Ground-State Atomic Nitrogen Measurements using fs-TALIF in High-Pressure NRP Discharges

Measurements of ground state atomic nitrogen inside of a nanosecond repetitively pulsed (NRP) discharge operating at pressures between 0.1-5 bar are performed using a femtosecond two-photon absorption laser induced fluorescence (fs-TALIF) technique. The main goal of this work is to develop a quench-free diagnostic technique which would allow measurements at elevated pressures with high spatial and temporal resolution. Quantitative information is extracted from the TALIF signal via a novel calibration technique based on direct absorption measurements performed in a low-pressure DC discharge. The VUV measurements were done at the Soleil synchrotron facility using their unique high-resolution Fourier-transform spectrometer ( (cid:1) (cid:2)(cid:1)⁄ = (cid:5)(cid:6) (cid:5)(cid:7) ). During this preliminary work, fs-TALIF measurements of N( 4 S) are demonstrated in the post-discharge of the NRP between 1-500 µs after the nanosecond pulse. A maximum number density of N-atoms of (cid:8) × (cid:5)(cid:6) (cid:5)(cid:7) (cid:11)(cid:12) (cid:13)(cid:14) was measured at 1 µs after the pulse when the discharge was operated at 1 bar in pure nitrogen. Importantly, the limit of detection of the fs-TALIF technique was determined to be (cid:15) (cid:16)( (cid:18) (cid:19) ) ~ (cid:5)(cid:6) (cid:5)(cid:22) (cid:11)(cid:12) (cid:13)(cid:14) . This is approximately two orders of magnitude lower than previously reported by ns-TALIF. The main goal of this work is to present a new diagnostic technique, based on femtosecond two-photon absorption laser-induced fluorescence (fs-TALIF), that enables spatially and temporally resolved measurements of ground-state population of atomic nitrogen in an NRP discharge at high pressures (p ≥ 1 bar). The first nitrogen TALIF experiment was performed by Bischel et al. 10 inside a flow discharge in which atomic nitrogen was obtained by N 2 dissociation in a He+SF 6 buffer gas mixture at ~10 Torr. Their proposed scheme involved the two-photon excitation of the 3p 4 D multiplet state at λ = 2 × 211 nm with fluorescence collection taking place in the NIR at 868 nm following the radiative decay: 3p 4 D ← 3s 4 P 5/2 . In our experiments, we are using a different TALIF excitation scheme involving the 3p 4 S 3/2 level at λ = 2 × 206.6 nm. This scheme was first proposed by Adams et al. and it has been shown in the past to present some distinct advantages compared to the scheme proposed by Bischel. Despite the higher photon energy, the 3p 4 S 3/2 state benefits from a lower quenching rate of the upper state by N 2 11 . Moreover, the collection of fluorescence takes place in the visible spectrum ( λ = 745 nm) where the quantum efficiency of the most detectors is notably higher. Previous work on ns-TALIF showed that it can be successfully employed for studying low pressure discharges ( below several tens of milibars 12 ). Above this pressure, quenching of the excited state becomes a major loss mechanism with a time scale even faster than the laser pulse length, i.e. below ns at atmospheric pressure 13 . Therefore, the main challenge of this study is to develop a “quenching free” diagnostic technique in the pressure range of interest (p=1-10 bar) using a femtosecond laser as the excitation source. In our study, the measurements are calibrated using VUV direct absorption performed in a DC calibration discharge that was characterized on the DESIR beamline at the Synchrotron Soleil facility in Saint Aubin, France. The manuscript focuses primarily on the description