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

Abstract

We present a detailed methodology for (i) correctly configuring a streak camera to capture raw picosecond two-photon absorption laser induced fluorescence signals (ps-TALIF) of H-atom in low- and atmospheric-pressure plasmas, and (ii) properly processing the recorded raw experimental data with a dedicated mathematical signal processing method to infer actual ps-TALIF signals of H-atom. The goal is the accurate determination of the decay time of the recorded ps-TALIF signals of H-atom. A ps-laser is used to excite atomic hydrogen produced in both plasmas and the raw fluorescence signals are detected by the streak camera using different time windows/ranges (TR). It is shown that the choice of the TR affects the shapes and the decay times of the recorded raw TALIF signals. This is defined as the instrumental function of the streak camera and has a Gaussian profile as determined by recording the ultrafast laser pulse at different TR. To remove this instrumental distortion and extract the actual shape of the TALIF signals, the captured raw TALIF signals were fitted using the mathematical procedure developed in this study, which involved an exponentially modified Gaussian function. The application of our methodology leads to more reliable measurements of hydrogen atoms decay times after respecting the following acquisition conditions: (i) the TR of the streak camera should be sufficiently large to capture the complete (raw) TALIF signal, and (ii) the time width of the instrumental function of the streak camera should be as small as possible compared to the actual decay time of the fluorescence, while ensuring an optimal signal-to-noise ratio. This work demonstrates the remarkable potential of the combination of ps-TALIF and streak cameras in state-of-the-art optical plasma diagnostics.