(laser vision correction) and semiconductor manufacturing.
Laser emission from an excimer molecule occurs due to the fact that it has an “attractive” (associative) excited state and a “repulsive” (non-associative) ground state - that is, molecules do not exist in the ground state. This is because noble gases such as xenon or krypton are highly inert and do not usually form chemical compounds. When excited (caused by electrical discharge), they can form molecules with each other (dimers) or with halogens such as fluorine or chlorine. Therefore, the appearance of molecules in an excited bound state automatically creates a population inversion between the two energy levels. Such a molecule, in an excited state, can give up its energy in the form of spontaneous or stimulated emission, as a result of which the molecule goes into the ground state, and then very quickly (within picoseconds) disintegrates into its constituent atoms.
Even though the term dimer refers only to the joining of identical atoms, and most excimer lasers use mixtures of noble gases with halogens, the name has stuck and is used for all lasers of a similar design.
The wavelength of an excimer laser depends on the composition of the gas used, and usually lies in the ultraviolet region:
Excimer lasers usually operate in pulsed mode with a pulse repetition rate from 1 Hz to several hundred Hz; in some models the frequency can reach 2 kHz; generation of single pulses is also possible. Radiation pulses usually have a duration from 10 to 30 ns and an energy from units to hundreds of mJ. The powerful ultraviolet radiation of such lasers allows them to be widely used in surgery (especially eye surgery), in photolithography processes in semiconductor production, in microprocessing of materials, in the production of LCD panels, as well as in dermatology. Today, these devices are quite bulky, which is a disadvantage for widespread medical use (see LASIK), but their size is constantly decreasing due to modern developments.
see also
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Links
- EXCIMER LASER - Physical encyclopedia. In 5 volumes. - M.: Soviet Encyclopedia. Editor-in-chief A. M. Prokhorov. 1988.
- Excimer lasers, ed. C. Rhodes, trans. from English, M., 1981
An excerpt describing the excimer laser
Balashev respectfully allowed himself to disagree with the opinion of the French emperor.“Every country has its own customs,” he said.
“But nowhere in Europe is there anything like this,” said Napoleon.
“I apologize to your Majesty,” said Balashev, “besides Russia, there is also Spain, where there are also many churches and monasteries.”
This answer from Balashev, which hinted at the recent defeat of the French in Spain, was highly appreciated later, according to Balashev’s stories, at the court of Emperor Alexander and was appreciated very little now, at Napoleon’s dinner, and passed unnoticed.
It was clear from the indifferent and perplexed faces of the gentlemen marshals that they were perplexed as to what the joke was, which Balashev’s intonation hinted at. “If there was one, then we did not understand her or she is not at all witty,” said the expressions on the faces of the marshals. This answer was so little appreciated that Napoleon did not even notice it and naively asked Balashev about which cities there is a direct road to Moscow from here. Balashev, who was on the alert all the time during dinner, replied that comme tout chemin mene a Rome, tout chemin mene a Moscow, [just as every road, according to the proverb, leads to Rome, so all roads lead to Moscow,] that there are many roads, and that among these different paths there is the road to Poltava, which Charles XII chose, said Balashev, involuntarily flushing with pleasure at the success of this answer. Before Balashev had time to finish the last words: “Poltawa,” Caulaincourt began talking about the inconveniences of the road from St. Petersburg to Moscow and about his St. Petersburg memories.
After lunch we went to drink coffee in Napoleon’s office, which four days ago had been the office of Emperor Alexander. Napoleon sat down, touching the coffee in a Sevres cup, and pointed to Balashev’s chair.
There is a certain after-dinner mood in a person that, stronger than any reasonable reason, makes a person be pleased with himself and consider everyone his friends. Napoleon was in this position. It seemed to him that he was surrounded by people who adored him. He was convinced that Balashev, after his dinner, was his friend and admirer. Napoleon turned to him with a pleasant and slightly mocking smile.
– This is the same room, as I was told, in which Emperor Alexander lived. Strange, isn't it, General? - he said, obviously without doubting that this address could not but be pleasant to his interlocutor, since it proved the superiority of him, Napoleon, over Alexander.
Balashev could not answer this and silently bowed his head.
“Yes, in this room, four days ago, Wintzingerode and Stein conferred,” Napoleon continued with the same mocking, confident smile. “What I cannot understand,” he said, “is that Emperor Alexander brought all my personal enemies closer to himself.” I do not understand this. Didn't he think that I could do the same? - he asked Balashev with a question, and, obviously, this memory pushed him again into that trace of morning anger that was still fresh in him.
“And let him know that I will do it,” said Napoleon, standing up and pushing his cup away with his hand. - I will expel all his relatives from Germany, Wirtemberg, Baden, Weimar... yes, I will expel them. Let him prepare refuge for them in Russia!
Balashev bowed his head, showing with his appearance that he would like to take his leave and is listening only because he cannot help but listen to what is being said to him. Napoleon did not notice this expression; he addressed Balashev not as an ambassador of his enemy, but as a man who was now completely devoted to him and should rejoice at the humiliation of his former master.
MSTU im. N.E. Bauman
Educational and methodological manual
Excimer lasers
N.V. Lisitsyn
Moscow 2006
Introduction
1. Theoretical foundations
1.1 Active medium
1.1.2 Inert gas oxide lasers
1.1.3 Lasers based on excimer molecules of pure noble gases
1.1.4 Diatomic halogen lasers
1.1.5 Metal vapor lasers
1.1.6 Cooling, ventilation and purification of working gas
1.2 Pumping
1.2.1 Electron beam pumping
1.2.2 Electric discharge pumping
1.2.2.1 Discharge circuits
1.2.2.2 Pumping by fast transverse electric discharge
2.2.3 Pumping by an electric discharge with preionization by an electron beam
1.2.2.4 Double electric discharge pumping
1.3 Output radiation parameters
2. Commercial models of excimer lasers
2.1 Laser LPXPro 305 from LAMBDA PHYSIK (Germany)
2.2 Laser eX5 BY gam lasers, inc (USA)
3. Applications
3.1 Photolysis excitation of laser media
3.2 Generation of short-wave radiation
3.2.1 Photolithography
3.2.2 Laser surgery. An example of recalculation of laser radiation parameters
Literature
Introduction
Excimer lasers are one of the most interesting types of lasers. The emission of sources belonging to this type in the spectral range occupies the range from 126 nm to 558 nm. Thanks to such a short wavelength, excimer laser radiation can be focused into a very small spot. The power of these sources reaches units of kW. Excimer lasers are pulsed sources. The pulse repetition rate can reach up to 500 Hz. This type of laser has a very high quantum yield and, as a result, a fairly high efficiency (up to 2 - 4%).
Due to such unusual characteristics, excimer laser radiation is used in many fields and applications. They are used in clinics during operations (on the iris and others) where tissue burning is necessary. Based on these lasers, microphotolithographic installations have been created for fine etching of materials when creating electronic printed circuit boards. Excimer lasers have found widespread use in experimental scientific research.
However, all these remarkable characteristics of excimer lasers entail some difficulties in their manufacture and the creation of installations based on them. For example, with such a high radiation power, it is necessary to prevent the formation of an arc in the active gas mixture. To do this, it is necessary to complicate the pumping mechanism in order to reduce the duration of its pulse. Short-wave radiation from excimer lasers requires the use of special materials and coatings in resonator structures, as well as in optical systems to convert their radiation. Therefore, one of the disadvantages of this type of source is its high cost compared to other types of lasers.
1. Theoretical foundations
1.1 Active medium
The active medium of an excimer laser is gas molecules. But, unlike CO, CO 2 or N 2 lasers, generation in excimer lasers occurs not on transitions between different vibrational-rotational states, but between different electronic states of molecules. There are substances that in the ground state cannot form molecules (their particles in an unexcited state exist only in monomer form). This occurs if the ground state of the substance corresponds to the mutual repulsion of atoms, is weakly bound, or is bound, but in the presence of large internuclear distances (Fig. 1).
Figure 1: a - sharply repulsive curve; b - flat curve; c - bound state curve at large internuclear distances
The molecules of the working substance of excimer lasers can be roughly divided into two types: those formed by particles of the same substance and particles of two different substances. In accordance with this, the active media themselves can be called “excimers” (excimer, excited dimer) and “exciplexes” (exciplex, excited complex).
It is convenient to consider the process of obtaining lasing in an excimer laser using Figure 2, which shows the potential energy curves for the ground and excited states of the diatomic A 2 molecule.
Figure 2. Excimer laser energy levels.
Since the potential energy curve of the excited state has a minimum, the A 2 * molecule can exist. This molecule is an excimer. In the process of relaxation of the excited medium, a certain trajectory of energy flow is established, which contains a jump that can only be overcome by the emission of radiation. If a fairly large number of such molecules are accumulated in a certain volume, then at the transition between the upper (bound) and lower (free) levels it is possible to obtain generation (stimulated emission) - a bound-free transition.
This transition is characterized by the following important properties:
When a molecule transitions to the ground state as a result of generation, it immediately dissociates;
There are no clearly defined rotational-vibrational transitions, and the transition is relatively broadband.
If population inversion is not achieved, then fluorescence is observed.
If the lower state is weakly bound, then the molecule in this state undergoes rapid dissociation either itself (predissociation) or as a result of the first collision with another molecule of the gas mixture.
At present, laser generation has been achieved on a number of excimer complexes - quasi-molecules of noble gases, their oxides and halides, as well as pairs of metal compounds. The generation wavelengths of these active media are given in Table 1.
Table 1
| Excimer complexes | Quasimolecules of noble gases | Oxides of noble gases | Pairs of metal connections | ||||
| Active quasimolecule | Xe 2* | Kr 2* | Ar 2* | ArO* | KrO* | XeO* | CdHg* |
| λ gene, nm | 172 | 145,7 | 126 | 558 | 558 | 540 | 470 |
| ∆λ, nm | 20 | 13,8 | 8 | 25 | |||
| R imp, MW (R avg, W) | 75 | 50 | |||||
| τ, ns | 10 | 10 | 4-15 | ||||
| Active quasimolecule | XeBr* | XeF* | ArF* | ArCl* | XeCl* | KrCl* | KrF* |
| λ gene, nm | 282 | 351 | 193 | 175 | 308 | 220 | 248 |
| ∆λ, nm | 1 | 1,5 | 1,5 | 2 | 2,5 | 5 | 4 |
| R imp, MW (R avg, W) | (100) | 3 | 1000 | (0,02) | (7) | 5(0,05) | 1000 |
| τ, ns | 20 | 20 | 55 | 10 | 5 | 30 | 55 |
To obtain quasi-molecules of noble gases, pure gases under pressure of tens of atmospheres are used; to obtain oxides of noble gases - a mixture of source gases with molecular oxygen or compounds containing oxygen in a ratio of 10,000: 1 under the same pressure; to obtain halides of noble gases - their mixture with halogens in a ratio of 10,000: 1 (for argon and xenon) or 10: 1 (for xenon or krypton) at a total pressure of 0.1 - 1 MPa.
1.1.1 Rare gas halide lasers
Let us consider the most interesting class of excimer lasers, in which an inert gas atom in an excited state combines with a halogen atom, which leads to the formation of an exciplex of inert gas halides. Specific examples include ArF (λ = 193 nm), KrF (λ = 248 nm), XeCl (λ = 309 nm), XeF (λ = 351 nm), which all generate in the UV range. Why noble gas halides are easily formed in an excited state becomes clear when we consider that in an excited state, noble gas atoms become chemically similar to alkali metal atoms, which readily react with halogens. This analogy also indicates that in the excited state the bond is ionic in nature: during the formation of the bond, the excited electron passes from the inert gas atom to the halogen atom. Therefore, such a bound state is also called a charge transfer state.
In inert gas halide lasers, photoabsorption processes have a significant effect on the state of the plasma. These include photodissociation of the original halogen, from which the inert gas halide F 2 + hν → 2F is formed; photodecay of the negative ion formed in the plasma F - + hν → F + e - ; photoionization of excited atoms and molecules of inert gas Ar * + hν → Ar + + e - ; photodissociation of dimers of inert gas ions Ar 2 + + hν → Ar + + Ar. As well as the absorption of inert gas halide molecules themselves.
Photoabsorption in the active medium of inert gas halide lasers can be divided into line and broadband. Line absorption occurs on bound-bound transitions present in a laser mixture of impurities of atomic and molecular gases, as well as free atoms and radicals formed under the action of a discharge either during the decomposition of impurity molecules or due to electron erosion. It has been shown that line absorption in some cases can quite significantly distort the emission spectrum, but, as a rule, does not lead to a noticeable decrease in its energy. Broadband absorption is mainly due to bound-free transitions occurring in processes such as photodissociation, photodetachment and photoionization.
Rare gas halide excimer lasers are typically pumped by an electrical discharge.
Efficient pumping of excimer lasers, i.e. creating a discharge that is optimal from the point of view of energy contribution to the active medium does not yet guarantee high lasing characteristics of the laser. It is equally important to organize the extraction of the light energy stored in it from the active medium.
In this article we will look at the advantages of excimer lasers. Today, medicine has a wide range of all kinds of laser equipment for the treatment of complex diseases in hard-to-reach areas of the human body. help to achieve the effect of minimally invasiveness and painlessness, which has a huge advantage over those surgical interventions that are performed manually during abdominal operations, which are very traumatic, fraught with high blood loss, as well as long-term rehabilitation after them.
What is a laser?
A laser is a special quantum generator that emits a narrow beam of light. Laser devices open up incredible possibilities for transmitting energy over different distances at high speed. Ordinary light, which can be perceived by human vision, consists of small beams of light that spread in different directions. If these beams are concentrated using a lens or mirror, a large beam of light particles will be obtained, but even this cannot be compared with a laser beam, which consists of quantum particles, which can only be achieved by activating the atoms of the medium that underlies the laser radiation.
Varieties
With the help of colossal developments by scientists around the world, excimer lasers are today widely used in many areas of human activity and have the following varieties:
Origin
This type is ultraviolet, which is widely used in the field of eye surgery. Doctors use this device to perform laser vision correction.
The term "excimer" means "excited dimer" and characterizes the type of material that is used as its working fluid. For the first time in the USSR, such a device was presented in 1971 by scientists V. A. Danilichev, N. Basov and Yu. M. Popov in Moscow. The working fluid of such a laser was a xenon dimer, which was excited by a beam of electrons in order to produce radiation with a certain wavelength. After some time, noble gases with halogens began to be used for this, and this was done in 1975 in one of the US research laboratories by scientists J. Hart and S. Searles.
People often ask why excimer lasers are used for vision correction.
Its uniqueness
It was found that the excimer molecule produces by being in an excited "attractive" state as well as a "repulsive" state. This effect can be explained by the fact that xenon or krypton (noble gases) are highly inert and, as a rule, never form chemical compounds. An electrical discharge causes them to become excited, so that they can form molecules either with each other or with halogens, such as chlorine or fluorine. The appearance of molecules in an excited state creates, as a rule, a so-called population inversion, and such a molecule gives up its energy, which is stimulated or spontaneous emission. After this, the molecule returns to its ground state and disintegrates into atoms. The excimer laser device is unique.
The term “dimer” is usually used when identical atoms are connected to each other, but most modern excimer lasers use compounds of noble gases and halogens. Nevertheless, these compounds, which are used for all lasers of a similar design, are also called dimers. How does an excimer laser work? We will look at this now.
Operating principle of excimer laser
This laser is the main player in PRK and LASIK. Its working fluid is inert and halogen gas. When high voltage is introduced into the mixture of these gases, one halogen atom and one inert gas atom combine to form a diatomic molecule. It is in an extremely excited state and after a thousandth of a second it disintegrates into atoms, which leads to the appearance of a light wave in the UV range.
This principle of operation of the excimer laser has found wide application in medicine, since ultraviolet radiation affects organic tissues, for example, the cornea, in such a way that the bonds between molecules are separated, leading to the transfer of tissues from a solid to a gaseous state. This process is called "photoablation".
Wave range
All existing models of this type operate in the same wavelength range and differ solely in the width of the light beam, as well as in the composition of the working fluid. The excimer laser is the most commonly used laser for vision correction. But there are other areas of its use.
The first had a light beam diameter that was equal to the diameter of the surface on which evaporation occurred. The wide range of the beam and its heterogeneity caused the same heterogeneity in the upper layers of the cornea, as well as an increase in temperature on its surface. This process was accompanied by damage and burns. This situation was corrected by the creation of the excimer laser. The MNTK Eye Microsurgery has been using it for a very long time.
New generation lasers have undergone a long process of modernization, during which the diameter of the light beam was reduced, and a special rotational scanning system for delivering laser radiation to the eye was created. Let's look at how excimer lasers are used by doctors.
Application in medicine
In cross-section, such a laser beam looks like a spot moving in a circle, removing the upper layers of the cornea, and also giving it a different radius of curvature. In the ablation zone, the temperature does not rise because the effect is short-term. As a result of the operation, a smooth and clear surface of the cornea is observed. The excimer laser is indispensable in ophthalmology.
The surgeon performing the surgery determines in advance what portion of energy will be supplied to the cornea, as well as to what depth the excimer laser will be applied. From here, the specialist can plan the course of the process in advance and assume what result will be obtained as a result of the operation.
Laser vision correction
How does an excimer laser work in ophthalmology? The method that is popular today is based on the so-called computer repurposing of the cornea, which is the main optical lens of the human eye. The excimer laser that is used on it smoothes the surface of the cornea, removing the upper layers and, thus, eliminating all defects present on it. At the same time, normal conditions appear for the eye to receive the correct images, creating the correct refraction of light. People who have had this procedure see like everyone else who has initially good vision.
The procedure for repurposing the cornea does not cause high temperatures on its surface, which can be detrimental to living tissue. And, according to most people, the so-called burning of the upper layers of the cornea does not occur.
The most important advantage of excimer lasers is that their use for vision correction allows you to get an ideal result and correct almost all existing corneal anomalies. These devices are so precise that they allow “photochemical ablation” of the upper layers.
For example, if this process is carried out on the central zone of the cornea, then its shape becomes almost flat, and this helps correct myopia. If, during vision correction, the layers of the cornea in the peripheral zone are evaporated, then its shape becomes more rounded, and this, in turn, corrects farsightedness. Astigmatism is corrected through dosed removal of the upper layers of the cornea in its various parts. Modern excimer lasers, which are widely used in refractive microsurgery of the eye, guarantee high quality surfaces that undergo photoablation.
Features of use in medicine
Excimer lasers in the form they have today appeared quite recently, but they are already helping people all over the world get rid of vision problems such as myopia, farsightedness, and astigmatism. This solution to the problem, for the first time in many years of creating such equipment, meets all the requirements of painlessness, maximum safety and efficiency.
Eye diseases that can be treated by using
The field of ophthalmic surgery that deals with the elimination of these anomalies of the human eye is called refractive surgery, and such disorders are called ametropic and refractive errors.
According to experts, there are two types of refraction:
Ametropia, in turn, includes several subtypes:
- myopia (nearsightedness);
- astigmatism - the eye receives a distorted image when the cornea has irregular curvature, and the flow of light rays becomes unequal on different parts of its surface;
- hypermetropia (farsightedness).
There are two types of astigmatism - hypermetropic, which is close to farsightedness, myopic, similar to myopia, and mixed.
In order to correctly imagine the essence of refractive manipulations, it is necessary to have a minimal knowledge of the anatomy of the human eye. The optics system of the eye consists of three main elements - the cornea, the lens, which are the light-refracting parts, and the retina, which is the light-receiving part. In order for the resulting image to become clear and sharp, the retina is in the focus of the ball. However, if it is in front of the focus, which happens with farsightedness, or behind it, which happens with myopia, the resulting image becomes unclear and significantly blurred.
In humans, the optics of the eye can change throughout life, in particular from the moment of birth until the age of 16-20, it changes due to the growth and increase in the size of the eyeball, as well as under the influence of certain factors that can lead to the formation of certain anomalies . Thus, the patients of the eye refractive surgeon most often become adults.
Contraindications to excimer beam vision correction procedure
Vision correction with an excimer laser is not indicated for all people suffering from visual impairments. The use of this procedure is prohibited:
Possible complications after use
All existing excimer laser treatment methods today are highly safe and particularly effective. However, there are a number of complications that can occur after surgery using such techniques. These include:
- Partial or incorrect growth of a part of the cornea, after which it is not possible to grow this part again.
- The so-called dry eye syndrome, when the patient experiences redness and pain in the eye. This complication can occur in cases where, during the process of vision correction, the nerve endings that are responsible for the production of tears have been damaged.
- Various vision disorders, for example, double vision or decreased vision in the dark, impaired color perception, or the appearance of a light halo.
- Weakening or softening of the cornea, which can occur either a few months after surgery or several years later.
Excimer laser in dermatology
The effect of low-frequency laser on the skin is extremely positive. This happens due to the following effects:
- anti-inflammatory;
- antioxidant;
- pain reliever;
- immunomodulatory.
That is, there is a certain biostimulating mechanism of action of laser radiation with low power.
Successfully undergoes excimer laser treatment for vitiligo. Pigment spots on the skin are smoothed out very quickly.
The excimer laser is the main protagonist of PRK and LASIK. It got its name from a combination of two words: excited - excited, dimer - double. The active body of such lasers consists of a mixture of two gases - inert and halogen. When high voltage is applied to a mixture of gases, an inert gas atom and a halogen atom form a diatomic gas molecule. This molecule is in an excited and extremely unstable state. After a moment, on the order of thousandths of a second, the molecule disintegrates. The disintegration of the molecule leads to the emission of a light wave in the ultraviolet range (usually 193 nm).
The principle of the effect of ultraviolet radiation on an organic compound, in particular on corneal tissue, is the separation of intermolecular bonds and, as a result, the transfer of part of the tissue from a solid to a gaseous state (photoablation). The first lasers had a beam diameter equal to the diameter of the evaporated surface, and were characterized by a significant damaging effect on the cornea. The wide profile of the beam, its heterogeneity, caused heterogeneity in the curvature of the corneal surface, rather high heating of the corneal tissue (by 15-20˚), which entailed burns and opacities of the cornea.
Since the surgeon knows in advance what portion of light energy is supplied to the object (cornea), he can calculate to what depth the ablation will be performed. And what result will he achieve in the process of refractive surgery. And finally, on the threshold of the third millennium, a new method has appeared to solve this problem - excimer laser correction, which relieves people of myopia, astigmatism and farsightedness. For the first time, laser correction meets all the requirements of a person with “poor” vision. Scientific validity, painlessness, maximum safety, stability of results - these are the unconditional factors that characterize it. The field of ophthalmic surgery that deals with the correction of these anomalies is called refractive surgery, and they themselves are called refractive errors or ametropia.
Experts distinguish two types of refraction:
- Emmetropia- normal vision;
- Ametropia- abnormal vision, including several types: myopia - myopia; hyperopia - farsightedness, astigmatism - image distortion when the curvature of the cornea is irregular and the path of light rays in different parts of it is not the same. Astigmatism can be myopic (nearsighted), hypermetropic (farsighted) and mixed. To understand the essence of refractive interventions, let us very briefly and schematically recall the anatomical physics of the eye. The optical system of the eye consists of two structures: the light-refracting part - the cornea and lens, and the light-receiving part - the retina, located at a certain (focal) distance. In order for the image to be sharp and clear, the retina must be in the focal point of the optical power of the ball. If the retina is in front of the focus, which happens with farsightedness, or behind the focus with myopia, the image of objects will be blurry and unclear. Moreover, from the moment of birth until the age of 18-20, the optics of the eye changes due to the physiological growth of the eyeball and under the influence of factors that often lead to the formation of certain refractive errors. Therefore, a patient of a refractive surgeon is often a person who has reached 18-20 years of age.
Excimer laser vision correction is based on a program of “computer repurposing” of the surface of the main optical lens of the human eye - the cornea. According to an individual correction program, a cold beam “smoothes” the cornea, eliminating all existing defects. This creates normal conditions for optimal refraction of light and obtaining an undistorted image in the eye, as in people with good vision. The process of “repurposing” is not accompanied by a destructive increase in the temperature of the corneal tissue, and, as many mistakenly believe, no “burning out” occurs. And most importantly, excimer laser technologies make it possible to obtain such an “ideal new specified profile” of the cornea that it makes it possible to correct almost all types and degrees of refractive error. Scientifically speaking, excimer lasers are high-precision systems that provide the necessary “photochemical ablation” (evaporation) of the layers of the cornea. If tissue is removed in the central zone, the cornea becomes flatter, which corrects myopia. If you evaporate the peripheral part of the cornea, its center will become steeper, which allows you to correct farsightedness. Dosed removal in different meridians of the cornea allows you to correct astigmatism. Modern lasers used in refractive surgery reliably guarantee high quality of the “ablated” surface.
Working on electronic transitions of excimer molecules (molecules that exist only in electronically excited states). Potential dependence The interaction energy of the atoms of an excimer molecule, which is in the ground electronic state, from the internuclear distance is a monotonically decreasing function, which corresponds to the repulsion of nuclei. For the excited electronic state, which is the top level of the laser transition, this dependence has a minimum, which determines the possibility of the existence of the excimer molecule itself (Fig.). The lifetime of an excited excimer molecule is limited
Dependence of the energy of an esimer molecule on distance R between its constituent atoms X and Y; The upper curve is for the upper laser level, the lower curve is for the lower laser level. The values correspond to the center of the gain line of the active medium, its red and violet boundaries. time its radiation. decay. Since the lower state of laser transition in electron beam. is devastated as a result of the scattering of atoms of the excimer molecule, the characteristic time of which (10 -13 - 10 -12 s) is significantly less than the radiation time. devastation top, laser transition states, gas containing excimer molecules is active medium with enhancement at transitions between excited bound and main expansion terms of the excimer molecule.
The basis of the active medium of E. l. They are usually composed of diatomic excimer molecules - short-lived compounds of inert gas atoms with each other, with halogens or with oxygen. Wavelength of radiation of E. l. lies in the visible or near UV region of the spectrum. Gain linewidth of laser transition E. l. is anomalously large, which is associated with the expanding nature of the lower transition term. Characteristic values of parameters of laser transitions for the most common electron beams. are presented in the table.
Excimer laser parameters
Optimal parameters of the active medium E. l. correspond to optimal conditions for the formation of excimer molecules. The most favorable conditions for the formation of dimers of inert gases correspond to the pressure range of 10-30 atm, when intensive formation of such molecules occurs in triple collisions involving excited atoms:
At such high pressures, the most effective. The method of introducing pump energy into the active medium of a laser involves passing a beam of fast electrons through the gas, which mostly lose energy. to ionize gas atoms. Conversion of atomic ions into molecular ions and subsequent dissociative recombination of molecular ions 
accompanied by the formation of excited atoms of an inert gas, provide the possibility of eff. converting the energy of a beam of fast electrons into the energy of excimer molecules. Lasers based on dimers of inert gases are characterized by an efficiency of ~1%. Basic The disadvantage of lasers of this type is the extremely high beat value. threshold energy input, which is associated with the short wavelength of the laser transition and, therefore, the width of the gain line. This imposes high demands on the characteristics of the electron beam used as a laser pumping source and limits the output energy of laser radiation to the level of fractions of a joule (per pulse) at a pulse repetition rate of no higher than several. Hz A further increase in the output characteristics of lasers based on noble gas dimers depends on the development of technology for electron accelerators with an electron beam pulse duration of the order of tens of nanoseconds and a beam energy of ~kJ.
E. l. have significantly higher output characteristics. on monohalides of inert gases RX*, where X is a halogen atom. Molecules of this type are effectively formed during pairwise collisions, for example or
These processes occur with sufficient intensity even at pressures on the order of atmospheric pressure, so the problem of introducing energy into the active medium of such lasers turns out to be technically much less complex than in the case of lasers based on inert gas dimers. Active medium E. l. on monohalides of inert gases consists of one or several. inert gases at a pressure of the order of atmospheric and a certain number (~10 -2 atm) of halogen-containing molecules. To excite the laser, either a beam of fast electrons or a pulsed electric beam is used. discharge. When using a beam of fast electrons, the output energy of laser radiation reaches values of ~ 10 3 J with an efficiency of several. percent and a pulse repetition rate well below 1 Hz. In case of using electric discharge, the output energy of laser radiation in a pulse does not exceed a fraction of a joule, which is due to the difficulty of forming a discharge that is uniform in volume, meaning a volume at atm. pressure for a time of ~ 10 ns. However, when using electric discharge, a high pulse repetition rate is achieved (up to several kHz), which opens up the possibility of a wide range of practical applications. use of lasers of this type. Naib. widespread among E. l. received a XeCl laser, which is due to the relative simplicity of operating in the high pulse repetition rate mode. Cp. The output power of this laser reaches a level of 1 kW.
Along with high energy. characteristics important attractive feature of E. l. is the extremely high value of the gain linewidth of the active transition (table). This opens up the possibility of creating high-power lasers in the UV and visible ranges with smooth wavelength tuning in a fairly wide range of the spectrum. This problem is solved using an injection laser excitation circuit, which includes a low-power generator of laser radiation with a wavelength tunable within the amplification line width of the active medium of the electron beam, and a broadband amplifier. This scheme makes it possible to obtain laser radiation with a linewidth of ~ 10 -3 HM, tunable along the wavelength in a range of width ~ 10 HM and more.
E. l. are widely used due to their high energy. characteristics, short wavelength and the possibility of its smooth tuning in a fairly wide range. Powerful single-pulse electron beams excited by electron beams are used in installations for studying laser heating of targets for the purpose of carrying out thermonuclear reactions (for example, a KrF laser with HM, output energy per pulse up to 100 kJ, pulse duration ~ 1 ns). Lasers with a high pulse repetition rate, excited by a pulsed gas discharge, are used in technology. purposes in the processing of microelectronics products, in medicine, in experiments on laser isotope separation, in sensing the atmosphere in order to control its pollution, in photochemistry and in experiments. physics as an intense monochromatic source. UV or visible radiation.
Lit.: Excimer lasers, ed. C. Rhodes, trans. from English, M., 1981; EletskyA. V.. Smirnov B. M., Physical processes in gas lasers, M.. 1985. A. V. Eletsky.
