Picosecond two-photon absorption laser-induced fluorescence (ps-TALIF) in krypton: The role of photoionization on the density depletion of the fluorescing state Kr 5p′[3/2]2

Abstract

The present study focuses on the application of a picosecond (ps) TALIF technique in krypton (Kr) at variable pressure (0.1-10 mbar). The laser intensity (I, units W.cm -2 ) is tuned between 1 and 480 MW.cm -2 , and the depletion of the density of the Kr 5p´[3/2]2 fluorescing state through photoionization (PIN) and amplified stimulated emission (ASE) is investigated. This is done by combining TALIF experiments with a simple 0D numerical model. We demonstrate that for a gas pressure of 3 mbar and 15 < 𝐼 ≤ 480 MW.cm -2 , a saturated fluorescence signal is obtained, which is largely attributed to PIN, ASE being negligible. Also, a broadening of the two-photon absorption line (i.e. 4p 6 1 S0→→5p´[3/2]2) is recorded due to the production of charged species through PIN, inducing a Stark effect. For I≤15 MW.cm -2 , though, PIN is significantly limited, the absorption line is noticeably narrowed, and the quadratic dependence of the TALIF signal intensity versus the laser energy is obtained. Thus, in this case, the investigated Kr TALIF scheme, using the 5p´[3/2]2→5s[3/2]1 fluorescence channel, can be used for calibration purposes in ps-TALIF experiments. These results are of interest for fundamental research since most ps-TALIF studies performed in Kr do not investigate in detail the role of PIN and ASE on the depletion of the Kr 5p´[3/2]2 state density. Moreover, this work contributes to the development of ps-TALIF for determining absolute densities and quenching coefficients of H and N atoms in plasmas. This is useful in numerous plasma-based applications (e.g. thin film synthesis, biomedical treatments, plasma-assisted combustion, …), for which the knowledge of the density/kinetics of reactive atoms is essential.

Validation of THz absorption spectroscopy by a comparison with ps-TALIF measurements of atomic oxygen densities

“…More details on this laser system can be found elsewhere. 26 Neutral density filters (NDFs) were placed in front of the laser output to attenuate the laser beam. The laser energy was monitored by a calorimeter (Coherent, J-10MB-LE) connected to an oscilloscope (Teledyne Lecroy, Wavesurfer 10).…”

Depopulation mechanisms of atomic hydrogen in the n = 3 level following two-photon excitation by a picosecond laser

“…TALIF measurements on atomic hydrogen were performed using a picosecond tuneable laser (EKSPLA ® composed of three units: PL3140, APL2100 and PG411, pulse width of ∼10 ps [54]. The laser excitation wavelength was tuned to the H 1s → 3s and 1s → 3d transitions at 205.08 nm and the fluorescence signal at 656.3 nm (Hα line) was observed.…”

Peculiarities of measuring fluorescence decay times by a streak camera for ps-TALIF experiments in reactive plasmas

“…Neutral density filters are placed after the diaphragm to adequately tune the laser energy. In principle, a suitable laser energy (E L ) for performing H-atom TALIF should be within a regime where the TALIF signal intensity scales quadratically with E L [16,17]. For our laser system, this is the case for E L ⩽ 14 µJ per pulse and for E L ⩽ 57 µJ per pulse for the low-and the atmosphericpressure plasma sources, respectively.…”

“…To perform ps-TALIF measurements in this work, a ps laser from EKSPLA ® was used. The laser system has been described in detail in [17] and therefore only a brief description is provided here. It is composed of three units: a Nd:YLF pump laser (PL3140), which produces a fundamental output at 1053 nm, a harmonic generator (APL2100) used for fundamental frequency doubling (527 nm) and tripling (351 nm), and a solid state optical parametric generator (PG411) generating ultrafast laser pulses (≈10 ps pulse duration; 60 µJ energy per pulse around 205 nm) in the spectral range 193-2300 nm with a repetition frequency of 5 Hz.…”