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Optics Express

  • Editor: J. H. Eberly
  • Vol. 8, Iss. 2 — Jan. 15, 2001
  • pp: 99–105
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Dependence on frequency of strong-field atomic stabilization

H. R. Reiss  »View Author Affiliations


Optics Express, Vol. 8, Issue 2, pp. 99-105 (2001)
http://dx.doi.org/10.1364/OE.8.000099


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Abstract

It is shown that strong-field atomic stabilization can occur at any frequency, that analytical methods exist that can describe all essential features of stabilization, that relativistic effects enhance the stabilization phenomenon, and that a simple physical picture exists that explains these properties. A necessary prelude is to show that the frequency properties of the three methods often conjoined by the KFR (Keldysh-Faisal-Reiss) label are quite different. Applicability of the SFA (Strong-Field Approximation) to stabilization at any frequency is shown, and verified by exhibiting close correspondence to numerical predictions by Popov et al. that also span both low and high frequencies.

© Optical Society of America

1. Introduction

The term “stabilization” has been employed with a variety of meanings by different authors. Stabilization is here defined to be that property of atoms in strong laser fields in which continued increases in field intensity will eventually lead to a decrease in the photoionization transition rate. This is consistent with the definition employed by Gavrila [1

1. M. Gavrila, “Atomic structure and decay in high frequency fields,” in Atoms in intense laser fields, M. Gavrila, ed. (Academic, Boston, MA1992).

], who identifies stabilization as a minimum in lifetime rather than a maximum in rate, the inverse of lifetime.

The concept of decreased efficacy of a field in causing transitions as field strength increases is far from new. For example, it was pointed out in the literature many years ago [2

2. H. R. Reiss, “Atomic transitions in intense fields and the breakdown of perturbation theory,” Phys. Rev. Lett. 25, 1149–1151 (1970). [CrossRef]

,3

3. H. R. Reiss, “Nuclear beta decay induced by intense electromagnetic fields: forbidden transition examples,” Phys. Rev. C 27, 1229–1243 (1983). [CrossRef]

] that some quantum systems will show a decline in transition rate as field strength increases. There are several qualitative reasons to expect this behavior. One of these reasons is explored in the final section below.

Analytical approaches to the stabilization problem are employed here, since analytical methods provide a unified way to explore the problem, as opposed to general patterns that may emerge from numerical approaches. We start from first principles in stating two related, exact statements of transition amplitudes that apply with complete generality to all perturbative or nonperturbative approaches. This allows us to examine, for the first time, the frequency dependence of approximations that follow from the exact transition amplitudes. The opinion has become widespread in the strong-field community that stabilization in the sense we have defined it is exclusively a high-frequency phenomenon. Our contention is that this notion has arisen only because the strong fields necessary to achieve stabilization exceed computational capacities for exploring low frequency interactions with atoms.

To establish the role of field frequency in nonperturbative analytical approximations, we examine the so-called KFR (Keldysh [4

4. L. V. Keldysh, “Ionization in the field of a strong electromagnetic wave,” Zh. Eksp. Teor. Fiz.47, 1945–1957 (1964) [Sov. Phys. JETP20, 1307–1314 (1965)].

], Faisal [5

5. F. H. M. Faisal, “Multiple absorption of laser photons by atoms,” J. Phys. B 6, L89–L92 (1973). [CrossRef]

], Reiss [6

6. H. R. Reiss, “Effect of an intense electromagnetic field on a weakly bound system,” Phys. Rev. A 22, 1786–1813 (1980). [CrossRef]

]) approximation. It is shown that the “K” method is based on a low-frequency approximation, the “F” method is founded on a high-frequency approximation, and the “R” approach requires only strong fields, with no limitation on the frequency. Hereafter, the “R” method will be referred to as the SFA, or Strong-Field Approximation. The method of Gavrila [1

1. M. Gavrila, “Atomic structure and decay in high frequency fields,” in Atoms in intense laser fields, M. Gavrila, ed. (Academic, Boston, MA1992).

] depends upon high frequencies, and is sometimes called the HFA (High-Frequency Approximation).

As employed here, high frequency means ω>>EB , where ω is the frequency of the laser field and EB is the binding energy of the atom. Low frequency refers to ω<<EB . Atomic units are used.

2. Fundamental transition amplitudes

The notation is adopted that quantum states with no interaction with the laser field have wave functions designated by Φ, and interacting states are given by Ψ. That is, these states satisfy the Schrödinger equations

itΦ=H0Φ,H0=122+V(r),
(1)
itΨ=(H0+HI)Ψ,HI=icA·+12c2A2.
(2)

Boundary conditions in laser experiments are such that the atom is initially and finally in a field-free environment. It is thus useful to define “in-states” [superscript (+)] and “out-states” [superscript (-)] as

limtΨ(+)=Φ,limt+Ψ()=Φ.
(3)

Subscripts i and f will identify initial and final states. The overlap of the fully evolving state Ψi onto some particular final non-interacting state Φf gives the quantum probability amplitude that that final state will occur. One can write, using Eq. (3) for the second element below,

Sfi=limt+(Φf,Ψi(+)),δfi=limt(Φf,Ψi(+)),
(4)

from which it follows that

(S1)fi=+dtt(Φf,Ψi(+))=i+dt(Φf,HIΨi),
(5)

upon using the equations of motion, Eqs. (1) and (2). In the last element of Eq. (5), the superscript (+) has been dropped as superfluous.

A transition amplitude fully equivalent to the above can be found from the time-reversed point of view that employs the overlap expression

Sfi=limt(Ψf(),Φi),
(6)

and yields the final transition amplitude

(S1)fi=idt(Ψf,HIΦi),
(7)

where the superscript (-) has been dropped.

3. Strong-field analytical approximations

3.1 Keldysh approximation

The Keldysh approximation [4

4. L. V. Keldysh, “Ionization in the field of a strong electromagnetic wave,” Zh. Eksp. Teor. Fiz.47, 1945–1957 (1964) [Sov. Phys. JETP20, 1307–1314 (1965)].

] is directly derivable from Eq. (7). Three steps are necessary. First, Keldysh uses the length gauge, so that

HIHIlength=E·r.
(8)

Second, on the grounds that the final continuum state of the photoelectron will have its motion dominated by the laser field, Coulomb effects are neglected and

ΨfΨfVolk,
(9)

where the Volkov solution for a free particle in the presence of the field is well known. The Volkov solution employed must be in the length gauge, which is a more complicated object than the velocity-gauge Volkov solution. This fact presents enough difficulty that Keldysh introduced his third step, which is to assume large photon orders from the outset. His approximation is therefore restricted to the tunneling case.

The constraint to tunneling for the Keldysh theory means both that it is confined to low frequencies and also that the stabilization domain cannot be explored with the Keldysh theory.

3.2 Faisal approximation

The Faisal approximation [5

5. F. H. M. Faisal, “Multiple absorption of laser photons by atoms,” J. Phys. B 6, L89–L92 (1973). [CrossRef]

] has been an enigma, since efforts to associate the fundamental assumptions with a strong-field low-frequency hypothesis have not borne fruit. Faisal’s work begins with the direct-time transition amplitude of Eq. (5). He assumes, as is true of all the strong-field analytical approximations, that the influence of the atomic binding potential can be neglected for the ionized electron, or

ΦfΦffree,
(10)

where the free-particle final state no longer has the Coulomb influences of the Φ of Eq. (1).

The use of the direct-time amplitude requires a knowledge of the initial bound state in the presence of the strong laser field, thus eschewing the advantage of the time-reversed amplitude, Eq. (7), that requires no such knowledge. The Kramers-Henneberger transformation [7

7. W. C. Henneberger, “Perturbation method for atoms in intense light beams,” Phys. Rev. Lett. 21, 838–841 (1968). [CrossRef]

]UKH (actually the inverse transformation) is applied to the initial state, giving

Ψi(r,t)=(UKH)1Φi(r+α,t),
(11)

Φi(r+α,t)Φi(r,t).
(12)

The approximation in Eq. (12) is a high-frequency approximation, in that it is justified only if the oscillation α occurs at too high a frequency to permit the atom to follow the motion [1

1. M. Gavrila, “Atomic structure and decay in high frequency fields,” in Atoms in intense laser fields, M. Gavrila, ed. (Academic, Boston, MA1992).

]. The final conclusion is that the Faisal transition amplitude,

(S1)fiF=idt(Φffree,HI(UKH)1Φi(r,t)),
(13)

is a strong-field, high-frequency approximation.

3.3 Strong-field approximation (SFA)

The SFA is based on the strong-field assumption that, in the time-reversed transition amplitude Eq. (7), one need only make the replacement

ΨfΨfVolk.
(14)

This requires only that the laser field be strong, and makes no reference to field frequency.

It is known [6

6. H. R. Reiss, “Effect of an intense electromagnetic field on a weakly bound system,” Phys. Rev. A 22, 1786–1813 (1980). [CrossRef]

] that the SFA transition amplitude yields the tunneling result in the low frequency limit, and that detailed behavior of strong-field, low-frequency photoionization is accurately predicted [8

8. H. R. Reiss, “Energetic electrons in strong-field ionization”, Phys. Rev. A 54, R1765–R1768 (1996). [CrossRef] [PubMed]

]. It is also easy to show that the SFA amplitude can be transformed to the Faisal amplitude (13) using the fact that the dipole-approximation Volkov solution can be written as a Kramers-Henneberger transformation of the free-particle wave function. The SFA thus possesses both low- and high-frequency behavior. This is further verified by the close correspondence between rates calculated with the HFA [9

9. M. Pont and M. Gavrila, “Stabilization of atomic hydrogen in superintense, high-frequency laser fields of circular polarization,” Phys. Rev. Lett. 65, 2362–2365 (1990). [CrossRef] [PubMed]

] and with the SFA [10

10. H. R. Reiss, “High-frequency, high-intensity photoionization,” J. Opt. Soc. Am. B 13, 355–362 (1996). [CrossRef]

].

4. Strong-field stabilization

4.1 Transition rates

Figure 1 shows monochromatic transition rates calculated with the SFA both relativistically and nonrelativistically for ionization of hydrogen by a circularly polarized laser field. Figure 1 gives two high frequency and two low frequency examples. The sharp drop in rate is clearly in evidence for all frequencies beyond a certain intensity that we shall call the stabilization intensity Istab . Matters are less clear for an actual experiment, since ionization will saturate before Istab is reached. Laboratory observation of stabilization will be considered elsewhere.

Relativistic effects are seen to enhance the stabilization effect. Earlier calculations [11

11. D. P. Crawford and H. R. Reiss, “Stabilization in relativistic photoionization with circularly polarized light,” Phys. Rev. A 50, 1844–1850 (1994). [CrossRef] [PubMed]

] that showed some ambiguity in the relativistic enhancement effect have been refined. It is now clear that relativistic rates are less than or equal to nonrelativistic rates at all intensities for circular polarization.

Relativistic SFA results follow from the Dirac version [12

12. H. R. Reiss, “Relativistic strong-field photoionization,” J. Opt. Soc. Am. B 7, 574–586 (1990). [CrossRef]

] of Eq. (7),

(S1)fi=icd4xΨf¯AμγμΦi,
(15)

where the bar over Ψ indicates the Dirac adjoint Ψγ0 the γµ are the Dirac gamma matrices; µ=0,1,2,3; A µ is the four-vector potential of the laser field; and a time-favoring real metric is used. Under relativistic conditions, the neglect of Coulomb effects on the ionized electron is a well-justified approximation. One then uses the Dirac Volkov solution for the final-state wave function, uses known solutions (or superpositions thereof) to the hydrogen-atom Dirac equation for the initial-state wave function, and obtains thereby a completely relativistic Dirac transition amplitude. Details of this procedure are given in Ref. [12

12. H. R. Reiss, “Relativistic strong-field photoionization,” J. Opt. Soc. Am. B 7, 574–586 (1990). [CrossRef]

]. Additional examples as well as an extension to linear polarization are to be found in Refs. [11

11. D. P. Crawford and H. R. Reiss, “Stabilization in relativistic photoionization with circularly polarized light,” Phys. Rev. A 50, 1844–1850 (1994). [CrossRef] [PubMed]

,13

13. H. R. Reiss and D. P. Crawford, “Relativistic photoionization,” in Ultrafast phenomena and interaction of superstrong laser fields with matter: nonlinear optics and high-field physics, M. V. Fedorov, V.M. Gordienko, V. V. Shuvalov, and V. D. Taranukhin, eds., Proc. SPIE3735, 148–157 (1998).

]. An important feature true for both relativistic and nonrelativistic calculations is that Istab is approximately the same for both circular and linear polarizations.

Fig. 1. Atomic hydrogen monochromatic ionization rates in a circularly polarized laser beam.

4.2 Comparison with alternative predictions of Istab

It has only been very recently that predictions for Istab have been calculated (by a method other than the SFA) for frequencies encompassing both the low and high frequency domains. Popov et al. [14

14. A. M. Popov, O. V. Tikhonova, and E. A. Volkova, “Applicability of the Kramers-Henneberger approximation in the theory of strong-field ionization,” J. Phys. B 32, 3331–3345 (1999). [CrossRef]

] have devised a one-dimensional numerical method for this task, and apply it to a hydrogenic state with a binding energy of 0.75 eV, or 0.0276 a.u. Theoretical studies of stabilization show that Istabω 3 for high frequencies [15

15. H. R. Reiss, “Frequency and polarization effects in stabilization,” Phys. Rev. A 46, 391–394 (1992). [CrossRef] [PubMed]

], and IstabEB3 for low frequencies [16

16. H. R. Reiss, “Physical basis for strong-field stabilization of atoms against ionization,” Las. Phys. 7, 543–550 (1997).

]. That is, the important feature is the energy. Hence, three-dimensional SFA predictions were calculated for the same binding energy as employed in Ref. [14

14. A. M. Popov, O. V. Tikhonova, and E. A. Volkova, “Applicability of the Kramers-Henneberger approximation in the theory of strong-field ionization,” J. Phys. B 32, 3331–3345 (1999). [CrossRef]

]. Comparison of the two sets of results is shown in Fig. 2.

The conditions for applicability of the SFA are Up >>EB , where Up is the ponderomotive energy. The values of Istab predicted by the SFA in the neighborhood of ω=EB do not satisfy that constraint. Only those stabilization intensities are plotted in Fig. 2 that satisfy SFA validity conditions, and hence a gap occurs. Plainly, the SFA and the results of Ref. [14

14. A. M. Popov, O. V. Tikhonova, and E. A. Volkova, “Applicability of the Kramers-Henneberger approximation in the theory of strong-field ionization,” J. Phys. B 32, 3331–3345 (1999). [CrossRef]

] are in excellent agreement for both low and high frequencies.

4.3 Physical explanation for stabilization

There are numerous ways to understand stabilization, but one of the simplest follows from Eq. (6). This equation gives the probability amplitude for ionization as the quantum overlap of the undisturbed initial state of the atom and a possible final continuum state for the photoelectron as projected back in time. When Coulomb effects on the continuum electron are negligible, as they are for fields strong enough to induce stabilization, the predominant part of the probability amplitude for limt Ψf is approximately just ΨfVolk (t) at any t. Consider the nonrelativistic circular polarization case. The initial atom can be viewed as a spherical Bohr atom, and the final photoelectron will be in a circular orbit around the atom with essentially the classical energy and angular momentum of a free electron in the laser field. The cylindrical symmetry of the final orbit is imposed by the symmetry of the problem, and the number of photons absorbed to provide the initially bound electron with its final energy in the laser field is also appropriate to provide the final angular momentum. This is illustrated by the sketch in Fig. 3(a). As the field intensity increases, the classical radius of the orbit grows much larger than the size of the atom, the wave function overlap decreases, and stabilization sets in.

Fig. 2. Value of the stabilization intensity as a function of field frequency for ionization from a hydrogenic atom with binding energy 0.75 eV. The Popov et al. data are from Ref. [14], and proportions in this figure mimic those of Fig. 10 of this reference. See text for explanation of the gap in SFA results.

4.4 Relativistic effects on stabilization

With circular polarization, the momentum of the absorbed photons displaces the plane of the circulating photoelectron in the forward direction, so that the circular orbit now lies on the surface of a circular cone with the atom at the vertex. The overlap of final and initial states is somewhat reduced. This effect is illustrated in Fig. 3(b), with the resulting diminution of overlap of Φ i and Ψ f leading to the reduced relativistic rate seen in Fig. 1.

For linear polarization, the relativistic path of the free electron is just the well-known “figure-8” as seen in the laboratory frame. The effect is to cause the ionized electron to “walk” away from the atom, thus reducing the wave function overlap expressed in Eq. (6).

The above conclusions differ from those obtained by Kylstra et al. [17

17. N. J. Kylstra, R. A. Worthington, A. Patel, P. L. Knight, J. R. V·zquez de Aldana, and L. Roso, “Breakdown of stabilization of atoms with intense, high-frequency laser pulses,” Phys. Rev. Lett. 85, 1835–1838 (2000). [CrossRef] [PubMed]

], who predict that magnetic field effects will suppress stabilization. This contrast poses an extremely interesting contradiction that needs to be resolved. In support of the conclusions given here, we can make several remarks. First, Kylstra et al. use a two-dimensional theory to calculate the explicitly three-dimensional phenomena that are a necessary consequence of coupled electric and magnetic fields. Second, it is not clear that it is consistent to introduce retardation effects into the nonrelativistic Schrödinger equation [18

18. H. R. Reiss, “Dipole-approximation magnetic fields in strong laser beams,” Phys. Rev. A 63, 013409 (2001). [CrossRef]

] as done by Kylstra. The procedures of Refs. [11

11. D. P. Crawford and H. R. Reiss, “Stabilization in relativistic photoionization with circularly polarized light,” Phys. Rev. A 50, 1844–1850 (1994). [CrossRef] [PubMed]

13

13. H. R. Reiss and D. P. Crawford, “Relativistic photoionization,” in Ultrafast phenomena and interaction of superstrong laser fields with matter: nonlinear optics and high-field physics, M. V. Fedorov, V.M. Gordienko, V. V. Shuvalov, and V. D. Taranukhin, eds., Proc. SPIE3735, 148–157 (1998).

] are three-dimensional, fully relativistic, and subject only to the mild (for intensities like Istab ) constraint of Eq. (14). Finally, we remark that the qualitative reasoning employed above and illustrated in Fig. 3 follows from the exact expression in Eq. (6), and is independent of any specific calculational method. Basically, Eq. (6) tells us that because the wave function overlap between the initial atom and a final relativistic free electron is reduced from the nonrelativistic situation, then as intensity increases the atom is denied an increasing portion of the probability of making a transition to an ionized state, and is thus more strongly stabilized relativistically.

Fig. 3. Ionization by circularly polarized light. (a) Nonrelativistically, the electron circulates around the atom in a circular orbit in a plane perpendicular to the propagation direction, with its classical energy and radius of motion. (b) Relativistically, the plane of the orbit is displaced forward by the momentum of the number of photons that must be absorbed to supply the ponderomotive energy of the electron.

References and Links

1.

M. Gavrila, “Atomic structure and decay in high frequency fields,” in Atoms in intense laser fields, M. Gavrila, ed. (Academic, Boston, MA1992).

2.

H. R. Reiss, “Atomic transitions in intense fields and the breakdown of perturbation theory,” Phys. Rev. Lett. 25, 1149–1151 (1970). [CrossRef]

3.

H. R. Reiss, “Nuclear beta decay induced by intense electromagnetic fields: forbidden transition examples,” Phys. Rev. C 27, 1229–1243 (1983). [CrossRef]

4.

L. V. Keldysh, “Ionization in the field of a strong electromagnetic wave,” Zh. Eksp. Teor. Fiz.47, 1945–1957 (1964) [Sov. Phys. JETP20, 1307–1314 (1965)].

5.

F. H. M. Faisal, “Multiple absorption of laser photons by atoms,” J. Phys. B 6, L89–L92 (1973). [CrossRef]

6.

H. R. Reiss, “Effect of an intense electromagnetic field on a weakly bound system,” Phys. Rev. A 22, 1786–1813 (1980). [CrossRef]

7.

W. C. Henneberger, “Perturbation method for atoms in intense light beams,” Phys. Rev. Lett. 21, 838–841 (1968). [CrossRef]

8.

H. R. Reiss, “Energetic electrons in strong-field ionization”, Phys. Rev. A 54, R1765–R1768 (1996). [CrossRef] [PubMed]

9.

M. Pont and M. Gavrila, “Stabilization of atomic hydrogen in superintense, high-frequency laser fields of circular polarization,” Phys. Rev. Lett. 65, 2362–2365 (1990). [CrossRef] [PubMed]

10.

H. R. Reiss, “High-frequency, high-intensity photoionization,” J. Opt. Soc. Am. B 13, 355–362 (1996). [CrossRef]

11.

D. P. Crawford and H. R. Reiss, “Stabilization in relativistic photoionization with circularly polarized light,” Phys. Rev. A 50, 1844–1850 (1994). [CrossRef] [PubMed]

12.

H. R. Reiss, “Relativistic strong-field photoionization,” J. Opt. Soc. Am. B 7, 574–586 (1990). [CrossRef]

13.

H. R. Reiss and D. P. Crawford, “Relativistic photoionization,” in Ultrafast phenomena and interaction of superstrong laser fields with matter: nonlinear optics and high-field physics, M. V. Fedorov, V.M. Gordienko, V. V. Shuvalov, and V. D. Taranukhin, eds., Proc. SPIE3735, 148–157 (1998).

14.

A. M. Popov, O. V. Tikhonova, and E. A. Volkova, “Applicability of the Kramers-Henneberger approximation in the theory of strong-field ionization,” J. Phys. B 32, 3331–3345 (1999). [CrossRef]

15.

H. R. Reiss, “Frequency and polarization effects in stabilization,” Phys. Rev. A 46, 391–394 (1992). [CrossRef] [PubMed]

16.

H. R. Reiss, “Physical basis for strong-field stabilization of atoms against ionization,” Las. Phys. 7, 543–550 (1997).

17.

N. J. Kylstra, R. A. Worthington, A. Patel, P. L. Knight, J. R. V·zquez de Aldana, and L. Roso, “Breakdown of stabilization of atoms with intense, high-frequency laser pulses,” Phys. Rev. Lett. 85, 1835–1838 (2000). [CrossRef] [PubMed]

18.

H. R. Reiss, “Dipole-approximation magnetic fields in strong laser beams,” Phys. Rev. A 63, 013409 (2001). [CrossRef]

OCIS Codes
(020.4180) Atomic and molecular physics : Multiphoton processes
(270.4180) Quantum optics : Multiphoton processes
(270.6620) Quantum optics : Strong-field processes

ToC Category:
Focus Issue: Quantum control of photons and matter

History
Original Manuscript: November 9, 2000
Published: January 15, 2001

Citation
Howard Reiss, "Dependence on frequency of strong-field atomic stabilization," Opt. Express 8, 99-105 (2001)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-8-2-99


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References

  1. M. Gavrila, "Atomic structure and decay in high frequency fields," in Atoms in intense laser fields, M. Gavrila, ed. (Academic, Boston, MA 1992).
  2. H. R. Reiss, "Atomic transitions in intense fields and the breakdown of perturbation theory," Phys. Rev. Lett. 25, 1149-1151 (1970). [CrossRef]
  3. H. R. Reiss, "Nuclear beta decay induced by intense electromagnetic fields: forbidden transition examples," Phys. Rev. C 27, 1229-1243 (1983). [CrossRef]
  4. L. V. Keldysh, "Ionization in the field of a strong electromagnetic wave," Zh. Eksp. Teor. Fiz. 47, 1945-1957 (1964) [Sov. Phys. JETP 20, 1307-1314 (1965)].
  5. F. H. M. Faisal, "Multiple absorption of laser photons by atoms," J. Phys. B 6, L89-L92 (1973). [CrossRef]
  6. H. R. Reiss, "Effect of an intense electromagnetic field on a weakly bound system," Phys. Rev. A 22, 1786-1813 (1980). [CrossRef]
  7. W. C. Henneberger, "Perturbation method for atoms in intense light beams," Phys. Rev. Lett. 21, 838-841 (1968). [CrossRef]
  8. H. R. Reiss, "Energetic electrons in strong-field ionization," Phys. Rev. A 54, R1765-R1768 (1996). [CrossRef] [PubMed]
  9. M. Pont and M. Gavrila, "Stabilization of atomic hydrogen in superintense, high-frequency laser fields of circular polarization," Phys. Rev. Lett. 65, 2362-2365 (1990). [CrossRef] [PubMed]
  10. H. R. Reiss, "High-frequency, high-intensity photoionization," J. Opt. Soc. Am. B 13, 355-362 (1996). [CrossRef]
  11. D. P. Crawford and H. R. Reiss, "Stabilization in relativistic photoionization with circularly polarized light," Phys. Rev. A 50, 1844-1850 (1994). [CrossRef] [PubMed]
  12. H. R. Reiss, "Relativistic strong-field photoionization," J. Opt. Soc. Am. B 7, 574-586 (1990). [CrossRef]
  13. H. R. Reiss and D. P. Crawford, "Relativistic photoionization," in Ultrafast phenomena and interaction of superstrong laser fields with matter: nonlinear optics and high-field physics, M. V.Fedorov, V. M. Gordienko, V. V. Shuvalov, V. D. Taranukhin, eds., Proc. SPIE 3735, 148-157 (1998).
  14. A. M. Popov, O. V. Tikhonova, and E. A. Volkova, "Applicability of the Kramers-Henneberger approximation in the theory of strong-field ionization," J. Phys. B 32, 3331-3345 (1999). [CrossRef]
  15. H. R. Reiss, "Frequency and polarization effects in stabilization," Phys. Rev. A 46, 391-394 (1992). [CrossRef] [PubMed]
  16. H. R. Reiss, "Physical basis for strong-field stabilization of atoms against ionization," Las. Phys. 7, 543-550 (1997).
  17. N. J. Kylstra, R. A. Worthington, A. Patel, P. L. Knight, J. R. V zquez de Aldana, L. Roso, "Breakdown of stabilization of atoms with intense, high-frequency laser pulses," Phys. Rev. Lett. 85, 1835-1838 (2000). [CrossRef] [PubMed]
  18. H. R. Reiss, "Dipole-approximation magnetic fields in strong laser beams," Phys. Rev. A 63, 013409 (2001). [CrossRef]

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