Geoscience Reference
In-Depth Information
whose contribution to the laser-driven RMPI and RMPD processes is not evident
a priori. In most cases, a rotation can increase the reaction rate by adding new
open channels as a consequence of the laser-induced nonadiabatic coupling between
the electronic and rotational degrees of freedom, which occurs when electronically
excited molecules are produced at the intermediate stage of the process. In other
situations, rotation has an inhibiting effect on the reaction, e.g., when RMPI of
diatomic
XY
molecules involves laser-induced bound states situated on the
X
C
Y
dissociative continuum background (Charron et al.
1994
).
The vast majority of results in the RMPI theory based on the concept of field-
dressed states pertain to atomic systems. As already noted, molecules are much
more difficult to study. Generally, the intermediate molecular states are degenerate
(e.g., with respect to the projection
M
of total angular momentum
J
when J
1).
Furthermore, their upper levels usually predissociate; that is, the corresponding set
of dissociation continua is more diverse than atomic continua. Molecules have the
vibrational and rotational degrees of freedom, which complicate the laser-driven
dynamics of even the simplest diatomic systems; i.e., analyses of photoinduced
processes must take into account rovibronic states. The best developed model of
interaction between a monochromatic field and a molecule is that of the two-photon
ionization and dissociation (Chen et al.
1993
), with a single intermediate state or a
few mutually interacting ones.
The field-induced coupling between resonances can be sufficiently strong that
the corresponding Rabi frequency becomes comparable with the rotational level
separations. Under such conditions, RMPI involves large groups of levels, that is,
the observed phenomenon has no analogue in atomic systems. It is obvious that the
positions and widths of these levels must manifest themselves in the photoelectron
spectra, each of which is a hybrid that is mixed with the other states. The radiative
scattering matrix method provides a very convenient formalism for analyzing the
combined effect of a weak probe field and a strong monochromatic one (Ivanov
et al.
1988
,
1995
,
1997b
,
c
,
1999
; Vartazaryan et al.
1989
; Golubkov et al.
1993
,
1999a
,
b
; Golubkov and Ivanov
1993
,
1994
,
1997
). In the ionization scheme, the
system is supposed to be excited to the intermediate (working) level by the probe (or
via off-resonant multiphoton processes induced by the strong field), and the strong
field drives a cascade of transitions between excited or high-lying excited states and
transitions to the continuum.
The rotational alignment of ions can be used to select energy-unresolved states
because the field-induced coupling strength between molecular states depends on
the projection
M
of total angular momentum on the laser polarization axis. As a
result, molecular ions with given
M
values can be produced by varying the frequency
and strength of the laser field.
When f
10
4
10
3
, the contribution from the groups of dressed states
gives rise to the strongly mixed “diatomic molecule
C
light field” hybrid states.
An analysis of the field-strength dependence of their widths has revealed a
nonmonotonic behavior (in particular, a decrease in width with increasing
f
), which
suggests molecular stabilization in a certain interval of
f
.
