The spectrophotometer is the core part of the ultraviolet visible spectrophotometer. It is mainly composed of an entrance slit, a collimating mirror, a grating, an objective lens and an exit slit. The incident slit plays the role of restricting the stray light to enter, and it is generally at the focus of the collimating mirror; The collimating mirror changes the composite light emitted from the incident slit into a parallel light; Grating is used for light splitting. The light splitting system has three functions: ① light splitting, which divides the composite light coming in from the incident slit and projecting onto the grating into monochromatic light; ② Turning, changing the direction of the parallel light from the collimating mirror and projecting it onto the objective lens; ③ Energy transfer: transfer the energy of the parallel light from the collimating mirror to the objective lens after changing the direction; If it is a type IV concave grating, it also has the function of imaging, focusing the divergent light coming in from the incident slit to the grating and imaging on the outgoing slit; The exit slit limits the spectral bandwidth. Each component of the beam splitter system is discussed below.
Grating is the core element of the spectrophotometer system. It is a very important optical element, which is related to the quality and level of the whole UV-VIS spectrophotometer. This book will be discussed in more detail.
（1） The Principle of Light Sharing of Grating
From the grating equation: d (sin α ± sin β) = m λ It can be seen that for the same spectral series m, the same incident angle α Different wavelengths projected on the grating λ 1 、 λ 2 、 λ 3 * For the mixed light, the interference poles generated by each wavelength are mostly located at different angles; That is, diffracted light of different wavelengths has different diffraction angles β Ejection. This means that for a given grating, the primary maximum or secondary maximum (
The spectral lines of different wavelengths in the same grating spectrum do not coincide, but are arranged in the order of wavelengths to form a series of discrete spectral lines. In this way, the composite light of different wavelengths, which is mixed together for incident, is separated from each other after being diffracted by the grating. This is the light splitting principle of the diffraction grating.
（2） Basic characteristics of grating
Grating can be divided into reflection grating and transmission grating. In the ultraviolet visible spectrophotometer, the reflective diffraction grating is the most widely used, usually called the reflective grating. Reflective diffraction gratings can be divided into planar reflective diffraction gratings and concave reflective diffraction gratings according to whether the shape of the grating base is planar or concave. According to whether the micro shape of the grating groove can make the diffracted light energy concentrate in a certain direction, there are blazed gratings and non blazed gratings. According to whether gratings are made by mechanical marking or holographic interferometry, they can also be divided into two categories: marking gratings and holographic gratings. According to the shape of the grating imaging, it can also be divided into ordinary gratings and flat field gratings (the image of a general planar or concave grating on the exit narrow peak is curved and uneven, while the image of a flat field grating on the exit narrow peak is a plane image). The following is a brief discussion of the spectral characteristics of the most commonly used planar reflective diffraction gratings.
1. Characteristics of grating spectrum
(1) The multi-level nature of the grating spectrum is the spectrum formed after prism dispersion, which is only arranged into a single spectrum according to the wavelength order. The spectrum formed after dispersion of diffraction grating is the sum of all levels of spectrum including m=0, ± 1, ± 2 and ± 3. The same grating can form a series of different orders of spectrum at different positions for the same incident composite light, and there are positive and negative orders of spectrum symmetrically distributed on both sides of m=0. Therefore, the multi-level nature of grating spectrum is theoretical, essential and inevitable. This characteristic of grating will cause many corresponding problems to the application of grating, and it will directly cause difficulties to the spectral resolution and spectral detection of UV-VIS spectrophotometer. This is a problem that all UV-VIS spectrophotometer designers, manufacturers and users must pay attention to.
(2) The order overlap of the grating spectrum is determined by the grating equation d (sin α ±sin β) = m λ It can be seen that the wavelength is λ The first order (m=1) spectral line and wavelength of λ/ Second order (m=2) spectral line of 2, wavelength: λ/ The third order (m=3) spectral lines of 3 all have the same diffraction angle, i.e βλ, 1 = βλ/ 2 , 2 = βλ/ 3 , 3 = βλ/ m , m 。 This is the order overlap of the diffraction grating spectrum. Namely, the diffraction grating has different orders of spectral lines of different wavelengths at the same position. When carrying out high-resolution spectral research or spectral analysis in a wide range of wavebands, the order overlap of grating spectra is very obvious, and strong measures must be taken to isolate or filter out the unwanted wavebands; For example, pre monochromator or filter of corresponding wave band are used. Only in this way can we avoid the interference that does not require the level spectrum, and ensure the resolution of the ultraviolet visible spectrophotometer and the accuracy and reliability of the analysis and test data.
(3) The uniformity of the grating spectrum is determined by the grating equation d (sin α ±sin β) = m λ It can be seen that, when the diffraction angle is not too large (such as in the primary spectrum, near the normal area of the grating), the positions of spectral lines of different wavelengths are basically proportional to their wavelength values. Therefore, the spectral lines of each wavelength in the grating spectrum are arranged evenly, and with the linear increase or decrease of the wavelength value, the corresponding position of the grating spectral line (such as the distance from the grating normal) also changes linearly. The grating spectra are arranged evenly, and the distance between two spectral lines with the same wavelength difference in different wavelength regions does not change much. The uniformity of grating spectrum not only makes the spectrum more neat and symmetrical, but also is convenient for preliminary judgment and estimation of the wavelength value of spectral lines in qualitative analysis.
In the prism spectrum, light of different wavelengths is dispersed due to different degrees of refraction. However, the refractive index change of prism material to different wavelengths is not linear with the wavelength. The change of the refractive index of the prism material in the short wave direction is much greater than that in the long wave region. Therefore, the arrangement of spectral lines in the prism spectrum is uneven. In the short wave region, due to d n/d λ Large, the spectral line arrangement is very sparse, while in the long wave region, the λ Small, the spectral line arrangement is very dense. Therefore, for wavelength difference of the same size, the distance between corresponding spectral lines at the short wavelength is greater than that at the long wavelength. Therefore, the dispersion of the prism in the ultraviolet region is larger than that in the visible and near-infrared regions. Therefore, some UV-VIS spectrophotometers (especially high-end UV-VIS spectrophotometers) use quartz prisms as pre monochromators, which is the reason.
In addition, grating and prism are different in the wavelength distribution order of spectral lines; In the grating spectrum, the longer the wavelength is, the larger the diffraction angle is, and the more the spectral line deviates from the normal of the grating. In the prism spectrum, the longer the wavelength is, the smaller the deflection angle is, and the closer the corresponding spectral line distribution is to the position in the direction of the incident angle. 2. Characteristic dispersion of plane diffraction grating
From the differential grating equation, we can get the role dispersion of the diffraction grating when the incident angle is fixed, β Is the diffraction angle; λ Is the wavelength; M is the spectral order of the grating; D is the grating constant.
It can be seen from equation (3-1) that the role dispersion of the grating has the following characteristics:
① The role dispersion of the grating is proportional to the spectral order m; That is to say, the higher subspectrum has a high dispersion rate. Therefore, higher subspectrum is more suitable for high resolution spectroscopy.
② The role dispersion rate of the grating is inversely proportional to the grating constant d, indicating that the higher the groove density of the grating, the higher the dispersion rate of the grating.
③ The role dispersion of grating is also related to cos β Is inversely proportional, so as the diffraction angle β As you increase, the character dispersion of the grating will also increase.
3. Resolution of plane diffraction grating
The definition of resolution of plane diffraction grating is: R= λ/ Δλ
4. Flashing of reflection diffraction grating
The reflection diffraction grating diffracts and distributes the composite light incident on it to different levels of spectrum. While the grating diffracts the incident light, each working surface on the grating groove is equivalent to a small mirror surface, which also reflects the incident light. When the diffraction direction formed by the grating on the incident light of a certain wavelength coincides with the specular reflection direction of the wavelength on the working surface, the outgoing light of this wavelength will be brighter or more dazzling than other wavelengths. This is called glitter. The grating blaze wavelength means that the grating has the strongest output energy at this wavelength.
It is often seen that some grating manufacturers, under the self collimation state in the catalog α=β) , Blaze angle of grating ε=α=β, The wavelength value is also marked. However, these data are under the condition of m=1 first order spectrum.
If a grating is the wavelength of the primary spectrum λ B shiny, then in the secondary spectrum λ The light of b/2 also has a shining effect. In the third order spectrum λ The light of b/3 also shines.
At the blazed wavelength, the relative light intensity of the grating is the highest. As the wavelength deviates from the blazed wavelength, the diffraction efficiency decreases gradually,
5. Bending of grating spectral line
Because the grating diffracts the light from the central point of the slit and the light from the non central point of the slit differently, the grating spectral line is bent. The bending of grating spectral lines is shown in Fig. 3-18. The greater the wavelength value, the more severely the spectral line bends (the opposite is true for prism spectral line bending). However, the bending degree of grating spectral line is smaller than that of prism spectral line.
6. Magnification of grating and holographic grating
(1) The magnification of the grating is the same as that of a dispersion prism that does not work at the minimum deflection angle. If the grating does not work at a symmetrical state where the diffraction angle is equal to the incidence angle, it will also produce an additional magnification for the beam
(2) Holographic grating Holographic grating is also a lot of gratings used at present. When two coherent parallel beams meet, a series of parallel and equidistant linear interference fringes will be formed. With the development of high energy, high monochromatic laser and high quality photoresist, the idea of using light interference fringes to make diffraction gratings has been realized. As shown in Fig. 3-19, the laser beam from the laser is converted into two coherent parallel beams by means of wavefront splitting or amplitude splitting, and they meet on the grating blank P uniformly coated with photoresist to produce uniform and equidistant parallel linear interference fringes, so that photoresist at each point on the grating blank P is exposed to different degrees. After proper development, rinsing and drying, a concave convex fine structure surface identical to the interference fringe can be obtained, and then aluminum (Al) can be vacuum plated on this surface, finally a reflection diffraction holographic grating with uniform spacing and parallel straight grooves can be obtained. The distance between interference fringes, that is, the grating constant d of the holographic grating, is determined by the angle 2 between two dry beams θ、 Laser wavelength λ And the refractive index n of the interference fringe recording space medium.
The refractive index of the recording space medium is generally air, n=1. Therefore, when the included angle between beams is maximum (i.e θ= Minimum grating constant dmin of holographic grating at 90 °)= λ/ 2。
7. Features of holographic grating
① It will not produce ghost lines and companion lines when working, which is the most popular among users.
② There is no micro irregularity or burr that marks the grating groove, so stray light is far smaller
The stray light that marks the grating.
③ When the fabrication conditions are properly changed, an aberration free holographic grating can be fabricated.
④ Holographic gratings of any size can be made.
⑤ The manufacturing cycle is short.
⑥ Low manufacturing cost.
Generally speaking, the basic requirements for grating of ultraviolet visible spectrophotometer are very high. It can be summarized as follows: ① The stray light should be small; ② The output energy curve should be smooth, that is, from long wave to short wave, the output energy curve should have small and smooth fluctuations; ③ The wavelength range should be wide. At present, the wavelength range of gratings used in the ultraviolet visible region can reach 190~900 or 1000 nm; ④ The resolution should be high.
According to Newton’s law, the light emitted from the focal plane of the lens or reflector will become parallel light when it hits the lens or reflector (collimator). The incident slit is located on the focal plane of the collimator. Therefore, the function of the collimating mirror is to change the divergent composite light emitted from the incident slit into parallel light. The relative position between the collimating mirror and the incident slit is very important (that is, the incident slit must be strictly located on the focal plane of the collimating mirror). It will directly affect the parallelism of the parallel light, thus affecting the monochromaticity of the monochromator.
3、 Objective lens
According to Newton’s law, a beam of parallel light incident on the lens or reflector will focus on the focal plane of the lens or reflector; The exit slit is located on the focal plane of the objective lens. Therefore, the function of the objective lens is to focus the parallel light that hits the objective lens on the exit slit. The relative position between the objective lens and the exit slit is very important (that is, the exit slit must be strictly on the focal plane of the objective lens), which will directly affect the parallelism of the parallel light. This affects the monochromaticity of the monochromator.
Monochromator is a kind of compound light source composed of various wavelengths Δλ An instrument for monochromatic light. The monochromatic light of a certain wavelength separated by the monochromator cannot be true monochromatic light, and always contains a narrow spectral range Δλ。 Because the wavelength range of this range is very small, it is considered as monochromatic light. Generally, monochromators are composed of incident slit, exit slit, dispersion element (grating or prism), collimator, imaging objective lens, etc. There are many kinds of monochromators, including special type, general type, grating type, prism type, etc. The designer and manufacturer of ultraviolet visible spectrophotometer generally select the type according to the use requirements. No matter what kind of monochromator, its main technical indicators generally include the following: ① Operating wavelength range Circumference; ② Wavelength accuracy; ③ Wavelength repeatability; ④ Spectral bandwidth; ⑤ Stray light; ⑥ Wavelength scanning speed; ⑦ Some monochromators also give the relative aperture of the objective lens, the field angle of the objective lens, the dispersion rate of the beam splitter, the type of monochromator, etc.
In UV-VIS spectrophotometer, grating, collimating lens, objective lens, slit and other elements are often combined as a component of UV-VIS spectrophotometer. The monochromator in the ultraviolet visible spectrophotometer is generally divided into prism monochromator and grating monochromator. The specific configuration and arrangement are introduced as follows.
（1） Prism monochromator
Common prism monochromators have the following four configurations.
1. Transmission prism monochromator
Abbe type constant deflection prism is generally used as the transmission prism monochromator, and its light path is shown in Figure 3-20. This prism is actually composed of two 30 ° beam splitters and a right angle prism. The right angle prism only reflects light and does not participate in light splitting. Therefore, Abbe prism can be regarded as two 30 ° dispersion prisms, which are equivalent to a 60 ° beam splitting prism.
2. Reflective Litt row prism monochromator
The light path diagram of the reflective Litt row prism monochromator is shown in Figure 3-21. The monochromator generally uses an off-axis parabolic mirror to simultaneously act as a collimating objective and an imaging objective. The dispersion can be doubled by passing the beam through the prism twice.
3. Self collimating 30 ° prism monochromator
When the resolution requirement is not high, a simple Littrow prism monochromator is often composed of a spherical mirror and a 30 ° prism. The optical path is shown in Fig. 3-22. The monochromator applies a reflective film on one side of the right angle side of the prism to make the light beam turn back through this side. When the self collimating prism is rotated, wavelength scanning can be realized. In order to reduce stray light, black matt paint is generally applied behind the prism film or on the edges without light.
4. Wadsworth prism monochromator
The dispersion prism of the monochromator is connected with a plane reflector to form a constant deflection device. During wavelength scanning, the prism and plane mirror rotate together around the midpoint C of the bottom edge of the prism. The system has good imaging quality.
（2） Grating monochromator
There are five types of common grating monochromators. 1. Light path of Litt row grating monochromator is shown in Fig. 3-24.
The incident angle of the light beam on the grating is nearly equal to the diffraction angle, and the collimating objective lens and the imaging objective lens share the same objective lens. This type of monochromator is also called self collimating grating monochromator. Because its entrance slit and exit slit are very close, its stray light is relatively large. 2. Czerny Turner (C-T type for short) grating monochromator
This grating monochromator is a system that uses two spherical mirrors as collimating mirrors and imaging objectives. Horizontal arrangement is commonly used. The two spherical mirrors can compensate each other for aberration and have good imaging quality. Moreover, increasing the slit height will not seriously affect the resolution of the instrument. At the same time, the processing of spherical mirror is relatively easy.
The optical system is shown in Fig. 3-25. In this system, the incident slit S1 and the outgoing slit S2 are symmetrically distributed on both sides of the dispersion element, and M1 and M2 are collimators and objective lenses respectively; The system is characterized by small aberration (coma). Its aberration is about 1/5 of that of the autocollimator system, so this system is often used in large quantities.
3. Setu Okaoka concave grating monochromator
The Laigu Bogan concave grating monochromator is a device outside the Roland circle. The entrance and exit slits are outside the Roland circle. The angle between the entrance axis and the exit axis on the grating is large, generally about i+ θ= 70°。
The spectral scanning can be completed by rotating the grating around the center of the grating while keeping both the entrance slit and the exit slit stationary. Generally, when the incident angle changes from 26 ° to 44 °, the defocusing amount is 0.1% of the distance from the grating center to the exit slit. The main aberrations are astigmatism and coma. However, there are concave gratings which can eliminate astigmatism and obtain high image quality.
4. Ebert grating monochromator
Ebert grating monochromator uses only two parts of a concave spherical mirror as collimating mirror and objective lens to replace two concave spherical mirrors of C-T grating monochromator. The optical path diagram is shown in Figure 3-27, which is simple in structure and low in cost.
5. Monk Gilison grating monochromator
（3） Arrangement and classification of monochromator light paths
1. Horizontal autocollimation system
The so-called autocollimator system is an asymmetric monochromator optical system; The so-called horizontal autocollimation system is an autocollimation system in which all optical element centers and slit centers are on the same plane.
Taking the grating type horizontal autocollimator system as an example, its working principle is explained. S1 is located on the focal plane of spherical mirror or parabolic mirror M1. When the light enters the slit S1, it hits M1 through a small plane reflector. M1 turns the incident light into a parallel light, shines on the grating G, and then the light after grating dispersion hits M1; It is still focused on the exit slit S2 by M1, and the spectrum is formed at S2.
2. Vertical autocollimation system
The amount of stray light and spherical aberration is the same as that of horizontal autocollimation system. However, the smart difference is different. The smart difference of the horizontal autocollimation system makes the spectral lines asymmetric wider, while the vertical autocollimation system makes the spectral lines asymmetric longer in the height direction. Therefore, for the same optical parameters, the resolution of the system is slightly better than the former.
（4） Plane grating monochromator for non parallel beams
The common plane grating monochromator is to work the grating in a parallel beam, so that the aberration generated by the grating is smaller, and the instrument can obtain higher resolution. However, in some special applications, it is not required that the resolution of the plane grating monochromator is very high, but the light transmission performance of the instrument is very good. Even the light transmission performance is more important than the resolution. At this time, the grating can be placed in a non parallel beam. In the entire optical system, except for the grating, there is only one concave reflector.
（5） Double monochromator
If two simple monochromators are connected together, a double monochromator is formed. The double monochromator has an entrance slit, an exit slit and a middle slit. The middle slit is both the exit slit of the first monochromator and the entrance slit of the second monochromator. There are two ways to connect two monochromators: one is to add the dispersion of two monochromators; The other is the dispersion subtraction of two monochromators.
How to determine whether the dispersion of two monochromators in a double monochromator is additive or subtractive? The following simple method can be used: suppose that one beam enters from the entrance slit of the first monochromator and the other beam enters from the exit slit of the second monochromator, and the two beams focus on the plane of the middle slit. If the positions of the long wave light and short wave light lines formed by the two beams are the same, it is a dispersion subtraction system; If the position of the formed long wave light and short wave light is opposite, it is a dispersion addition system.