Species composition in a CO2laser discharge

Recent experimental studies have shown that small quantities of nitrogen oxides can seriously effect discharge stability and power output in flowing CO
2
discharge lasers. In the present study, the formation of these oxides was measured in a sealed-off discharge tube under operating conditions similar to those in CO
2
lasers. The gas composition was sampled with a modulated molecular beam, low-voltage ionizer, quadrupole mass spectrometer. The combined use of low ionizer voltage and phase sensitive detection eliminated extraneous mass peaks caused by background gases and dissociative ionization in the analyzer itself. Equilibrium and time-varying measurements were made of neutral and ionix species. Two initial gas compositions were used 1) 71.8 percent He, 15.5 percent N
2
, 12.7 percent CO
2
, and 2) 87.2 percent He, 12.8 percent CO
2
. Measurements were made in a discharge tube 2.2 cm inside diameter, 26 cm between electrodes. The system was pumped to approximately 10
^-6
mm Hg between data runs. Total pressure was varied from 1 to 8 mm Hg and discharge current from 0 to 80 mA. Typical CO
2
dissociation data at 40 mA and 4 mm Hg indicate an initial rate of
7(\pm0.5) \times 10^{-2}
s
^-1
with an equilibrium value of approximately 65 percent dissociation in the He/ N
2
/CO
2
mixture and slightly lower vatues in the He/CO
2
mixture. Under the same conditions, nitric oxide (NO) exhibited a buildup rate of
8(\pm1) \times 10^{-2}
s
^-1
with an equilibrium value of approximately 0.1 percent of total gas composition. Nitrogen diozide (NO
2
) was detected by comparing the relative amounts of mass numbers 45 and 46. Data indicate that in the He/N
2
/ CO
2
mixture the NO
2
component of mass 46 can be as high as 0.12 percent of total mixture with an initial buildup rate of
7(\pm2) \times 10^{-2}
s
^-1
at 80 mA discharge current. Measurement of the ion spectra at the cathode indicate that CO
2
+ is the major initial ion in the He/N
2
/CO
2
mixture, but decays with a rate of
2.5 (\pm.5) \times 10^{-1}
s
^-1
and is replaced by NO
⁺
at approximately the same rate. In the He/CO
2
mixture CO
2
+ is replaced as the major ion by O
2
+ , with with slightly lower rates than those measured in the mixtures containing N
2
.