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.

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

“…The fluorescent radiation, with a wavelength of approximately 845 nm, was detected perpendicular to the direction of the laser beam by a streak camera (HAMAMATSU, C10910-05) to record the spatiotemporal development of the TALIF signal. Technical details on this method of detection can be found in the work of Invernizzi et al 27 The streak camera was synchronized with the laser pulses (repetition rate: 5 Hz) by a delay generator (Stanford Research, DG645) and connected to a digital CMOS camera for readout (HAMAMATSU, ORCA-Flash4.0 V3). The fluorescent radiation was focused on the horizontally oriented entrance slit (width: 100 lm) with a magnification of 0.25.…”

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

“…The laser beam diameter at the measurement location was experimentally determined to be ∼250 µm. The laser energy was manually adjusted by adding one or several neutral density filters after the output of the laser unit, and monitored with a joulemeter located at the end of the optical path [35].…”

“…The fluorescence decay times of H(n = 3) atoms extracted from TALIF measurements served two purposes: to determine the atomic hydrogen absolute densities in conjunction with measured time-integrated fluorescence signal intensities, and to study the de-excitation pathways of H(n = 3) atoms. We developed an algorithm that fits the measured fluorescence signal to the convolution of a theoretical TALIF signal and a Gaussian apparatus function [35].…”

“…Even if one manages to use an acquisition system with sub-nanosecond resolution, it is complicated to estimate τ fluo H when using a laser having a larger pulse duration such as that of a typical nanosecond laser (∼10 ns). In other words, measurement of τ fluo H for most laboratory conditions would only be feasible by using a combination of ultrafast lasers such as picosecond [22,23,35] or femtosecond [31] lasers and a fast acquisition system [22,35].…”