Determination of N2(X) vibrational level populations and rotational temperatures using CARS in a D.C. low pressure discharge

Abstract

For the first time, vibrational level population distributions of a low pressure D.C. flowing discharge were measured up to ν = 14 using Coherent Anti-Stokes Raman Scattering (CARS). The vibrational populations of the ground state N2(X 1Σ+g, ν) were obtained by recording rotationally resolved CARS spectra of the different vibrational bands. In the discharge region, the vibrational distributions have a non Boltzmann behaviour, as expected. The vibrational parameters were found to lie in the range 4 000-5 000 K, and rotational temperature to be about 550 K. Similar results were observed in the post-discharge region up to ν = 10. The vibrational and rotational temperatures were about 3 000 and 350 K respectively. The problems experienced while recording CARS spectra in low density media are discussed and comparisons with previously published theoretical models are presented

Spontaneous Raman scattering: a useful tool for investigating the afterglow of nanosecond scale discharges in air

“…This spectrum shows that high vibrational levels (up to v = 16) are populated in the first stage of the pulse discharge and that signals from all these levels are acquired. This example illustrates the interest of spontaneous Raman scattering and its synergy with more classical CARS measurements [11][12][13] as it is possible to describe the vibrational excitation of molecules, and especially N 2 molecules, over a wide range of vibrational levels, and perform simultaneous measurements along a line [17]. As shown further on, the comparison of Spontaneous Raman scattering spectra with theoretical spectra allows characterizing the energy transfers from the rotational and vibrational energy distributions measured versus time throughout the post-discharge.…”

“…Two other advantages of spontaneous Raman scattering are that it allows line-wise probing and measuring various species with the same experimental setup. All these features make spontaneous Raman scattering complementary to Coherent Anti-Stokes Raman Spectroscopy (CARS), frequently used to investigate pulsed discharge [11][12][13][14]. The good spectral resolution provided by CARS makes it possible to resolve rotational structures [14], however the measurements are mostly limited to v ≤ 6 [11,13,14], and CARS measurements are limited to point-wise measurements with a spatial resolution of around 1 mm in the longitudinal direction.…”

CARS investigations of a N2 planar-ICP discharge

“…To do this it could be necessary to increase the CARS interaction lenght at the expenses of spatial resolution. The change of configuration has been found necessary, for instance in [8], using the boxCARS configuration for level v = 0 and collinear arrangement for the following level up to v = 12 ÷ 14 in a DC glow discharge at p = 2 Torr.…”

Gas Density Distributions and Reduced Field Strenghts of the Positive Column of Low Pressure XeCl* Glow Discharges

“…However, the plasma can interfere with the thermocouple. A non-intrusive method of LPGD temperature diagnostic is CARS, which has been used to investigate LPGD in nitrogen [6]. Though this costly technique delivers both rotational and vibrational temperatures as well as particle densities with high spatial resolution, it can be applied successfully only to molecular gases with well known susceptibilities of third order.…”