Gas temperature measurements in a high pressure argon pulsed dielectric barrier discharge using diode laser absorption spectroscopy

Summary form only given. A tunable diode laser absorption spectroscopic technique has been used to quantify the high-pressure (p= 100-500 Torr) scaling of gas temperature in an argon dielectric barrier discharge (DBD) excited using two different applied unipolar voltage pulses from 1.7-6.0 kV and <;800μJ/pulse maximum deposited energy. The diode laser tuning range was sufficient to capture both collision-broadened "wings" of the 1s
₃
-2p
₂
(772.42 nm) and 1s
₅
-2p
₇
transitions simultaneously in a single scan. The diagnostic technique is based upon Lindholm-Foley theory
¹
which is particularly applicable for highly nonequilibrium discharge conditions in which the Lorentzian line shape dominates over the Doppler component. Along with the ideal gas law, Lindholm-Foley theory can be used to express both "shift" and "collision width" temperatures as T
_s
=T
_o
(β
_o
p/β)
^10/7
and T
_c
=T
_o
(2Γ
_o
p/2Γ)
^10/7
, respectively, where T
_s
=300 K, β
_o
, and 2Γ
_o
are the reduced shift and broadening coefficients for a specified transition. In order to use this method to measure small gas temperature changes, it is important to obtain accurate values of β
_o
and 2Γ
_o
for each transition. Measurements of 2Γ
_o
were not found in literature
²
, while β
_o
was found to be systematically biased
²
. Estimates of these coefficients were performed using a self-consistent in situ cross-calibration incorporating both transitions, (i) low pressure measurements of 2Γ
_o
and (ii) high pressure measurements of the absolute shifts and line widths. Low pressure (p=5-50 Torr) measurements gave 2Γ
_o
(1s
₃
-2p
₂
)=22.8±1.0 MHz/Torr and 2Γ
_o
(1s
₅
-2p
₇
)=21.6±0.3 MHz/Torr. Frequency shift measurements were performed by comparing absorption peaks obtained simultaneously from the DBD and a 1 Torr argon dc discharge. The 2Γ measurements were obtained from Voigt profile fitting. Lindholm-Foley theory predicts that β/2Γ=β
_o
/2Γ
_o
≈ -0.3627. Our measurements indicate that β
_o
2Γ
_o
=-0.333±0.025 for both transitions so that within the specified uncertainties, β
_o
(1s
₃
-2p
₂
)≈-7.6±0.7 MHz/Torr and β
_o
(1s
₅
-2p
₇
)≈-7.2±0.5 MHz/Torr. Over the range of pressure and voltages, the measured gas temperature was nearly constant at 330±30 K.