fragments with a probability of 1. Nonadiabatic processes can occur during this
state decay. A similar effect is observed in the A absorption band of alkyl- and
perfluoroalkyl iodides.
Let in the Franck–Condon zone, the state correlating with fragments having
lower excitation energy lies higher than the state correlated to products with higher
excitation energy (Fig. 4.32a). In the process of decay of these states nonadiabatic
processes are very likely, i.e., transitions from one PES to another occur.
The probabilities of these processes depend on interaction matrix elements and
time of passage of an image point through the region of convergence of the states.
Qualitative dependence of the probability of nonadiabatic processes on the photon
energy, i.e., in fact, from the velocity of motion of the image point, obeys the
Landau-Zener rule (see Sect. 3.6). If the ‘intersection’ of repulsive states occurs in
the Franck–Condon zone, oscillations can be observed on partial absorption
cross-sections corresponding to transitions to states dissociating through different
channels. The curve of the total absorption cross-section remains smooth. The
mechanism of this effect is illustrated in (Fig. 4.32b).
Suppose that the oscillator strengths of optical transitions to the interacting states
A and B converging diabatically to A 2 + B 2 and A 1 + B 1 , respectively, are
approximately the same, and the PESs of these states in the vertical transition region
approach and repel along the decay coordinate, i.e., the probabilities of transition
from point a to point b and from point c to point d are small. Then the spectral
dependencies of the partial absorption cross-sections corresponding to transitions to
states dissociating on A 2 + B 2 (1) and A 1 + B 1 (2) have an oscillating character.
Indeed, the optical transition to the e
B state PES corresponds to a wider absorption
spectrum with a lower maximum value r i (k) (the left side of curve 1 and the right
side of curve 2). Since the partial absorption cross-section corresponding to the
decay into A 2 + B 2 is due to the transition of the state e
B to the intersection zone,
and state e
A after the intersection zone on the PESs, on curve 1 at the point corresponding to the intersection zone, a sharp change in the absorption cross-section
Fig. 4.32 Interaction of repulsive states in the process of their dissociation
4.8 Dissociation of Polyatomic Molecules …
147
state decay. A similar effect is observed in the A absorption band of alkyl- and
perfluoroalkyl iodides.
Let in the Franck–Condon zone, the state correlating with fragments having
lower excitation energy lies higher than the state correlated to products with higher
excitation energy (Fig. 4.32a). In the process of decay of these states nonadiabatic
processes are very likely, i.e., transitions from one PES to another occur.
The probabilities of these processes depend on interaction matrix elements and
time of passage of an image point through the region of convergence of the states.
Qualitative dependence of the probability of nonadiabatic processes on the photon
energy, i.e., in fact, from the velocity of motion of the image point, obeys the
Landau-Zener rule (see Sect. 3.6). If the ‘intersection’ of repulsive states occurs in
the Franck–Condon zone, oscillations can be observed on partial absorption
cross-sections corresponding to transitions to states dissociating through different
channels. The curve of the total absorption cross-section remains smooth. The
mechanism of this effect is illustrated in (Fig. 4.32b).
Suppose that the oscillator strengths of optical transitions to the interacting states
A and B converging diabatically to A 2 + B 2 and A 1 + B 1 , respectively, are
approximately the same, and the PESs of these states in the vertical transition region
approach and repel along the decay coordinate, i.e., the probabilities of transition
from point a to point b and from point c to point d are small. Then the spectral
dependencies of the partial absorption cross-sections corresponding to transitions to
states dissociating on A 2 + B 2 (1) and A 1 + B 1 (2) have an oscillating character.
Indeed, the optical transition to the e
B state PES corresponds to a wider absorption
spectrum with a lower maximum value r i (k) (the left side of curve 1 and the right
side of curve 2). Since the partial absorption cross-section corresponding to the
decay into A 2 + B 2 is due to the transition of the state e
B to the intersection zone,
and state e
A after the intersection zone on the PESs, on curve 1 at the point corresponding to the intersection zone, a sharp change in the absorption cross-section
Fig. 4.32 Interaction of repulsive states in the process of their dissociation
4.8 Dissociation of Polyatomic Molecules …
147