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Laser-induced breakdown spectroscopy (LIBS) is a powerful technique for a rapid,nondestructive, and sensitive determination of elemental composition of solids, liquids andgases with a little or no sample preparation.LIBS is an attractive technique and has showngreat promise in number of applications.However, it still lacks the sensibility, accuracy andprecision of other analytical techniques.Thus LIBS is primarily used to detect the majorelements of a sample.Much of LIBS limits come from the continuum emissionsuperimposed to the line emissions.Due to the high level of continuum emission, the linesare not well resolved in conventional LIBS.Although several studies attempted to eliminateor reduce the continuum emission in the spectra and thereby increase spectral resolutionhave been conducted, they have been proved expensive and with adverse impact on thespectra.
In the light of increasing LIBS sensitivity and resolution, in this study, a new techniquecalled Polarization Resolved LIBS (PRLIBS) is investigated.PRLIBS technique suggeststhe use of plasma polarization state to improve LIBS performance.The instruments used inPRLIBS are similar to those used in conventional LIBS except that a polarizer is placed infront of the detection system.
Our PRLIBS has been performed using a femtosecond laser on Al plate in open air.Theplasma polarization degree study reveals that the continuum emission is strongly polarized(＞80％) while lines characteristic of Al appeared much less polarized (＜30％).Therefore, byintroducing a polarizer in detection system and adjusting its axis, the continuum emission isgreatly reduced while signal emission remains almost intact.Consequently, signal to noise(S/N) and signal to background(S/B) ratios for the most intense lines increase and someweak lines that cannot be detected in conventional LIBS become well resolved in PRLIBS.The objective of this study is to acquire a detailed understanding of PRLIBS technique andtry to find the optimum parameters for PRLIBS experiments. To optimize PRLIBS performance, the effects of pulse energy, focal position, detectionangles and polarization plane of the laser on plasma polarization degree are systematicallyinvestigated.Our results indicate that plasma polarization degree P is reduced by increasingpulse energy from 75μJ to 300μJ, focusing the pulse laser exactly on the sample surface orby increasing the detection angle from 30° to 60°.In addition, it is also revealed that theplasma polarization state, which is mostly in vertical direction, is insensitive to thepolarization state of the ablating laser pulse.As PRLIBS application, a comparison of fs-PRLIBS and ns-PRLIBS performance, to the best of our knowledge, for the first time hasbeen conducted in this work.According to our results, the introduction of polarizer at itsminimum transmission angle brings more S/N (and S/B) enhancements in fs-LIBS than thatin ns-LIBS.It is also noted that, at the same fluence, more spectral lines are detected with ns-PRLIBS than fs-PRLIBS.
Besides, this study examines the influence of ambient environment such as pressure andnature of buffer gas on fs-LIBS of solid sample.Three kinds of gas (Ar, air and He) atdifferent pressures are used as ambient conditions.No special device is used to reduce thecontinuum emission in the spectra.The results show that emission intensities and linesdetection are significantly influenced by the ambient conditions.As the pressure of all gasesincreases, the emission intensity initially increases, attains its maximum value and thendecreases with further increase in pressures.It is also observed that line detection is thehighest at relative low pressure in Ar while it increases with pressure in air and He.Knowingthat the plasma shielding effect is absent in fs-LIBS, the mechanisms through which theambient conditions affect fs-LIBS signal are presented for the first time in this work.
In addition, plasma parameters such as electron densities, electron temperature and linewidth broadening have been calculated in the three gases at different pressures.A strongdependence of electron densities and temperature on ambient conditions is also observed.Hotter and denser plasma has been observed in Ar than in air and He.In fact, the plasmaparameters at relative low pressure of Ar are similar to those obtained at relative highpressures He.As consequence, Ar provides the best environment of fs-LIBS only at relativelow pressures while He constitutes good environment only at relative high pressures.For thefirst time, the optimum conditions of the use of Ar and He as ambient gases have beendetermined in this work.Contrary to the Ar and He, the plasma characteristics generated inair as ambient gas are found affected by chemical reactions due to N2 and O2.