Page 97 - High Power Laser Handbook

66 G a s , C h e m i c a l , a n d F r e e - E l e c t r o n L a s e r s Chemical Lasers 67

–1
Transition sec
3-4 5.0
3-3 2.1
3-2 0.6
2-3 2.4
2-2 3.0
2-1 2.3

Table 3.3 Einstein A
Coefficients

for upper and lower population densities N and N the gain is pro-
U
portional to
N – (g /g )N (3.20)
U
L
L
U
Then, for the 3 to 4 transition, gain is proportional to
(7/12)[P 1/2 ] – (7/9)(9/24)[P 3/2 ] = (7/12)([P 1/2 ] – 1/2[P 3/2 ]) (3.21)

This implies that a partial inversion also produces gain in COIL
devices.
The relatively narrow line width of the single-line COIL devices,
as compared with other types of chemical lasers, has advantages in
some applications. At very low pressures, for example, COIL devices
are substantially Doppler broadened. At moderate pressures, pres-
sure broadening also becomes important—and even potentially
helpful from a hole-burning standpoint

Energy Pumping Reactions: Singlet Delta E-E Transfer
The source of energy for COIL devices is near-resonant energy trans-
fer from electronically excited singlet delta oxygen [O ( ∆)] to ground-
1
2
2
state iodine atoms, yielding P 1/2 iodine atoms and ground state
O ( Σ) oxygen also denoted simply as O . The electronic energy level
3
2
2
to electronic energy level (E-E) transfer energetics are illustrated in
Fig. 3.22. Although the vibrational and rotational levels of oxygen are
not shown in Fig. 3.22, vibrational levels may play some role in deac-
tivation processes and iodine dissociation. The kinetic equation that
describes the E-E transfer process is as follows:
1
I + O ( ∆) → I* + O (3.22)
2
2
The reverse reaction is
I* + O → I + O ( ∆) (3.23)
1
2
2