Time and space-resolved laser-induced breakdown spectroscopy on molybdenum in air

Abstract

In the present work, laser-induced plasma (LIP) of Mo is studied using both time and space resolved laser-induced breakdown spectroscopy (LIBS) in air as a function of incident laser energy. For this a second harmonic Q-switched Nd:YAG laser having pulse width of 7 ns and repetition rate of 1 Hz is used. The Boltzmann plot method is employed to estimate the plasma temperature of LIP using MoI and MoII lines separately. The stark-broadened profile of MoI-313.2 nm is exploited to measure the electron density. The temporal study in the time delay range of 0.5–5.0 μs shows that both the parameters, decay with the increase in delay time but increases with the increase in laser energy. It is observed that the plasma temperature estimated for MoII lines is higher than that of the MoI lines during the initial stage of plasma formation, delay time of 0.5–1.0 μs, but at a later time scale, both these species are found to possess nearly the same values of the temperature, indicating the coexistence of thermal equilibrium among the Mo atoms and ions in LIP. Therefore, in the second part of the experiment, time-integrated spatial evolution of the LIP of Mo is studied at a fixed delay of 2 μs as a function of axial distance normal to the target. It is found that emission intensity (MoI and MoII both), plasma temperature, and electron density initially increase with the increase in distance from the target, attains maximum value and then falls down. The Mc-Whirter criteria is applied to test the validity of local thermodynamic equilibrium (LTE). The relaxation time and diffusion length are estimated in time and space resolved studies respectively to take care of the transient and inhomogeneous nature of the LIP. The optical thin condition of LIP is verified by employing the branching ratio method. From these studies, a suitable spatio-temporal is identified where the LTE and optically thin plasma condition hold along with a high signal to noise ratio.