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arXiv:1608.07211 · Physical Review A

Dynamics of atomic spin-orbit-state wave packets produced by short-pulse laser photodetachment

S. M. K. Law, G. F. Gribakin

Published Nov 2, 2016• 1 citation
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physreva.94.053402

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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.

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A series of calculations are presented showing the response of an atom to a picosecond laser pulse that simultaneously excites a number of Rydberg states. A coherent electron wave packet is formed. The packet may disperse and reform repeatedly as its center moves along a classical orbit. The repeated acceleration and deceleration of the wave packet as it moves around the orbit leads to a corresponding variation in the radiation rate of the atom. The result is a complicated pattern of multilevel quantum beats. Examples are shown for various excitation frequencies and pulse durations. Experimental implications are discussed.

Annual Meeting Optical Society of America•Jan 1, 1985•0 citations
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 C\ensuremath−,Si\ensuremath−{\text{C}}^{\ensuremath{-}},\phantom{\rule{0.16em}{0ex}}{\text{Si}}^{\ensuremath{-}}C\ensuremath−,Si\ensuremath−, and Ge\ensuremath−{\text{Ge}}^{\ensuremath{-}}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.
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.
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.
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.
A series of calculations are presented showing the response of an atom to a picosecond laser pulse that simultaneously excites a number of Rydberg states. A coherent electron wave packet is formed. The packet may disperse and reform repeatedly as its center moves along a classical orbit. The repeated acceleration and deceleration of the wave packet as it moves around the orbit leads to a corresponding variation in the radiation rate of the atom. The result is a complicated pattern of multilevel quantum beats. Examples are shown for various excitation frequencies and pulse durations. Experimental implications are discussed.