Double ionization

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Double ionization is a process of formation of doubly-charged ions when laser radiation is exerted on neutral atoms or molecules. Double ionization is usually less probable than single-electron ionization. Two types of double ionization are distinguished: sequential and non-sequential.

Sequential double ionization[edit]

Sequential double ionization is a process of formation of doubly-charged ions consisting of two single-electron ionization events: the first electron is removed from a neutral atom/molecule (leaving a singly-charged ion in the ground state or an excited state) followed by detachment of the second electron from the ion.[1]

Examples of sequential double ionization
Sequential double ionization 
Sequential double ionization via an ionic exited state 

Non-sequential double ionization[edit]

Non-sequential double ionization is a process whose mechanism differs (in any detail) from the sequential one. For example, both the electrons leave the system simultaneously (as in alkaline earth atoms, see below), the second electron's liberation is assisted by the first electron (as in noble gas atoms, see below), etc.

The phenomenon of non-sequential double ionization was experimentally discovered by Suran and Zapesochny for alkaline earth atoms as early as 1975.[2] Despite extensive studies, the details of double ionization in alkaline earth atoms remain unknown. It is supposed that double ionization in this case is realized by transitions of both the electrons through the spectrum of autoionizing atomic states, located between the first and second ionization potentials.[3] [4] [5] [6] [7] [8]

Non-sequential double ionization in alkaline earth atoms

For noble gas atoms, non-sequential double ionization was first observed by L'Huillier.[9][10]  The interest to this phenomenon grew rapidly after it was rediscovered[11][12] in infrared fields and for higher intensities. Multiple ionization has also been observed.[13][14]  The mechanism of non-sequential double ionization in noble gas atoms differs from the one in alkaline earth atoms. For noble gas atoms in infrared laser fields, following one-electron ionization, the liberated electron can recollide with the parent ion.[15][16] This electron acts as an "atomic antenna",[16] absorbing the energy from the laser field between ionization and recollision and depositing it into the parent ion. Inelastic scattering on the parent ion results in further collisional excitation and/or ionization. This mechanism is known as the three-step model of non-sequential double ionization, which is also closely related to the three step model of high harmonic generation.

Dynamics of double ionization within the three-step model strongly depends on the laser field intensity. The maximum energy (in atomic units) gained by the recolliding electron from the laser field is \sim 3.2 U_p,[15] where U_p=\frac{F^2}{4\omega^2} is the ponderomotive energy, F is the laser field strength, and \omega is the laser frequency. Even when 3.2 U_p is far below ionization potential I_p experiments have observed correlated ionization.[13][14][17][18][19]  As opposed to the high-U_p regime (3.2 U_p > I_p)[20][21][22][23] in the low-U_p regime (3.2 U_p < I_p) the assistance of the laser field during the recollision is vital.

Classical and quantum analysis[24][25][26] of the low-U_p regime demonstrates the following two ways of electron ejection after the recollision: First, the two electrons can be freed with little time delay compared to the quarter-cycle of the driving laser field. Second, the time delay between the ejection of the first and the second electron is of the order of the quarter-cycle of the driving field. In these two cases, the electrons appear in different quadrants of the correlated spectrum. If following the recollision, the electrons are ejected nearly simultaneously, their parallel momenta have equal signs, and both electrons are driven by the laser field in the same direction toward the detector [27] . If after the recollision, the electrons are ejected with a substantial delay (quarter-cycle or more), they end up going in the opposite directions. These two types of dynamics produce distinctly different correlated spectra (compare experimental results [13][14][17][18][19] with .[22][23]

Non-sequential double ionization of noble gas atoms
The high-U_p regime 
The low-U_p regime 

See also[edit]

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References[edit]

  1. ^ Delone, N. B.; Krainov, V. P. (2000). Multiphoton Processes in Atoms. Springer. ISBN 3540646159. , chapter 8.
  2. ^ Suran, V. V.; Zapesochny, I. P. (1975). "Observation of Sr2+ in multiple-photon ionization of strontium". Sov. Tech. Phys. Lett. 1 (11): 420. 
  3. ^ Lambropoulos, P.; Tang, X.; Agostini, P.; Petite, G.; L'Huillier, A. (1988). "Multiphoton spectroscopy of doubly excited, bound, and autoionizing states of strontium". Physical Review A 38 (12): 6165–6179. Bibcode:1988PhRvA..38.6165L. doi:10.1103/PhysRevA.38.6165. PMID 9900374.  edit
  4. ^ Bondar', I. I.; Suran, V. V. (1993). "The two-electron mechanism of Ba2+ ion formation in the ionization of Ba atoms by YAG-laser radiation". JETP 76: 381. Bibcode:1993JETP...76..381B. 
  5. ^ Bondar’, I. I.; Suran, V. V. (1998). "Resonance structure of doubly-charged-ion production during laser dielectronic ionization of atoms". Journal of Experimental and Theoretical Physics Letters 68 (11): 837. doi:10.1134/1.567802.  edit
  6. ^ Bondar, I. I.; Suran, V. V.; Dudich, M. I. (2000). "Resonant structure in doubly charged ion formation during multiphoton ionization of Sr and Ba atoms by infrared laser radiation". Journal of Physics B: Atomic, Molecular and Optical Physics 33 (20): 4243. doi:10.1088/0953-4075/33/20/304.  edit
  7. ^ Liontos, I.; Bolovinos, A.; Cohen, S.; Lyras, A. (2004). "Single and double ionization of magnesium via four-photon excitation of the 3p^{2}^{1}S_{0} autoionizing state: Experimental and theoretical analysis". Physical Review A 70 (3). Bibcode:2004PhRvA..70c3403L. doi:10.1103/PhysRevA.70.033403.  edit
  8. ^ Liontos, I.; Cohen, S.; Lyras, A. (2010). "Multiphoton Ca2+production occurring before the onset of Ca+saturation: Is it a fingerprint of direct double ionization?". Journal of Physics B: Atomic, Molecular and Optical Physics 43 (9): 095602. doi:10.1088/0953-4075/43/9/095602.  edit
  9. ^ l'Huillier, A.; Lompre, L.; Mainfray, G.; Manus, C. (1982). "Multiply Charged Ions Formed by Multiphoton Absorption Processes in the Continuum". Physical Review Letters 48 (26): 1814. Bibcode:1982PhRvL..48.1814L. doi:10.1103/PhysRevLett.48.1814.  edit
  10. ^ l'Huillier, A.; Lompre, L. A.; Mainfray, G.; Manus, C. (1983). "Multiply charged ions induced by multiphoton absorption in rare gases at 0.53 μm". Physical Review A 27 (5): 2503. Bibcode:1983PhRvA..27.2503L. doi:10.1103/PhysRevA.27.2503.  edit
  11. ^ Walker, B.; Mevel, E.; Yang, B.; Breger, P.; Chambaret, J.; Antonetti, A.; Dimauro, L.; Agostini, P. (1993). "Double ionization in the perturbative and tunneling regimes". Physical Review A 48 (2): R894–R897. Bibcode:1993PhRvA..48..894W. doi:10.1103/PhysRevA.48.R894. PMID 9909791.  edit
  12. ^ Walker, B.; Sheehy, B.; Dimauro, L.; Agostini, P.; Schafer, K.; Kulander, K. (1994). "Precision Measurement of Strong Field Double Ionization of Helium". Physical Review Letters 73 (9): 1227–1230. Bibcode:1994PhRvL..73.1227W. doi:10.1103/PhysRevLett.73.1227. PMID 10057657.  edit
  13. ^ a b c Rudenko, A.; Zrost, K.; Feuerstein, B.; De Jesus, V.; Schröter, C.; Moshammer, R.; Ullrich, J. (2004). "Correlated Multielectron Dynamics in Ultrafast Laser Pulse Interactions with Atoms". Physical Review Letters 93 (25). arXiv:physics/0408065. Bibcode:2004PhRvL..93y3001R. doi:10.1103/PhysRevLett.93.253001.  edit
  14. ^ a b c Zrost, K.; Rudenko, A.; Ergler, T.; Feuerstein, B.; Jesus, V. L. B. D.; Schröter, C. D.; Moshammer, R.; Ullrich, J. (2006). "Multiple ionization of Ne and Ar by intense 25 fs laser pulses: Few-electron dynamics studied with ion momentum spectroscopy". Journal of Physics B: Atomic, Molecular and Optical Physics 39 (13): S371. doi:10.1088/0953-4075/39/13/S10.  edit
  15. ^ a b Corkum, P. (1993). "Plasma perspective on strong field multiphoton ionization". Physical Review Letters 71 (13): 1994–1997. Bibcode:1993PhRvL..71.1994C. doi:10.1103/PhysRevLett.71.1994. PMID 10054556.  edit
  16. ^ a b Kuchiev, M. Y. (1987). "Atomic Antenna". JETP Letters 45: 404. Bibcode:1987JETPL..45..404K. 
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