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64 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 65

Thus, current activity in HF and DF chemical lasers is limited to low-
power laboratory devices and specialty applications, which typically
use electrical discharge, rather than combustion, to dissociate SF to
6
produce fluorine atoms and which range in power from tens of watts
to a few hundred watts.

3.4 Chemical Oxygen Iodine Laser (COIL)

The possibility of making a COIL device was first suggested by
12
Derwent and Thrush in 1971. The first lasing demonstration was
made by McDermott and his team at the Air Force Weapons Lab
13
(AFWL) in 1978. Truesdell, Helms, and Hager summarized
14
development efforts within the United States through 1995. Tech-
nology improvements and scaling to large devices have continued
at a variety of sites.
COIL devices are based on a very different chemistry from HF and
DF lasers. Figure 3.20 shows a simplified block diagram of the opera-
tional approach. First, chlorine gas reacts with liquid basic hydrogen
peroxide (BHP) in a gas-liquid reaction. The reaction then produces
electronically excited singlet delta oxygen, which has a very long life-
time as compared with most electronically excited states. The reactor
is denoted a singlet delta oxygen generator (SOG). Next, the excited
oxygen, along with a suitable diluent, is transported to supersonic
expansion nozzles, which mix the oxygen with molecular iodine and
a suitable carrier gas. The singlet delta oxygen chemically dissociates
the molecular iodine to produce atomic iodine, which has electronic
states that favor the resonant transfer of energy from the singlet delta
oxygen to the iodine. The resultant gain allows lasing on an atomic
iodine transition. Typically, the ratio of oxygen to iodine atoms is rela-
tively large, and many energy transfers occur per iodine atom until
the singlet delta fraction is sufficiently depleted. As the block diagram
in Fig. 3.20 shows, a practical device must also include provisions to
circulate the BHP, remove the reaction heat, and recover pressure to
ambient conditions.

3.4.1 Energy Levels
The lasing species in COIL devices is the iodine atom. In contrast to
most other chemical lasers, the transition energy levels are electronic
rather than molecular. The lasing transition is a magnetic dipole tran-
sition between two spin orbit terms of the ground state 5 P 1/2 →
2
2
5 P 3/2 . Because this transition is forbidden, it has a relatively long
radiative lifetime of approximately 125 milliseconds (ms). The upper
level is split into two hyperfine levels—total angular momentum
quantum numbers, F = 2 and 3. The lower level is split into four
hyperfine levels, F = 1, 2, 3, and 4. Associated degeneracies are 2F + 1.
The energy levels are shown schematically in Fig. 3.21, in which the