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

The present study focuses on the application of a picosecond (ps) two-photon absorption laser-induced fluorescence (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., 4p6 1S0 → → 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 vs 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. The use of ps-TALIF can allow sub-ns measurements of quenching rates that are necessary for absolute atomic densities determination in atmospheric pressure plasmas. Thus, the present study is linked to many experimental works focused on plasma physics and applications. In fact, the investigation of the application of ps-TALIF in Kr and the definition of regimes where the TALIF signal intensity scales quadratically with the laser energy is essential to calibrate H and N atom densities in reactive plasmas.