Page 91 - Tunable Lasers Handbook

72 Charles Freed

so that during these near-resonant collisions the excited N, molecules can read-
ily transfer their vibrational energy to the CO, molecules and excite the vibra-
tional levels of the v3 mode in CO,.
Excitation of CO, molecules to the upper laser levels may also occur by
means of electron impact excitation from the ground state, from recombinations.
or from cascades from levels above the (0001) upper laser level. We already men-
tioned in Section 2 and also illustrated in Fig. 2 that the levels in the v3 mode
form an almost equally spaced ladder so that during a collision an excited CO,
(OOOu,) molecule can lose one quantum of v3 vibrational energy and become a
CO, (OOOu3-1) molecule, while the CO, (0000) molecule in the ground state
gains that quantum of energy and becomes an excited CO, (0001) molecule in the
upper laser level [1,2.4,5]. As Pate1 was the first to point out, this type of process
is resonant in the sense that there is a redistribution of the energy of the excited
molecule without any loss of the total internal energy by its conversion into
kinetic, or thermal, energy [ 1,2.4.5]. Similarly, resonant redistribution of energy
can also occur in the vibrational ladder of the excited N, molecules. Thus the
excitation of the CO, molecules to the required upper laser level may be very effi-
ciently accomplished by electron impact in the gas discharge of a CO, laser. Note
that CO plays a role similar to N2 in the gas discharge. CO may be present in the
gas discharge as a result of dissociation of CO,, or it may be initially added to the
laser gas fill in order to reduce the deleterious-buildup of 0, that also occurs due
to dissociation of CO, in the laser gas discharge.
During laser operation the excited CO, molecules in the (0001) upper laser
level will go to the [1000, 02001, and [1000, 0200],, lower laser levels while
emitting photons in the lasing transitions belonging to the 10.4- and 9.4-pm reg-
ular bands of CO,, respectively. The molecules in the lower laser levels are then
deexcited through collisions with other molecules. The possibility of resonant
vibrational energy transfer again plays an important role in vacating the 10001,
0200],,11 lower laser levels, because molecules in these levels have nearly twice
the energy required to excite a CO, molecule in the (0000) ground state to the
(0100) vibrational level. Thus a c6llision involving a molecule in one of the
11000. 02001 lower laser levels with a molecule in the (0000) ground state will
efficiently redistribute the vibrational energy between the two molecules by
exciting both of them to the CO, (0100) level. Because of the resonant nature of
this collision, the vibrational deexcitation of the lower laser level can be also
very efficient.
Finally, however, the CO, molecules in the (0100) level still must be deex-
cited to the (0000) ground state before they can be reutilized in the laser gas mix-
ture. This deexcitation of CO, molecules in the (0100) level is governed by colli-
sions with other CO,, or other gas particles. or the walls of the laser tube.
Because of the nonr&onant nature of this vibrational energy conversion into
kinetic energy, the deexcitation of the CO, molecules in the (0100) level can be
relatively slou and cause a “bottleneck” in the overall cycle of excitation and