Page 87 - Tunable Lasers Handbook

68 Charles Freed

The most readily obtainable and widely utilized lasing transitions in CO,
lasers are the so-called regular band transitions. These occur between the (0001)
upper laser level and the (1000) and (0200) lower laser levels as indicated by the
solid-line arrows interconnecting those levels in Fig. 2. The (1000) and (0200)
levels have nearly the same energy in spite of belonging to different vibrational
modes, and are “accidentally degenerate” [31]. As was first recognized by Fermi
[32] in the case of CO,, such “Fermi resonance” leads to a perturbation of the
energy levels. Thus for-CO, one of the two previously mentioned energy levels
is shifted up and the other iown so that the separation of the two levels is much
greater than expected, and a mixing of the eigenfunctions of the two states
occurs. Thus the correct regular band transition assignments are denoted by
[0001-(1000,0200),] for the 10.4-pm band and by [0001-(1000,0200),,] for the
9.4-pm band, respectively. This follows Amat’s notation [33] in which the com-
ponents of a Fermi multiplet are labeled with Roman subscripts in order of
decreasing energy. It has been determined [33-351 relatively recently that the
[lOOO, 02001, level is to be identified with the unperturbed (0200) level in
IT160 2’ 12C180,. and 13C18O,, and with the unperturbed (1000) level in 13C1602.
Note that this identification is the reverse of the traditional notation for 12C160,
in many older publications (including the pioneering ones by Patel). Femii reso-
nances similar to the one observed for the (1000) and (0200) vibrational levels
also exist between certain higher levels and will be similarly designated in other
lasing transitions of the C02 system in later portions of this chapter.
Two additional aspects of the CO, laser system should be emphasized in the
energy-level diagram of Fig. 2. The first of these relates to the 4.3-pm fluores-
cence, indicated by a dotted mow, that emanates from the (0001) upper laser
level of the regular band into the (0000) ground state due to spontaneous emis-
sion. This spontaneous emission at 4.3 pm plays a most important role in the
long-term line-center stabilization of the lasing transitions that constitute a
salient portion of this chapter.
Finally, attention is drawn to the very small energy difference (AE= 18 cm-1)
that exists between the (0001) upper laser level of 12C1607 and the (U = 1) level
of IJN,. Nitrogen molecules can be very efficiently excitedfrom the (U = 0) level
to the <v = 1) level by electron impact in a low-pressure discharge. Because the
energy of excitation of the N, (u = 1) molecule nearly equals the energy of exci-
tation of the CO, (0001) molecule. an efficient transfer of vibrational energy
takes place from h, to CO, in collisions between N, (U = 1) molecules and CO,
(0000) molecules. In such a collision, the nitrogen molecule returns from the (6
= 1) level to its ground state by losing one quantum of its vibrational energy,
thereby exciting the carbon dioxide molecule from its ground state to the (0001)
level. The CO, can then radiatively decay to either the [lOOO, 02001, or [lOOO,
0200],, levels, and emit infrared light at 10.4 or 9.4 pm, respectively, during this
process [ 1,2,4,5].