The intensity of light propagating in a medium can decrease due to its absorption and scattering by the molecules (atoms) of the substance.
By absorbing light called the weakening of the intensity of light when passing through any substance due to the conversion of light energy into other forms of energy.
P
Rice. 24.1
absorption of a light quantum occurs during its inelastic collision with a molecule (atom), leading to the transfer of photon energy to matter, and is a random event. The probability of absorption of a light quantum by a sample of a substance with a thickness l(Fig. 24.1) is estimated by the value of the absorption coefficient 1 T, equal to the ratio of the intensities of the absorbed light I n = I 0 I to the intensity of the incident I 0
(24.1)
where I is the intensity of the transmitted light, 
- transmission coefficient.
Let us derive the law of absorption of light by matter. Select a thin layer of matter d x, perpendicular to a beam of monochromatic light with intensity i (I 0 i I), and we will proceed from the assumption that the attenuation of light (the fraction of absorbed quanta) -d i/i such a layer does not depend on the intensity (if the intensity is not too high), but is determined only by the thickness of the layer d x and proportionality factor k :
D i/i = k d x. (24.2)
Coefficient k is different for different wavelengths and its value depends on the nature of the substance. Integrating (24.2) and substituting the limits of integration for X from 0 to l and for i from I 0 to I, we get
whence, potentiating, we have
(24.3)
This formula expresses Bouguer's law of absorption of light. Coefficient k is called the natural absorption index, its value is the reciprocal of the distance at which the light intensity is attenuated as a result of absorption in the medium in e once.
Since the absorption of light is due to interaction with molecules (atoms), the law of absorption can be associated with certain characteristics of molecules. Let n- concentration of molecules (the number of molecules per unit volume) that absorb light quanta. Let us denote by the letter s the effective absorption cross section of the molecule - a certain area, when a photon enters into which it is captured by the molecule. In other words, a molecule can be represented as a target of a certain area.
If we assume that the cross-sectional area of a rectangular parallelepiped (Fig. 24.1) is equal to S, then the volume of the selected layer S d x, and the number of molecules in it nS d x; the total effective cross section of all molecules in this layer will be snS d x. Fraction of the absorption cross-sectional area of all molecules in the total cross-sectional area
(24.4)
We can assume that the same as (24.4), part of the quanta that hit the layer is absorbed by the molecules, because the area ratio determines the probability of interaction of one quantum with the molecules of the selected layer. The fraction of quanta absorbed by the layer is equal to the relative decrease in intensity (d i/ i) Sveta. Based on the above, one can write
(24.5)
whence, after integration and potentiation, we have
I = I 0e- snl . (24.6)
This equation, similar to (24.3), includes the parameter s, which reflects the ability of molecules to absorb monochromatic light of the wavelength used.
More accepted molar concentrations C =n/ N A , whence n = CN A. Let's transform the product sn = sCN A = C, where = sN A is the natural molar absorption rate. Its physical meaning is the total effective absorption cross section of all molecules of one mole of a substance. If the molecules that absorb quanta are in a solvent that does not absorb light, then (24.6) can be written as
(24.7)
This formula expresses Bouguer-Lambert-Ver law . In laboratory practice, this law is usually expressed in terms of an exponential function with base 10:
(24.8)
The Bouguer-Lambert-Beer law is used for the photometric determination of the concentration of colored substances. To do this, directly measure the fluxes of incident and passed through the solution of monochromatic light ( concentrationcolorimetry), but the transmittance thus determined T(or absorption 1 - T, see (24.1)) is inconvenient, because due to the probabilistic nature of the process it is related to the concentration non-linearly [see (24.8) and fig. 24.2, a]. Therefore, in quantitative analysis, it is usually determined optical density (D) solution representing the decimal logarithm of the reciprocal of the transmittance,
(24.9)
Rice. 24.2
Optical density is convenient in that it is linearly related to the concentration of the analyte (Fig. 24.2, b).
The Bouguer-Lambert-Beer law is not always fulfilled. It is valid under the following assumptions: 1) monochromatic light is used; 2) the molecules of the solute in the solution are evenly distributed; 3) when the concentration changes, the nature of the interaction between the dissolved molecules does not change (otherwise the photophysical properties of the substance, including the values of s and , will change); 4) in the process of measurement, chemical transformations of molecules under the action of light do not occur; 5) the intensity of the incident light should be sufficiently low (so that the concentration of unexcited molecules does not practically decrease during the measurement). Dependencies s, , or D on the wavelength of light is called the absorption spectra of a substance.
Absorption spectra are sources of information about the state of matter and about the structure of the energy levels of atoms and molecules. Absorption spectra are used for the qualitative analysis of solutions of colored substances.
Takeover (absorption )light is called the loss of energy by a light wave passing through a substance.
Light is absorbed when the transmitted wave expends energy on various processes. Among them: the transformation of wave energy into internal energy - when the substance is heated; energy costs for secondary radiation in a different frequency range (photoluminescence); energy costs for ionization - in photochemical reactions, etc. When light is absorbed, the oscillations die out and the amplitude of the electrical component decreases as the wave propagates. For a plane wave propagating along the axis x, we have
Here E(x) is the amplitude value of the electric field strength of the wave at points with the coordinate x; is the amplitude at the point with the coordinate x = 0; t is the time it takes the wave to travel a distance equal to x; β is the vibration damping coefficient; absorption coefficient, depending on the chemical nature of the medium and on the wavelength of the transmitted light.
The intensity of the wave will change according to Bouguer's law (P. Bouguer (1698 - 1758) - French scientist):
where is the intensity of the wave at the entrance to the medium.
At , . Consequently, absorption coefficient – physical quantity,numerically equal to the reciprocal value of the thickness of the substance layer, in which the wave intensity decreases in e = 2,72once.
The dependence of the absorption coefficient on the wavelength determines the absorption spectrum of the material. In a substance (for example, in a gas) there may be several types of particles participating in oscillations under the action of a propagating electromagnetic wave. If these particles interact weakly, then the absorption coefficient is small for a wide frequency spectrum, and only in narrow regions does it increase sharply (Fig. 10.7, a).
 |
|
| a | b |
These regions correspond to the frequencies of natural vibrations of optical electrons in atoms of different types. The absorption spectrum of such substances is lined and represents dark bands on the iridescent color of the spectrum, if this is the visible region. As the gas pressure increases, the absorption bands broaden. In the liquid state, they merge, and the absorption spectrum takes the form shown in Fig. 10.7, b. The reason for the broadening is the strengthening of the bonding of atoms (molecules) in the medium.
The absorption coefficient, which depends on the wavelength λ (or frequency ω), is different for different substances. For example, monatomic gases and metal vapors (i.e., substances in which atoms are located at considerable distances from each other and can be considered isolated) have an absorption coefficient close to zero, and only for very narrow spectral regions (approximately m) are sharp maxima (the so-called line absorption spectrum). These lines correspond to the frequencies of natural oscillations of electrons in atoms. The absorption spectrum of molecules, determined by vibrations of atoms in molecules, is characterized by absorption bands (approximately m).
The absorption coefficient for dielectrics is small (approximately 
), however, they exhibit selective absorption of light in certain wavelength ranges, when α sharply increases and relatively broad absorption bands (about m) are observed, i.e. dielectrics have a continuous absorption spectrum. This is due to the fact that there are no free electrons in dielectrics and the absorption of light is due to the phenomenon of resonance during forced vibrations of electrons in atoms and atoms in dielectric molecules.
The absorption coefficient for metals is large (approximately ), and therefore metals are practically opaque to light. In metals, due to the presence of free electrons moving under the action of the electric field of a light wave, rapidly alternating currents arise, accompanied by the release of Joule heat. Therefore, the energy of the light wave rapidly decreases, turning into the internal energy of the metal. The higher the conductivity of the metal, the stronger the absorption of light in it.
On fig. 10.8 shows a typical dependence of the absorption coefficient α on the frequency of light ν and the dependence of the refractive index n on ν in the region of the absorption band. It follows from the figure that an anomalous dispersion is observed inside the absorption band ( n decreases with increasing ν). However, the absorption of the substance must be significant in order to affect the course of the refractive index.
The dependence of the absorption coefficient on frequency(wavelength)explains the coloration of absorbing bodies. For example, glass that weakly absorbs red and orange rays and strongly absorbs green and blue rays will appear red when illuminated with white light. If green and blue light is directed at such glass, then due to the strong absorption of light of these wavelengths, the glass will appear black. This phenomenon is used for the manufacture of light filters, which, depending on the chemical composition (glasses with additives of various salts; plastic films containing dyes; dye solutions, etc.), transmit light only of certain wavelengths, absorbing the rest. The variety of selective (selective) absorption limits for various substances explains the diversity and richness of colors and colors observed in the surrounding world.
Spectral analysis allows you to obtain information about the composition of the Sun, since a certain set of spectral lines characterizes a chemical element extremely accurately. So, with the help of observations of the spectrum of the Sun, helium was discovered.
The visible part of solar radiation, when studied with the help of spectrum-analyzing instruments, turns out to be inhomogeneous - there are observed in the spectrum absorption lines, first described in 1814 by I. Fraunhofer.
With the help of spectral analysis, we learned that stars are composed of the same elements that are found on Earth.
The phenomenon of absorption is widely used in the absorption spectral analysis of gas mixtures based on measurements of the frequency spectra and intensities of absorption lines (bands). The structure of absorption spectra is determined by the composition and structure of molecules; therefore, the study of absorption spectra is one of the main methods for the quantitative and qualitative study of substances.
When passing through to-l. environment due to interaction with it, as a result of which light energy passes into other types of energy or into optical. radiation of other spectral composition. Main the law of P. s., linking the intensity I a beam of light that has passed through a layer of an absorbing medium with a thickness l s incident beam intensity I 0 , is Bouguer's law called absorption index, and, as a rule, is different for different wavelengths. This law was experimentally established by P. Bouguer (P. Bouguer, 1729) and subsequently theoretically derived by I. Lambert (J. H. Lambert, 1760) under very simple assumptions that when passing through any layer of matter, the intensity decreases by a certain fraction, depending only on and the thickness of the layer l, i.e. dI/l= The solution to this equation is Booger - Lambert - Bera law. Phys. its meaning lies in the fact that the very process of loss of beam photons in the medium, characterized does not depend on them in the light beam, i.e. on the light intensity, and on the thickness of the absorbing layer l. This is true for not too high radiation intensities (see below).
The dependence on the wavelength of light is called the absorption spectrum of a substance. Absorption spectrum isolated. atoms (for example, rarefied gases) has the form of narrow lines, i.e., it is different from zero only in certain narrow wavelength ranges (hundredths - thousandths of nm), corresponding to the frequencies of their own. electrons inside atoms. The absorption spectrum of molecules, determined by the vibrations of atoms in them, consists of much wider wavelength regions (the so-called absorption bands, tenths - hundreds of nm; see. Molecular spectra). The absorption of solids is characterized, as a rule, by very wide regions (hundreds and thousands of nm) with a large value of ; qualitatively, this is explained by the fact that in the condenser. media between particles leads to a rapid transfer to the entire collective of particles of the energy given off by the light of one of them.
Qualities. a picture of the processes of interaction of radiation with matter occurring at the atomic level and leading to P. s., can be obtained within the framework of the quasi-classical. approach. It is based on a model that considers atoms as a set of harmonics. oscillators : electrons in atoms (molecules) oscillate around the equilibrium position. Such a model is acceptable for rarefied gases and metal vapors, where the influence of neighboring atoms can be ignored. For liquid and solid bodies, such a model is unsuitable, because the behavior of the electrons that determine the optical. properties of an atom changes dramatically under the action of the fields of neighboring atoms.
Spontaneous emission of atoms of the oscillatory model corresponds to free (damped) oscillations of electrons. Own the frequencies of these vibrations v nm are given by the 2nd postulate of Bohr: where and are the energy levels of the atom, between which a quantum transition takes place with the emission of light at a frequency v nm.
During propagation in a medium of light falling on it from the outside, the oscillations of electrons in atoms are of a forced nature and occur at the frequency of the incident light wave. With this approach, P. s. is associated with the loss of wave energy due to forced oscillations of electrons. (The energy absorbed by an atom can be re-emitted or converted into other types of energy.) The light field incident on the medium causes electron oscillations described by the equation
Here t 0 and e 0 - mass and electron, X- its displacement from the equilibrium position, - coefficient characterizing the attenuation. The first term in (1) describes the force of inertia, the second - the braking force, proportional. oscillating speed. the motion of an electron and causing the damping of its oscillations (similar to the force of friction), the third term is the elastic force, proportional. displacement of an electron from the equilibrium position; the right side of equation (1) is the driving force. The solution to this equation
at nonzero, there is a complex quantity, which indicates the absorption of the wave energy by the atom. With a complex connection of the driving force and the deflection of the electron, the integral quantities turn out to be complex, respectively, and the integral quantities: dielectric. permeability ( - conductivity, - substances, part of the dielectric constant) and the refractive index The imaginary part of the quantity is directly related to the characteristic of the absorbing properties of the medium - the absorption index: the main indicator of absorption. The introduction of complex quantities made it possible to apply the formal description developed for transparent media to absorbing media as well. Since light absorption anomalous dispersion is associated, which takes place inside the absorption band (see Fig. Light dispersion).
When considering P. s. from a quantum point of view, such a characteristic is introduced energetically. levels like population level N n,m- the number of atoms in a given energetic. condition. In this case, the expression for can be represented as
where is the level population difference P and tN m - (g m /g n)N n(here g m and gn- statistic. level population weights). The dependence on the frequency difference - called. absorption line contour. In the classic approximation, the width of the absorption line at a level of 0.5 from the maximum 
This is the so-called. natural line width. In real media, there are a number of reasons that increase the width of the absorption line, sometimes by many times. Ch. The reason for the broadening of the absorption line in gases is , which arises due to the random motion of atoms (see Fig. line broadening).
With special conditions of excitation is possible so-called. inverse population, when, i.e., when the population of the upper level is greater than the population of the lower one. In this case, as can be seen from (2), the sign and the absorption index change - the medium is characterized by the so-called. negative uptake. Light passing through such a medium is not weakened, but, on the contrary, is enhanced. Media in which it is possible to create (in one way or another) an inverse population of levels are used to create lasers and light amplifiers.
Since the absorption of a photon leads to the transfer of an atom from the lower level to the upper one, the absorption process affects the energy population. levels. At commonly observed light intensities, the number of absorbed photons is much less than the number of absorbing atoms, and therefore does not depend on the light intensity. Accordingly, does not depend on it and However, if the intensity of the light incident on the medium is sufficiently large, then it can go into an excited state means. fraction of absorbing atoms. This will lead to the fact that and will depend on the intensity of light - there will be a so-called. nonlinear absorption. In this case, Bouguer's law ceases to be valid. In the limit, at a very high intensity of the incident light, the population is up. and lower levels are aligned and the medium ceases to absorb light - it becomes enlightened, i.e., light passes through such a medium without experiencing absorption at all (see Fig. self-induced transparency).
At very high light intensity, one more feature of P. s is also possible. - multiphoton absorption, when several ( i) photons of lower frequencies under the condition
P. s. used in various fields of science and technology. So, many are based on it. especially highly sensitive quantity methods. and qualities. chem. analysis, in particular absorption spectral analysis, spectrophotometry, colorimetry. Spectrum type P. s. can be associated with chem. the structure of a substance, by the form of the absorption spectrum, one can investigate the nature of the movement of electrons in metals, find out the band structure, and many others. others
Lit.: Landsberg G. S., Optics, 5th ed., M., 1976; Sokolov A. V., Optical properties of metals, M., 1961; Elyashevich M. A., Atomic and molecular spectroscopy, M., 1962; Korolev F. A., Theoretical optics, M., 1966; Born M., Wolf E., Fundamentals of optics, trans. from English, 2nd ed., M., 1973.
A. P. Gagarin.
light absorption.
Light, passing through any substance, is absorbed in it to some extent. Usually, absorption is selective, that is, light of different wavelengths is absorbed differently. Since the wavelength determines the color of light, therefore, rays of different colors, generally speaking, are absorbed differently in a given substance.
Transparent uncolored bodies are bodies that give a small absorption of light of all wavelengths related to the interval of visible rays. Thus, glass absorbs in a layer with a thickness of 1 cm only about 1 %
visible rays passing through it. The same glass strongly absorbs ultraviolet and far infrared rays.
Colored transparent bodies are bodies that exhibit selectivity of absorption within the limits of visible rays.
- For example, "red" is glass that weakly absorbs red and orange rays and strongly absorbs green, blue, and violet.
- If white light, which is a mixture of waves of different wavelengths, falls on such glass, then only longer waves will pass through it, causing a sensation of red color, while shorter waves will be absorbed.
- When the same glass is illuminated with green or blue light, it will appear "black" because the glass absorbs these rays.
From the point of view of the theory of elastically bound electrons, the absorption of light is caused by the fact that a passing light wave excites forced oscillations of electrons. To maintain the oscillations of electrons, energy is used, which then passes into the energy of other types.
If, as a result of collisions between atoms, the energy of electron oscillations is converted into the energy of random molecular motion, then the body heats up.
The absorption of light can be described in general terms from the energy point of view, without entering into the details of the mechanism of interaction of light waves with atoms and molecules of the absorbing substance.
Let a beam of parallel rays propagate through a homogeneous substance (Fig.).
Let us isolate in this substance an infinitely thin layer of thickness dl bounded by parallel surfaces perpendicular to the direction of light propagation.
The energy flux density and will change with the passage of rays through this layer by the value −du. It is natural to put this reduction −du proportional to the value of the energy flux density itself in a given absorbing layer and its thickness dl:
−du=kudl. (1)
Coefficient k determined by the properties of the absorbing substance, it is called the absorption coefficient. Coefficient constancy k indicates that in each layer the same proportion of the flow that reached the layer is absorbed.
To obtain the law of decreasing energy flux density in a layer of finite thickness l we rewrite expression (1) in the form:
du/u=-kdl
and then integrate it within 0
before l:
0 l ∫(du/u) = −k 0 l ∫dl.
Let at the beginning of the layer ( l = 0) the flux density is u0. Denote by u the value that it acquires when the flow passes through the thickness of the substance l. Then as a result of integration we get:
lnu − lnu o = −kl or ln(u/u o) = −kl,
where
u = u o e −kl, (2)
where e− base of natural logarithms.
The greater the absorption coefficient k the more light is absorbed. At l = 1/k, according to (2):
u = u o /e = u o /2.72;
Thus, a layer whose thickness is equal to 1/k, weakens the energy flux density in 2,72
times.
For various substances, the numerical value of the absorption coefficient k varies over a very wide range. In the visible region for air at atmospheric pressure k approximately equal to 10 −5 cm −1 for glass k = 10 −2 cm −1, and for metals k is in the order of tens of thousands. For all substances, the absorption coefficient k depends to some extent on the wavelength.
A tinted window can absorb, for example, visible light from 0
before 100 %
. For example, tinting windows in an apartment often becomes a very simple and convenient way out if the windows face the sunny side - thus harmful ultraviolet rays in large quantities do not penetrate into the apartment. As a result, in the hot summer, a pleasant coolness remains in the room, and interior items do not lose their colors due to the bright sun.
On fig. dependency is presented lgk from the wavelength λ
for gaseous chlorine at 0 °C and atmospheric pressure. As you can see, the coefficient is large in the purple region, then it falls off steeply in the yellow-green region and rises again in the red region. 
Experience shows that when light is absorbed by substances dissolved in a transparent solvent, the absorption is proportional to the number of absorbing molecules per unit length of the path of the light beam in the solution. Since the number of molecules per unit length is proportional to the concentration of the solution FROM, then the absorption coefficient k proportional FROM from where you can put k = xC, where X is a new constant coefficient that does not depend on the concentration of the solution, but is determined only by the properties of the molecules of the absorbing substance. Substituting this value k into the absorption formula (2), we obtain
u = u o e -xCl. (3)
The statement that the coefficient X does not depend on the concentration of the solution, is called the law beera. This law is satisfied under the condition that the presence of neighboring molecules does not change the properties of each given molecule. At significant concentrations of the solution, the mutual influence of molecules affects, and then Beer's law ceases to be fulfilled. In those cases where it takes place, relation (3) makes it possible to determine the concentration of the solution from the degree of light absorption in the solution.
In addition to the considered "true" absorption, in which the energy of light waves is converted into energy of other types, a decrease in the energy flux density in the beam of rays is possible due to the scattering of energy to the sides.
According to the basic law of photochemistry, which is a consequence of the law of conservation of energy, only the light that is absorbed by a given system can have a photochemical effect. The light that is not absorbed by this system will not cause photochemical reactions. Therefore, to consider the energy of a photobiological process, it is necessary to know the absorptivity of the system. In this regard, two factors are most significant:
- the total amount of absorbed energy or the number of quanta absorbed per unit time (the first factor). This indicator is usually estimated using the optical density of the object;
- the value of the absorbed quantum (second factor)
The first factor determines the possible number of reactions that occur per unit time, i.e., the rate of the process. The second factor determines the energy of the photoreaction itself, i.e., determines which reaction is possible.
The flow of light quanta, passing through a system containing molecules of a substance, is weakened. The weakening of the flow of quanta occurs due to the fact that some of the quanta are absorbed (captured) by molecules.
Let I be the intensity of the light flux, i.e., the number of quanta passing through a given sample per unit time. The weakening of the light intensity dI will depend on the number of collisions of quanta with the molecules of the substance. Obviously, the number of these collisions is proportional to the number of molecules in the path of the light flux, i.e., proportional to the concentration C of the substance. On the other hand, it must also be proportional to the number of quanta themselves passing through the system per unit time, i.e., the intensity of the light flux I. If we take a sufficiently small distance dl at which absorption occurs, then the attenuation of the flux intensity dI will be proportional to this distance. The established dependencies can be expressed by the equation: DI=k · I · C · dl, (3) where k is the coefficient of proportionality; minus sign before dI indicates that the luminous flux is decreasing. Equation (3) is a first order linear differential equation. Let's write it in the following form: DI/I=k · C · dl Integrating the left and right sides, we get: LnI = k · C · l=B, where l is the sample thickness (optical path length); B is the integration constant to be determined. Let l \u003d 0, then B \u003d -lnI o, lnI o - lnI = k ·
C ·
l, or I \u003d I about e - kCl, (5) where e is the base of natural logarithms. Equations (4) and (5) are an expression of the Lambert-Beer law: the intensity of the light flux passing through the substance decreases exponentially depending on the length of the optical path and the concentration of the substance in the sample. In equation (4), we replace the natural logarithm with a decimal one and denote the new proportionality coefficient by ε. Then lg I o / I = ε · C · l(6) The decimal logarithm of the ratio of the intensity of the incident light to the intensity of the light leaving the sample is called the optical density. Denoting it by D, we get: lg I o / I = D = ε · C · l(7) In this case, the Lambert - Beer law can be formulated as follows: the optical density of the sample is directly proportional to the concentration of the substance in the sample and the length of the light path. In equation (7), ε is called the molar absorption coefficient. If l=1 and C=1, then ε=D, i.e., is the optical density of a sample with a thickness of one unit (1 cm) at a substance concentration of 1 mol/l. Optical density shows the absorption capacity of a substance. The greater the absorption, the greater the ratio I o / I, i.e., the greater the optical density. |
A substance absorbs light of different wavelengths differently. The dependence curve of the optical density of a substance on the wavelength of the absorbed light is called the absorption spectrum.
Usually, the absorption spectra of molecules are continuous, but they show maxima at the wavelength of light where there is maximum absorption of light quanta. In Fig.1. the absorption spectra of some biologically important compounds that absorb light in the visible and ultraviolet regions of the solar spectrum are given. Proteins have an absorption maximum at a wavelength of 280 nm, nucleic acids - in the region of 260 nm, rhodopsin - 500 nm, chlorophyll a has two absorption maxima: 430 and 680 nm.
As can be seen from the figure, the absorption spectra sometimes have a rather complex form, which is characteristic of a given substance and depends on the structure and properties of the molecules of a given substance.
The study of the absorption spectra of any photobiological process makes it possible to find out which substance is responsible for the absorption of light in this process. This is achieved by comparing the spectra of the process under study and the spectra of known substances. In addition, the position of the maxima on the wavelength scale can be used to determine the wavelength of light predominantly absorbed by this substance.
Knowing the wavelength of the absorbed light makes it possible to determine the energy of the absorbed quanta. And by the magnitude of the energy of the absorbed quanta, one can calculate the location of the electronic and vibrational energy levels of the molecule, as well as the transitions of molecules from one energy state to another.
In addition to all this information, the optical density value gives information about the concentration of a substance in the test sample. Based on the magnitude of the absorption maxima, on the basis of equation (7), one can draw conclusions about the concentration of the substance in the object under study.
The method of studying photobiological processes using absorption spectra is called absorption spectrophotometry. Absorption spectra are obtained using special instruments - spectrophotometers. On fig. 2 shows a diagram of the structure of the spectrophotometer.
Light from a light source L enters the monochromator M, which gives radiation of a strictly defined wavelength. Light enters the cuvette from the monochromator To with a solution of the test substance.
From the cuvette, the attenuated flux of quanta is directed to PMT- a photomultiplier that converts the energy of quanta into electrical energy and amplifies it. In some cases, an ordinary photocell with an amplifier can be used instead of a PMT.
From the PMT, the electric current is supplied to the recording device G, calibrated in units of optical density. It can be a galvanometer or a recorder.
By turning the handle of the monochromator, light of various wavelengths is sent to the object and readings are taken from the recording device.
In modern spectrophotometers, the spectrum of the monochromator is deployed automatically and readings are also automatically recorded on a moving tape recorder. In this case, the cuvette with the solution is placed in the chamber, the device is turned on and a ready-made curve is obtained - the absorption spectrum.
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