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Light Sources

General Aspects

The choice of the light source is probably the most important decision when photoreactions shall be conducted. Ideally the light source emits light with a wavelength identical to the wavelength of the absorption maximum of the photochemical active species. In special cases it can also be necessary to consider the absorption spectra of the product. This is required if the product also absorbs light emitted by the light source and undergoes e.g. photodegradation or another consecutive photochemical reaction. Beside the emission spectra the power of the light source determines the performance of a photochemical reaction. Hence, if a high reaction rate is required, a light source with an high optical power is required. In general the luminescent efficiency of artificial light sources is below 50 % resulting an additional engineering demands if high power light sources are used. Especially the heat generated by the light sources has to be removed by an appropriate heat management. This is especially crucial if reactions are conducted which are temperature sensitive. It is evident that the characteristics of the light source dictate to a certain extent the design of the photoreactor as well as the required periphery like temperature control systems or safety measures.

In contradiction to the importance of the choice of the light source only a low number of different light sources is available. In general light sources can be classified be the primary process of light generation. Hence, thermal and non-thermal light sources can be distinguished. Table 1 provides an overview of the different types of light sources and their typical application.

LST2

  Table 1: Light sources and their properties[1].

 LS1Figure 1: Emission spectra of a ideal black body emitter for different temperatures.

Thermal Light Sources

Thermal light sources generate light by heating a material, e.g. a tungsten filament, causing a thermal excitation of the electrons which consecutively relax to the ground state which is accompanied with the emission of a photon. Popular examples are incandescent lamps or candles. The emission spectrum of thermal light sources is continuous and depends on the temperature of the heated material. The distribution and the intensity of the emission can be described by Planck’s law for an ideal black body emitter: LSeq1

The spectral irradiance M0 is a function of the wavelength λ and the temperature T. It becomes clear, that the main emission wavelength is shifted to shorter wavelength as the temperature is increased. This is illustrated in figure 0.1. Because the black body radiation is only a function of temperature, the maximum peak λmax can be calculated as:

 

Non-Thermal Light Sources

For non-thermal light sources the energy for exciting atoms or molecules is provided by a nonthermal energy source. Among this group of light sources are discharge lamps, light emitting diodes and fluorescence lamps. Due to the quantization of the energy levels of atoms and

molecules non-thermal lamps exhibit discontinuous spectra. With this the choice of the light source for initiating photochemical reactions becomes even more important.

 LST1

Table 2: Persistent ultraviolet emission lines of mercury.[3]

The most important light sources for chemical reactions are mercury arc lamps which are available with different Hg pressures. Low pressures Hg lamps typically operate with a pressure of 10·10−3 mbar to 10mbar, medium pressure lamps with a pressure of over1bar and high pressure lamps with a pressure of about 100 bar.[2] Table 0.2 gives an overview on the different persistent spectral lines and the corresponding energy levels which are emitted by mercury. Persistent in this case means the emission lines which are still measured with a minimal amount of the investigated element. In spectroscopy these lines are of special importance because they can be observed over a broad range of experimental conditions. The most dominant spectral lines of Hg lamps are at 254 nm, 297 nm, 313 nm, 334 nm, 365nm, 404nm, 435nm, 546nm and 576nm. In figure 2 typical emission spectra for Hg lamps are shown exemplarily. While the emission lines are very sharp for low and medium pressure lamps, the continuous fraction of the spectrum increases with increasing operating pressure (line broadening). Due to this characteristic, the spectral distribution characteristics of mercury lamps is often only represented by a relative spectral distribution. With this the emitted spectral power can only be calculated if the optical power output of the light source is known.

Low pressure mercury arc lamps emit predominantly (80%) at 253.7nm being the resonance

 LS2

Figure 2: Normalized emission spectra of a low pressure Hg lamp, a medium pressure Hg lamp (gray), and a high pressure Hg lamp (dashed).

line of mercury. The higher pressure in medium and high pressure lamps causes the so called autoabsorption phenomenon where the 253.7nm emission band is absorbed by mercury atoms. With this the 253.7nm emission appears in the form of two lines separated by a narrow dark zone, when the mercury pressure reaches 1bar (inverted radiation).[4] Further the higher Hg pressure as well as the higher temperature lead to excitation of the mercury atoms to additional energy levels as for low pressure lamps. This leads to additional photon emissions at longer wavelength. Further, tuning of the emission spectra can be achieved by adding metal salts as dopants.[2]

Another way to produce UV radiation on a non-thermal way is the excimer emission. For this purpose, gases are excited with accelerated electrons. The electrons are generated by an electrode in the middle the lamp and shift to a second mesh electrode which wraps the capacity filled with gas. The most common geometry for excimer lamps is tubular. On their way, electrons excitate the gas molecules. Due to distinct excitated states, im most cases only one emission peak with a narrow half width occurs.[5] Figure 0.3 shows the emission spectra of three excimer lamps with emission in the UV-range. Xe∗2 excimer lamps emit light with a peak wavelegth of 172nm, KrCl∗ excimer lamps with a peak wavelength of 222nm and XeCl∗ excimer lamps with a peak wavelength of 308nm. Due to the mentioned single and narrow emission, excimer lamps can be chosen for photochemical reactions where different reaction pathways can be provoked with different wavelengths of excitating light. For example, the formation of an unwanted side product can be suppressed. On the other hand, light sources with only one emission peak can only be used for some fitting reactions.

 

 LS3

 Figure 3: Emission spectra of a Xe2 excimer lamp a KrClexcimer lamp a XeClexcimer lamp. Pel = 700 W.

 

 One of the newest light sources with one distinct emission peak are the light emitting diodes (LED). The functional principle is called electroluminescence and is shown in Figure 4. The setup is similar to a pn-semiconductor diode. On big difference is the position of the energetic levels and the band gaps. When an electron is transferred from the n- to the p-semiconductor, it will fall from the conduction band to the valence band. This process is also described as the recombination of electrons and electron holes. The energy of the band gap between conduction and valence band will be released. For a LED this amount of energy is in the range of UV visible and infrared light. Like for other light sources where the difference between energy levels is used, the emission of LEDs possesses a narrow half width. In principle, a large variety of wavelengths is possible by combining the appropriate semiconductors, however, the efficiency of LEDs with different wavelengths still differs a lot and research is still in progress for optimization.[6] The most efficient LEDs emit radiation between 365nm to 465nm, with 30% to 50% the efficiency is comparable to classical discharge lamps as Hg lamps or excimer lamps.

 

 LS4

 Figure 4: Functional principle of a LED, showing the recombination of electrons and electron holes between the  conduction and the valence band.

Figure 5 shows the emission spectra of multiple LEDs between 280nm to 520nm. To enable comparison of the intensity of the spectra in Figure 3, the electrical power Pel is scaled to 700 W. LED emitting UV radiation below 350nm show a much lower efficiency.

 LS5

 Figure 5: Emission spectra of LEDs with the emission wavelengths of 365nm, 385nm, 395nm, 405nm, 450nm, 465nm and 520nm. Pel = 700 W.

 

  1.  BÖTTCHER, H.: Technical Applications of Photochemistry, 1991, Deutscher Verlag fuer Grundstoffindustrie, ISBN 978–3–3420–0627–5.
  2.  BRAUN, A. M.; MAURETTE, M.-T.; OLIVEROS, E.: Photochemical technology, 1991, Wiley, ISBN 0471926523, DOI: 10.1002/ange.19921041147.
  3. Persistent Lines of Neutral Mercury ( Hg I ), 2016, http://physics.nist.gov/ PhysRefData/Handbook/Tables/mercurytable3.htm.
  4. GÜNTHER PESCHL: Informationen zu Hg-Mitteldruckstrahlern.
  5. KOGELSCHATZ, U.: SPIE Proceedings, 2012, SPIE, pp. 272–286, DOI: 10.1117/12.563006.
  6. HIRAYAMA, H.; MAEDA, N.; FUJIKAWA, S.; TOYODA, S.; KAMATA, N.: Recent progress and future prospects of AlGaN-based high-efficiency deep-ultraviolet light

 



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