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Dissociative Ionization of Molecules in Intense Laser Fields
| Content Provider | Semantic Scholar |
|---|---|
| Author | Dundas, Daniel McCann, James F. Taylor, Kenneth T. |
| Abstract | We present the results of simulations of the dynamics of the hydrogen molecular ion subjected to intense ultrashort laser pulses. We study the dissociative ionization process for one-color and two-colour fields. We calculate multiphoton ionization rates and harmonic generation rates including the vibrational nuclear motion quantally and without making the Born-Oppenheimer approximation. We find twocolor high-order wavemixing processes are strongly affected by the phase difference between the light sources. We demonstrate that the dissociation process greatly amplifies high-order harmonic generation, and thus that nuclear dynamics are essential in the understanding of this process. INTRODUCTION The energy and angular momentum carried by light readily disturbs the electronic states of molecules. Electronic excitations produced in this interaction are transferred to the nuclear motion and result in rotational heating and large-amplitude molecular vibration of the system often leading to dissociation. At high frequency or high intensity, electron ejection becomes a dominant process. Photoionization will occur rapidly over timescales much shorter than vibrational relaxation times, and the chemical stability of the molecule is destroyed in a Coulomb explosion. Although the molecule in unstable under such conditions, the interaction with the oscillating radiation field gives rise to laser-molecule dressed states which have characteristics of both the laser and the molecular bound and continuum states. The decay of these states, observed by particle energy spectroscopy, is a key to understanding their nature and behaviour. However, even in simple diatomic molecules the complexity of the interactions leads to rich variety in ionic fragment charges and energies [1,2] which are dependent on laser pulse intensity and duration. Inevitably the analysis of the data produced in experimental studies is obscured by the light intensity and particle density variations over the focal volume of the interaction region. And although a great deal of data has been acquired for large molecules [3], an understanding of the detailed underlying mechanisms in simple diatomic systems has remained elusive. The problem is inherently difficult, for experimentalists and theorists, owing to the proliferation of many-body continuum channels. So far laboratory studies have concentrated on simple diatomic systems through the observation of fragment ions using techniques such as covariance mapping [1]. While theory has been less well developed, nonetheless one can make some progress with the interpretation of such data with simplified models which treat the ionization process as a sequence of electron release processes, neglecting the electron-electron interactions. The treatment of multielectron systems is a formidable task and only recently have the details of the dynamics begun to emerge from three-dimensional simulations for atoms [4]. Examples of the multiphoton dynamics of the helium atom are presented in this volume [5]. Simulation of the dynamics of multiphoton dissociation has been extensively studied by theory and is reviewed in other papers [2,6]. In this paper we study dissociative ionization by ultrashort ultra-intense laser pulses, with a view to the important mechanisms and to produce quantitative data that can be used to compare with experimental studies. We consider the simplest molecule, the hydrogen molecular ion, and approach the problem by numerical solution of the time-dependent Schrödinger equation (TDSE). MODEL Let us begin with some well-known facts [7] regarding the molecule in question. The ground-state of the hydrogen molecular ion has the electronic configuration Σg . Using atomic units, this state is characterized by a bond length Re = 2.0, with vibrational constants h̄ωe = 1.05×10 −2 and h̄ωexe = 2.9×10 , and rotational constant Be = 1.36 × 10 . The dissociation energy is D0 = 0.597, while the ionization potential of the molecular ion at equilibrium internuclear separation is 0.602 . We solve the TDSE for the interaction of an ultrashort intense laser pulse interacting with the hydrogen molecular ion. The nuclear vibrational motion is described quantally and the electronic motion is considered in its full dimensionality. If the laser pulse duration is relatively short compared with rotational timescales for this molecule (Trot ∼ 1/Be ∼ 10 ), then laser-molecule interaction be regarded as ‘sudden’ in comparison to the rotation of the system. The neglect of rotation is reasonable on such timescales which are characteristic of vibrational dynamics and often termed ultrafast in chemical or biological contexts. In this paper, we consider parallel electronic transitions from the ground state, that is we consider the field component along the internuclear axis. At low frequencies the process is dominated by the large dipole moment along the molecular axis giving rise to molecular alignment [2]. Essentially this is a result of the large energy gap between the ground (Σg) and first excited state (Σu) so that for low frequencies the most strongly offresonant processes are driven by parallel transitions. This simplifies the electronic dynamics since only Σ symmetries are coupled. Thus the electronic wavefunction has cylindrical symmetry and the evolution occurs in only two-dimensions. Specifically we construct a 2D-grid in the radial and axial coordinates and discretize the wavefunction on this grid. Following Fujimura and co-workers [8–10], we use scaled cylindrical co-ordinates and adopt atomic units. Thus the electron position vector is given by r = ρ cosφi + ρ sinφj + zk. (1) As in the paper of Kono [8] we choose ` = 3/2 so that the resulting wavefunction is analytic at ρ = 0. Since, for Σ-symmetry, the azimuthal (φ) dependence does not arise and the Hamiltonian has the form H = − 1 mp ∂ ∂R2 − 1 2μ { ∂ ∂z2 + 1 `2ρ2 ∂ ∂ρ ρ ∂ ∂ρ } + V (ρ, z, R) + U(z, t), (2) where μ = 2mpme/(2mp +me) is the reduced mass, me and mp are respectively the electron and nuclear masses, V (ρ, z, R) represents the coulomb interactions V (ρ, z, R) = − Z1 |
| File Format | PDF HTM / HTML |
| Language | English |
| Access Restriction | Open |
| Subject Keyword | Above threshold ionization Alignment AngularJS Apache Axis Apache Continuum Approximation Auriculo-Ventricular Dissociation Axis vertebra Complexity Concentrate Dosage Form Control theory Covariance mapping Dimensions Discretization Electron tomography Energy, Physics Excited state FOCAL (programming language) Ground state Hamiltonian (quantum mechanics) Heating Helium Hydrogen Interaction Ionic Ions Iontophoresis Light dressed state Linear programming relaxation Many-body problem Numerical partial differential equations Optic axis of a crystal Paper Photoelectrochemical process Pulse duration Radial (radio) Radiation Schrödinger Simulation Triune continuum paradigm Unstable Medical Device Problem light intensity |
| Content Type | Text |
| Resource Type | Article |
