Bond softening and Coulomb explosions

As already remarked in section 2.31, H2 has an exceptionally high ionisation potential amongst molecules, and a Rydberg series extending far into the vacuum ultraviolet range of the spectrum. Consequently, it is very suitable for strong field studies, especially as its stability means it is a permanent gas. 

the harmonic spectrum of H2 in a strong laser field has been studied (Tisch, private communication), and there have been similar experiments on other diatomics [500], although few to date on polyatomics 

Multiphoton effects in molecules are, as one might expect, similar to those in atoms but even richer. As for atoms, more than the minimum number of photons required to produce ionisation can be absorbed, so that ATI is observed [499, 501]. Many other effects are possible, for example, Coulomb explosions [502] which arise when two charged fragments fly apart. 

When the laser field becomes comparable to the internuclear binding field of a molecule, which clearly depends on the orientation of the internuclear axis with respect to the electric field vector of the radiation, the molecular potential is deformed potential wells become more shallow and, as a consequence, the number of vibrational states which are observable is reduced. This effect is called molecular bond softening [503]. 

Another effect is found near and above the photodissociation thresholds. Just as in the case of ionisation, molecules may continue to absorb excess photons over and above the minimum required, and this results in extra peaks in the spectra this effect is called ATD by analogy with ATI. 

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