Role of laser fluence on ionic emission characteristics from steel plasmas induced in atmospheric air

Abstract

We report on spatially-integrated spectral emission measurements of singly-ionized species excited in laser-generated steel plasmas in air at atmospheric pressure via laser induced breakdown spectroscopy (LIBS). We follow up the behavior of emission parameters of selected ionic spectral lines including signal and background intensities as well as signal-to-background and signal-to-noise ratios, while varying the incident effective laser energy fluence. The plasma was induced onto a standard low alloy steel target employing Q-switched Nd:YAG laser pulses emitting at the fundamental wavelength of 1064 nm. Spectral features emitted were detected using an echelle spectrograph coupled with CCD detector (EMCCD) fixed at 2 μs delay time of integration. Signal intensities exhibited monotonical increase with laser fluence up to a certain value of about 30 J cm−2, as was confirmed by corresponding signal-to-background measurements for all elements under test. The gradual rise in the spectral signal intensity was attributed to an increase in mass ablation rate of the target up to 30 J cm−2, but not due to a corresponding rise in excitation temperature of the plasma. Signal-to-noise ratios of all spectral lines exhibited anomalous behavior in which two distinctive maxima can be identified, with the favourable value located at the higher range of the laser energy fluence.