Chemical laser

Chemical laser is a laser that is based on a mostly exothermic chemical reaction . This means that the energy released during the chemical reaction taking place in the laser reaction tube is converted into electromagnetic energy (light). These electromagnetic waves are reflected on mirrors. Enhanced in this way, they leave the resonator as laser radiation.

The three main components of the chemical laser are:

• the laser medium , a molecular gas,
• the pump source, the chemical reaction that supplies energy,
• the resonator : two reflecting mirrors which, as feedback elements, generate the laser radiation.

Working principle

Chemical lasers are of the infrared or ultraviolet laser type, since the molecules are excited to vibrate in the infrared range of the spectrum or to electronic transitions in the ultraviolet range. The theoretical basis for lasers in general is the more frequent occupation of higher energy levels than the corresponding occupation in the ground state of the molecules. One speaks of population inversion . The laser radiation is then generated as a stimulated emission .

Chemical lasers use the reaction energy of a chemical reaction, mostly between gaseous media, which is mostly stored in the form of vibration energy of the molecules (see molecular oscillation ). The laser transitions are therefore often vibration-rotation transitions within the electronic ground state in the corresponding wavelength range between 3 and 10 µm. The chemical energy is converted into coherent radiation. This is generated by an exothermic chemical reaction with little or no supply of electrical energy.

Practical laser systems, on the other hand, are usually not “pure” chemical lasers, since the reacting atoms or molecules are often prepared by an electrical discharge, photolysis , electron beam excitation, etc. The laser emission is achieved by a mirror resonator perpendicular to the gas flow.

In America, research on chemical lasers was stopped in 2012 because of the poorly environmentally friendly source chemicals, and research on alkali lasers pumped by laser diodes was started .

Types

Chlorine-hydrogen laser

An example of a chemical laser is the chlorine-hydrogen laser (HCl laser), which is based on the following reaction sequence:

${\displaystyle \mathrm {Cl_{2}+h\nu \longrightarrow 2Cl^{*}} }$

${\displaystyle \mathrm {Cl+H_{2}\longrightarrow HCl+H} }$

${\displaystyle \mathrm {H+Cl_{2}\longrightarrow HCl+Cl} }$

(where hν is the photon from the UV light beam of the flash tube )

The basic structure of the chlorine-hydrogen laser consists of a gas flow device into which hydrogen and the halogen radicals generated by a gas discharge flow through nozzles. The excited molecules are formed in the reaction tube. Two reflecting mirrors are arranged perpendicular to this, which, as a resonator, generate the laser emission, which is coherent radiation.

Examples

Laser emitter	Wavelength in μm
Hydrogen fluoride (HF)	1.3
Hydrogen chloride (HCl)	2.6-3.5
Deuterium Flouride (DF)	3.5-4.1
Hydrogen bromide (HBr)	4.0-4.2

Fluorine-hydrogen laser

Chemical reactions can take place in such a way that the end product is a molecule in an excited oscillation state of the basic electron level. So is z. B. the reaction that leads to the formation of hydrogen fluoride is exothermic:

${\displaystyle \mathrm {F+H_{2}\longrightarrow H+HF} }$

with ΔH = 132 kJ / mol

Almost 70% of the excess energy ΔH goes into the excitation of the vibration levels of the HF molecule. The chemical reaction generates radiation through transitions between these levels with different quantum numbers v. The excitation is selective, so that population inversion between the vibration levels is achieved.

The laser output is at a wavelength of 2.6-3.5 μm, consisting of a number of different wavelengths that are generated by rotation-vibration transitions. With a reaction enthalpy (ΔH) of 132 kJ / mol, the frequency distribution for the vibration energy levels 0, 1, 2, 3 is 1: 2: 10: 8, at ΔH = 410 kJ / mol for the vibration energy levels from v = 1 to v = 10 a distribution of 6: 6: 9: 16: 20: 33: 30: 16: 9: 6: 6.

The hydrogen fluoride laser works in a similar way to the reaction cycle of the HCl laser, and analogously to this the DF laser, in which hydrogen is exchanged for deuterium. The main difference to the HCl laser is that in the initial reaction the fluorine free radicals are generated during an electrical discharge by electron bombardment of a substance that is less dangerous than F ₂ , such as SF ₆ . Oxygen gas, which is also in the reaction mixture, converts the sulfur released into SO ₂ . Only about 1% of the reaction gas flows through the laser.

RF lasers have been built for missile defense applications, but have rarely been used for this purpose. Applications in the field of spectroscopy are also possible.

Iodine laser

A typical gas mixture for the iodine laser is 1-iodoheptafluoropropane, which is stored in an ampoule and filled into a silicate laser tube at a pressure of 30 to 300 mbar. The following reaction sequence takes place:

${\displaystyle \mathrm {C_{3}F_{7}I+h\nu \longrightarrow C_{3}F_{7}+I*} }$

${\displaystyle \mathrm {I*\longrightarrow I+h\nu } }$

${\displaystyle \mathrm {C_{3}F_{7}+I+M\longrightarrow C_{3}F_{7}I+M} }$

Iodine lasers are types of lasers with which nuclear fusion experiments would be conceivable.

Chemical oxygen-iodine laser (COIL)

The chemical oxygen Iodlaser ( English chemical oxygen iodine laser , COIL), a variation of the iodine laser, emits laser radiation at a wavelength of 1.315 microns.

First, singlet oxygen ( ¹ ΔO ₂ ) is generated by an electrical discharge . This supplies the pump energy for the iodine atoms via an energy transfer. These are excited to oscillate, resulting in population inversion between the ground state I ( ² P3 / 2) and the first excited electronic state I ( ² P1 / 2). In addition, the energy is sufficient to dissociate the gaseous iodine molecules.

The entire laser-active mixture in the cavity resonator has a flow speed that corresponds to twice the speed of sound . This achieves efficient population inversion at low temperature and low pressure. In addition, the oxygen-iodine mixture has laser-active properties that do not impair the radiation field in the resonator. The chemical oxygen-iodine laser system is characterized by its high beam quality and high laser power.

literature

• Entry to chemical lasers. In: Römpp Online . Georg Thieme Verlag, accessed on September 11, 2015.
• David L. Andrews: Lasers in Chemistry. 3rd edition. Springer, Berlin et al. 1997, ISBN 3-540-61982-8 , pp. 46-47.
• Marc Eichhorn: Laser Physics. Basics and applications for physicists, mechanical engineers and engineers. Springer Spectrum, Berlin et al. 2013, ISBN 978-3-642-32647-9 .
• Jürgen Eichler , Hans Joachim Eichler : Laser. Designs, beam guidance, applications. 7th, updated edition. Springer, Berlin et al. 2010, ISBN 978-3-642-10461-9 .
• Fritz Peter Schäfer , Alexander Müller: Applications of the laser. Spectrum of Science Publishing Company, Heidelberg 1988, ISBN 3-922508-47-2 .
• Donald J. Spencer, Theodore A. Jacobs, Harold Mirels, Rolf WF Gross: Continuous-Wave Chemical Laser. In: International Journal of Chemical Kinetics. Vol. 1, No. 5, 1969, pp. 493-494, doi: 10.1002 / kin.550010510 .
• Carsten Pargmann, Thomas Hall, Frank Duschk, Karin Maria Grünewald, Jürgen Handke: COIL emission of a modified negative branch confocal unstable resonator. In: Applied Optics. Vol. 46, No. 31, 2007, pp. 7751-7756, doi : 10.1364 / AO.46.007751 .

Individual evidence

1. ^ Fritz Kurt Kneubühl, Markus Werner Sigrist: Laser (= Teubner study books. Physics. ). 4th revised edition. Teubner, Stuttgart 1995, ISBN 3-519-33032-6 .
2. ↑ ^{a b c} Entry on chemical lasers. In: Römpp Online . Georg Thieme Verlag, accessed on September 11, 2015.
3. ↑ United States Army - Space and Missile Defense Command: Directed Energy Master Plan. United States Army - Space and Missile Defense Command, Huntsville AL 2000.
4. ^ ^{A b c} David L. Andrews: Lasers in Chemistry. Springer, 1997, ISBN 3-540-61982-8 , pp. 46-47.
5. ^ ^{A b c} Hans-Joachim Eichler, Jürgen Eichler: Laser. Designs, beam guidance, applications. 7th, updated edition. Springer, 2010, ISBN 978-3-642-10461-9 , pp. 63, 81.
6. ↑ Chemical oxygen-iodine laser (COIL) . DLR Institute of Technical Physics, accessed on September 7, 2015.