Quantitative fs-TALIF in high-pressure NRP discharges: calibration using VUV absorption spectroscopy
Ciprian Dumitrache, Arnaud Gallant, N. de Oliveira, Christophe O. Laux, Gabi-Daniel Stancu
Abstract
Abstract This work presents a femtosecond two-photon absorption laser-induced fluorescence (fs-TALIF) diagnostic for measuring ground-state atomic nitrogen in nanosecond repetitively pulsed (NRP) discharges. Absolute atom density is obtained from the TALIF signal via a novel calibration technique based on one-photon direct absorption measurements performed in a low-pressure DC discharge. The Vacuum Ultraviolet (VUV) measurements were done at the Soleil synchrotron facility using the high-resolution Fourier-transform spectrometer (minimum linewidth Δ <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:mrow> <mml:mover accent="true"> <mml:mrow> <mml:mi>ν</mml:mi> </mml:mrow> <mml:mo>~</mml:mo> </mml:mover> </mml:mrow> </mml:math> = 0.08 cm −1 ). The main goal of this work was to develop a quench-free diagnostic technique, which would allow measurements at elevated pressures with high spatial and temporal resolution. Here fs-TALIF measurements of N( 4 S) are demonstrated in the NRP post-discharge between 1–500 μ s after the nanosecond high-voltage pulse. A maximum number density of N-atoms of 5 × 10 16 cm −3 was measured at 1 μ s after the pulse when the discharge was operated at 1 bar in pure nitrogen. This corresponds to a dissociation fraction of ∼0.1%. The fs-TALIF technique at high laser intensity regime (>1 TW cm −2 ) calibrated using VUV absorption was compared with the fs-TALIF at low laser intensity regime (<100 MW cm −2 ) calibrated via the well-established non-saturated TALIF technique using krypton as an etalon gas. It was found that the two measurements of N( 4 S) in the NRP post-discharge agree within a factor of 3. Importantly, the limit of detection of the fs-TALIF at high laser intensity regime was determined to be <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>n</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">N</mml:mi> <mml:mrow> <mml:mo stretchy="false">(</mml:mo> <mml:mrow> <mml:mmultiscripts> <mml:mrow> <mml:mi mathvariant="normal">S</mml:mi> </mml:mrow> <mml:none/> <mml:none/> <mml:mprescripts/> <mml:none/> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> </mml:mmultiscripts> </mml:mrow> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:mrow> </mml:msub> <mml:mo>∼</mml:mo> <mml:mn>1</mml:mn> <mml:msup> <mml:mrow> <mml:mn>0</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>12</mml:mn> </mml:mrow> </mml:msup> <mml:mspace class="nbsp" width="0.3333em"/> <mml:mi mathvariant="normal">c</mml:mi> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">m</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> </mml:mrow> </mml:msup> </mml:math> . This is approximately one order of magnitude better than previously reported by ns-TALIF in low-pressure discharges.