Dissociative ionization of molecules in intense laser fields: a method of predicting ion kinetic energies and appearance intensities

Abstract

We pment a model lo describe certain aspens of the multiphoton multielectron dissociative ionization of simple molecules m tnteme laser fields. We obtain fragment ion dissociation energies and appearance Intensities that are in good agreement with experiment.The multielectron dissociative ionization of diatomic and triatomic molecules in intense laser fields (focal intensities W cm-*) is not understood. Recent experiments indicate that all fragment ions emerge with dissociation energies which are specific fiactions (-70% for 12. -45% for NI, etc) of the Coulomb explosion energy, regardless of their charge state (see, for example, Frasinski et , Hatherly et a[ 1994, Schmidt er al 1994. This Coulomb explosion energy, E,, is given, in the diatomic case, by the expression E, = Ql Q2/RP (in atomic units), where Q, and Q 2 are the atomic ion charges and Re is the equilibrium internuclear separation of the neutral molecule. One might argue that the reduction in Coulomb energy is caused by different screening at each ionization stage (a reduction in Ql and Q2) at Re. A simpler explanation is that the molecule (molecular ion) relaxes to a critical distance, R,, where it stabilizes and subsequently ionizes, with little or no change in inter-ion separation for the different ionization stages. For example, for I1 (Re = 2.67 A) a value of R, of 3.8 %, (= 2.67/70%) would explain the observations. We have developed a simple model which allows us to predict the value of R, for any ionization stage of a diatomic (or linear triatomic) molecule. The only data required by the model are the atomic ionization potentials. We find, quite surprisingly, that the value of Rc is virtually independent of the stage of ionization of the molecule and that using this value of R, reproduces the experimental values for the dissociation energies of the various fragment ion pairs. Moreover, the model is able to predict appearance (threshold) intensities for 12 that agree well with the experimental data of Strickland er al (1992).Consider the molecular ion I: dissociating in a laser electric field of steadily rising amplitude, with the molecular axis lying along the field direction. We focus our attention on the outer electron, and combine the remaining electrons and nuclei into two point-like atomic ions. Initially the outer electron can move freely in the double well, but as the ions move apart, the inner barrier rises and at a certain, critical internuclear distance it starts to impede the motion of the electron: the electron can no longer follow the field and the system makes a transition to a non-dissociative state (see later discussion). The situation that

Dissociative ionization and Coulomb explosion of CH3I in intense femto second laser fields

“…It was inferred that CE occurs at a larger critical bond length R C due to bond softening induced by the laser field [10,15,16]. Various models have been developed to explain this reduction in Coulomb energy, for example, the two-step model [10], the Thomas-Fermi-Dirac (TFD) model [17], the charge transfer enhanced ionization (CREI) model [15,16,[18][19][20][21] and the Molecular Ammosov-Delone-Krainov (MO-ADK) model [22][23][24]. The temporal rise of the E-field amplitude laser pulse brings about a weakening of the binding energy of the valence electrons to the molecules.…”

“…Consequently, the molecule ionizes predominantly at R ≈ R C . The fragment ions born out of such a CE usually give rise to an anisotropic angular distribution which favors the direction of the polarization vector of the laser field [18,[25][26][27][28][29][30][31][32][33][34]. Such anisotropic angular distributions are characteristic for a CE based on a dynamic or a geometric alignment mechanism or on a combination of these two.…”

“…The interaction between this induced dipole moment and the E-field amplitude of the laser field pre-aligns the parent molecular axis to the laser's Efield amplitude until the molecular bond stabilizes. Subsequently a Coulomb Explosion occurs after which the ionic fragments recoil direction is parallel to the E-field amplitude of the laser pulse [18,[25][26][27][28][29].…”

Counterintuitive Alignment ofH2+in Intense Femtosecond Laser Fields

“…It was concluded that even in the heavy I 2 molecule a dynamic alignment must take place, or at least the laser must impart a substantial impulse to the fragments [4,5]. However, these ideas were formulated prior to the discovery of enhanced ionization at the critical internuclear separation [6][7][8]. As the molecule dissociates with about twice the equilibrium separation, the ionization threshold drops by about an order of magnitude if the molecule is parallel to the field but stays almost constant if the molecule is in an orthogonal orientation.…”

Photon–ion collisions and molecular clocks

“…The double ionization of molecular hydrogen through enhanced ionization is well documented in the literature and heavily studied theoretically [11][12][13][14]. In recent years two additional double ionization processes have been identified and modelled: rescattering ionization [15][16][17][18][19][20][21] and sequential ionization [22,23].…”