Vibrational energy transfer in CO2under laser conditions with and without water vapor

It is shown that in the case of a dry 1.5-torr CO
2
gas fill the upper laser level is indirectly excited by vibrationally excited CO produced during the discharge, whereas in the case of a 1.5-torr CO
2
and 0.2-torr H
2
O mixture the upper laser level is directly excited by the electrons in the discharge. The collision relaxation times measured under laser conditions for the symmetric valence vibration of CO
2
in a CO
2
-H
2
O mixture and in a CO
2
-CO mixture as produced during a discharge of an initially pure CO
2
fill were 19 and 73 μs, respectively. If the reasonable assumption was taken that half of the CO
2
was dissociated into CO then this result shows that H
2
O was 14 times more effective in depopulating the lower laser level than CO. The growth in laser intensity for the dry fill was shown to be due to the CO (
\nu = 1
) transfer of energy to the asymmetric vibration of CO
2
(00°1) with a characteristic increase that was exponential strictly only for a time short compared with the relaxation time of the symmetric vibration. The characteristic transfer time for excitation of the asymmetric vibration was dependent upon the fraction of CO present. If we make the assumption of 50 percent dissociation, the intermolecular energy transfer time between CO and CO
2
was found to be 40 torr-μs. Results obtained with N
2
and He added to the laser mixture indicated that He was not more effective in relaxing the lower laser level than N
2
or CO and was less effective than H
2
O.