RU2131629C1 - Electrooptical diffraction measuring device - Google Patents

Abstract

FIELD: electronic equipment, which displays electron-diffraction pattern with pico- or femtosecond resolution, in particular, for investigations in physics, chemistry, biology, medicine, instruments and industry. SUBSTANCE: device has vacuum chamber which contains serial circuit of photo cathode, focusing system, anode diaphragm, electronic shutter, travelling wave deviation system, unit of targets, detector of electron image and reading system using CCD matrix. In addition to accelerating and focusing electrodes, focusing system has correcting electrode. Parameters of electrodes are in fixed proportion to each other, in order to provide optimal section of electron beam in plane of target under investigation. Planes of target and crossover are joined. This results in focusing of photo cathode image and diffraction pattern in plane of output screen. Housing also has optical windows for supply of laser beams in synchronization with electron pulse to target, which is located in unit of targets which is designed as device with sockets for mounting samples under investigation. EFFECT: increased temporal resolution (down to hundreds of femtoseconds), increased functional capabilities. 4 cl, 1 dwg

Description

 The invention relates to electronic equipment, in particular to devices operating in an electronographic mode with a picofemtosecond time resolution, and is used to study the structures of matter and their changes when conducting research in the field of physics, chemistry, geology, biology, medicine, materials science, in the electronic industry, in instrument and mechanical engineering.

 An electron-optical diffractometric device (EDI) can serve as a photoelectron diffractometer for conducting experiments on ultrafast electron diffraction on solid-state or gas targets with the possibility of synchronously heating the target with laser radiation and measuring the duration of the flux of electrons incident on the target, allowing the diffraction to be recorded on the EDI screen pictures, the analysis of which gives information about the processes occurring inside the investigated medium at the atomic and molecular levels.

 Electrons are known [1-3], using the phenomenon of electron diffraction for structural analysis of matter. In such electronographs, a source (a cold tip cathode or a thermal cathode) creates a continuous stream of electrons that are scattered on the test sample, and with the help of an electrostatic or electromagnetic electronic lens they focus in the image plane (luminescent screen, photographic plate, CCD matrix, etc.) as primary and diffracted electron beams.

 In electronographs, microdiffraction of high-energy electrons (20 - 100 keV) from selected sections of the test sample is most widely used [4-5]. These devices use sources of ultrathin (several tens of angstroms) electron beams and multi-element electron lenses that provide accurate focusing of both the initial and diffracted beams (nanodiffraction), including with an increase in the scale of diffraction patterns.

 The closest in technical essence to the proposed solution is an electron-optical diffractometric device containing a flat photocathode in the vacuum volume, followed by an electron beam conical electrostatic focusing system, including an accelerating and focusing electrodes to form an electron beam, an anode diaphragm, a target, luminescent screen and microchannel brightness amplifier [6].

 However, the known electron-optical device cannot operate in the registration mode of diffraction patterns with a femtosecond time resolution and does not allow measuring the duration of the electron flux acting on the target.

 An object of the present invention is to increase the time resolution (up to hundreds of femtoseconds) and expand the functionality of the device.

 The task is achieved by the fact that in the known electron-optical diffraction device containing a photocathode in a vacuum housing, followed by an electrostatic focusing system, an anode diaphragm, a target, a luminescent screen and a microchannel brightness amplifier, sequentially located along the electron beam, behind the anode diaphragm along the electronic A high-speed electronic shutter was introduced, and after the electronic shutter a symmetrical meandering deflecting system of the "traveling wave" type (OSBV) was introduced. On both ends, the OSBV is aligned with coaxial, 50-ohm connectors, and control deflecting electrical pulses with a front duration of 50-100 ps are connected to the connectors located at the beginning of the system, and to the connectors located at the end of the system (along the direction of the electron beam) connects the load.

 Further along the movement of the electron beam is a block of targets, which is a device with sockets for installing several interchangeable samples, the introduction of which into the plane of the "crossover" (plane of the smallest section of the electron beam) is carried out using a vacuum rod controlled by an electromagnet. The target block has one empty slot for unhindered passage of the electron beam in order to operate the EDI in linear scanning mode (streak camera) for direct measurement of the duration of the electron pulse acting on the target. A luminescent screen is installed in the second slot for spatial information on the target of the electron beam and heating laser radiation. The remaining nests are used to install the studied samples of the substance.

 The vacuum case has special optical windows installed on both sides of the case: one for summing up the heating laser radiation, and the other for visual observation when the electron beam and the heating laser radiation are brought to the target. It should be noted that an optical delay is introduced into the optical line of the heating laser radiation, which makes it possible to synchronize the arrival time of the heating radiation to the target with an electronic pulse.

 The registration of photoelectron pulses and diffraction patterns from the luminescent screen of the diffractometer is carried out on a reader based on a CCD matrix through a microchannel brightness amplifier. The brightness amplifier is installed outside the vacuum diffractometer housing and is attached to the luminescent screen and the CCD through optical fiber disks. The sensitivity of the system ensures the registration of single photoelectrons emitted by the input photocathode of the EDI.

The electrostatic focusing system of the EDI was substantially redesigned so that the crossover position remained in the plane of the target installation when the anode voltage and the voltage between the photocathode and the accelerating electrode on the EDI changed within 20–40 kV and 5–10 kV, respectively, and focusing on the output screen as the image of the photocathode, and electron beams diffracted on the target. It is made in the form of three axisymmetric cylinder-electrodes, including sequentially arranged accelerating (length L ₁ ), focusing (length L ₂ ) and correcting (length L ₃ ) electrodes, and the ratio of the lengths of the electrodes and the distances between them (l ₁₂ and l ₂₃ ) , as well as the distance between the correction electrode and the anode (l) to the diameter of the accelerating electrode (D) is within
5.3≤L ₁ /D≤6.5, 1.9≤L ₂ /D≤2.4, 0.31≤L ₃ /D≤0.38, 0.78≤l ₁₂ /D≤0.96, 2.4≤l ₂₃ /D≤3.0, 1.2≤l / D ≤1.4.

 Comparative analysis with the prototype and analysis of information sources shows that the claimed electron-optical diffractometric device is in accordance with the criterion of "novelty."

 When comparing the claims with other technical solutions in the art, no solutions are found that have similar features and solve similar technical problems, which allows us to conclude that this solution meets the criterion of "inventive step".

 The drawing shows a diagram of the EDI.

 It contains a photocathode 2 located in the vacuum housing 1, an electrostatic focusing system consisting of an accelerating electrode 3 made in the form of a cylinder with a fine-grained mesh, focusing 4 and correcting 5 cylindrical electrodes of the same diameter as the accelerating electrode 3, anode diaphragm 6, electronic a shutter 7, a traveling-wave type deflecting system 8, a target block 9, a luminescent screen 11, as well as a microchannel brightness amplifier 12 and a reader based on a CCD matrix 13, which are installed I am outside the vacuum volume. Special windows 10 are made in the housing for supplying a heating laser pulse to interchangeable targets located in the target block 9.

 In the focusing electronic EDI system, the sizes and relative positions of the electrodes are in a fixed proportion to each other, which ensures a minimum cross section of the electron beam in the plane of the target under test within a wide range of changes in the energy of the electron beam, with the target and crossover planes being combined. A special form of the focusing system allows you to provide the required focusing modes when the voltage on the accelerating electrode and the anode voltage are set between 5-10 kV and 20-40 kV, respectively.

 To ensure the operation of the device in the spectral ranges from infrared to ultraviolet, an appropriate photocathode can be installed.

 The introduction to the device of a reading system based on a CCD matrix 13 with an output to a computer automates the operation of the device: the diffraction pattern from the target in the target block 9 is read, written, stored and processed, and then visualized. An electron-sensitive CCD array can be used, which is installed inside the housing of the EDI instead of the luminescent screen 11.

 EDI works as follows.

 Optical radiation in the form of femtosecond pulses, incident on the photocathode 2, causes the emission of photoelectrons, which are accelerated by the electric field formed between the photocathode 2 and the accelerating electrode 3; in this case, the scatter of the initial electron velocities is relatively aligned. The electron beam is formed by an electrostatic focusing system formed by electrodes 3,4 5 and the anode diaphragm 6, and is directed to the test sample located in the block of targets 9, which is synchronously “heated” by laser radiation. Behind the anode diaphragm 6, an electronic shutter 7 is installed, transmitting electronic pulses only for a period of time determined by the time the shutter opens. Interacting with the target in the target block 9, the electrons diffract on it, creating a diffraction pattern of the electrons scattered on the target on the luminescent screen 11 (or CCD matrix 13). Varying the delay between the arrival of the laser and electron beams on the target allows us to obtain diffraction images corresponding to these changes. From a reader based on a CCD matrix 13, the information is transmitted to a computer and processed. An analysis of the diffraction pattern of electron scattering on molecules or atoms of the target material allows one to judge its internal structure, molecular temperature, phase transitions, photodissociation processes, etc. In the target block 9, an empty slot is installed to measure the duration of the electron beams and the EDI operates in the photochronograph mode with by scanning the electronic flow with a deflecting system on the luminescent screen 11, while the reading device 13 together with the computer is used to determine the duration of the probe ruyuschego electron beam. In the second window of the target block 9, a luminescent screen is installed, designed to visualize the spatial reduction process on the target of the electron beam and the heating laser pulse. The remaining windows in the block of targets 9 are occupied by the studied targets. When conducting experiments on the excitation of targets, a "heating" laser pulse is synchronously with the electron beam supplied to the targets located in the target block 9 through special windows 10.

толщины. Угловая расходимость электронного пучка в кроссовере составляла 10^-3 - 2•10^-2 радиан, размер пятна на мишени - 0.9 мм. Отклоняющая система 8 при работе в режиме дифрактометра могла отключаться, а с помощью электронного затвора 7 можно было "выделять" требуемое количество фотоэлектронных импульсов.The design of the electron-optical device was simulated on a computer with the subsequent development, manufacture and testing of experimental samples of EDI. Under the influence of 60 fs laser pulses, the device made it possible to form single photoelectron pulses with a duration of no more than 550 fs containing 10 ² - 10 ³ electrons with an energy of 30 keV, and to ensure the interaction of such photoelectron pulses with a target of 300 aluminum foil

thickness. The angular divergence of the electron beam in the crossover was 10 ^–3 - 2 • 10 ^–2 radians, and the spot size on the target was 0.9 mm. The deflecting system 8, when operating in the diffractometer mode, could be turned off, and using the electronic shutter 7, it was possible to “select” the required number of photoelectron pulses.

 Visualization and processing of diffraction patterns obtained on the screen of the device was carried out by a reader based on a cooled CCD matrix with the number of elements 1040x1160 and an absolute sensitivity of 300 photons per pixel at a wavelength of 530 nm. The CCD was docked to the EDI screen through fiber optic plates. Oxygen-silver-cesium (S1) and multi-alkaline (S20) photocathodes were introduced into the sealed volume of the EDI using the vacuum manipulator method. The possibility of the operation of EDI in the UV, visible and IR spectral ranges was shown.

 The following are comparative data obtained in the prototype and in the claimed invention when interacting with an aluminum target of 30 keV photoelectron pulse obtained from a laser pulse having a duration of 60 fs.

,
длительность лазерного импульса на фотокатоде - 15 пс,
длительность фотоэлектронного импульса на мишени - <100 пс,
размер электронного пятна на мишени - 3 мм,
число электронов в одиночной вспышке - 10³-10⁵
В заявляемом решении:
тип фотокатода - S1/S20
тип мишени - A1,
толщина мишени - 300

,
длительность лазерного импульса на фотокатоде - 60 фс,
длительность фотоэлектронного импульса на мишени - <550 фс
размер электронного пятна на мишени - 0,5-1 мм
число электронов в одиночной вспышке - 10²-10³
1. Разработанное устройство может работать в режиме дифрактометра или в режиме электронографа и позволяет измерять длительность зондирующих мишени фотоэлектронных импульсов в фемтосекундном диапазоне при проведении экспериментов по сверхбыстрой дифракции электронов на твердотельных и газообразных мишенях. Повысилось временное разрешение - двумерные картины исследуемых процессов получены на дифрактометре с разрешением 550 фс, что почти в 200 раз превосходит прототип.In the prototype:
photocathode type - A1 photocathode,
target type - A1 foil,
target thickness - 150

,
the duration of the laser pulse at the photocathode is 15 ps,
the duration of the photoelectron pulse on the target is <100 ps,
the size of the electron spot on the target is 3 mm,
the number of electrons in a single flash - 10 ³ -10 ⁵
In the claimed decision:
Photocathode Type - S1 / S20
target type - A1,
target thickness - 300

,
photocathode laser pulse duration - 60 fs,
the duration of the photoelectron pulse on the target is <550 fs
the size of the electron spot on the target is 0.5-1 mm
the number of electrons in a single flash - 10 ² -10 ³
1. The developed device can operate in the diffractometer mode or in the electron diffraction mode and allows measuring the duration of the probing photoelectron pulses in the femtosecond range during experiments on ultrafast electron diffraction on solid and gaseous targets. The temporal resolution increased - two-dimensional pictures of the processes under study were obtained on a diffractometer with a resolution of 550 fs, which is almost 200 times higher than the prototype.

 2. The width of the spectral region, depending on the type and sensitivity of the photocathode used in the device, covered the UV, visible and infrared ranges.

 3. The technical capabilities of the device have been expanded thanks to a developed computer-compatible reader that allows you to automate the processing of data obtained on the basis of a cooled CCD matrix operating in slow scanning mode and providing registration of single photoelectrons.

Literature:
1. 3. G. Pinsker. Electron diffraction. M .; L. Publishing house of the Academy of Sciences of the USSR, 1949, 404 p.

 2. Weinstein. Structural electron diffraction. M., Publishing House of the Academy of Sciences of the USSR, 1956, 320 p.

 3. B. B. Zvyagin. Electron diffraction methods and the tasks they solve. In the book "Methods of structural analysis" M., Publishing house "Science", 1989.

 4. Fifty years of electron diffraction. / Ed. P. Goodman. Dordrecht: Reidel, 1981, 440 p.

 5. B.B. Zvyagin, A.N. Gorshkov. Electron microscopy and electron diffraction (microdiffraction). Methods of electron microscopy of minerals. M .; Publishing House "Science", 1969, p. 207 - 310.

 6. G. Mourou, S. Williamson. Picosecond electron diffraction. Appl. Phys. Lett. 41 (1), 1 July 1982, pp. 44-45. Prototype.

Claims (4)

 1. Electron-optical diffraction device containing a photocathode sequentially arranged along the electron beam in the vacuum housing, an electrostatic focusing system, an anode diaphragm, a target and an electronic image recorder, characterized in that an electronic shutter is inserted into it, a traveling wave-type deflecting system and a reader based on a CCD matrix, wherein the electronic shutter is located behind the anode diaphragm, the deflecting system is located between the electronic shutter and the target, and A CCD based luminaire is installed behind a microchannel brightness amplifier outside the casing. 2. The device according to claim 1, characterized in that the focusing electrostatic system is made in the form of three axisymmetric cylinder-electrodes and includes, in addition to sequentially accelerating length L ₁ and focusing length L ₂ electrodes, a correction electrode of length L ₃ , the ratio of the lengths of the electrodes and the distances between them l ₁₂ and l ₂₃ , as well as the distance between the correction electrode and the anode l to the diameter of the accelerating electrode D is within
5.3≤L ₁ / D ≤6.5, 1.9≤L ₂ / D≤2.4, 0.31≤L ₃ / D≤0.38, 0.78≤l ₁₂ / D≤0, 96, 2.4 ≤l ₂₃ / D≤3.0, 1.2≤l / D≤1.4,
this ensures focusing in the plane of the output screen both the image of the photocathode and the diffraction pattern, and the geometrical position of the crossover remains in the plane of the target location regardless of the change in accelerating voltages.  3. The device according to claim 1, characterized in that additional targets are assembled in a block, which is a device with sockets for installing several interchangeable targets, the introduction of which into the plane of the crossover is carried out using a vacuum rod controlled by an electromagnet, with one socket left empty for the unhindered passage of the electron beam and the subsequent measurement of its duration, a luminescent screen is installed in the second slot for spatial information of the electron beam and laser radiation, and the remaining nests are used to establish the studied samples.  4. The device according to claim 1, characterized in that the same optical windows are made in the vacuum casing, one of which serves to bring the laser radiation synchronously with the electronic transmission pulse to the target located in the target block, the other for visual observation.