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.

中文翻译：

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考虑激光诱导等离子体中的光学飞行时间测量

摘要 如果在距目标几毫米的距离记录 OTOF 信号，我们考虑了激光诱导等离子体中光学飞行时间 (OTOF) 的一些细节。我们证明了 OTOF 信号的形状受激发态之间的激发转移过程的影响。在激发转移占主导地位的情况下，与飞行时间 (TOF) 技术的类比，为脉冲解吸流入真空而开发，是没有意义的。结果表明，对于两条不同的谱线，至少需要记录两个 OTOF 信号，以便估计激发转移是否有问题。假设等离子体的膨胀是各向同性的，我们得出了一个简单的表达式，该表达式考虑了伴随 OTOF 信号的几何细节。结果表明，光谱仪相对于激光束位置的适当对准对于正确解释实验数据至关重要，并且 OTOF 信号的渐近行为取决于狭缝的垂直开口和透镜系统。我们证明，如果 OTOF 信号是在距目标不同距离处记录的，则所提出的 OTOF 信号表达式可为重粒子的温度 T 和质心速度 vc 提供一致的数据。对于相同的 OTOF 信号，Shifted Maxwell-Boltzmann (SMB) 函数未能正确再现 T 和 vc。我们还讨论了时间和长度尺度中零位置的不准确估计对重粒子的评估温度 T 和质心速度 vc 的影响。考虑为入口狭缝的不同垂直开口记录的两个 OTOF 信号，而其余参数完全相同，证明了几何细节的重要性。重粒子的温度 T 和质心速度 vc ，根据所提出的表达式进行评估，无论是从大或小的垂直狭缝开口的 OTOF 信号推断出来的，都具有一致的值。对于相同的 OTOF 信号，Shifted Maxwell-Boltzmann 函数未能正确再现重粒子的质心速度和温度 T。此外，对于在目标不同位置记录的 OTOF 信号估计的 T 和 vc，应用提出的表达式，一致支持假设膨胀在选定的实验条件下主要是各向同性和无碰撞的。