Geoscience Reference
In-Depth Information
developed by Krol and Watson (
1973
). In those studies, collisions were treated as the
instantaneous elastic scattering events, the spontaneous emission being neglected.
Because we are interested in the processes involving electrons, we must first dis-
cuss the free electron dressing effects. It is known, that in the strong electromagnetic
field with a frequency !
f
we can use the semiclassical approximation to describe
a motion of the nonrelativistic electron with the energy E
p
and momentum
p
,in
which the corresponding wave function has the form (
„D
m
e
D
e
D
1):
mDC
X
m
exp
i
E
p
m!
f
t
C
i.
p
m
k
/
r
ˆ
p
.
r
;t/
D
(2.4)
mD1
where
k
is the photon momentum. Expression (
2.4
) is a superposition of terms
corresponding to the number of virtual absorbed (or emitted) photons in the external
field. The properties of the expansion coefficients
m
were discussed in references
(Nikishov and Ritus
1964
; Brown and Ribble
1964
; Leubner
1981
;Reiss
1962
,
1980
).
When the target has an internal structure, the atomic system dressing effects
should be taken into account (Ehlotzky
1985
). A consistent treatment of field-
dressed electron and target states is primarily based on two approaches. One of
these approaches uses perturbation series in the coupling between the laser field and
the atom-electron system (Francken et al.
1988
). In the other, the laser-electron
interaction is described exactly while the target dressing is treated perturbatively
(Martin et al.
1989
; Dimou and Faisal
1987a
).
Theoretical studies of intense laser radiation effects on the scattering of slow
electrons by the ion targets were carried out (Francken et al.
1988
; Martin et al.
1989
; Dimou and Faisal
1987a
; Giusti-Suzor and Zoller
1987
;Heetal.
1989
;
Ivanov et al.
1988
; Vartazaryan et al.
1989
; Golubkov et al.
1993
). In three studies
(Ivanov et al.
1988
; Vartazaryan et al.
1989
; Golubkov et al.
1993
), the laser-induced
continuum-continuum coupling between free electron states was ignored. In terms
of the quasi-energy states, this approximation discards the contributions to Eq.
2.4
from the harmonics corresponding to changes in total photon number .m
D
0/,
which is justified if the following condition is realized:
fp
e
!
f
D
1
(2.5)
(p
e
is the electron momentum). This result means that the oscillation amplitude of
the laser-driven electron is smaller than its wavelength. The condition in Eq.
2.5
implies that the electron can be treated as the free particle with well-defined
momentum and energy at an asymptotic region. If the target is an unstructured
particle, then the only channel at small is the elastic scattering. Therefore, the
laser radiation can have a significant effect on the electron-target system under the
condition (
2.5
) only if the target has a internal structure. If the target is a positive
molecular ion, this structure causes by formation of a Rydberg complex
XY
**.
