Geoscience Reference

In-Depth Information

and Zoller
1981
). This work required the use of a special procedure developed

to describe the continuum coupling because the widespread relaxation parameter

matrix method had proved to be inapplicable. Both RMPI and RMPD characteristics

revealed nonmonotonic variation in molecular dynamics with laser intensity (Joli-

card and Atabek
1992
). This behavior is most clearly manifested in above-threshold

dissociation (ATD), which is significant at laser intensities I
10
13
W=cm
2

(Jolicard and Atabek
1992
; Giusti-Suzor et al.
1990
;Zavriyevetal.
1990
). The

process competing with ATD is the ordinary single-photon absorption, and their

relative rates depend on the molecule's initial vibrational state and the laser intensity

and frequency. An increase in intensity to I
10
14
W=cm
2
has a considerable

bond-softening effect because the molecular potential is distorted by the laser

field (Zavriyev et al.
1990
; Yao and Chu
1992
). Details of the intensity-dependent

dynamics underlying these phenomena were examined by Yang et al. (
1991
). Laser

effects on molecular ions were investigated by Zavriyev et al. (
1993
) and Aubanel

et al. (
1993
). It should also be noted that a bichromatic driving field (the fundamental

harmonic and a phase-shifted higher one of the same laser) provides a means for the

coherent control of dissociation (Charron et al.
1994
; Abrashkevich et al.
1998
),

in particular for the isotope separation (Atabek et al.
1994
; Charron et al.
1995
).

These experimental observations stimulated theoretical studies intended to explain

and predict the laser-driven phenomena. Let us discuss these matters in some detail.

Two essentially different approaches were developed to solve problems of this kind

(e.g., see Giusti-Suzor et al.
1995
;Guo
1998
).

In one of these, the stationary equations of the Floquet theory are used (Shirley

1965
), which is valid to describe processes driven by dye lasers with typical pulse

durations of 10
10
to 10
9
s, when the field is switched on at
t
D
0 and applied

for a sufficiently long time. The approach is true if the laser pulse is sufficiently

long and the process dynamics does not vary from cycle to cycle (
l
r
,where

r
is a reaction time). The total wave function is represented as a superposition of

dressed states, and the system of coupled equations is solved numerically (Chu

1991
;Heetal.
1990
; Bandrauk et al.
1993
). For short laser pulses .
l
<10
12
s/,

the Schr odinger equation describing the time evolution of the emerging wave packet

is generally computed by using a Gaussian pulse shape. Finding of such solutions

is a formidable task. However, a number of assumptions can be made that greatly

simplify theoretical analysis, such as the one-dimensional approximation where

the internuclear vector is collinear to the laser polarization vector. The resulting

solution is only slightly different from those of the three-dimensional problem

(Charron et al.
1994
).

2.5

The Role of Rotational Degrees of Freedom

One of the issues most widely discussed in the literature is the role of molecular

rotation (Ivanov et al.
1996
,
1997b
; Charron et al.
1994
; Giusti-Suzor et al.
1995
;

McCann and Bandrauk
1992
; Banerjee et al.
1994
; Sukharev and Krainov
1998
),