Engineering Chemistry S Datta

V 2
V 1V0
PHOTOCHEMISTRY 551
E 1
E 2
Potential energy
r 2
A
V 2
V 1
V 3
V 2
V 1
V 0
V 0
r 0
r 1
Internuclear distance
Fig. 25.2 Energy curves for a diatomic molecule. Two possible transitions are shown. When an
electron has been excited to the point marked A, the molecule may cleave.
(2) When the promotion is to a lower vibrational level v 1
or v 2
which lies within the curve
E 2
, the molecule may cleave. From the curve in Fig. 25.2, we find that the equilibrium distance
is greater for excited state than in ground state.
Franck-Condon principle states that promotion of an electron (10 –15 sec) is much
faster than the single vibration (10 –12 sec). Hence when an electron is suddenly promoted the
distance between the atoms remains unchanged and hence the bond behaves like a compressedspring,
the condition is suddenly relieved by breaking the bond.
(3) In some cases as in Fig. 25.3, the excited state entirely leads to dissociation. E.g., is
the σ – σ* promotion in hydrogen molecule leading to cleavage. The photolytic cleavage may
break the molecule into two smaller molecules or into free radicals. Free radicals behave in a
similar way like free radicals produced in other processes, except that the former are in excited
state. Rarely the molecule breaks into two ions.
E 1
E 2
Potential energy
Internuclear distance
Fig. 25.3 Promotion to a dissociative state results in bond cleavage.