

Multiphoton selective dissociation of a HF molecule in intense laser fields by the Lie-algebra approach

The multiphoton selective dissociation of a HF molecule is studied by means of the Lie-algebra method using a quadratic anharmonic Hamiltonian. The averaged absorbed energy in a shaped laser pulse is calculated. The selective dissociation of the target states in chirped and shaped infrared laser fields is discussed. The paper shows that Lie algebra can be employed to discuss multiphoton dissociation.

Lie algebraic approach to multiphoton rovibrational transition of diatomic molecule in intense laser fields

The multiphoton rovibrational excitation of diatomic molecule in intense laser fields is studied by using simulated potential and Lie algebraic approach. The rovibrational transition probability as a function of external field frequency and time, respectively, for the molecules HF, NO and LiH is calculated. The influences of the rotation on the multiphoton processes are investigated.

A Theory of Coulomb Explosion of Molecules

AbstractIn this paper, a theory of Coulomb explosion of molecules in intense laser fields based on the Auger effect will be presented. We first show how to calculate the rate of Auger process and then show that multiple ionization processes can be induced by the Auger effect alone or by the Auger effect followed by autoionization. For the case of molecules, multiple ionization is in general followed by dissociation.

Spatial alignment of diatomic molecules in intense laser fields: II.Numerical modelling

The angular distributions of ionic fragments produced bymulti-electron dissociative ionization of diatomicmolecules are calculated using a field-ionization Coulombexplosion model that includes dynamic rotation of themolecule in the laser field. The majority of dynamicalignment occurs on the leading edge of the laser pulseat low intensities before the laser intensity reaches thedissociative ionization threshold. This makes the degreeof alignment sensitive to the precise value of thedissociative ionization threshold. Measuring the totalion signal for different pulse-lengths enablesangle-dependent dissociative ionization to bedistinguished from dynamic alignment. Increasedalignment with long pulses is an unambiguous sign thatthe molecules are forced into alignment with laser field.

Spatial alignment of diatomic molecules in intense laser fields: I.Experimental results

We present measurements of the laser induced spatialalignment of two diatomic molecules, iodine (I2) andbromine (Br2). Dynamic alignment is inferred from theangular distribution of the ionic fragments frommulti-electron dissociative ionization (MEDI). Theangular distributions were determined for different pulsedurations and energies of the short infrared laser pulsesthat induce the MEDI, and were measured using a velocitymap ion imaging detector. The width of the angulardistribution of the fragments with respect to the laserpolarization axis depends only weakly on the laser pulseenergy, and decreases rapidly for longer pulse lengths (afew picoseconds) at constant pulse energy. Theinterpretation of this result in terms of dynamicalignment is supported by an extended field ionizationCoulomb explosion model that includes the rotation of themolecule induced by the laser field.

Angular distributions of the fragments from coulomb explosions of diatomic molecules in intense laser fields

The angular distributions of the fragments from a Coulomb explosion of a diatomic heteronuclear molecule during multielectron dissociative ionization in a superintense field are considered in terms of classical mechanics. The patterns of angular distributions of the Coulomb explosion fragments are shown to differ in different ranges of laser pulse parameters. In particular, there are two distinct modes of fragment separation: separation in a Coulomb field and separation in the field of an effective “fragment + field” potential. The effective potential includes both the force of Coulomb repulsion between the fragments and the period-averaged force exerted on the system by the field; it can be determined by using the Kramers-Henneberger method. The limits of applicability of the Kramers-Henneberger method to the problem in question are discussed. These limits specify the range of field parameters in which the fragments fly apart in a direction perpendicular to the field for the initially arbitrary orientation of the molecular axis relative to the field.

Moving adaptive grid methods for numerical solution of the time-dependent molecular Schrödinger equation in laser fields

We present a moving adaptive grid method for solving the time-dependent Schrödinger equation, TDSE, for molecules in intense laser fields, applicable in the nonperturbative nonlinear regime where dissociation ionization occurs. The method is based on a Lagrangian, moving coordinate system. In this representation, the reference system is moving with the laser pulse so that the classical movement of free particles in the field, i.e., in the asymptotic region where electron–molecule potentials are negligible but the laser field is still present, is exactly described. As a consequence, the asymptotic quantum wave functions are exact in presence of a laser pulse. We have tested several discrete propagator methods for the TDSE in different gauges in a Born–Oppenheimer simulation of H2+ in a short, intense laser pulse. Our comparison of convergence between the same discretization methods for different gauges have demonstrated the superiority of the present Lagrangian adaptive grid method to treat the response of molecules to intense time-dependent electromagnetic fields.

Generalized space translation and new numerical methods for time-dependent Schroedinger equations of molecules in intense laser fields

We present new numerical methods, based on a Generalized Space Translation representation (GST), for solving the Time-Dependent Schroedinger equation (TDSE), to treat the nonlinear, nonperturbative interaction of molecules with intense laser pulses. Adopting a Lagrangian, moving coordinate system, we generalize the previous Space Translation method (ST), used in atom–laser interaction problem. In this representation (gauge), the reference system is moving with the laser pulse so the classical movement of free particles in the field, i.e. in the asymptotic region where electron-molecule potentials are negligible but the laser field is still present, is exactly described. As a consequence, the asymptotic quantum wave functions are exact in presence of the laser pulse. To solve numerically the GST–TDSE, all standard discrete propagators for the time discretization and all efficient space discretization methods can be applied. In order to illustrate different possibilities for the choice of discretization methods, we have tested several types of split-operator (SO) and alternating direction implicit (ADI) methods combined with adaptive finite difference methods for the 3D Born–Oppenheimer simulation of H 2 + in a short intense laser pulse field. Our comparison of convergence between the same discretization methods for different gauges have demonstrated the superiority of the GST method. As examples, we present for the first time the ionization of H 2 + exposed to a circularly or linearly polarized intense laser pulse perpendicular to the internuclear axis by an exact 3D simulation.

Intense-field laser ionization rates in atoms and molecules

We discuss experiments on the ionization of atoms and molecules in intense laser fields, designed to facilitate a comparison of theory and experiment especially in the case of complex molecules. We concentrate on ionization in the regime where single ionization dominates. We describe an approach which allows a direct measurement of ionization efficiencies, absolute ionization rates, and well-defined saturation intensities. In addition, intensities where multiple ionization and Coulomb explosion set in are determined. We also analyze how ion yields are sensitive to any dependence of ionization rates on molecular alignment. Application of these procedures to molecular ionization is demonstrated in detail for the organic molecules benzene and cyclohexane.

Intense Optical Field Science. Dynamics of Molecules in Intense Laser-Light Fields: New Research Directions.

In intense laser light fields, molecules undergo a variety of characteristic processes such asalignment along laser polarization direction, dressed state formation, ultrafast structural deformation, multiple ionization, and Coulomb explosion. The investigation of the dynamical behavior of molecules in intense laser fields has afforded us invaluable opportunities to understand fundamentals of interaction between molecules and light fields as well as to manipulate molecules using characteristics of laser light fields. In the present article, I would like to describe the important aspects of this rapidly growing interdisciplinary research fields, called Molecular Science in Intense Laser Fields, and propose new directions for the next frontiers by introducing our recent studies based on (i) a quasi-stationary Floquet approach, (ii) pulsed gas electron diffraction measurements, and (iii) tandem TOF mass spectroscopy.

Fragment-emission patterns from the Coulomb explosion of diatomic molecules in intense laser fields

We have systematically studied emission patterns of fragments arising from the Coulomb explosion of ${\mathrm{N}}_{2},$ ${\mathrm{Cl}}_{2},$ and ${\mathrm{I}}_{2}$ molecules when exposed to intense, linearly polarized laser fields. The experiments are performed at 395, 610, and 790 nm, using two different pulse durations (130 fs and 2 ps) and choosing two different peak intensities ${(10}^{15}$ and $2\ifmmode\times\else\texttimes\fi{}{10}^{16}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}).$ We show that the anisotropy of the emission patterns increases with the number of absorbed photons necessary to produce the parent molecular ion. The set of experimental results suggest that the alignment process occurs in the leading edge of the laser pulse at moderate intensities $(<{10}^{14}{\mathrm{W}\mathrm{}\mathrm{cm}}^{\mathrm{\ensuremath{-}}2})$ where the multiphoton regime prevails. We explain these observations in terms of the rotational pumping model, as demonstrated by statistical simulations of multiphoton absorption and emission. The present study confirms that, in most cases, the confined emission of fragments along the laser electric field is due to the alignment of the transient molecular ions.

Lie algebraic approach to multiphoton excitation of diatomic molecules in intense laser fields

The quadratic anharmonic oscillator Lie algebraic model is used to study the multiphoton transition of the diatomic molecule placed in intense laser fields. The multiphoton excitation of vibration and vibration-rotation of diatomic molecules in intense laser fields are discussed. In the pure vibration transition we calculate the transition probability versus the frequency of the laser fields for the CO molecule. We also investigate the roles of rotational motion in multiphoton processes and compare with pure vibration for the LiH molecule. The influences of the angular quantum number l and the molecular orientations in laser fields on the multiphoton processes are discussed. The averaged absorb energy changing with the laser field's frequency is calculated. ©1999 John Wiley & Sons, Inc. Int J Quant Chem 71: 201–207, 1999

Observing the Transition from a Multiphoton-Dominated to a Field-Mediated Ionization Process for Polyatomic Molecules in Intense Laser Fields

The photoelectron spectra for benzene, naphthalene, and anthracene ionized using a 780 nm, 170 fs laser pulse of intensity $3.8\ifmmode\times\else\texttimes\fi{}{10}^{13}\mathrm{W}{\mathrm{cm}}^{\ensuremath{-}2}$ reveal a decrease in the discrete features associated with above threshold ionization with increasing molecular size. A corresponding exponential increase in the intensity of a broad featureless distribution indicates that field ionization dominates the coupling mechanism for the larger molecules. A coupling model based on a zero-range potential predicts that only multiphoton ionization should occur. This model, incorporating both the ionization potential and electronic delocalization of the molecule, correctly predicts the observed transition.