Excimer equipment. What is an excimer laser

In modern refractive surgery, 2 types of laser systems are used for laser vision correction: these are excimer and femtosecond devices, which have a number of distinctive features and are used to solve various problems.

Excimer lasers

Excimer laser refers to gas laser devices. The working medium in this laser is a mixture of inert and halogen gases. As a result of special reactions, the formation of excimer molecules occurs.

The word excimer is an abbreviation that can be literally translated as an excited dimer. This term refers to an unstable molecule that is formed when stimulated by electrons. With further transition of molecules to the previous state, photons are emitted. In this case, the wavelength depends on the gas that is used in the device. In medical practice, excimer lasers are usually used, which emit photons in the ultraviolet spectrum (157-351 nm).

For medical purposes, a high-power pulsed light flux is used, which leads to tissue ablation in the affected area. So the excimer laser in some cases can replace the scalpel, as it causes photochemical destruction of surface tissues. At the same time, the laser does not lead to an increase in temperature and subsequent thermal destruction of cells, which affects deeper tissues.

History of excimer lasers

In 1971, an excimer laser was presented for the first time at the P.N. Lebedev Physical Institute. in Moscow by several scientists (Basov, Popov, Danilichev). This device used bi-xenon, which was excited by electrons. The laser had a wavelength of 172 nm. Later, mixtures of various gases (halogens and inert gases) began to be used in the device. It was in this form that the laser was patented by the Americans Hart and Searles from the Navy laboratory. At first, this laser was used to engrave computer chips.

Only in 1981, the scientist Srivanson discovered the property of the laser to produce ultra-precise tissue cuts without causing damage to surrounding cells by high temperatures. When tissues are irradiated with a laser with a wavelength in the ultraviolet range, intermolecular bonds are broken, as a result of which tissues from solids become gaseous, that is, they evaporate (photoablation).

In 1981, lasers began to be introduced into ophthalmological practice. In this case, the laser was used to influence the cornea.

In 1985, the first laser correction was carried out using the PRK method using an excimer laser.

All excimer lasers that are used in modern clinical practice operate in a pulsed mode (frequency 100 or 200 Hz, pulse length 10 or 30 ns) with the same wavelength range. These devices differ in the shape of the laser beam (flying spot or scanning slit) and the composition of the inert gas. In cross section, the laser beam looks like a spot or a slit, it moves along a certain trajectory, removing the specified layers of the cornea. As a result, the cornea acquires a new shape, which has been programmed taking into account individual parameters. There is no significant (more than 6-5 degrees) temperature increase in the photoablation zone, since the duration of laser irradiation is insignificant. With each pulse, the laser beam vaporizes one layer of the cornea, the thickness of which is 0.25 microns (about five hundred times less than a human hair). This accuracy allows you to get excellent results when using an excimer laser for vision correction.

Femtosecond lasers

Ophthalmology, like many other areas of medicine, has been actively developing in recent years. Thanks to this, methods of performing operations on the eyes are being improved. About half of the success of the operation depends on modern equipment that is used during the diagnosis and directly during the intervention. During laser vision correction, a beam is used that contacts the cornea and changes its shape with high precision. This allows you to make the operation bloodless and as safe as possible. It was in ophthalmology that, earlier than in other areas of medical practice, they began to use a laser for surgical interventions.

In the treatment of eye diseases, laser devices of a special type are used, which differ in the source of study, wavelength (krypton lasers with a red-yellow emission range, argon lasers, helium-neon installations, excimer lasers, etc.). Recently, femtosecond lasers have been widely used, which are distinguished by a short luminescence pulse of only a few (sometimes several hundred) femtoseconds.

Advantages of femtosecond lasers

Femtosecond lasers have a number of advantages that make them indispensable for use in ophthalmology. These devices are highly accurate, so you can get a very thin layer of the cornea with predetermined flap parameters.

During the operation, the contact lens of the unit is in contact with the cornea for a moment, as a result of which a flap is formed from the surface layers. The unique capabilities of the femtosecond laser help to create a flap of any shape and thickness, depending on the needs of the surgeon.

The area of application of the femtosecond laser in ophthalmology is the correction of ametropia (astigmatism, myopia, hypermetropia), corneal transplantation and the creation of intrastromal rings. It is the operations in which the femtosecond laser is used that make it possible to obtain a stable and high result. After the surgical intervention, the flap is placed in its original place, so the wound surface heals very quickly without suturing. Also, when using a femtosecond laser, discomfort during surgery and pain after it are reduced.

7 facts in favor of the femtosecond laser

During the surgical operation, the use of a scalpel is not required, and the manipulation itself takes place very quickly. It takes only 20 seconds to create a flap with a laser. The laser scale is ideal for ophthalmic interventions. During and after the procedure, the patient does not experience pain, because the tissues are practically not damaged (the layers of the retina exfoliate under the influence of air bubbles).
Immediately after the removal of the corneal flap, direct vision correction can be started by evaporating the stromal substance. In this case, the entire operation takes no more than six minutes for one eye. If you use another laser, it may take time for all air bubbles to disappear (about an hour).
The operation is carried out under the control of Eye-tracking, which is a tracking system for the displacement of the eyeball. Thanks to this, all the pulses of the laser beam fall exactly at the point where it was programmed. As a result, vision after surgery is restored to high values.
Visual acuity in the dark during surgery with a femtosecond laser also reaches high values. Dark vision is restored especially well after correction according to the FemtoLasik method, which takes into account the individual parameters of the cornea and pupil of the patient.
Fast recovery. After laser vision correction, you can immediately go home, but experts recommend staying at the clinic for at least a day. This will reduce the risk of infection and injury to the cornea along the way. Visual function is restored as quickly as possible. Already the next morning, visual acuity reaches its maximum values.
Disability only for a day. Complete healing of the cornea lasts about a week, but in most cases the patient can return to work the very next day after femtosecond laser surgery. During the recovery period, special drops should be instilled, as well as physical activity and increased visual stress should be excluded.
Technical excellence in the performance of FemtoLasik becomes possible thanks to the rich experience in such operations. The femtosecond laser has been used since 1980, and during this time all the errors and inaccuracies of the technique have been corrected.
The predictability of results with this type of laser vision correction reaches 99%. It is extremely rare, due to the individual characteristics of the patient, that there is an undercorrection after the operation, which requires repeated intervention or spectacle correction.

(laser vision correction) and semiconductor manufacturing.

Laser radiation of 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, there are no molecules in the ground state. This is because noble gases such as xenon or krypton are highly inert and do not normally form chemical compounds. In an excited state (caused by an 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 two energy levels. Such a molecule, which is 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) decays into its constituent atoms.

Even though the term dimer refers only to the bonding 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 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 a pulsed mode with a pulse repetition rate from 1 Hz to several hundred Hz, in some models the frequency can reach 2 kHz; it is also possible to generate single pulses. Radiation pulses usually have a duration of 10 to 30 ns and an energy of a few to hundreds of mJ. The powerful ultraviolet radiation of such lasers allows them to be widely used in surgery (especially in the eye), in photolithography processes in semiconductor production, in the micromachining of materials, in the production of LCD panels, and also in dermatology. Today, these devices are rather bulky, which is a disadvantage in wide medical applications (see LASIK), but their size is constantly decreasing due to modern developments.

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An excerpt characterizing 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 else in Europe is there anything like it,” 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 by Balashev, hinting at the recent defeat of the French in Spain, was later highly appreciated, according to Balashev's stories, at the court of Emperor Alexander and very little appreciated now, at Napoleon's dinner, and passed unnoticed.
It was clear from the indifferent and perplexed faces of the gentlemen of the marshals that they were perplexed, what was the witticism, which was hinted at by Balashev's intonation. “If she was, then we did not understand her or she is not witty at all,” said the facial expressions of the marshals. This answer was so little appreciated that Napoleon did not even notice it resolutely and naively asked Balashev about which cities there was a direct road to Moscow from here. Balashev, who was on his guard all the time of dinner, answered that comme tout chemin mene a Rome, tout chemin mene a Moscou, [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 is the road to Poltava, which was chosen by Charles XII, said Balashev, involuntarily flushing with pleasure at the success of this answer. Before Balashev had time to say the last words: "Poltawa", Caulaincourt was already talking about the inconvenience of the road from Petersburg to Moscow and about his Petersburg memories.
After dinner we went to drink coffee in Napoleon's study, which four days earlier had been the study of Emperor Alexander. Napoleon sat down, touching the coffee in a Sevres cup, and pointed to a chair meanly to Balashev.
There is a certain post-dinner mood in a person, which, stronger than any reasonable reasons, makes a person be pleased with himself and consider everyone his friends. Napoleon was in this location. 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 not doubting that this appeal 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, Winzingerode 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 him. I do not understand this. Did he think that I could do the same? - he asked Balashev with a question, and, obviously, this memory pushed him back into that trail of morning anger, which 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 drive out of Germany all his relatives, Wirtemberg, Baden, Weimar ... yes, I will drive them out. Let him prepare a 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 but listen to what he is told. 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.

In this article, we will consider 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 is capable of being perceived by human vision, is a small beams of light that spread in different directions. If these beams are concentrated using a lens or a 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 laser radiation.

Varieties

With the help of the colossal developments of scientists around the world, excimer lasers are now widely used in many areas of human activity and have the following varieties:

Origin

This variety is ultraviolet, which is widely used in the field of eye surgery. With the help of this device, doctors 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 body of such a laser was a xenon dimer, which was excited by an electron beam in order to obtain 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 an excimer laser is used for vision correction.

Its uniqueness

It was found that the excimer molecule produces due to the fact that it is in an excited "attractive" state, as well as in a "repulsive" state. This action can be explained by the fact that xenon or krypton (noble gases) have a high inertness and, as a rule, never form chemical compounds. An electrical discharge puts them in an excited state, as a result of which they can form molecules either among themselves or with halogens, for example, chlorine or fluorine. The appearance of molecules in an excited state creates, as a rule, the so-called population inversion, and such a molecule gives up its energy, which is stimulated or spontaneous emission. After that, this molecule returns to the ground state and breaks up into atoms. The excimer laser device is unique.

The term "dimer" is usually used when the same atoms are connected to each other, however, compounds of noble gases and halogens are used in most modern excimer lasers. Nevertheless, these compounds, which are used for all lasers of this design, are also called dimers. How does an excimer laser work? This is what we will now consider.

The principle of operation of the excimer laser

This laser is the main protagonist of PRK and LASIK. Its working fluid is an inert and halogen gas. When a 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 a highly excited state and after a thousandth of a second it decays 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 bonds between molecules are disconnected, 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 only 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 for vision correction. But there are other areas of its use.

The former had a light beam diameter that was equal to the diameter of the surface on which the evaporation took place. The wide range of the beam and its inhomogeneity caused the same inhomogeneity of the upper layers of the cornea, as well as an increase in temperature on its surface. This process was accompanied by injuries and burns. This situation was corrected by the creation of an excimer laser. The MNTK "Eye Microsurgery" has been using it for a very long time.

Lasers of the new generation have undergone a long process of modernization, during which the diameter of the light beam has been reduced, and a special rotational-scanning system for delivering laser radiation to the eye has also been created. Consider 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, as well as giving it a different radius of curvature. In the ablation zone, the temperature does not rise, since 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 surgical intervention determines in advance what portion of the energy that will be supplied to the cornea, and also to what depth the excimer laser will be exposed. 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 technique popular today is based on the so-called computer reprofiling of the cornea, which is the main optical lens of the human eye. The excimer laser, which acts on it, smoothes the surface of the cornea, removing the upper layers and, thus, eliminating all defects present on it. In this case, normal conditions appear for obtaining the correct images by the eye, creating the correct refraction of light. People who have undergone such a procedure see as everyone who has initially good vision.

The procedure for reshaping the cornea does not cause high temperatures on its surface, which can be detrimental to living tissues. And, as most people believe, there is no so-called burning of the upper layers of the cornea.

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 for "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 to correct myopia. If the layers of the cornea in the periphery zone evaporate during vision correction, then its shape becomes more rounded, and this, in turn, corrects farsightedness. Astigmatism is corrected by dosed removal of the upper layers of the cornea in its various parts. Modern excimer lasers, which are widely used in refractive eye microsurgery, guarantee the high quality of the surface that is subjected to photoablation.

Features of use in medicine

Excimer lasers in the form they have today appeared quite recently, but now they help people all over the world to get rid of such vision problems as myopia, hyperopia, astigmatism. Such a 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.

Diseases of the eye that are treated with

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 ametropia and refraction anomalies.

According to experts, there are two types of refraction:

Ametropia, in turn, includes several subspecies:

myopia (nearsightedness);
astigmatism - obtaining a distorted image by the eye, when the cornea has an irregular curvature, and the flow of light rays becomes uneven in different parts of its surface;
hypermetropia (farsightedness).

Astigmatism is of two types - hypermetropic, which is close to farsightedness, myopic, similar to myopia and mixed.

In order to correctly present the essence of refractive manipulations, it is necessary to know the anatomy of the human eye minimally. The optical 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-perceiving part. In order for the resulting image to become clear and sharp, the retina is in 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 fuzzy and significantly blurred.

In humans, the optics of the eye can change throughout life, in particular, from the moment of birth and up to 16-20 years, it changes due to the growth and increase in the size of the eyeball, as well as under the influence of some factors that can lead to the formation of certain anomalies . Thus, the patients of the eye refractive surgeon most often are adults.

Contraindications for Excimer Beam Vision Correction

Vision correction with an excimer laser is not indicated for all people suffering from visual impairments. The prohibition on the use of this procedure are:

Possible complications after application

All existing methods of treatment with an excimer laser today are highly safe and especially effective. However, there are a number of complications that can occur after surgery using these 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 has redness and pain in the eye. This complication can occur in cases where the nerve endings that are responsible for the production of tears have been damaged in the process of vision correction.
A variety of visual disorders, for example, double vision or decreased vision in the dark, impaired perception of colors or the appearance of a light halo.
Weakening or softening of the cornea, which can occur both a few months after surgery, and after a few years.

Excimer laser in dermatology

The effect of a low-frequency laser on the skin is extremely positive. This is due to the following effects:

anti-inflammatory;
antioxidant;
anesthetic;
immunomodulatory.

That is, there is a certain biostimulating mechanism of action of laser radiation with low power.

Vitiligo is being successfully treated with an excimer laser. Pigmented spots on the skin are very quickly smoothed out.

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 a 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 highly 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 to separate intermolecular bonds and, as a result, transfer part of the tissue from a solid state to a gaseous state (photoablation). The first lasers had a beam diameter equal to the diameter of the evaporated surface, and were distinguished by a significant damaging effect on the cornea. The wide profile of the beam, its inhomogeneity, caused the inhomogeneity of the curvature of the cornea surface, rather high heating of the corneal tissue (by 15-20˚), which led to burns and opacities of the cornea.

New generation lasers have been upgraded. The beam diameter was reduced, and a rotational-scanning system for delivering laser radiation to the eye was created to treat the entire necessary surface of the cornea. In fact, this system was created in the late 50s, and is still successfully used in scanning homing missiles. All excimer lasers operate in the same wavelength range, in a pulsed mode, and differ only in the modulation of the laser beam and the composition of the active body. The laser beam, which in cross section is a slit or spot, moves around the circumference, gradually removing the layers of the cornea and giving it a new radius of curvature. The temperature in the ablation zone practically does not increase due to short-term exposure. The smooth surface of the cornea obtained as a result of the operation allows to obtain an accurate and stable refractive result.

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 appeared to solve this problem - this is the excimer laser correction, which saves people from myopia, astigmatism and farsightedness. Laser correction for the first time 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 dealing with the correction of these anomalies is called refractive surgery, and they themselves are refractive anomalies or ametropias.

Specialists distinguish two types of refraction:
- Emmetropia- normal vision;
- Ametropia- abnormal vision, including several types: myopia - myopia; hypermetropia - farsightedness, astigmatism - image distortion, when the curvature of the cornea is incorrect and the course of light rays in different parts of it is not the same. Astigmatism is myopic (nearsighted), hyperopic (farsighted) and mixed. To understand the essence of refractive interventions, let us 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 focus 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 fuzzy. At the same time, from the moment of birth and up to 18-20 years, 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 more often becomes a person who has reached 18-20 years of age.

Excimer laser vision correction is based on the program of "computer reprofiling" of the surface of the main optical lens of the human eye - the cornea. According to an individual correction program, the cold beam "smoothes" the cornea, eliminating all existing defects. In this case, normal conditions are formed for optimal refraction of light and obtaining an undistorted image in the eye, as in people with good vision. The process of "re-profiling" is not accompanied by a fatal increase in the temperature of the corneal tissues, and as many mistakenly believe, no "burning" occurs. And most importantly, excimer laser technologies make it possible to obtain such an "ideal new set profile" of the cornea, which made it possible to correct almost all types and degrees of refractive errors with them. In scientific terms, excimer lasers are high-precision systems that provide the necessary "photochemical ablation" (evaporation) of the corneal layers. If the tissue is removed in the central zone, then the cornea becomes flatter, which corrects myopia. If you evaporate the peripheral part of the cornea, then its center will become more "steep", 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 the high quality of the "ablated" surface.

Excimer lasers represent an interesting and important class of molecular lasers based on transitions between different electronic states. Consider a diatomic

molecule, the potential energy curves for the ground and excited states of which are shown in Figs. 6.25. Since the ground state corresponds to the mutual repulsion of atoms, the molecule does not exist in this state (i.e., in the ground state, the particles exist only in the monomeric form A). However, since the excited state potential energy curve has a minimum, the molecule can exist in an excited state (i.e., in an excited state, particles exist in a dimeric form. Such a molecule A is called an excimer (an abbreviation of the English words for an excited dimer). Let us now assume that in in some volume a large number of excimers are somehow created.Lasting can then be obtained on the transition between the upper (bound) and lower (free) states (bound-free transition).The corresponding laser is called excimer.These lasers are characterized by two unusual, but important properties due to the fact that the ground state corresponds to the mutual repulsion of atoms 1) As soon as the molecule passes into the ground state as a result of generation, it immediately dissociates. This means that the lower laser level will always be empty. 2) There are no clearly defined rotational-vibrational transitions, and the transition is relatively broadband. However, it should be noted that in some excimer lasers the potential energy curve of the ground state does not correspond to pure mutual repulsion, but has a shallow minimum. In this case, the transition occurs between an upper bound state and a lower (weakly) bound state (bound-bound transition). However, since the ground state is only weakly bound, the molecule in this state undergoes rapid dissociation, either by itself (predissociation) or as a result of the first collision with another molecule in the gas mixture.

Rice. 6.25. Energy levels of the excimer laser.

Let us now consider the most interesting class of excimer lasers, in which an atom of an inert gas (for example, ) in an excited state combines with a halogen atom, which leads to the formation of an excimer of rare gas halides. As specific examples, we point out , which generate everything in the UV range. Why rare gas halides are easily formed in the excited state becomes clear when one considers that in the excited state the rare gas atoms become chemically similar to the alkali metal atoms, which are known to readily react with halogens. This analogy also indicates that in the excited state the bond has an ionic character; in the process of bond formation, an excited electron passes from an inert gas atom to a halogen atom. Therefore, such a bound state is also called a charge-transfer state. Let us now consider the γ-laser in more detail, since it is one of the most important lasers in this category. Figure 6.26 shows a diagram of the potential energy of a molecule. The upper laser level is a state with charge transfer and ionic bonding, which at corresponds to the state of a positive ion and the state of 5 of a negative ion. Therefore, the energy at is equal to the ionization potential of the krypton atom minus the electron affinity of the fluorine atom, At large internuclear distances, the energy curve obeys Coulomb's law. Thus, the interaction potential between two ions extends over a much greater distance than in the case when the covalent interaction prevails (cf., for example, with Fig. 6.24), The lower state has a covalent bond and at corresponds to the state of the krypton atom and the state of the fluorine atom, Thus Thus, in the ground state, the atomic states of the inert gas and the halogen are reversed. As a result of the interaction of the corresponding orbitals, the upper and lower states at small internuclear distances split into states

Turning to the mechanisms of excitation, we note that electrical excitation leads mainly to the formation of excited atoms and ions. Both particles immediately lead to the formation of excited molecules. Indeed, an excited atom can react with a molecule according to the following reaction:

Using the analogy discussed above between excited atoms of an inert gas and atoms of alkali metals, one can immediately assume that the rate of reaction (6.12) will be comparable with the rate of the reaction between (an alkali metal atom, corresponding to and a molecule

Rice. 6.26. Potential energy curves reflecting molecular structure

The ion, on the contrary, reacts with ions that are formed in the reaction of electron addition with dissociation:

Note that for the simultaneous fulfillment of the laws of conservation of energy and momentum, the recombination of two ions must proceed through a three-particle collision:

where M is an atom of the buffer gas (in this case, as a rule, it is helium). Due to the large interaction distance between the two ions, this reaction also proceeds at a very high rate if the pressure of the buffer gas is high enough (the gas mixture usually consists of at a pressure of about 120 mbar, at a pressure of 6 mbar, and He at a pressure of 2400 mbar).

Excimer lasers based on rare gas halides are usually pumped by an electric discharge in accordance with the general scheme shown in Fig. 6.21.

Rice. 6.27, Energy per pulse emitted by a TEA laser with UV preionization of an electric discharge. Each of these lasers used the same laser tube as in Fig. 6.21 but filled with the appropriate gas.

Preionization is usually achieved, as in Fig. 6.21, emitting spark discharges in the UV range. Since the depth of penetration of UV radiation into the gas mixture is limited, for large installations (the transverse dimensions of the discharge are greater than 2–3 cm), X-ray preionization is sometimes used. For laboratory devices and the largest installations, pumping by an external electron beam is also sometimes used. In all cases, the gain turns out to be very large, so that in a laser cavity, an uncoated standard is usually installed as a mirror at one end, and a mirror with 100% is used at the other end. reflector (for example, the rear mirror in Fig. 6.21), Since the lifetime of the upper level is relatively short, and also to avoid arcing, it is necessary to provide fast pumping (pump pulse duration 10-20 nsec). In the case shown in Fig. 6.21, this is achieved, as in the nitrogen laser, by reducing the inductance of the circuit as much as possible and using

non-inductive capacitors connected to the discharge electrodes by short conductors. In fact, the same laser of the type shown in Fig. 6.21 can be used as a TEA laser, a nitrogen laser, or an excimer laser simply by changing the gas mixture. 6.27 shows the output energies of a single pulse obtained in this way for various lasers. There are excimer lasers with a repetition rate of up to about 500 Hz and an average output power of up to 100 W. Larger installations with an average power of more than 1 kW are also currently being created. Due to the large quantum yield (see Fig. 6.26) and high efficiency pumping processes, the efficiency of these lasers is usually quite high (2-4%).

Excimer lasers are used for very precise etching of various materials in electronic printed circuit applications, as well as for tissue burning in biology and medicine (for example, radial keratomy of the iris). Excimer lasers are also widely used in scientific research and are likely to find numerous applications where a source of high-power UV radiation with high efficiency is required (for example, in photochemistry).