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Chair experimental physics V "atom and laser physics" at the faculty of physics and astronomy at Ruhr University Bochum

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Time evolution of CO2 ro-vibrational excitation in a nanosecond discharge measured with laser absorption spectroscopy

The results in this dataset have been published in J. Phys. D: Appl. Phys. 54 (2021) 365201.
CO2 dissociation stimulated by vibrational excitation in non-equilibrium discharges has drawn lots of attention. Nanosecond (ns) discharges are known for their highly non-equilibrium conditions. It is therefore of interest to investigate the CO2 excitation in such discharges. In this paper, we demonstrate the ability for monitoring the time evolution of CO2 ro-vibrational excitation with a well-selected wavelength window around 2289.0 cm−1 and a single
continuous-wave quantum cascade laser with both high accuracy and temporal resolution. The rotational and vibrational temperatures for both the symmetric and the asymmetric modes of CO2 in the afterglow of CO2 + He ns-discharge were measured with a temporal resolution of 1.5 µs. The non-thermal feature and the preferential excitation of the asymmetric stretch mode of CO2 were experimentally observed, with a peak temperature of Tv3, max = 966 ± 1.5 K, Tv1,2, max = 438.4 ± 1.2 K and Trot = 334.6 ± 0.6 K reached at 3 µs after the nanosecond pulse.
In the following relaxation process, an exponential decay with a time constant of 69 µs was observed for the asymmetric stretch (001) state, consistent with the dominant deexcitation mechanism due to VT transfer with He and deexcitation on the wall. Furthermore, a synchronous oscillation of the gas temperature and the total pressure was also observed and can be explained by a two-line thermometry and an adiabatic process. The period of the oscillation
and its dependence on the gas components is consistent with a standing acoustic wave excited by the ns-discharge.

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Plasma Source Name
Plasma Source Application
Plasma Source Specification
Plasma Source Properties
The applied voltage by the DC high voltage generator is 3 kV with a voltage pulse length of 150 ns and a repetition rate of 2 kHz. The measured voltage at the powered electrode and current waveforms are shown in the publication (J. Phys. D: Appl. Phys. 54 (2021) 365201). The voltage rise time from 10% to 90% amplitude is 13 ns. The measured peak voltage and current are 2.1 kV and 8.4 A, respectively.
Plasma Source Procedure
The discharge consists of two molybdenum electrodes with a cross-section of 20 × 1 mm placed vertically with a distance of 1 mm. Two glass plates are pressed tightly to the electrodes at the front and the back, yielding a well-confined discharge volume of 20 mm × 1 mm × 1 mm. Nanosecond voltage pulse generated by combining a DC power supply (Heinzinger, LNC 6000-10 neg) with a fast high-voltage switch (Behlke HTS-81) is applied to one electrode with the other one grounded. A delay generator (Stanford Research Systems DG535) with internal triggering is used to trigger the switch with a repetition rate of f p = 2 kHz and an on-time of 150 ns. To allow easy discharge operation, the value of the repetition frequency is chosen within a range determined by the energy input to the system (to limit gas heating) and remaining seed electrons from the previous pulse (to ease discharge ignition). The incoming gas consists of 10% CO2 + He by mixing pure CO2 and He gas flow using two mass flow controllers (MKS instruments) with a total flow rate of 30 sccm. The pressure in the discharge chamber is monitored by a pressure gauge (Pfeiffer vacuum) and is kept constant at 145 mbar by fine adjusting the mechanical needle valve at the gas outflow.
Plasma Medium Name
Plasma Medium Properties
10% CO2+He gas mixture, total flow rate: 30 sccm, pressure: 145 mbar
Contact Name
Yanjun Du
Contact Email
Plasma Diagnostic Properties
A single-mode continuous-wave QCL (Alpes) with a coverage from 2276 to 2290 cm−1 was used to scan the CO2 absorption transitions. The collimated output of the laser is guided in and out of the discharge chamber through two inserted wedged CaF2 windows to reduce the CO2 absorption outside of the discharge along the beam pass. A combination of a polarizer and a quarter-waveplate is used to adjust the laser intensity to avoid saturation. Two irises help to prevent potential etalon effects in the optical pass by shielding the back reflection from the beam splitter and the CaF2 windows. After traversing the discharge chamber, the laser beam is focused onto a DC coupled photovoltaic detector (Vigo, PVI-3TE-5, bandwidth 1 MHz) by an off-axis parabolic mirror. A narrow bandpass filter (Thorlabs, FB4250-500) and a third iris are mounted in front of the detector to reduce the infrared thermal emission from the discharge. The laser frequency is characterized by a silicon Fabry–Perot interferometer (LightMachinery) with a free spectral range of 0.0176 cm−1 mounted on a flipping mount.
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Data and Resources

  • Fig 1btxt

    Voltage, current and coupled energy waveforms for the investigated discharge...

  • Fig 2txt

    Comparison of the absorption calculated with the model in this publication...

  • Fig 3txt

    Fit to a non-equilibrium CO2 absorption from the literature (Dang C, Reid J...

  • Fig 4btxt

    The simulated absorption spectra in the selected wavelength range at room...

  • Fig 5txt

    Example of the detected spectra for a time series measurement. The QCL is...

  • Fig 6txt

    Absorbance of the CO2 gas mixture without discharge and before the discharge...

  • Fig 7txt

    Time evolution of the measured absorbance in the afterglow of the nanosecond...

  • Fig 8txt

    Examples of fitted absorption spectra together with the corresponding...

  • Fig 9txt

    Time evolution of the best-fit rotational temperature and vibrational...

  • Fig 10txt

    Validation of the best-fit rotational temperature and pressure from the...

  • Fig 11txt

    The area ratio of peak ‘3’ and ‘0’ deduced from the single-peak fitting with...

Yanjun Du