Excimer Lamps
Author: the photonics expert Dr. Rüdiger Paschotta (RP)
Definition: gas discharge lamps where ultraviolet radiation is generated by spontaneous emission from excited dimers
Alternative term: excilamps
Category: 
Related: excimer lasers
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DOI: 10.61835/t1w Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn
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What are Excimer Lamps?
Excimer lamps (excilamps) are a special type of gas discharge lamps, where the generation of ultraviolet light involves excited dimers (or more generally exciplex molecules). In contrast to excimer lasers, they do not contain a laser resonator and thus exploit only spontaneous emission. They are therefore diffusely emitting lamps, e.g. long tubes emitting to the side, not generating directed spatially coherent laser beams.
After spontaneous emission, the de-excited dimers rapidly dissociate. Therefore, there is basically no reabsorption of the generated radiation (which is known to be a problem e.g. in mercury vapor lamps, where it substantially reduces the power conversion efficiency). This feature is particularly beneficial for lamps operated with a relatively high gas density.
Working pressures of excimer lamps can vary in a wide range, from low-pressure lamps with a few millibars to high-pressure lamps with more than one atmosphere. Even high-pressure excimer lamps are not operated at particularly high plasma temperatures, since the power density is moderate and the radiation efficiency is relatively high. Since high plasma temperatures are not required, there is no substantial warm-up time.
The envelope glass of such a lamp must of course be well transparent to the emitted UV light. For example, UV-grade fused silica can be used for wavelengths down to somewhat below 200 nm.
Typical Characteristics of Excimer Lamps
Geometric Shapes
Excimer lamps often have the shape of long cylindrical tubes which radiate to the side. However, it is also possible to produce them with various other shapes. For example, there are excimer lamps with a design which is arranged around a flow tube, through which gases or liquids to be irradiated can be sent. Such a geometry is useful for water purification, for example.
Emission Wavelengths
The following table shows the emission wavelengths of common excimer and exciplex species:
| Excimer | Wavelength |
|---|---|
| NeF* | 108 nm |
| Ar2* | 126 nm |
| Kr2* | 146 nm |
| F2* | 158 nm |
| ArBr* | 165 nm |
| Xe2* | 172 nm |
| ArCl* | 175 nm |
| KrI* | 190 nm |
| ArF* | 193 nm |
| KrBr* | 207 nm |
| KrCl* | 222 nm |
| KrF* | 248 nm |
| XeI* | 253 nm |
| Cl2* | 259 nm |
| XeBr* | 282 nm |
| Br2* | 289 nm |
| XeCl* | 308 nm |
| I2* | 342 nm |
| XeF* | 351 nm |
For example, Ar2 and Xe2 are true dimers, while NeF and ArCl should strictly speaking be called exciplex molecules. The star (*) indicates that we are dealing with electronically excited species.
The list contains various values in the vacuum UV range (below ≈ 200 nm), where not many alternative solutions are available.
The obtained radiation is essentially quasi-monochromatic; naturally, one UV emission line is clearly dominating. The emission bandwidth is usually a few nanometers. If required, some additional unwanted weaker spectral lines may be filtered out.
Only small amounts of visible and infrared light are emitted. During operation one usually sees some visible glow from additional spectral lines, which however carries a radiant flux which is much weaker than that of the generated UV light.
Continuous or Pulsed Emission
While excimer lasers are basically always pulsed lasers, excimer lamps are often operated in continuous-wave mode. This is because a very high excitation density is not required, and it is no problem to make the emitting volume a bit larger. Typically, the power density is moderate — of the order of 1 W/cm2 for high-pressure lamps and much lower for low-pressure lamps.
The used electrode types and geometries are adapted to the operation mode and excimer type. For example, dielectric barrier discharges (driven with radio frequency) are common for continuously emitting lamps. A beneficial property of that technology is that the electrodes do not need to be in direct contact with the plasma, so that long lamp lifetimes are possible.
Efficiency
The radiant efficiency of excimer lamps can easily reach values of several tens of percent, depending on the excimer species used, but also on details like gas pressure, power density and electrode geometry. The achieved efficiency values are quite favorable in comparison to those of competing technologies.
Lamp Lifetime
With the common dielectric barrier discharge lamps, lifetimes of several thousand hours are achievable. However, particularly for lamps with a relatively short emission wavelength, there may be a degradation of transparency of the envelope glass (and thus the light output) due to the formation of color centers.
Applications of Excimer Lamps
Excimer lamps are used in various industries where short-wavelength ultraviolet light is required. For example, they are useful for printing processes, photolithography, UV curing of optical adhesives, surface cleaning and UV surface modification, ozone generation and sterilization (disinfection). Often, photochemical processes induced by the ultraviolet light are exploited. In some cases, the deactivation of germs (microbes) is relevant; this can work by the destruction of DNA and RNA, but also by the general mineralization of organic substances at higher radiant exposure.
Mercury vapor lamps are also often used as UV sources; in comparison with those, excimer lamps are environmentally more benign, since the used emitting species are less toxic, if at all problematic. The concrete choice of lamp, however, is often dominated by the required emission wavelength.
Comparison with Mercury Vapor Lamps
Excimer lamps differ significantly from mercury vapor lamps, which are the most common alternative UV sources. Key differences include:
- Spectral output: Mercury lamps emit a broad spectrum with multiple peaks (dominantly 254 nm and 185 nm for low-pressure types), whereas excimer lamps emit quasi-monochromatic light centered at a specific wavelength determined by the gas mixture (e.g., 172 nm, 222 nm, 308 nm).
- Switching behavior: Excimer lamps can be switched on and off instantly without a warm-up or cool-down phase, allowing for pulsed operation and energy savings during standby. Mercury lamps typically require warm-up time and cannot be restarted immediately when hot.
- Geometry: While mercury lamps are restricted to tubular shapes due to the discharge physics, excimer lamps (specifically dielectric barrier discharge types) can be manufactured in various shapes, including flat panels for large-area irradiation.
- Temperature: Excimer lamps transfer significantly less infrared heat to the substrate (“cold UV”), which is crucial for processing heat-sensitive materials.
Frequently Asked Questions
This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).
What is an excimer lamp?
An excimer lamp is a gas discharge lamp that produces ultraviolet light from the spontaneous emission of excited dimers (excimers) or exciplex molecules. Unlike a laser, it does not have a resonator and emits light diffusely.
What is the main advantage of excimer lamps over mercury lamps?
Excimer lamps can be more efficient because the generated UV light is not reabsorbed by the gas. They are also environmentally friendlier as they do not contain toxic mercury and offer instant-on operation without a warm-up period.
What kind of light do excimer lamps produce?
They produce quasi-monochromatic ultraviolet radiation at specific wavelengths, depending on the gas used, with an emission bandwidth of a few nanometers. They emit very little visible or infrared light.
Can excimer lamps operate continuously?
Yes, unlike most excimer lasers, excimer lamps are often operated in continuous-wave mode. A common method for this is using a dielectric barrier discharge, which also contributes to a long lamp lifetime.
What are excimer lamps used for?
They are used in various industrial processes requiring short-wavelength UV light, such as printing, photolithography, UV curing of adhesives, surface cleaning, sterilization (disinfection), and ozone generation.
How long do excimer lamps last?
Lamps based on a dielectric barrier discharge can achieve lifetimes of several thousand hours. However, for very short emission wavelengths, the lamp's glass envelope may degrade over time, reducing the light output.
Suppliers
Sponsored content: The RP Photonics Buyer's Guide contains seven suppliers for excimer lamps. Among them:
Hamamatsu Photonics excimer lamp light sources contribute to surface modification, dry cleaning, oxidative decomposition, TOC measurement, and VOC decomposition.
The optimum product can be selected based on the product size and irradiation area.
Bibliography
| [1] | J.-Y. Zhang and I. W. Boyd, “Lifetime investigation of excimer UV sources”, Appl. Surface Science 168, 296 (2000) |
| [2] | T. Oppenländer, “Mercury-free sources of VUV/UV radiation: application of modern excimer lamps (excilamps) for water and air treatment”, J. Environ. Eng. Sci. 6, 253 (2007); doi:10.1139/S06-059 |
(Suggest additional literature!)
Questions and Comments from Users
2020-08-14
Are there also known medium or high pressure excimer lamps for the visible spectrum?
The author's answer:
I am not aware that something like that exists. Basically, these are meant to be ultraviolet sources only.
2021-02-19
KrCl is reported as having minimal or no environmental impact. Are the quantities of gas contained in a KrCl excimer lamp hazardous if a person is exposed to it, for example, if an excimer lamp is damaged in a in-duct system?
The author's answer:
Although chlorine is quite poisonous, I think that the quantities should usually be so small that there is no serious hazard. However, particularly in the case of high-pressure lamps with substantial volume, one should check that more closely. In case of doubt, ask the manufacturer.
2020-05-02
Would it be possible to produce an excimer lamp with a structure like a fluorescent lamp by filling it with excimers like KrCl or other gases?
The author's answer:
In principle yes, but various details needs to be observed — for example, the lamp envelope must be UV-transmitting, the electrodes should be made with appropriate materials and geometry, the write operation voltage and current needs to be applied, etc. The gas filling must be an appropriate mixture e.g. of Kr and Cl2; the excimer molecules are formed from that only during operation.