Vibrational temperatures of CO 2 are studied in a pulsed glow discharge by means of time-resolved in situ Fourier transform infrared spectroscopy, with a 10 μs temporal resolution. A method to analyze the infrared transmittance through vibrationally excited CO 2 is presented and validated on a previously published CO 2 spectrum, showing good agreement between fit and data. The discharge under study is pulsed with a typical duty cycle of 5-10 ms on-off, at 50 mA and 6.7 mbar. A rapid increase of the temperature of the asymmetric stretch vibration (T 3 ) is observed at the start of the pulse, reaching 1050 K, which is an elevation of 550 K above the rotational temperature (T rot ) of 500 K. After the plasma pulse, the characteristic relaxation time of T 3 to T rot strongly depends on the rotational temperature. By adjusting the duty cycle, the rotational temperature directly after the discharge is varied from 530 to 860 K, resulting in relaxation times between 0.4 and 0.1 ms. Equivalently, as the gas heats up during the plasma pulse, the elevation of T 3 above T rot decreases strongly.

It is experimentally demonstrated that a narrow band continuous wave ͑cw͒ light source can be used in combination with a high-finesse optically stable cavity to perform sensitive, high-resolution direct absorption and optical rotation spectroscopy in an amazingly simple experimental setup, using ideas from the field of cavity ring down spectroscopy. Light from a scanning narrow band cw laser is coupled into the cavity via accidental coincidences of the laser frequency with the frequency of one of the multitude of modes of the cavity. The absorption and polarization rotation information is extracted from a measurement of the time-integrated light intensity leaking out of the cavity as a function of laser wavelength.

The excitation and relaxation of the vibrations of CO 2 as well as the reduction of CO 2 to CO are studied in a pulsed glow discharge. Two diagnostics are employed: (1) time-resolved in situ Fourier transform infrared spectroscopy and (2) spatiotemporally resolved in situ rotational Raman spectroscopy. Experiments are conducted within a pressure range of 1.3-6.7mbar and a current range of 10-50mA. In the afterglow, the rate of exponential decay from the asymmetric stretch temperature (T 3 ) to the rotational temperature (T rot ) is found to be only dependent on T rot , in the conditions under study. The decay rate r -

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A rotational Raman study under non-thermal conditions in a pulsed CO 2 glow discharge. Plasma Sources Science and Technology, 27(4), [045009].

This work employs in situ rotational Raman spectroscopy to study the effect of N 2 and O 2 addition to CO 2 in pulsed glow discharges in the mbar range. The spatiotemporally resolved measurements are performed in CO 2 and 25%, 50% and 75% of N 2 or O 2 admixture, in a 5-10ms on-off cycle, 50mA plasma current and 6.7mbar total pressure. The rotational temperature profile is not affected by adding N 2 , ranging from 400to 850K from start to end of the discharge pulse, while the addition of O 2 decreases the temperature at corresponding time points. Molecular number densities of CO 2 , CO, O 2 and N 2 are determined, showing the spatial homogeneity along the axis of the reactor and uniformity during the cycle. The measurements in the N 2 containing mixtures show that CO 2 conversion factor α increases from 0.15 to 0.33 when the content of N 2 is increased from 0% to 75%, demonstrating the potential of N 2 addition to enhance the vibrational pumping of CO 2 and its beneficial effect on CO 2 dissociation. Furthermore, the influence of admixtures on CO 2 vibrations is examined by analysing the vibrationally averaged nuclear spin degeneracy. The difference between the fitted odd averaged degeneracy and the calculated odd degeneracy assuming thermal conditions increases with the addition of N 2 , demonstrating the growth of vibrational temperatures in CO 2 . On the other hand, the addition of O 2 leads to a decrease of α, which might be attributed to quenched vibrations of CO 2 , and/or to the influence of the back reaction in the presence of O 2 .

This work uses in situ narrowband quantum cascade laser (QCL) absorption spectroscopy to study the effect of N 2 on the time evolution of gas temperature, rotational temperature and the vibrational temperatures of CO 2 and CO in a pulsed glow discharge. Three colinear QCLs are used to scan three regions of about 1 cm −1 between 2179.20 and 2253.51 cm −1 , includingCO 2 transitions up to the asymmetric stretch level v 3 = 6, as well as (v CO ) → (v CO + 1) CO transitions up to v CO = 1. A fitting routine is used to extract temperatures from the measured absorption spectra. The time resolved measurements are performed in CO 2 , admixed with up to 90% N 2 , with the plasma operated with a 5-10 ms on-off cycle, a discharge current of 50 mA and a pressure of around 6.7 mbar. The time evolution of the gas temperature has been measured and agrees well with the time evolution of the rotational temperature. The asymmetric stretch vibrational temperature T 3 of CO 2 reaches a maximum of 1060 K at 0.7 ms for pure CO 2 , while T 3 goes up to 2250 K for a N 2 content of 90% and stays constant until the plasma is switched off. Both T 3 and the vibrational temperature of CO T CO show a clear non-equilibrium with respect to the rotational temperature T rot . Both do not equilibrate with the rotational temperature T rot between consecutive plasma cycles for a N 2 content above 70%, although T 3 and T CO always equilibrate with each other in the afterglow. The symmetric stretch and bending mode temperature T 12 is elevated more with respect to the rotational temperature for increasing N 2 content, while the maximum of the rotational temperature decreases for increasing N 2 admixtures, which might be attributed to the energy stored in the vibrational modes of N 2 , CO 2 and CO. Additionally, an indication of an increase in the total pressure during the plasma on-time due to heating and a subsequent decrease in the afterglow due to cooling was found for a pure CO 2 plasma.

Over the past few decades many diagnostics have been developed to study the non-equilibrium nature of plasma. These developments have given experimentalists the possibility to measure in situ molecular and atomic densities, electron and ion densities, temperatures and velocities of species in the plasma, to just name a few. Many of the diagnostic techniques are based on the ‘photon-in, photon-out’ principle and were at first developed to perform spectroscopy on atoms and molecules. Much later they were introduced in the research of plasmas. In this foundation paper we will focus on optical-based diagnostics that are now for quite some time common use in the field of low-temperature plasma physics research. The basic principles of the diagnostics will be outlined and references will be given to papers where these techniques were successfully applied. For a more comprehensive understanding of the techniques the reader will be referred to textbooks.