Single-Photon Ionization (SPI)

This is the simplest photoionization process, and the most extensively studied. It is basically the ejection of one electron from its orbital in the free space, or into the conduction band of the material. Only photons that posses an energy corresponding to an ionization transition of the system are absorbed, with a certain probability. This makes SPI very wavelength sensitive.

The probability of a SPI event is given by the extinction coefficient, or if you are interested in the microscopic picture, by the cross section. In condensed matter, the distance between the free electron and the geminate ion varies with the dynamics involved. It depends on the wavelength of the laser light, and on the effects of the optoelectric field on the free electron, from its birth and until the end of the laser pulse. In most applications SPI is induced at low intensities, and in this case the effects of the optoelectric field on the free electron can be neglected. The distance between the free electron and the geminate ion determines the recombination kinetics, and can have an important influence on the nature and the relative yields of long-term, or stable, photolytic species.

Radiative and non-radiative internal conversions of the resulting ion might follow the ionization event. Molecular ions can also relax thorough dissociation channels. As opposed to MPI and AI, coherence doesn't play an important role in SPI. Usually UV lamps are used, that are based on non-coherent plasma emission.

In amorphous materials (like liquid water for example), ionization can also occur at energies bellow the band-gap of the material, and in this case the electron is never totally free, but it occupies localized states [1-5].

Control parameters

Wavelength

SPI is very wavelength dependent. This sensitivity confers this process an important use: it is possible to selectively ionize or dissociate molecules from a mixture/solution.

Average intensity

The local intensity affects directly the ionization yield. In theory, this process is observed at any intensity level - it doesn’t exhibit an intensity threshold. Being possible at very low intensities can be seen as an advantage, as the free electrons and other primary photolytic species are not significantly affected by the presence of the electric field. For higher intensities, multiphoton processes can take place simultaneously (see next section), and the primary photolytic species can be further perturbed by the presence of the electric field.

Pulse duration

SPI is an "instantaneous" process, however, it is obvious that for longer pulse durations the probability of ionization of a given atom/molecule per pulse increases. Moreover, the pulse duration becomes important for the after effects of the SPI. The fate of the primary photolytic species can be affected by the pulse duration if this parameter is longer then the characteristic relaxations times of the material, and if the light can interfere with the relaxation mechanisms.

Polarization

SPI is also sensitive to the polarization of light. In the case of crystals the net effect is clear, the ionization yield depends on the relative orientation between the lattice axes of the material and the polarization of the light. For amorphous materials, the ionization yield is not affected by the orientation, but SPI selects the molecules with a specific orientation, creating heterogeneity.

Material impurities

SPI depends strongly on the target system. This fact makes SPI a very selective process; impurities can be ionized leaving host material intact. For materials non-absorbing on a specific wavelength domain, introducing impurities absorbing on that same domain induces SPI.

Reference

[1] On the electronic structure of liquid water: conduction-band tail revealed photoionisation data; T. Goulet, A. Bernas, C. Ferradini et J.-P. Jay-Gerin; Chemical Physics Letters, vol. 170, no.5-6, p. 492-496, 1990

[2] Search for Urbach Tails in the Optical Spectra of Solvated Electron in Alcohols and Wather; C.Houé-Lévin, C. Tannous, and J-P Jay-Gerin; The jurnal of physical chemestry 1989, 93, 7074

[3] Two photon ionization and dissociation of liquid water by powerful laser UV radiation; D. N. Nikogosyan, Alexander A. Oraevsky et Valery I. Rupasov; Chemical Physics, vol. 77, p. 131-143, 1983

[4] Photoionization Yield vs Energy in H2O and D2O; D. M. Bartels et R. A. Crowell; J. Phys. Chem. A, vol. 104, p. 3349-3355, 2000

[5] Femtochemistry of the Hydrated Electron at Decimolar Concentration; S. Pommeret, F. Gobert, M. Mostafavi, I. Lampre, et J.-C. Mialocq; J. Phys. Chem. A, vol. 105, p. 11400-11406, 2001