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Physical Review A

Control of electron localization in the dissociation of H 2+ using orthogonally polarized two-color sequential laser pulses

Feng He

Published Dec 20, 2012• 34 citations
Publisher

Abstract

Orthogonally polarized two-color sequential laser pulses are used to control the electron localization in the dissociation of H${}_{2}{}^{+}$. The first single attosecond pulse, whose polarization axis is perpendicular to the molecular axis, excites H${}_{2}{}^{+}$ from $1s{\ensuremath{\sigma}}_{g}$ to $2p{\ensuremath{\pi}}_{u}$, and the time-delayed infrared pulse, whose polarization axis is parallel to the molecular axis, steers the electron between two nuclei. The simulation of the time-dependent Schr\"odinger equation predicts the control degree of the electron localization can be up to 90$%$ with the current laser technology. In this article, we reveal that the mechanism for this asymmetric localization is due to the mixture of $2p{\ensuremath{\pi}}_{g}$ and $2p{\ensuremath{\pi}}_{u}$, instead of $1s{\ensuremath{\sigma}}_{g}$and $2p{\ensuremath{\sigma}}_{u}$ in the previous studies.

DOI
10.1103/physreva.86.063415

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Orthogonally polarized two-color sequential laser pulses are used to control the electron localization in the dissociation of H2+{}_{2}{}^{+}2​+. The first single attosecond pulse, whose polarization axis is perpendicular to the molecular axis, excites H2+{}_{2}{}^{+}2​+ from 1s\ensuremathσg1s{\ensuremath{\sigma}}_{g}1s\ensuremathσg​ to 2p\ensuremathπu2p{\ensuremath{\pi}}_{u}2p\ensuremathπu​, and the time-delayed infrared pulse, whose polarization axis is parallel to the molecular axis, steers the electron between two nuclei. The simulation of the time-dependent Schr"odinger equation predicts the control degree of the electron localization can be up to 90% with the current laser technology. In this article, we reveal that the mechanism for this asymmetric localization is due to the mixture of 2p\ensuremathπg2p{\ensuremath{\pi}}_{g}2p\ensuremathπg​ and 2p\ensuremathπu2p{\ensuremath{\pi}}_{u}2p\ensuremathπu​, instead of 1s\ensuremathσg1s{\ensuremath{\sigma}}_{g}1s\ensuremathσg​and 2p\ensuremathσu2p{\ensuremath{\sigma}}_{u}2p\ensuremathσu​ in the previous studies.
The single ionization of H2{\mathrm{H}}_{2}H2​ in strong laser fields creates the correlated electron-H2+{{\mathrm{H}}_{2}}^{+}H2​+ pair. Based on such a correlation, we conceive a strategy to control the energy spectra of the freed electron or dissociative fragments by simulating the time-dependent Schr\"odinger equation. Two attosecond pulses in a train produce the replica of electron-H2+{{\mathrm{H}}_{2}}^{+}H2​+ pairs, which are to be steered by a time-delayed phase-stabilized (mid)infrared laser pulse. By controlling the behavior of the freed electron, the dissociation of H2+{{\mathrm{H}}_{2}}^{+}H2​+ can be controlled even though there is no direct laser-H2+{{\mathrm{H}}_{2}}^{+}H2​+ coupling. On the other hand, the photoelectron energy spectra can be manipulated via laser-H2+{{\mathrm{H}}_{2}}^{+}H2​+ coupling. This study demonstrates the entanglement of molecular quantum wave packets, and affords a route to remotely control molecular dissociative ionization.
Within the Born-Oppenheimer approximation, we build a multichannel model to simulate the dissociation of H2+{\mathrm{H}}_{2}{}^{+}H2​+ in strong laser fields. Two applications of this model are presented: first, orthogonally polarized UV and infrared pulses are used to dissociate H2+{\mathrm{H}}_{2}{}^{+}H2​+ into $2p{\ensuremath{\pi}}_{u}$ and $3d{\ensuremath{\pi}}_{g}$ states, whose superposition results in asymmetric electron localization after the dissociation; second, two orthogonally polarized UV pulses are introduced to trigger competing Rabi oscillations among $1s{\ensuremath{\sigma}}_{g}$, $2p{\ensuremath{\sigma}}_{u}$, $3d{\ensuremath{\sigma}}_{g}$ and $2p{\ensuremath{\pi}}_{u}$ states. This study shows the dissociation of H2+{\mathrm{H}}_{2}{}^{+}H2​+ can be steered into selected electronic states with controllable probabilities with state-of-the-art laser technologies.
Quantum Dynamics of H2+H_2^+H2+​ in Orthogonal Two-Color Fields
We present full-dimensional quantum simulations of H2+H_2^+H2+​ dissociative ionization driven by strong orthogonal laser fields. We consider equal-frequency orthogonal components, which generate elliptical or circular polarization depending on their relative phase and amplitude, as well as orthogonal 800800800- and 400400400-nm two-color fields. These two-dimensional fields strongly modify the fragmentation dynamics. Most notably, we identify a high-energy peak in the proton kinetic-energy-release (KER) spectrum at approximately 4−54-54−5 eV that is absent from the corresponding single-color, linearly polarized calculations. The yield of this peak can be coherently controlled by varying the relative carrier-envelope phase of the perpendicular field component. The perpendicular field also disrupts the clear electron-proton energy-sharing pattern observed in the main 3−3.53-3.53−3.5 eV dissociation channel, indicating more complex multichannel dynamics. Time-dependent state projections and calculations initiated from individual excited states attribute the additional peak to laser-induced vibrational excitation of H2+H_2^+H2+​. Furthermore, the perpendicular field rotates the fragment angular distributions, causing the most probable proton and electron emission directions to deviate substantially from the principal zzz axis. These findings demonstrate that the spatial and temporal geometry of orthogonal laser fields provides an additional degree of freedom for controlling ultrafast electron-nuclear dynamics.
Laser Induced Dissociation Control of H2+H_2^+H2+​ and H32+H_3^{2+}H32+​
Laser induced dissociation control of the symmetric diatomic molecular ion H2+H_2^+H2+​ and the triatomic molecular ion H32+H_3^{2+}H32+​ is discussed. The simulation results demonstrate that a long-wavelength terahertz or mid-infrared laser pulse can be used to control the electrons of the dissociative states after the excitation of an ultrashort ultraviolet (UV) laser pulse. For H2+,H_2^+,H2+​, there exists an effective time, which increases with increasing pulse duration of the UV laser pulse, for controlling the molecular dissociation. For the electrons of the 1sσg1sσ_g1sσg​ state, they move along the polarization direction of the dissociation control electric field. In contrast, for the electrons of the 2pσu2pσ_u2pσu​ state, they move in the opposite direction to that of the electric force. And for the triatomic molecule H32+H_3^{2+}H32+​, the electron dissociation control can also be realized by changing the central wavelength of the exciting UV pulse.
We demonstrate sub-cycle control of frustrated double ionization (FDI) in the two-electron triatomic molecule D3+_3^+3+​ when driven by two orthogonally polarized two-color laser fields. We employ a three-dimensional semi-classical model that fully accounts for the electron and nuclear motion in strong fields. We control FDI triggered by a strong near-infrared laser field with a weak mid-infrared laser field. This control as a function of the time delay between the two pulses is demonstrated when the FDI probability and the distribution of the momentum of the escaping electron along the mid-infrared laser field are considered in conjunction. We find that the momentum distribution of the escaping electron has a hive-shape with features that can accurately be mapped to the time one of the two electrons tunnel-ionizes at the start of the break-up process. This mapping distinguishes consecutive tunnel-ionization times within a cycle of the mid-infrared laser field.
Summary form only given. Angstrom and attosecond control of free electron wave packets is one of the pinnacles of attosecond science. Orthogonally polarized two-color (OTC) laser fields allow to control the motion of field-ionizing electronic wave packets both in time and space[1]. In OTC pulses time and space are connected and thus an attosecond time scale is established in the polarization plane for both the emitted and the re-colliding wave packets[2,3].In this submission, we report on experiments that use OTC pulses for studying atomic single and double ionization. The three-dimensional momentum vector of electrons and ions created by single and double ionization of neon atoms was measured with the COLTRIMS technique as a function of the sub-cycle shape of the OTC pulses. We introduce a concept similar to attosecond streaking that allows reading sub-cycle timing information from the momentum vector of emitted electrons. The concept is then applied to gaining information about the influence of the long-range Coulomb potential on the trajectories of tunneling electrons.Fig.1 (a) shows the measured momentum distributions of electrons correlated with singly ionized neon in the polarization plane of the OTC field with relative phase Δφ between the two color components. The spectra show that the electron emission direction is sensitive to Δφ. Furthermore, the spectra feature a prominent x-shaped central structure and weaker fine-scale modulations due to wave packet interferences. To compare with simulated spectra with strong field approximation (SFA) and solving two-dimensional time-dependent Schrödinger equation (TDSE) with single active electron approximation, we conclude that Coulomb effect plays important role on the spatial distribution of released electron wave packets. In the conference, we will furthermore demonstrate control over the correlation between the two electrons emitted during double ionization upon electron recollision by tuning the shape of the electric field of the OTC pulses on the sub-cycle scale. The results of these experiments are in agreement with theoretical predictions [4].