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Physical Review A

Orbital alignment in atoms generated by photodetachment in a strong laser field

Mikael Eklund, Hannes Hultgren, D. Hanstorp, Igor Yu. Kiyan

Published Aug 26, 2013• 5 citations
Publisher

Abstract

A pump-probe laser scheme is employed to investigate orbital alignment and its dynamics in the ground state of laser-generated neutral atoms. The alignment is initiated by electron photodetachment of an atomic negative ion in a strong laser pulse. The electron density distribution in the ground state of the residual atom is probed by means of strong-field ionization in a second laser pulse at a delayed time. The principle of the probe method relies on the fact that the portion of the electron density distribution oriented along the laser polarization axis constitutes the ionization yield in the high-energy jets of emitted electrons. A systematic study is carried out on C, Si, and Ge atoms, which possess two electrons in an open $p$ shell. A pronounced temporal modulation in the yield of high-energy electrons is observed for C and Si, revealing a periodic spatial rearrangement of the electron density distribution in these atoms. Its period is defined by the beat between the $J=1$ and $J=2$ spin-orbit components of the ground state.

DOI
10.1103/physreva.88.023423

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Journal of Physics B Atomic Molecular and Optical Physics•Jan 28, 2011•39 citations
A pump-probe laser scheme is employed to investigate orbital alignment and its dynamics in the ground state of laser-generated neutral atoms. The alignment is initiated by electron photodetachment of an atomic negative ion in a strong laser pulse. The electron density distribution in the ground state of the residual atom is probed by means of strong-field ionization in a second laser pulse at a delayed time. The principle of the probe method relies on the fact that the portion of the electron density distribution oriented along the laser polarization axis constitutes the ionization yield in the high-energy jets of emitted electrons. A systematic study is carried out on C, Si, and Ge atoms, which possess two electrons in an open ppp shell. A pronounced temporal modulation in the yield of high-energy electrons is observed for C and Si, revealing a periodic spatial rearrangement of the electron density distribution in these atoms. Its period is defined by the beat between the J=1J=1J=1 and J=2J=2J=2 spin-orbit components of the ground state.
Photoelectron imaging techniques are used to experimentally investigate photodetachment of negative ions in a strong laser-field as well as the orbital alignment dynamics in the residual neutral.\n\nStrong-field photodetachment in a linearly polarized pulsed laser with 100 fs pulse length is used to create a wave packet in the neutral atoms of carbon, germanium and silicon. The negative ions of these three atomic species are isoelectronic with a half-filled ppp-shell. The photodetachment rate is highly dependent on the magnetic quantum number $m_\\ell$ with the orbital with $m_\\ell = 0$ having a detachment rate of an order of magnitude larger than orbitals with $m_\\ell$ non-zero, with the polarization axis acting as a quantization axis. The residual neutral is thus left in an aligned state after photodetachment. The aligned state constitutes of a coherent superposition of fine-structure states and the population in the different orbitals will accordingly display a time-dependent spatial oscillation. The created wave packet is subsequently probed through photoionization by a second strong-field laser pulse.\n\nSecondly, a tomographic method is implemented to observe the three-dimensional distribution of photoelectrons produced in strong-field photodetachment processes. The method makes no assumptions of the existence of any symmetry axes in the photoelectron distribution and is thus applicable to any laser polarization ellipticity. Traditional imaging methods are dependent on the photoelectron distribution to be cylindrically symmetric by using either linearly or circularly polarized light, a requirement which can be lifted by using a tomographic technique. The method is applied to multiphoton detachment of the negative ion of silver.\n\nFinally, strong-field photodetachment of homonuclear diatomic negative ions is performed. The possibility of the additional electron being ejected from either of the two atomic centers can significantly alter the photoelectron distribution compared to an atomic negative ion with the same detachment energy. Experiments are performed on C2−_2^-2−​, Si2−_2^-2−​ and Ag2−_2^-2−​ and compared to theoretical models.
We analyze the experiment by Hultgren et al. [Phys. Rev. A 87, 031404 (2013)] on orbital alignment and quantum beats in coherently excited atomic fine-structure manifolds produced by short-pulse laser photodetachment of ${\text{C}}^{\ensuremath{-}},\phantom{\rule{0.16em}{0ex}}{\text{Si}}^{\ensuremath{-}}$, and ${\text{Ge}}^{\ensuremath{-}}$ negative ions, and derive a formula that describes the beats. Analysis of the experimental data enables us to extract the noncoherent background contribution for each species, and indicates the need for a full density-matrix treatment of the problem.
The goal of the presented work is to investigate double detachment of\nnegative ions on nano- and femtosecond time scales. I have performed\nthree types of experiments.\nFemtosecond time resolved pump-probe experiments have been performed\non electron dynamics in the valence shell of carbon, silicon\nand germanium atoms. The atom under investigation was prepared\nby strong field photodetachment of its parent negative ion in a linearly\npolarized femtosecond laser pulse. The detachment process coherently\npopulates the fine structure levels of the atomic ground state\ngiving rise to an electronic wave packet moving in the valence shell\nof the atom. The wave packet is probed by strong field ionization at a\ntime delay controlled on a femtosecond time scale. The ionized electrons\nare detected in an electron imaging spectrometer. The energy\nand angular distribution of the ionized electrons provide information\non the phase of the wave packet. The results reveal a periodic spatial\nrearrangement of the electron cloud in the atom. The experimental\nprocedure and detection scheme developed provide the first direct visualization\nof an electronic wave packet moving in the ground state of\nan atom.\nThe process of rescattering in strong field detachment has also been\ninvestigated. The detached electron is accelerated in the laser field.\nWhen the E-field changes its direction the electron follows and may\nreturn and collide with the core. I have investigated the rescattering\neffect in negative fluorine ions and compared with previous results on\nbromine ions. Furthermore, strong field detachment experiments of\nF \n2 were conducted and the results from molecular fluorine ions are\ncompared to results from atomic fluorine ions.\nPhotodetachment cross sections and doubly excited states in K and\nCs have been investigated through double detachment. A resonant\nionization scheme was used where the ion beam and laser beams were\nin a collinear configuration allowing for measurements of partial cross\nsection channels individually with nearly 100% detection efficiency.\nA new threshold behavior has been found in photodetachment into\natomic states having a large and negative polarizability. A new model\nwas developed to explain the observed phenomena. Furthermore, two\nnew resonances stemming from doubly excited states were found below\nthe K(7 2P) channel opening in K and a set of new resonances\nwas observed in Cs below the Cs(10 2P) channel.
We investigate orbital alignment dynamics within the valence shell of atoms in coherently excited j=3/2,1/2j=3/2,1/2j=3/2,1/2 fine-structure manifolds generated by short-pulse photodetachment of F−^-−, Cl−^-− and Br−^-− anions. Using Keldysh-type theory, we calculate the density matrix of the residual atoms generated by few-cycle pulses, whose elements determine the populations and coherence among the electronic states. Our calculations demonstrate that the degree of atomic coherence can be represented by a near universal function of the ratio between the pulse duration τ~p\tildeτ_pτ~p​ and the beat period τj′jτ_{j'j}τj′j​ of the atomic system, which allows one to characterize the coherence generated in atomic states.
The precession of orbital and spin angular momentum vectors has been observed in a pump-probe study of the 4P fine-structure states of atomic potassium. A femtosecond pump pulse prepared a coherent superposition of the two fine-structure components. A time-delayed probe pulse then ionized the system after it had been allowed to evolve freely. Oscillations recorded in the ion signal reflect the evolution of the orientation of the orbital and spin angular momentum due to spin-orbit coupling. This interpretation gives physical insight into the cause of the half-period phase shift observed when the relative polarizations of the laser pulses were changed from parallel to perpendicular. Finally, it is shown that these changes in the orientation of the spin momentum vector of the system can be utilized to produce highly spin-polarized free electrons on the femtosecond scale
Using sequential strong-field double ionization in a pump–probe scheme we show through calculations how electronic dynamics can be prepared and imaged. Electronic dynamics may arise whenever multiple states of the ion are accessed in the ionization step. The dynamics in the cation influence the rate of the second ionization step and the momentum distribution of the ejected electron, allowing their detailed characterization. We show how the probe step is controlled through spatial propensities of the ionizing orbitals and the energy level structure of the dication. Both the final electronic state of the dication and the spin state of the ejected electron pair can be controlled through the time delay between the two ionizing pulses. We discuss how our results will extend to the preparation and measurement of attosecond electron dynamics.