Multiphoton Ionization (MPI)

The term multiphoton ionization is a generic one. It includes many individual processes that depend on the wavelength, field strength, and the polarization state of the laser pulse. In essence, more then one photon participate in the ionization process, in sequence or simultaneously. Coherence between photons becomes an important factor, not only because this makes it possible to achieve very high intensity levels, but also because certain MPI involve resonant processes. The electronic structure of the material is also very important. MPI processes are also very selective in terms of which atomic or molecular species is ionized, and how [1].

In this sections we will look at MPI processes that occur in conditions relevant to the applications that we have in mind (material processing - biology, medicine, microelectronics and semiconductors, and nanotechnology). What we realy need to understand is:

• what are the MPI processes at play in a given experimental situation,

• what is the nature and relative yields of the photolytic species generated,

• what is the spatial distribution of photolytic species generated, and

• how can we control all the elements above, by controlling the characteristics of the laser pulses?

Models on MPI usually enable us to calculate the probabilities of ionization, considering the electronic structure of the material, and the characteristics of the laser pulse. Some more powerful models provide more information by predicting even the angular distribution, and the residual energy of photoelectron. These models can be further complexified in order to follow in time the photoelectron from its birth, until the end of the laser pulse.

MPI processes

A number of MPI processes have been empirically identified:

• Resonance enhance multi-photon ionization (RE-MPI)

• Non-resonant multi-photon ionization (NR-MPI)

• Field ionization (FI)

• Above threshold ionization

• Multiple non-resonant ionization, or recollision ionization: occurs only with linearly polarized light

• Non-resonant multiphoton ionisation of inner-valence electrons

• Photoionization through superexcited states.

Everyone of the aforementioned MPI processes occurs within a certain range of laser parameters, for a given material. Some of these MPI processes occur at very high intensities. In the case of material surface processing in vacuum, any intensity level is relevant. But in the case of bulk material processing, powers beyond the petawatt range are not sustainable within the material, and all MPI processes occurring over this range are irrelevant in this case. The purpose of this section is to briefly describe the above MPI processese, and to put them in the context of laser material processing. MPI processes are responsible for plasma formation during the interaction between laser pulses and materials. They also dictate the nature of the primary photolytic species generated, as well as their relative yields.

Every MPI process refers to a material system (atom, molecule, lattice) described by its electronic structure (being subjected to a optoelectric field), and by the spatial orientation of its electronic bounds. It also refers to the properties of the optoelectric field, which are the wavelength, the polarization state, and the field strength, as well as to the time variation of these parameters.

Control Parameters

Pulse wavelength

To be completed !

Average pulse intensity

To be completed !

Pulse duration

MPI processes are extremely fast, in the fs range. The pulse duration parameter merely dictates the time that MPI remains active. We have to mention here that the time MPI is active is actually the time the intensity seen by an atom/molecule stays above MPI threshold, and this time is always shorter then the duration of the entire pulse.

Moreover, in a real experiment other processes are normally in competition with MPI, and the pulse duration can operate a selection between them, and dictate their relative importance. For example, the local intensity can be limited inside the material at values that are related to the pulse duration. This in turn dictates which MPI processes are active.

To be completed !

Polarization

MPI of molecules is sensitive to the polarization of light. In the case of crystals the net effect is clear, the ionization yield depends on the orientation. For amorphous materials, the ionization yield is not affected by the orientation, but MPI selects the molecules with a specific orientation, creating heterogeneity.

To be completed !

Material impurities

MPI depends strongly on the target system. This fact makes MPI a very selective process; impurities can be ionized leaving the host material intact.

To be completed !

References

[1] Single photon and femtosecond multiphoton ionisation of the dipeptide valyl-valine; N.P. Lockyer, J.C. Vickerman; International Journal of Mass Spectrometry 197 (2000) 197–209