Consideration of optical time of flight measurement in laser induced plasmas

Abstract

We considered some specifics of Optical Time Of Flight (OTOF) in Laser Induced Plasma if the OTOF signal is recorded at a distance of a few millimeters from the target. We demonstrated that the shape of OTOF signal is influenced by process of excitation transfer among the excited states. In case when the excitation transfer is dominant analogy with Time Of Flight (TOF) technique, developed for pulsed desorption flows into vacuum, is not meaningful. It is shown that at least two OTOF signals, for two different spectral lines, are necessary to record in order to estimate whether the excitation transfer is an issue or not. Assuming that expansion of the plasma is isotropic we derived a simple expression that takes into account geometric details concomitant for OTOF signal. It is shown that appropriate alignment of the spectrograph in respect to the laser beam position is essential for correct interpretation of experimental data and that asymptotic behavior of OTOF signal depends on a vertical opening of the slit and on the lens system. We demonstrated that proposed expression for OTOF signal provides consistent data for temperature T of heavy particles and center of mass velocity vc if OTOF signals are recorded at different distances from the target. For the same OTOF signals Shifted Maxwell-Boltzmann (SMB) function failed to correctly reproduce the both T and vc. We also discussed impact of inaccurate estimation of zero position in time and length scale on evaluated temperature of heavy particles, T, and center of mass velocity, vc. Importance of the geometric details is demonstrated considering two OTOF signals recorded for different vertical openings of the entrance slit while the rest of parameters were exactly the same. Temperature of the heavy particles T and the center of mass velocity vc, evaluated relying on the proposed expression, had a consistent values no matter if they were inferred from OTOF signal for large or small vertical slit opening. For the same OTOF signals Shifted Maxwell-Boltzmann function failed to correctly reproduce the both - center of mass velocity and temperature T of heavy particles. Also, T and vc estimated for OTOF signals recorded at different positions from the target, applying proposed expression, are consistent supporting assumption that expansion is dominantly isotropic and collisionless at chosen experimental conditions. We also discussed impact of inaccurate estimation of zero position in time and length scale on evaluated temperature of heavy particles, T, and center of mass velocity, vc.