THE THEORY

• LIBS is an atomic/molecular spectroscopic technique that uses pulses of intense laser light to vaporize a small amount of a target. 

• The vaporization creates a high-temperature (10,000 K – 20,000 K) plasma plume in which the constituent elements or molecules of the target are excited and ionized. 

• As the plasma cools the atoms, ions, and molecules lose energy via the spontaneous emission of optical wavelength photons.  This light is collected, dispersed by a spectrometer, and the resulting emission peaks are analyzed to accurately identify all the elements that were present in the target.

•  This technique can be used to qualitatively identify constituent elements (what exactly was in the target?) or to quantitatively analyze the target (exactly how much of something was in the target?)

HOW DOES IT WORK EXACTLY?

LIBS of solids occurs in a complicated series of physical processes which consists of several basic steps roughly grouped as:

1) laser interaction with the solid

2) removal of samples mass (ablation)

3) plasma formation (breakdown)

Figure (a): The process is initiated by absorption of energy by the solid from a pulsed radiation field.  Typical pulse durations are on the order of nano-seconds, but LIBS has been performed with pico- and femto-second laser pulses.

Figure (b): The absorbed energy is rapidly converted into heating, resulting in vaporization of the sample (ablation) when the temperature reaches the boiling point of the material. The removal of particulate matter from the surface leads to the formation of a vapor above the surface

Figure (c): The laser pulse continues to illuminate the vapor plume.  The vapor tends to condense into sub-micrometer droplets that lead to absorption and scattering of the laser beam, inducing strong heating, ionization and plasma formation. 

The high electron temperature of such laser plasmas necessitates the use of time-resolved spectroscopy.  During the early stages of the plasma, the electron density is particularly high and the spectra are characterized by a non-specific continuum emission due to ion-electron interactions (recombination and bremmstrahlung).

Figure (d): The dynamical evolution  of the plasma plume is then characterized by a fast expansion and subsequent cooling.  Approximately  1 microsecond after the ablation pulse, spectroscopically narrow atomic/ionic emissions may be identified in the spectrum.  It is also possible to identify long lived molecular transitions in these spectra.  In this way, all elements present in the target are observed simultaneously.