Correlated Electron and Nuclear Dynamics in Strong Field Photoionization ofH2+

Abstract

We present a theoretical study of H þ 2 ionization under strong IR femtosecond pulses by using a method designed to extract correlated (2D) photoelectron and proton kinetic energy spectra. The results show two distinct ionization mechanisms-tunnel and multiphoton ionization-in which electrons and nuclei do not share the energy from the field in the same way. Electrons produced in multiphoton ionization share part of their energy with the nuclei, an effect that shows up in the 2D spectra in the form of energy-conservation fringes similar to those observed in weak-field ionization of diatomic molecules. In contrast, tunneling electrons lead to fringes whose position does not depend on the proton kinetic energy. At high intensity, the two processes coexist and the 2D plots show a very rich behavior, suggesting that the correlation between electron and nuclear dynamics in strong field ionization is more complex than one would have anticipated. DOI: 10.1103/PhysRevLett.110.113001 PACS numbers: 33.20.Xx, 33.60.+q, 33.80.Rv The interaction of atoms and molecules with intense infrared laser pulses has been the object of continuous research for more than two decades [1][2][3][4][5][6][7][8][9]. Since the potential induced by such lasers on the electrons is comparable to or even stronger than that generated by the nuclei, the resulting electron dynamics is significantly different from that of the isolated system, which makes these lasers ideal tools to achieve electronic control [10][11][12][13]. Strong fields can efficiently excite and ionize atoms and molecules. The electrons, which can be ejected following either multiphoton absorption or tunneling, can either directly reach the detector after having been repeatedly accelerated and decelerated by the field [direct electrons (DE)] or recollide with the ionic core within an optical cycle [rescattered electrons (RE)] [14,15]. Only a small fraction of the ejected electrons rescatter, but this fraction is responsible for important nonlinear phenomena such as high-harmonic generation (HHG). In this process, high-energy photons are emitted as a result of electron recombination with the ionic core. HHG is currently used to produce ultrashort extreme ultraviolet laser pulses and trains of these pulses [16][17][18][19], and also to uncover multielectron dynamics in atoms and molecules [13,20] or the structure of atomic and molecular orbitals in the so-called orbital tomography [10,21,22].Rescattered electrons that do not recombine with the ion also leave their signature in the photoelectron spectra at relatively high energies, typically between 2U p and 10U p [23,24], where U p ¼ I=4! 2 is the electron ponderomotive energy (in a.u.), I is the laser intensity, and ! its frequency. Because of their high energy, in contrast with that of direct electrons which is 2U p , RE can be used as signal and DE as reference to image atomic and molecular structure by photoelectron holography [20,25].Compared to atoms, the study of strong-field electron dynamics in molecules, in particular ...

Molecular resolvent-operator method: Electronic and nuclear dynamics in strong-field ionization

“…We have additionally checked that the total ionization probability obtained with our method is almost identical to those obtained with other methods such as the virtual detector method [49]. The differential ROM will allow us to track the transition from the multiphoton regime (γ > 1) to the tunneling regime (γ 1) [16]. This is shown in Fig.…”

“…As shown in Ref. [16], this is due to electrons resulting from tunnel ionization, where the molecular character is partly lost. The intermediate situation [ Fig.…”

“…A recently proposed alternative to those two methods is the ROM for molecules [16]. From the ROM one can extract the differential energy spectra for electrons and nuclei, or, as we show here, the absolute total probability density as a function of the total energy of the system.…”

“…As one of the properties of these harmonics is coherence, the harmonic spectrum contains information about the molecular structure and the induced dynamics [12,13]. The measurement of the kinetic energy of the electrons that escape forever (i.e., do not recombine) and the associated molecular cation (EKE and KER) allows one to study time-dependent processes such as the nuclear dynamics in, e.g., H 2 [14,15] or H 2 + [16][17][18][19]. The KER spectra for H 2 + ionization show peculiar structures, for which different mechanisms, such as CREI or above-threshold Coulomb explosion, have been proposed [20][21][22][23].…”

“…The measured ionization yield plotted as a function of both the electron-and ion-kinetic energies [hereafter called for short correlated kinetic energy (CKE) spectrum] thus exhibits the electronic and nuclear dynamics, as well as the connection between them [14,15,19,33]. While most theoretical works on molecules interacting with strong IR fields have reported either KER or EKE spectra, recently, few works (experimental and theoretical) have shown CKE spectra resulting from H 2 + and H 2 photoionization [16,19,34]. These CKE spectra show clearly how the energy is shared between the electron and the protons.…”