Momentum-resolving photoelectron spectrometer and method for momentum-resolved photoelectron spectroscopy

Abstract

The invention relates to the field of physics and relates to an impulse-resolving photo-electron spectrometer, by means of which the physical properties can be determined. The aim of the invention is to provide an impulse-resolving photo-electron spectrometer enabling the device components to have a simple structure with a significantly reduced overall volume. The aim of the invention is achieved by means of an impulse-resolving photo-electron spectrometer comprising components arranged one behind the other in the direction of the optical axis at least in a vacuum and which are each at least one electron emission sample and a focusing system, wherein the focusing system consists of at least one electron lens and at least one detector, wherein the electron lens consists of three cylindrical elements, wherein the first cylindrical element has a potential=0 and the two subsequently arranged cylindrical elements have a potential of ≠0, and wherein the detector is one or more spatially resolved detectors which are arranged in the focal plane of the electron lens.

Claims

1. A momentum-resolved photoelectron spectrometer, containing components disposed in succession along the direction of the optical axis at least within a vacuum, said components respectively being at least one electron emission sample and a focusing system, wherein the focusing system consists of at least one electron lens and at least one detector, wherein the electron lens consists of three cylindrical elements which are disposed in succession and at a distance from one another along the direction of the optical axis, wherein the first cylindrical element has a potential equal to 0 and the two subsequently disposed cylindrical elements have a potential not equal to 0, with these two cylindrical elements not having the same potential, and wherein the focusing system focuses and detects electrons which respectively have substantially the same kinetic energy and, of these electrons, those which have left the electron emission sample with the same momentum are focused at substantially one point in the focal plane of the focusing system for this same kinetic energy, and wherein the detector is one or more spatially resolved detectors which are disposed in the focal plane of the focusing system, and wherein a lower limit of the kinetic energy of the electrons to be focused and detected is adjustable up to the Fermi energy in the focusing system by way of applying an altered voltage to the cylindrical elements of the electron lens and/or the detector.

2. The momentum-resolved photoelectron spectrometer as claimed in claim 1, wherein the components are disposed in a chamber in which there is a high vacuum or an ultra-high vacuum, at least during the measurements.

3. The momentum-resolved photoelectron spectrometer as claimed in claim 1, wherein the electron emission sample consists of the material to be examined.

4. The momentum-resolved photoelectron spectrometer as claimed in claim 1, wherein the electron lens of the focusing system generates an electric field, by means of which a focal plane for a given kinetic energy of electrons is generated, in which the focusing of the electrons with this given and same kinetic energy and with the same momentum is realized.

5. The momentum-resolved photoelectron spectrometer as claimed in claim 1, wherein the electron lens of the focusing system consists of a container having a cylindrical entrance opening and two further cylindrical elements disposed in succession therein.

6. The momentum-resolved photoelectron spectrometer as claimed in claim 1, wherein the at least one detector is disposed in the focal plane of the electron lens as a microchannel plate.

7. The momentum-resolved photoelectron spectrometer as claimed in claim 6, wherein the detector or detectors is/are in disposed in the container transversely to the optical axis and downstream of the three cylindrical elements.

8. The momentum-resolved photoelectron spectrometer as claimed in claim 1, wherein meshes are disposed upstream of the detectors, said meshes advantageously also being disposed in the container and/or advantageously also being disposed in the focal plane of the electron lens.

9. The momentum-resolved photoelectron spectrometer as claimed in claim 1, wherein the electron lens and/or the detector in the focal plane of the electron lens are embodied to be alterable, by applying a voltage, in respect of the detectability of the kinetic energy of the electrons to be focused and detected.

10. A method for momentum-resolved photoelectron spectroscopy, wherein electrons are released from an electron emission sample and guided through a focusing system, wherein the focusing system generates an electric field, by means of which the focusing of electrons is realized in a focal plane of the focusing system which is assigned to a given kinetic energy, from a desired kinetic energy to the Fermi energy, and wherein all electrons with this desired kinetic energy and substantially the same momentum, i.e., substantially the same emission direction from the electron emission sample, are focused and detected substantially at one point on a detector in the focal plane of the focusing system.

11. The method as claimed in claim 10, wherein electrons are released from the surface of the electron emission sample by means of a photon beam in the form of synchrotron radiation or radiation from a helium lamp.

12. The method as claimed in claim 10, wherein only electrons with substantially the Fermi energy are focused and detected by the focusing system.

13. The method as claimed in claim 10, wherein the desired kinetic energy of the electrons to be focused, up to the Fermi energy, is set by applying a different voltage to the electron lens and/or the detector in the focal plane of the focusing system.

14. The method as claimed in claim 10, wherein, upstream of the detector, the focusing system brakes substantially all electrons which have a kinetic energy below the desired kinetic energy of the electrons to be detected and consequently said braked electrons are not detected.

15. The method as claimed in claim 10, wherein, upstream of the detector, the focusing system accelerates and detects substantially all electrons which have the desired kinetic energy up to the Fermi energy of the electrons to be detected.

16. The method as claimed in claim 15, wherein the acceleration of the electrons to be detected, which have the desired kinetic energy up to the Fermi energy, is realized by means of meshes upstream of the detector in the focal plane of the electron lens.

17. The method as claimed in claim 10, wherein, for the purposes of ascertaining the momentum distribution of electrons at a desired kinetic energy below the Fermi energy, electrons with the desired kinetic energy up to the Fermi energy are focused and the momentum distribution is ascertained and, subsequently, electrons with a higher kinetic energy up to the Fermi energy are focused and the momentum distribution is ascertained, and subsequently the momentum distribution at the higher energy is subtracted from the momentum distribution of the desired kinetic energy.

18. The method as claimed in claim 10, wherein the momentum distribution of the emitted electrons as a function of their kinetic energy is ascertained as a pictorial representation.

19. The method as claimed in claim 10, wherein statements about the physical properties of the electron emission sample are derived from the ascertained values of the momentum distribution of the emitted electrons depending on their energy.

20. The method as claimed in claim 10, wherein electrons are released from the surface of the electron emission sample by means of a photon beam, said photon beam being a monochromatic photon beam.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows the electron emission sample, electron lens and detector of the impulse-resolving photo-electron spectrometer

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(2) The invention is explained in more detail below using an exemplary embodiment.

Example 1

(3) In a vacuum chamber which is evacuable to a vacuum of 10.sup.−10 hPa, an electron emission sample and a focusing system are disposed in succession in the direction of the optical axis, proceeding from the electron emission sample.

(4) The electron emission sample consists of TaSe.sub.2 and has the following dimensions: 1 mm surface diameter and 0.2 mm height.

(5) The focusing system consists of an electron lens and a detector.

(6) The electron lens consists of a cylindrical container with a length of 108 mm and a diameter of 140 mm and a cylindrical entrance opening with a 30 mm diameter and 15 mm length.

(7) Two cylindrical elements, each with a radius of 49 mm, are disposed in succession in the container along the direction of the optical axis and are spaced apart 5 mm; the first cylinder has a length of 35 mm and the second cylinder has a length of 42 mm. The cylindrical element next to the entrance opening of the container is located at a distance of 11 mm from the inner edge of the cylindrical entrance opening.

(8) These two cylindrical elements and the cylindrical entrance opening of the container together form the electron lens.

(9) The sample is disposed at a distance of 28 mm from the container opening.

(10) The detector is a circular microchannel plate with a diameter of 75 mm, which is disposed in the container, transversely to the optical axis and at a distance of 130 mm from the sample, i.e., still within the second cylindrical element, and which is coupled to a phosphor screen disposed therebehind (standard design; so-called MCP assembly).

(11) The electrons are emitted from the sample surface by way of the radiation of the He lamp with a photon energy of 21.2 eV. Due to the work function of approximately 4.2 eV of TaSe.sub.2, the electrons have the highest kinetic energy of ˜17 eV, depending on the temperature of the sample. This energy is the Fermi energy and the corresponding momentum distribution is the so-called Fermi surface. To record the Fermi surface of TaSe.sub.2, the following voltages are applied to the focusing elements:

(12) Container V.sub.G=0 V;

(13) First cylinder V.sub.1=−16.8 V;

(14) Second cylinder V.sub.2=−16.65 V;

(15) Detector surface V.sub.D=−17 V.

(16) Using these settings, all electrons with energies of less than 17 eV are braked and do not reach the detector. The electrons with the Fermi energy are focused in the focal plane by the electron lens and strike the surface of the microchannel plate. All electrons with Fermi energy, which have left the surface of the sample, are focused here, with all electrons with the same emission direction, i.e., the same momentum, being focused on a certain point on the detector surface.

(17) Since the momentum of the electrons is defined at the Fermi energy as a result of this emission direction, the intensity distribution at the surface of the detector (MCP) directly corresponds to the Fermi momentum distribution or the Fermi surface of TaSe.sub.2. This intensity distribution is amplified by the detector (MCP) and is visible on the coupled phosphor screen. It can be recorded by the CCD camera from outside of the vacuum camera through the window flange.

Example 2

(18) To record the momentum distribution of the electrons with energies below the Fermi energy (e.g., 16.98 eV), all negative voltages are reduced proportionally (V.sub.G=0 V, V.sub.1=−16.78 V, V.sub.2=−16.63 V, V.sub.D=−16.98 V).

(19) In this case, all electrons with an energy of 16.98 eV and more, up to the Fermi energy, reach the detector. To ascertain the momentum distribution at 16.98 eV, the momentum distribution at the Fermi energy of the electrons of the same sample is subtracted in order to obtain the desired momentum distribution of the electrons with a kinetic energy of 16.8 eV.

LIST OF REFERENCE NUMERALS

(20) 1 Electron emission sample 2 Electron lens made up of three cylindrical elements 3 Detector