Molecular LIBS

Let's look at [@fig1] an carbon spectrum measured from as pure of a carbon anthracite sample as we had available.

[FIG1-molecularcarbon]

Analyzing this with the tools of elemental analysis we have built before we expect to see the following in our spectrum (1) carbon from the sample, (2) oxygen and (3) nitrogen from the air and possibly (4) hydrogen from moisture. We do as usual and label the peaks matching these elements in [@fig2].

[FIG2-molecularcarbonidentified]

After this there seems to be multiple set of peaks still not accounted for. To make this more let's cut off the identified elemental peaks and reduce the continuum and we get [@fig3].

[FIG3-molecularcarbonpruned]

These peaks look a bit different than we are used to. They come in bands of multiple peaks and do not seem to match our usual Lorentzian peak shape. These kind of less boring shapes are characteristic to molecular vibration emissions. To identify these, we must consult our references for molecular vibrations. In the plasma we have C, so from the list of commong molecular LIBS vibrations we can expect at least C2 and CN bands. In [fig4] we draw to match the theoretically calculated emission bands (from Parigger et al [@pariggercomputation2015]) with our measurement spectrum. Calculating the strenght of the matches we get and then removing these from the spectrum the leftover is [@fig5] which shows that we are finished as the leftover is pretty much flat. And we get the full elemental+molecular result in table [@table1]:

[FIG4-molecularcarbontheoretical] [FIG5-molecularcarbonprunedflat] [table1-carbonelementalmolecularresults]

This tells us what is present in the plasma(C,O,N,C2,CN) and therefore we can verify the sample to be mostly just carbon. But what is the value of these molecular signals in addition to the elemental ones? Well in this specific example not much, but sometimes they are crucial. In some cases the elemental peaks of an element we are interested can be weaker than their molecular peaks or can be masked by other overlapping peaks making the elemental peaks invisible to us. Molecular vibration can also give us crucial information on the material based on whether or not these molecules form in the plasma. For example: carbon measured from carbonates often shows no molecular behaviour whereas carbon from organic or graphite sources does, and thus allows us to differentiate carbonates from other carbon sources by comparing the intensities of carbon elemental and molecular signals! Other important application is isotope analysis which is shortly presented at the end of this chapter.

Carbon plus air plasma here is a clear example on molecular LIBS but be aware that there are other materials, for example halogens[@fluoride-spectrum] or aluminium[@aluminium-spectrum], which have strong molecular emissions in LIBS plasma.

If molecular features are important to your application, there are techniques to focus on the molecular emissions such as timing the measurement window to reduce the elemental and thermal emissions or to enhance them by using dual laser setup to excite the plasma after ablation. But even in conventional LIBS setups the molecular emissions can turn out surprisingly well visible and useful.

Isotopes revealed by molecular LIBS i.e. laser ablation molecular isotopic spectrometry LAMIS

Molecular LIBS is specifically fitted for isotope analysis because of isotope based shift on molecular peaks is relatively larger than the one on elemental peaks. More on this by [@bolshakovlaser2016]