Single-Photon mode (SP)

This photoionization mode is obtained at small wavelengths (UV, X-ray), and at low intensity levels. The only photoionization process involved is single-photon ionization (SPI).

The theories needed to understand the most important features of the SP mode are:

• the physics of low-intensity optoelectric field interaction with matter, to account for the photoionization, excitation, and dissociation of material systems.

• the geometrical optical theory, and linear propagation, to account for the spatial intensity distribution, and the dose distribution.

Applications

The SP mode is very wavelength sensitive, which means that we can use it to affect specific components of a homogeneous mixture, a solution for example, being able to control how the targeted species are going to be affected. In other words, we can selectively ionize atoms an molecules, and excite them to specific states, by tuning the wavelength of the UV radiation. The SP mode can be used in material processing, stimulating certain chemical reactions by exciting/ionizing reactants, generating radicals, or more specifically in curing/polymerization. It can also be used in diagnosis: UV spectroscopy (absorption, transmission, and fluorescence); and in imaging/microscopy (fluorescence microscopy for example).

In biology, UV and X-rays are used in the SP mode to study DNA and protein damage formation, or the damage repair machinery. Phototherapy is also an important application, where the UV light is used to activate a certain therapeutic agent at a specific location. It should be mentioned that for these specific applications the coherence of the UV or soft X-ray radiation is not necessary. Due to its ionizing power, and to its high rate of lethal photoinduced damage (direct or indirect), the UV radiation is very toxic at the cellular level. For this reason, in situ UV imaging is to be avoided if the surviving of the living system is important. For this reason different techniques such as fluorescence microscopy have been developed, where the excitation of the fluorophore is achieved by multiphoton absorption in the (near-)IR. However, UV light is used extensively in the SP mode for sterilization.

Range of operation of the SP mode

The SP mode exists within a domain of laser parameters defined by an important event, which is the occurrence of multiphoton ionization (MPI) processes, and/or avalanche ionization (AI). In short, SP is possible for any combination of laser parameters for which SPI processes occur, but AI and MPI do not.

Wavelength

The SP mode operates at very high frequencies (small wavelengths), in the UV and X-ray. It relies on SPI processes, which are very wavelength sensitive. Ionization, dissociation, and excitation of atoms/molecules is possible only if the wavelength is tuned to a specific absorption band of the material system. The absorption bands are directly related to the electronic structure of the material system.

This wavelength sensitivity confers this mode an important characteristic: it is possible to selectively ionize or dissociate atoms or molecules from a mixture/solution.

Average local intensity

As in the case of SPI processes, SP doesn’t exhibit an intensity threshold. However, we can define an upper intensity limit for this mode, in its pure state. This value is the MPI threshold in a given experimental condition (depending on the type of material, and on the characteristics of the laser light). Above the MPI intensity threshold, we have the emergence of mixed modes SP-B/OB, or SP-OB.

We must also consider that the ionization yield in the SP mode depends on the local intensity level. For higher intensity levels, we have a higher ionization density.

Pulse duration

SPI is an "instantaneous" process, therefore this parameter is not very important for the SP mode. However, it is obvious that for longer pulse durations, the probability of ionization of a given atom/molecule per pulse increases, simply because the material system is exposed to the light for a longer period time.

Polarization

SPI of molecules is very sensitive to the polarization of light, and so is the SP mode. 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 SPI selects molecules with a specific orientation, creating heterogeneity.

Material impurities

SPI is a very selective process, and so is the SP mode: we can chose to ionize impurities, leaving the host material intact.

Control of primary photolytic species

In the following sections we discuss the effects of different controllable parameters on the relative yields of primary photolytic species generated in the SP mode.

Average local intensity

The local intensity affects directly the ionization yield. In turn, the physical distance between primary photolytic species, which is related to the ionization density, affects the chain of chemical reactions that follows, and ultimately it affects the long-term photolytic effects.

Operating 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 electromagnetic field. For higher intensities, multiphoton processes can take place simultaneously, and the situation changes.

Wavelength

The wavelength has a selective effect on ionization/dissaciation/excitation channels, and can influence the photoelectron (electron freed by the ionization process) energy spectrum, as well as the distance between photoelectrons and their geminate ions. This in turn affects the yields of primary photolytic species.

Pulse duration

The pulse duration becomes important for the after effects sought by photolysis in the SP mode. The fate of the primary photolytic species can be affected by the pulse duration if this parameter is longer then the characteristic relaxation times of the material, and if the light can interfere with the relaxation mechanisms. In other words, the pulse duration can operate a selection on fast occurring processes, and can influence the path of relaxation or dissociation of atoms or molecules. Therefore, this parameter can be instrumental to control photolysis yields (stable species that result from irradiation).

The role of the pulse duration in determining the relative yields of long-term photolytic species also appears at another level. In solutions for example, the primary photolytic species generated by the physical processes (ionization, excitation, and dissociation) take part in a complex chemistry, which forms new stable products. The dynamic of these chain reactions is very sensitive to the way in which primary photolytic species are supplied into the system. For a longer pulse duration (above the ns range), primary photolytic species are still being supplied long after the chemistry has started. Although in the case of a femtosecond laser pulse, we can safely consider that all the initial species are generate in the same time, before the chemical phase. In conclusion, even if the laser light doesn't interfere with the relaxation processes, the pulse duration can still affect the outcome of the photolysis, by controlling the dynamics of the chain reactions in the chemical phase.

Control of dose distribution

The dose distribution is related to the spatial distribution of the primary photolytic species. There is a linear correlation between the spatial intensity distribution and the dose distribution.

Moreover, in the SP mode the wavelength can be tuned to affect (ionize, dissociate, or excite) only certain species within the material, which can be considered impurities. The dose is then spatially distributed according to the global spatial intensity distribution, and at the microscopic level, it is centered on every impurity atoms/molecules affected by the laser light. In other words, the ptotoionization density can also be controlled by the density of absorbers. For relatively high intensity levels, or for a very small concentration of absorbers/impurities, the absorption can be locally saturated, meaning that all potential absorbers are excited or ionized, and the material becomes totally transparent to the remaining light. This effect is called photobleaching.

In the case where photobleaching doesn't occur, the dose distribution according to the depth of penetration within the sample is given by the simple equation of Beer-Lambert.

Were l is the length/depth of absorption, Io is the initial intensity, I(l) is the intensity at depth l within the sample, A is called absorbance, c is the concentration of the absorbent entity, and alpha is the absorption coefficient.

A typical depth-dose distribution is illustrated in the figure below.

References

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