I have been doing some literature review on species ionisation to understand a particular mechanism that occurs when a high-intensity femtosecond laser interacts with molecular nitrogen to produce fluorescence. The literature state that the laser causes a tunnel ionisation of the molecule rather than photodissociation despite the ionisation potential of N2 (15.6 eV) being higher than the disassociation energy (9.8 eV).
The literature that I have found on this process, tends to introduce the Keldysh parameter (gamma) which gives a measure of the regime of ionization and has the plot that I have attached.
Based on my current inference, I believe the graph represents the energy of an electron along with the distance from the nucleus and I assume that the amount of energy to either drive the electron closer or away from the nucleus. I could, however, also be wrong.
I was hoping if someone could explain how I should interpret the various cases in the plot. For (b) to (d), why does the slope on the left-hand side of each curve increase and the right-hand side decrease, and is it right to assume that the laser somehow reduces the ionisation potential of the atom resulting in a lower number of photons ionizing the species?
Since I do not have a background in quantum physics, I was hoping that someone could give me an explanation of what happens in the various cases, especially with tunnel ionization where I notice the line is drawn through the curve. I would appreciate it if you could provide me with a more 'physical' sense of the phenomenon that I could use to visualize.
Thank you in advance.
The image is taken from the following literature:
Amini, Kasra & Biegert, Jens & Calegari, Francesca & Chacon, Alexis & Ciappina, Marcelo & Dauphin, Alexandre & Efimov, Dmitry & Figueira de Morisson Faria, Carla & Giergiel, Krzysztof & Gniewek, Piotr & Landsman, Alexandra & Lesiuk, Michał & Mandrysz, Michał & Maxwell, Andrew & Moszyński, Robert & Ortmann, Lisa & Pérez-Hernández, Jose & Picón, Antonio & Pisanty, Emilio & Lewenstein, Maciej. (2018). Symphony on Strong Field Approximation.
