Ultrafast Electronuclear Dynamics ofH2Double Ionization

Abstract

The ultrafast electronic and nuclear dynamics of H2 laser-induced double ionization is studied using a time-dependent wave packet approach that goes beyond the fixed nuclei approximation. The double ionization pathways are analyzed by following the evolution of the total wave function during and after the pulse. The rescattering of the first ionized electron produces a coherent superposition of excited molecular states which presents a pronounced transient H + H − character. This attosecond excitation is followed by field-induced double ionization and by the formation of short-lived autoionizing states which decay via double ionization. These two double ionization mechanisms may be identified by their signature imprinted in the kinetic-energy distribution of the ejected protons.PACS numbers: 33.80. Rv, 33.80.Eh, 42.50.Hz Laser-induced non-sequential double ionization of atoms has been extensively studied since its discovery in the early 1980s [1]. This strong field effect as well as highorder harmonic generation (HHG) in rare gases [2] gave birth to the so-called rescattering model which gives a unified picture of the atomic response [3]. Moreover, this model constitutes the natural framework for the production of attosecond pulses and for the control of ionization in atoms and molecules [4]. On the molecular side, new effects in strong laser fields involve the coupling of the electronic and nuclear motions such as bond softening [5] or charge resonance enhanced ionization [6]. Recently, this coupling was shown to play a determinant role in HHG [7,8,9]. In particular, Lein predicted that more intense harmonics are generated in heavier molecular isotopes [7]. This effect has been demonstrated experimentally using H 2 , D 2 , CH 4 and CD 4 [8]. In addition, molecular vibrations were recently detected with a mÅ sensitivity using HHG in SF 6 [9]. Following a recent theoretical proposal [10], ground state vibrational wave packets of D 2 where also mapped with 20 mÅ sensitivity using selective depletion via tunnel ionization [11].These recent advances show that nuclear motions are fast enough to exercise a decisive influence on the overall electronic response of the molecule in strong fields although the vibrational time scale only belongs to the femtosecond domain. Moreover, the hydrogen molecule exhibits the shortest vibrational period: 7.5 fs. From the theoretical point of view it is therefore highly desirable to develop a unified and realistic theoretical framework where the nuclear dynamics is treated at the same quantum level as the electronic excitation. By treating the internuclear distance R as a full quantum variable, we show that impulsive vibrational excitation also takes place in H 2 . In addition, the rescattering dynamics is basically a two-electron problem which demands a twoactive-electron approach. This approach allows for the description of the interplay between electronic rescattering and nuclear vibrations. Using a single-optical-cycle pulse, we show that the electron rescattering leads during ...

Intensity enhancement in the molecular ionization and dissociation dynamics in the presence of noise

“…This 1+1D model has been tested by others [26][27][28][29][30][31] and by ourselves [32], showing a qualitatively agreement with the experimental measurements. Using this model, we carried out a detailed investigation when the diatomic molecular ion (H 2 + ) is irradiated by an intense infrared laser field in combination with a noise field where either a Gaussian white noise or a color noise is considered.…”

“…The most important feature of this potential is that at large z it falls off like the true Coulomb potential (a0 b00 is the true Coulomb potential), and it was proved previously that the physical characteristics of this model have allowed realistic investigations of the behavior of a H 2 + ion in a laser field [26,29]. But as demonstrated in Ref.…”

Vibrational-state and isotope dependence of high-order harmonic generation in water molecules

“…Full-dimensional calculations have been achieved in the case of H 2 + with the direction of the molecular axis restricted to the laser polarization axis [28]. In most cases, however, reduced dimensional models have been used to study coupled electronic and nuclear motion in strong laser fields [29][30][31][32][33].…”

High-order harmonic generation in vibrating two-electron molecules

“…We study a one-dimensional H 2 model molecule similar to the one used in [33]: the direction of the molecular axis is fixed along the laser polarization axis, and also both electrons are allowed to move along this direction only. We follow here the strategy of Saugout et al [34,35] who adjusted the electron-nuclear and electron-electron interaction potential in this model such that the ground-state Born-Oppenheimer potentials of H 2 and H þ 2 are reproduced. Neglecting mass-polarization terms, the Hamiltonian for the laser-driven molecule with internuclear distance R and electron coordinates x 1 ; x 2 then reads (atomic units are used throughout)…”