LIBS Basics, Part II: Hardware - - Spectroscopy
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LIBS Basics, Part II: Hardware


Spectroscopy
Volume 29, Issue 4, pp. 26-31

Choice of Laser Wavelength

Given a particular pulse duration for the LIBS system, the laser wavelength also has a large influence on the laser–material interaction that ensues. This influence is more pronounced for nanosecond lasers, because (as previously discussed) the pulse length is long enough to excite phonons in the material and interact with the plasma as the plasma forms. Hence, here I will focus on a few effects that are known to occur for nanosecond lasers. As we will see, the laser wavelength can influence both the ablation and plasma when using a nanosecond laser.


Figure 2: Optical absorption depth as a function of wavelength. Data were obtained and adapted from reference 6.
A primary effect in the laser–material interaction is the laser absorption depth, which changes as a function of wavelength. As shown in Figure 2, for some materials such as silicon or germanium, the optical absorption depth at longer wavelengths is measured in tens or hundreds of micrometers or more. Such large characteristic absorption lengths mean that much of the laser pulse is absorbed in the depth of the material. In-depth absorption either causes large and unpredictable ablation, or the energy is deposited at depth and is lost, unable to participate in the ablation and plasma formation process. The rule of thumb that ensues from this consideration is that UV laser wavelengths — for example, 193-nm ArF excimer and 213- and 266-nm Nd:YAG — are required for the application of LIBS to materials that may be optically thin at longer wavelengths. Often, these are materials that appear transparent or translucent to the eye, but many polymers that appear opaque to the eye are still translucent at the fundamental 1064-nm wavelength of the Nd:YAG laser.

In general, shorter wavelengths are better for ablation using nanosecond lasers. Shorter wavelengths have smaller absorption depths in essentially all materials, which concentrates ablation energy and promotes efficient ablation. Also, the diffraction-limited spot size is proportional to the wavelength of the laser. As a result, short-wavelength lasers can be focused into smaller spot sizes, which is helpful for microanalysis.

When it comes to plasma formation, however, there are some drawbacks to shorter wavelengths. As the electron density increases in the plasma, there is increased absorption in the plasma because of inverse Bremsstrahlung. The strength of the overall absorption can be described by the interaction of the incoming laser light and the rapid oscillations of electron density in the plasma known as "Langmuir waves." If the laser frequency is lower than the frequency of the Langmuir waves, known as the "plasma frequency," then there is a sharp increase in the absorption in the plasma. Fundamentally speaking, if the electrons are oscillating faster than the laser light, they can keep up with the electromagnetic wave and absorb it. If the laser frequency is higher, there is much lower absorption.

Because the LIBS plasma is forming above a solid sample, the electron density increases from zero to a number on the order of 1017 electrons/cm3. Considering the evolution of the plasma during the 6–8 ns duration of a typical Nd:YAG laser pulse, the beginning portion of the pulse starts the ablation process, ejecting material into a plume and initiating plasma formation through other mechanisms such as multiphoton ionization. The latter portion of the pulse has the opportunity to interact with the expanding plume from the ablation. Depending on the wavelength of the laser, which for example could be 266 nm or 1064 nm for a Nd:YAG laser, the trailing portion of the pulse may be either more absorbed (in the case of a 1064-nm pulse) or less absorbed (in the case of a 266-nm pulse) in the plasma. A longer wavelength (lower frequency) laser will more efficiently couple in to the forming plasma and be absorbed by the plasma at an earlier stage in the plasma lifetime, corresponding with lower electron densities required for absorption. This improved coupling with longer-wavelength lasers results in a more robust plasma, with typically a larger volume and somewhat higher temperature and electron density. Hence, while the shorter-wavelength lasers have an advantage in ablation efficiency and spot size, the longer-wavelength lasers have an advantage in plasma formation.

Laser Energy Influence on the Sample

The total laser energy has an influence on both the material ablation and on the plasma. For a given sample, the amount of energy per unit area (laser fluence) is the relevant important parameter related to energy. It is the fluence that most directly influences the ablation volume. For some samples, increased fluence is too much of a good thing; for example, too much fluence in the analysis of gemstones can cause cracking or unsightly marks. Similarly, pressed pellets or tablets may crack or crumble if the fluence is too high. Therefore, it is not always important to have a high energy, and typically systems have adjustable energy and spot size so that the fluence can be tailored to the material being measured.

Another interesting point is that at any given time the radius of the initial plasma is proportional to energy to the 1/5 power. In particular, R(t) ˜≈ E (1/5).t (2/5), where R is the radius, E is the energy, and t is time. The time, electron density, and temperature (T) progression can be shown to follow a roughly self-similar path (based on hydrodynamic theory) with T(t) ≈˜ E 2/3 (7). The upshot of this self-similarity in the decay time of the plasma is that the delay (after the laser) and the gate time (open shutter) of the LIBS detector can be adjusted in predictable ways when the energy is changed in a LIBS experiment.


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