Geoscience Reference
In-Depth Information
2.4
Effects of Monochromatic Laser Radiation
Even though the basic mechanisms of the interaction between monochromatic
laser radiation and autodissociating intermediate states are known and can be
described by well-developed mathematical methods (e.g., see Geller and Popov
1981
; Andryushin et al.
1982
,
1985
;Ivanoveta .
1995
; Yakovlenko
1982
;
Lambropoulos and Zoller
1981
; Alber and Zoller
1983
; L'Huillier et al.
1988
), the
theory of the continuum-intermediate-state-continuum-type processes is far from
complete. As already mentioned, this problem is substantially different from those
generally solved in the theory of atom/molecule-field interaction. It also differs
from the problems solved in the theory of the radiative collisions (Dubov et al.
1982
; Yakovlenko
1984
), where the semiclassical approach has proved to be most
successful in the study of collisions between atoms in a radiation field. Quantum
mechanical analysis of the relative motion using conventional methods of the
theory of radiative collisions is a more complicated task, particularly when particle
structure is changed by reaction. In particular, the rotational and vibrational degrees
of freedom have been found to manifest themselves in the resonant multiphoton
ionization (RMPI) and the dissociation (RMPD) of diatomic molecules, when their
bound or autodissociating states are significantly populated at the intermediate stage
(before transition to a continuum state). These findings motivate a detailed study of
the laser-induced coupling of the bound or quasibound states between themselves
and to continuum states.
The quantum scattering formalism (including multichannel quantum defect
theory (Golubkov and Ivanov
2001
)) is applicable because the molecule-field
interactions in e
C
XY
C
and X
C
Y configurations are strong only at the XY
formation stage, when the laser-induced coupling
V
f
can be treated as comparable
to the electrostatic interaction
V
e
, which formally increases the number of channels
in the scattering problem. The spectrum of the resulting unperturbed Hamiltonian
consists of noninteracting states with quasi-energies defined as modulo !
f
;this
provides a simple method for constructing a
T
-matrix and wave functions that can
be used to calculate RMPI and RMPD processes involving large groups of states
(including infinite Rydberg resonance series) while keeping precise track of the
coupling of the intermediate states to the continua and transitions between them.
When the process occurs via a single intermediate resonant state (as in
k
*
-
photon absorption for k
<k
m
,where
k
m
is the minimum number of !
f
photons
required for ionization), the photoelectron spectrum can be described by the Breit-
Wigner-type formula with resonance width corresponding to the .k
k
m
/-photon
transition from this state to the continuum (Letokhov
1987
). In other words, the
single-resonance approximation can be applied to the simplest transition state that
is formed at an intermediate stage.
The next step in theory focused on systems where resonance conditions are
satisfied simultaneously for two intermediate states, such as those populated by
.k
1/ and k
photon absorption. Processes of this kind were examined by
analyzing equations for the intermediate-state population amplitudes when a cou-
pling field was rapidly switched on (Andryushin et al.
1982
,
1985
; Lambropoulos
