CN113871287A - Device and method for preparing large amount of cold molecular ions - Google Patents
Device and method for preparing large amount of cold molecular ions Download PDFInfo
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Abstract
The invention relates to a device and a method for preparing a large number of cold molecular ions, and belongs to the technical field of cold molecular ion preparation. According to the method, a large number of cold atoms are prepared in a magneto-optical trap through laser cooling, then a large number of ions are generated through continuous photo-ionization of the cold atoms, the ions collide with the cold atoms to generate cold molecular ions and are trapped by the ion trap, therefore, a large number of cold molecular ions are obtained, and finally, clear and identifiable molecular ion signals are obtained through ion flight time mass spectrometry. The invention solves the problem of difficult preparation and discrimination of cold molecular ions.
Description
Technical Field
The invention belongs to the technical field of preparation of cold molecular ions, and particularly relates to a device and a method for preparing a large number of cold molecular ions.
Background
The spatially trapped cold molecular ions have the characteristics of spatial localization, long-time storage, easier regulation and control of charged particles and the like, so that the method has a very great application prospect in the aspects of cold controllable chemical reaction, formation and evolution processes of low-temperature ion interstellar clouds, quantum computation and quantum simulation under the interaction of external fields and the like. At present, three experimental schemes exist for preparing the space-trapped cold molecular ions, and the technology and the defects are as follows:
1. the collision of ions of the coulomb crystal with cold atoms generates molecular ions: in the mixed trap system, ions trapped in the ion trap are cooled by laser to form coulomb crystals, and the coulomb crystals collide with cold atoms/ultra-cold atoms in the atom trap to generate molecular ions. The disadvantages are that: firstly, coulomb crystal is formed, extra laser cooling ions need to be introduced, and the coulomb crystal with high enough density needs to be formed, and the requirements on laser parameters of an ion trap and the cooling ions are high; secondly, the type of ions is required, the ions are required to have proper cyclic transition for laser cooling, and alkali metal ions are in a full shell structure and cannot be cooled by laser; the geometric center of the ion trap and the geometric center of the atom trap are required to be highly coincident; and fourthly, the existing device can not directly screen reaction products and needs theoretical calculation assistance. [ Phys.Rev.Lett.107,243202(2011), mol.Phys.111,2020(2013), mol.Phys.111,1683(2013) ].
2. The ions are compounded with the high-density ultra-cold atomic cloud trisome to generate molecular ions: and (3) immersing single ions into the cold atoms or ultra-cold atom clouds condensed by the glass-Einstein, and carrying out ion-atom three-body composite reaction to generate molecular ions. The disadvantage is that the experimental setup for preparing the bose einstein condensation/supercooled atoms is complex and only one molecular ion can be generated. [ Phys. Rev.Lett.109,123201(2012), Phys Rev Lett.116,193201(2016), Phys.Rev.A 94,030701(R) (2016), Phys.Rev.A 102,041301(R) (2020), Phys.Rev.Lett.126,033401(2021) ].
3. Photo-ionizing cold molecules generate cold molecular ions: a small amount of cold molecules can be generated by cold atom collision in a magneto-optical trap, or the cold molecules are generated by adopting a light association method, and then the cold molecules are ionized by adopting pulse laser two-photon to generate molecular ions. The disadvantages are that: because the probability of cold molecules generated by cold atom collision or light association is very small, the efficiency of the nonlinear process of two-photon ionization is low, and the generated molecular ions are fewer; there is also a need for dye lasers that provide the high intensity pulsed laser light required for two-photon ionization.
[PhysRevLett.99.043003(2007),PhysRevLett.117.213002(2016)]。
Disclosure of Invention
Aiming at the defects in the prior art, the invention aims to provide a method and a device for preparing a large amount of cold molecular ions, and solve the problem of difficulty in preparation and discrimination of the cold molecular ions.
In order to achieve the above purposes, the invention adopts the technical scheme that:
an apparatus for preparing a large number of cold molecular ions, comprising an ultra-high vacuum system, a hybrid trap, an optical path system, a timing control system and a signal detection system, wherein:
the ultrahigh vacuum system is used for providing an ultrahigh vacuum environment for trapping atoms and ions, and comprises a scientific cavity, a three-stage sputtering ion pump, a mechanical pump, a molecular pump and an atom source, wherein the three-stage sputtering ion pump is communicated with the cavity of the scientific cavity through a main pipeline; the main pipeline is provided with a plurality of branch pipelines, the mechanical pump and the molecular pump are communicated with the main pipeline through a molecular pump valve arranged at the tail end of the second branch pipeline and are used for pre-pumping the vacuum system to a set vacuum degree, and then the vacuum degree of the system is maintained through the three-stage sputtering ion pump; the atom source is arranged at the tail end of the first branch pipeline and is communicated with the main pipeline through an atom source valve arranged on the first branch pipeline so as to provide neutral atoms for preparing cold molecular ions;
the scientific cavity is of a polyhedral flat metal cavity structure, the side face of the scientific cavity is provided with at least six vacuum observation windows, and the upper end face and the lower end face of the scientific cavity are respectively provided with one vacuum observation window;
the hybrid trap is arranged in the center of the scientific cavity and comprises a magneto-optical trap and a linear Paul trap, wherein the magneto-optical trap is used for cooling and confining neutral atoms, the linear Paul trap is used for confining ions, the center of the magneto-optical trap coincides with the center of the linear Paul trap, and the hybrid trap is characterized in that:
the magneto-optical trap consists of an optical field formed by three pairs of laser beams and a gradient magnetic field formed by a pair of reverse Helmholtz coils, the laser beams are combined beams formed by cooling light and re-pumping light, and the three pairs of laser beams are respectively emitted along three coordinate axis directions of a three-dimensional Cartesian coordinate system and respectively pass through four vacuum observation windows on the side surface of the scientific cavity and an upper vacuum observation window and a lower vacuum observation window to be converged at the center of the scientific cavity; the pair of reverse Helmholtz coils are respectively and symmetrically arranged above and below the scientific cavity and provide a gradient magnetic field required by the magneto-optical trap; the magneto-optical trap cools neutral atoms and traps the neutral atoms in the center of the magneto-optical trap to form a cold atom cloud;
ionizing light is emitted to the center of the scientific cavity through a vacuum observation window on the side surface of the scientific cavity, acts on the cold atom cloud, and generates ions through two-step photoionization together with the cooling light;
the linear Paul trap comprises four radio frequency electrode rods which are parallel to each other and two end electrodes, two pairs of diagonals of the radio frequency electrode rods are loaded with radio frequency signals in opposite phases, the two end electrodes are loaded with the same positive voltage, and the ions are trapped in the center of the linear Paul trap to form an ion cloud;
the ions and cold atoms are reacted and collided to generate cold molecular ions, and the cold molecular ions are trapped in the center by the linear Paul trap, so that a large number of cold molecular ions are obtained;
the optical path system comprises a laser system and a laser frequency stabilization and shift system, wherein the laser system is used for providing cooling light and re-pumping light required by the magneto-optical trap and ionizing light for ionizing cold atoms to generate ions; the laser frequency stabilizing and shifting system is used for regulating and controlling the frequency and stability of laser;
the time sequence control system is used for controlling experiment time sequence and experiment parameters, and the controlled experiment parameters comprise magneto-optical trap gradient magnetic field coil current, cooling light and re-pumping light switch and frequency shift quantity, ionizing light switch, ion trap radio frequency switch and terminal voltage switch.
Further, according to the device for preparing a large amount of cold molecular ions, the ultrahigh vacuum system further comprises an ionization gauge for measuring the vacuum degree of the system, the ionization gauge is installed in the third branch pipeline, and the vacuum degree index of the ultrahigh vacuum system is better than 10-7Pa。
Further, according to the device for preparing a large amount of cold molecular ions, the scientific cavity is made of 316L non-magnetic stainless steel, twelve vacuum observation windows with the types of CF35 are arranged on the side face of the scientific cavity, vacuum observation windows with the types of CF200 are respectively arranged on the upper end face and the lower end face of the scientific cavity, and the vacuum observation windows and the scientific cavity body are sealed through full knife edges, so that the sealing performance of the scientific cavity is guaranteed.
Further, in the apparatus for preparing a large amount of cold molecular ions as described above, the diameter of the radio frequency electrode rod is 3mm, the length of the radio frequency electrode rod is 10cm, and the rod interval is 15 mm; the inner diameter of the end electrode is 10 mm; the end electrodes are in a circular ring shape, the circular ring is disconnected by a notch with the angle of 1 degree in the axial direction, so that an eddy current electric field is avoided, and the distance between the axial centers of the two end electrodes is 10 cm.
Further, according to the device for preparing a large number of cold molecular ions, the included angle between the radio frequency electrode rod and the laser beam in the horizontal direction is 45 degrees.
Further, the device for preparing a plurality of cold molecular ions as described above, wherein the laser beam is circularly polarized light, the intensity of the laser beam is the same for each pair of opposite rays, and the polarization directions are respectively σ+And σ-。
Further, in the apparatus for preparing a large amount of cold molecular ions as described above, the total power of the cooling light is 95mW, the total power of the re-pumping light is 10mW, and the diameter of a light spot formed by the cooling light and the re-pumping light after beam expansion is 18 mm; the magnetic field gradient generated by the reverse Helmholtz coil was 10G/cm.
Further, in the apparatus for preparing a large amount of cold molecular ions as described above, the signal detection system includes a microchannel plate disposed at one end of the linear Paul trap, a photodetector and two vertically disposed CCD cameras, and an ion time-of-flight spectrum is obtained through the microchannel plate; and collecting cold atom fluorescence through the photoelectric detector and the CCD camera to obtain cold atom cloud related parameters.
Based on the device, the invention provides a method for preparing a large number of cold molecular ions, which comprises the following steps:
step S1, starting the magneto-optical trap, continuously working for a first preset time, and cooling and trapping atoms in the center of the magneto-optical trap under the action of the optical field and the gradient magnetic field;
step S2, after the magneto-optical trap works for a first preset time, the ionization light and the linear Paul trap are simultaneously opened, the magneto-optical trap works for a second preset time, the ionization light continuously ionizes cold atoms to generate cold ions, and the cold ions are imprisoned in the center of the linear Paul trap; the cold ions react and collide with atoms, so that a large amount of cold molecular ions are generated;
step S3, after the ionized light and the linear Paul trap continuously work for a second preset time, the ionized light and the end electrode close to the microchannel plate are closed at the same time, the radio frequency electrode rod is kept continuously work for a third preset time, and ions are prevented from radially diffusing; keeping the end electrode far away from the microchannel plate to continuously work for a third preset time, and guiding the ions to the microchannel plate along the axial direction under the action of an electric field pointing to the microchannel plate; and (3) the ions are driven to the microchannel plate, and an ion flight time mass spectrum signal is obtained through an electron multiplication effect.
Further, according to the method for preparing a large amount of cold molecular ions, the relationship between the voltage signal of the microchannel plate and the number of ions is calibrated, and then the number of the molecular ions is directly obtained according to the intensity of the ion flight time mass spectrum signal.
The invention has the beneficial technical effects that:
according to the device and the method for preparing a large number of cold molecular ions, the magneto-optical trap for cooling and trapping neutral atoms and the linear Paul trap for trapping ions are integrated into an integrated mixed trap experiment platform, continuous laser light is used for photoionizing the cold atoms to generate ions, cold molecular ions can be generated by the collision of the ions and the cold atoms, and the molecular ions of reaction products are trapped by the ion trap, so that a large number of cold molecular ions are obtained. And the compact time-of-flight spectrum device is combined, so that the reaction product can be accurately discriminated and used for researching molecular ion energy level and dynamics. The difference of the invention from the prior scheme is that:
(1) compared with the molecular ions generated by the collision of the ions of the coulombic crystal and the cold atoms, the device provided by the invention is simpler, and the ions do not need to be cooled by laser to form the coulombic crystal; meanwhile, the existing scheme adopts a resonance fluorescence mass spectrometry method, and the charged particles can be discriminated by combining theoretical calculation, but the flight time mass spectrometry method adopted by the invention is the most accurate method for discriminating the charged particle ions.
(2) The device and the method provided by the invention have lower requirements on the vacuum degree and the density of cold atoms, the device is simple, and a large amount of molecular ions can be generated.
(3) Compared with the scheme of generating molecular ions by cold molecules through two-photon ionization, the invention uses a common semiconductor laser to perform continuous photoionization and can generate a large amount of molecular ions; the scheme of two-photon ionization of cold molecules to generate molecular ions needs a high-performance dye laser, and the nonlinear process is low in efficiency and the number of generated molecular ions is small.
Drawings
FIG. 1 is a schematic diagram of an apparatus for producing a large number of cold molecular ions according to an embodiment of the present invention;
FIG. 2 is a schematic diagram of the structure of the science chamber in FIG. 1;
FIG. 3 is a schematic diagram of the structure of the hybrid trap of FIG. 1;
FIG. 4 is a schematic diagram of the two-step photoionization ion generation and the energy level diagrams involved;
FIG. 5 is a schematic diagram of the signal detection system of FIG. 1;
FIG. 6 is a flow diagram of a method for producing a plurality of cold molecular ions according to an embodiment of the present invention;
FIG. 7 is a timing diagram illustrating a method for preparing a plurality of cold molecular ions according to an embodiment of the present invention;
FIG. 8 is a schematic view of ion extraction;
figure 9 is an ion time-of-flight mass spectrum.
Detailed Description
The invention is further described with reference to the following figures and detailed description.
The core idea of the method for preparing a large amount of cold molecular ions provided by the invention is as follows: a large number of cold atoms are prepared in the magneto-optical trap through laser cooling, continuous laser is utilized to ionize the cold atoms to generate a large number of ions, cold molecular ions can be generated due to collision of the ions and the cold atoms, and the molecular ions of reaction products are imprisoned by the ion trap, so that a large number of cold molecular ions are obtained.
Based on the thought, the invention provides a device for preparing a large number of cold molecular ions, the core equipment of the device is a mixed trap platform, namely a magneto-optical trap for cooling trapped neutral atoms and a linear Paul trap for trapping ions are integrated into a whole, the center of the magneto-optical trap is coincided with the center of an ion trap so as to ensure that the center of an ion cloud is coincided with the center of an atom cloud, and meanwhile, a microchannel plate is used as a detection device of an ion flight time mass spectrum. The device prepares a large amount of cold atoms in a magneto-optical trap through laser cooling, then generates a large amount of ions through continuous photo-ionization of the cold atoms, cold molecular ions are generated by collision of the ions and the cold atoms and are imprisoned by the ion trap, so that a large amount of cold molecular ions are obtained, and finally, clear and identifiable molecular ion signals are obtained through ion flight time mass spectrometry.
The technical scheme of the invention is explained in detail by taking rubidium as an example.
The structure schematic diagram of the device for preparing a large amount of cold molecular ions provided by the invention is shown in figure 1, and the device comprises five parts: ultrahigh vacuum system 1, mixed trap 2, light path system, time sequence control system and signal detection system 3, wherein:
ultra-high vacuum system 1: the device is used for providing an ultrahigh vacuum environment for trapping atoms and ions so as to avoid the collision of background gas to cold atoms/ions as much as possible and prolong the service life of the cold atoms/ions. The degree of vacuum is set according to the contents of the study, and in the present embodiment, the degree of vacuum is set for the Rb atomIs 10-7Pa。
The ultrahigh vacuum system comprises a scientific cavity 11, a mechanical pump (not shown in the figure), a molecular pump (not shown in the figure), a three-stage sputtering ion pump 12, an atom source 13, an ionization gauge 16, an atom source valve 14 and a molecular pump valve 15, wherein the three-stage sputtering ion pump 12 is communicated with the cavity of the scientific cavity 11 through a main pipeline, and three branch pipelines are arranged on the main pipeline; the mechanical pump and the molecular pump are communicated with the main pipeline through a molecular pump valve 15 arranged at the tail end of the second branch pipeline and are used for pre-pumping the vacuum system to high vacuum and then maintaining the ultrahigh vacuum of the system through the three-stage sputtering ion pump 12; the atom source 13 is communicated with the main pipeline through an atom source valve 14 arranged on the first branch pipeline, and further communicated with the cavity of the scientific cavity 11 to provide an atom sample required by an experiment; an ionization gauge 16 is mounted in the third branch conduit for measuring the vacuum level of the system.
In this embodiment, the atom source is rubidium element, and the rubidium element is communicated with the cavity of the scientific cavity 2 through an atom source valve, so as to provide a rubidium atom sample required by an experiment.
During the experiment, the mechanical pump and the molecular pump are started firstly, and the vacuum system is pre-pumped to high vacuum to 10-4And after Pa, starting the three-stage sputtering ion pump, then closing a molecular pump valve between the molecular pump and the system, removing the mechanical pump and the molecular pump, and continuously optimizing and maintaining the vacuum degree of the system through the three-stage sputtering ion pump. The three-stage sputtering ion pump belongs to a capture pump, is completely isolated from the outside during use, and can ensure the safety of personnel and environment.
Scientific cavity 11: the structure schematic diagram is shown in fig. 2, the scientific cavity 11 is a polyhedral flat metal cavity structure, the material is 316L nonmagnetic stainless steel, 12 vacuum observation windows with the model of CF35 are arranged on the side surface, the upper end surface and the lower end surface are respectively provided with a vacuum observation window with the model of CF200, and the vacuum observation windows and the scientific cavity main body are sealed by a full knife edge, so that the scientific cavity 11 is guaranteed to have ultrahigh sealing performance.
Mixing trap 2: arranged in the center of a scientific chamber 2, the structure of which is shown in fig. 3, the hybrid trap 2 integrates a magneto-optical trap 21 for cooling trapped neutral atoms with a linear Paul trap 22 for trapping ions.
Magneto-optical trap 21: the light field formed by the three pairs of laser beams 211 and the gradient magnetic field formed by the pair of opposing Helmholtz coils 212; wherein, the laser beam is a beam combining light composed of cooling light and re-pumping light, three pairs of laser beams 211 are respectively emitted along three coordinate axis directions of the three-dimensional Cartesian coordinate system and respectively converged at the center of the scientific cavity 11 through four vacuum observation windows and an upper vacuum observation window and a lower vacuum observation window on the side of the scientific cavity 11; each pair of oppositely irradiated laser beams has the same intensity and is circularly polarized light with the polarization direction of sigma+And σ-(ii) a A pair of opposing helmholtz coils 212 are symmetrically disposed above and below the scientific cavity 11, respectively, to provide the gradient magnetic fields required for the magneto-optical trap 21. The magneto-optical trap 21 cools and traps neutral atoms to form a cold atom cloud in the center of the magneto-optical trap.
The ionized light is emitted to the center of the science chamber 11 through a vacuum observation window on the side surface of the science chamber 11, acts on the cold atom cloud, and generates ions through two-step photoionization together with the cooling light.
Linear Paul trap 22: consists of four mutually parallel radio frequency electrode rods, two end electrodes and a plurality of fixing pieces. The diameter of the radio frequency electrode rod is about 3mm, the length of the radio frequency electrode rod is 10cm, and the rod interval is 15 mm; the inner diameter of each terminal electrode is 10mm, a notch of about 1 degree is formed in the axial direction of each terminal electrode, so that an eddy current electric field is avoided, and the distance between the two terminal electrodes is 10 cm. Two pairs of diagonals of the radio-frequency electrode rod are loaded with radio-frequency signals in opposite phases, and two end electrodes are loaded with the same positive voltage, so that ions are trapped in the center of the linear Paul trap to form an ion cloud.
The center of the linear Paul trap and the center of the magneto-optical trap are coincided with the origin, and the centers of the ion cloud and the cold atom cloud are also coincided with the origin. The included angles between the four parallel radio-frequency electrode rods and the laser beams in the horizontal direction are 45 degrees.
In this embodiment, the total power of the cooling light (before splitting into 6 beams) is 95mW, the total power of the re-pumping light (before splitting into 6 beams) is 10mW, and the diameter of a light spot formed by expanding the cooling light and the re-pumping light is 18 mm; the magnetic field gradient produced by the reverse Helmholtz coil was 10G/cm.
Figure 4 is a schematic diagram of the two-step photoionization ion generation and the energy level diagrams involved. To be provided with87Rb atom as an example, firstCooling the light to the ground state 52S1/2(F-2)87Excitation of Rb atoms to excited state 52P3/2(F' ═ 3), and is in an excited state87Rb atoms spontaneously radiate to the ground state, thereby realizing cyclic transition and cooling87Purpose of the Rb atom. The energy level transition corresponding to the cooling light is 52S1/2(F＝2)→5 2P3/2(F' ═ 3). To prevent optical pumping, a re-pumping light is introduced which exactly coincides with the cooling light, the corresponding energy level transition being 52S1/2(F＝1)→5 2P3/2(F ═ 2). In an excited state87The Rb atom ionization threshold is 2.588eV, and the corresponding laser wavelength is 479.059 nm. In this example, a continuous laser with a wavelength of 450nm was used to ionize the excited state with a power of 40mW and a spot diameter of 5mm87Rb atom to produce rubidium ion87Rb+。
An optical path system: the optical path system comprises a laser system and a laser frequency stabilization and shift system, wherein the laser system is used for providing cooling light and re-pumping light required by the magneto-optical trap and ionizing light for ionizing cold atoms to generate ions; the laser frequency stabilizing and shifting system is used for regulating and controlling the frequency and the stability of laser and meets the requirements of cooling laser and re-pumping light on the accuracy and the stability of the frequency.
The time sequence control system comprises: for specific control of experimental timing and measurements. The controlled experimental parameters comprise magneto-optical trap gradient magnetic field coil current, cooling light and re-pumping light switching and frequency shift quantity, ionizing light switching, ion trap radio frequency switching, terminal voltage switching and the like.
The signal detection system 3: the schematic structural diagram is shown in fig. 5, and the signal detection system 3 includes a microchannel plate 31 disposed at one end of the linear Paul trap 22, a photodetector 32, and two vertically disposed CCD cameras 33. The front end plate of the microchannel plate is applied with negative high voltage, and a grounding grid is added between the front end plate and the end electrode close to the microchannel plate, so that the negative high voltage is prevented from influencing trapping of ions in the linear Paul trap. Ions are impacted on the micro-channel plate under the action of an electric field, and an ion flight time spectrum is obtained through an electron multiplication effect.
Cold atom fluorescence is collected through a photoelectric detector, two CCD cameras are used for imaging the center of the mixed trap (the photos of the cold atoms and the detection light, and the photos of the cold atoms with the detection light and the background are not used), and information such as the cold atom cloud size, the cold atom number, the cold atom temperature and the like can be obtained by comparing the photos of the three conditions. A photoelectric detector or a CCD camera can also be used for observing the cold atom cloud through a window on the side surface of the scientific cavity to obtain related information.
Based on the above device, the present invention also provides a method for preparing a large amount of cold molecular ions, the flow chart of the method is shown in fig. 6, and the method comprises the following steps:
step S1, loading cold atoms
The method comprises the steps of firstly, starting a magneto-optical trap, continuously working for a first preset time, and realizing cooling and trapping of atoms.
Step S2, photoionization cooling atoms and evolution
After the magneto-optical trap continuously works for a first preset time, the ionization light and the linear Paul trap are simultaneously opened, the magneto-optical trap continuously works for a second preset time, and cold atoms are continuously ionized and photoionized at the stage to generate cold ions and imprison the cold ions; the cold ions react and collide with the atoms to generate a large number of cold molecular ions. To be provided with87For example, the chemical reaction formula for the Rb atom is:87Rb++87Rb→87Rb2 ++ h ν, wherein h ν is the photons generated with the reaction collisions.
Because the ion cloud generated by ionization coincides with the cold atom cloud center in space, the probability of reaction collision is very high, and thus a large amount of rubidium molecular ions can be obtained. Due to the limited lifetime of cold molecular ions in the ion trap, a larger number of cold molecular ions can be obtained by selecting an appropriate evolution time.
Step S3, ion derivation
After the evolution is finished, the ionizing light and the end electrode close to the microchannel plate are closed at the same time, the radio frequency electrode rod is kept to work continuously for a third preset time, and the ions are prevented from diffusing radially; keeping the end electrode far away from the microchannel plate to continuously work for a third preset time, wherein ions are subjected to the action of an electric field pointing to the microchannel plate, so that the ions are guided to the microchannel plate along the axial direction; ions are emitted to the microchannel plate, and a stronger ion flight time mass spectrum signal is obtained through an electron multiplication effect, and an ion derivation schematic diagram is shown in fig. 7. The relationship between the voltage signal of the microchannel plate and the ion number is calibrated, and then the molecular ion number is directly obtained according to the intensity of the ion flight time mass spectrum signal.
Axial direction is the direction which passes through the center of the ion trap and is along the radio frequency electrode rod of the ion trap; radial refers to a direction from the foot to another point on the plane perpendicular to the central axis of the ion trap.
The timing diagram of one embodiment of the method is shown in fig. 7, and the method is divided into three phases: firstly, loading cold atoms; photoionization cooling atoms and evolution; and thirdly, ion leading-out. To be provided with87For example, the method comprises the following steps:
the method comprises the following steps: the magneto-optical trap 50s is first turned on, cooled and imprisoned87Rb atoms, number of cold atoms stabilized at 5X 107About (cold atom number density of 1X 10)11cm-3)。
Step two: then, ionizing light and ion trap are opened simultaneously, and cold atoms are continuously photoionized to generate rubidium ions87Rb+And imprisoning; the ionization produces an ion cloud that coincides with the cold atom cloud center. In the evolution process, rubidium ions87Rb+And87reactive collision of Rb atoms87Rb++87Rb→87Rb2 ++ h v, generating large amount of rubidium molecular ion87Rb2 +. Because the number of cold atoms and ions in the experimental device is large, and the ion cloud and the cold atom cloud are overlapped in space, the probability of reaction collision is high, and a large amount of rubidium molecular ions can be generated. While taking into account rubidium molecular ion87Rb2 +The lifetime in the ion trap is limited (3-4s), so that a larger number of rubidium molecular ions can be obtained by selecting a proper evolution time87Rb2 +The evolution time chosen in this experiment was 250 ms. The charge-to-mass ratios of the rubidium ions and rubidium molecular ions are different, and therefore the trapping conditions in the ion trap are different (trap depth and trap depth for both ions)The Mathieu imprisoning parameters are all different). For the linear Paul trap of the present invention, the optimal conditions for trapping rubidium molecular ions are: the radio frequency is 425kHz, the radio frequency voltage amplitude is 140V, and the terminal voltage is 90V. At the moment, for rubidium atom ions, Mathieu trapping parameter a is 0.428, q is 0.333, and the trap depth is 5.819 eV; for rubidium atom ions, Mathieu trapping parameter a is 0.214, q is 0.166, and the trap depth is 2.909 eV.
Step three: in order to verify the generation of rubidium molecular ions in the experiment87Rb2 +Ions in the ion trap are measured using a microchannel plate. The front end plate of the microchannel plate is added with a high voltage of minus 1700V, and a grounding grid is added between the front end plate and the end electrode close to the microchannel plate, so that the negative high voltage is prevented from influencing trapping of ions in the ion trap. After the evolution is completed, the end electrodes close to the microchannel plate are closed, as shown in fig. seven. The radio frequency electrode rod is kept open, and ions cannot diffuse radially; keeping the end electrodes away from the microchannel plate open, the ions are subjected to an electric field directed at the microchannel plate and are thus directed towards the microchannel plate, and the ion extraction diagram is shown in fig. 8. Ions are ejected onto the microchannel plate, and a stronger ion flight time mass spectrum signal can be obtained through an electron multiplication effect, as shown in fig. 9.
In fig. 9, the horizontal axis of the ion time-of-flight mass spectrum is time, and the vertical axis is the ion signal intensity reaching the microchannel plate at a certain time. The ion time-of-flight mass spectrum obtained in this example has two signal peaks, the peak position t of the first signal peakpeak10.0267ms, peak position t of the second signal peakpeak2In the order of 0.0383ms, the time of the start,
this is because rubidium ions87Rb+And rubidium molecular ion87Rb2 +The energy obtained in the electric field is the same; but rubidium molecular ion87Rb2 +Of mass rubidium ion87Rb+Twice of that of rubidium molecule ion, therefore87Rb2 +At a rate of rubidium ions87Rb+Is/are as follows
Times, the flight time to the microchannel plate is rubidium ions87Rb+Is/are as follows
And (4) doubling. Thereby judging that the first signal peak of the ion flight time mass spectrum is rubidium ions87Rb+The second signal peak is generated by rubidium molecular ion87Rb2 +Produced, in this example, large quantities of rubidium molecular ions were produced87Rb2 +. After calibrating the corresponding relationship between the voltage signal of the ion time-of-flight mass spectrum and the number of ions, the number of cold molecular ions obtained in this embodiment is about 25000.
The environment (density, temperature, etc. of particles) in the mixed trap is similar to that of the interplanetary medium, and chemical reactions occur in the process of preparing molecular ions, so that the understanding of the chemical reactions of ion-atoms in the space can be promoted; after the molecular ions are prepared, the method can be used for researching the energy level and the kinetic characteristics of the molecular ions; the molecular ions can also be applied to the fields of quantum precision measurement, quantum information, quantum computation and the like.
The invention is based on a hybrid trap platform, cold atoms are ionized by continuous laser to generate ions, and cold molecular ions generated by the collision of the ions and the cold atoms are trapped by an ion trap, so that a large amount of cold molecular ions are obtained. The difference of the invention from the prior scheme is that:
(1) compared with the molecular ions generated by the collision of the ions of the coulombic crystal and the cold atoms, the device provided by the invention is simpler, and the ions do not need to be cooled by laser to form the coulombic crystal; meanwhile, the existing scheme adopts a resonance fluorescence mass spectrometry method, and the charged particles can be discriminated by combining theoretical calculation, but the flight time mass spectrometry method adopted by the invention is the most accurate method for discriminating the charged particle ions.
(2) The device and the method provided by the invention have lower requirements on the vacuum degree and the density of cold atoms, the device is simple, and a large amount of molecular ions can be generated.
(3) Compared with the scheme of generating molecular ions by cold molecules through two-photon ionization, the invention uses a common semiconductor laser to perform continuous photoionization and can generate a large amount of molecular ions; the scheme of two-photon ionization of cold molecules to generate molecular ions needs a high-performance dye laser, and the nonlinear process is low in efficiency and the number of generated molecular ions is small.
The above-described embodiments are merely illustrative of the present invention, which may be embodied in other specific forms or in other specific forms without departing from the spirit or essential characteristics thereof. The described embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. The scope of the invention should be indicated by the appended claims, and any changes that are equivalent to the intent and scope of the claims should be construed to be included therein.
Claims (10)
1. An apparatus for preparing a large number of cold molecular ions, comprising an ultra-high vacuum system (1), a hybrid trap (2), an optical path system, a timing control system and a signal detection system (3), wherein:
the ultrahigh vacuum system (1) is used for providing an ultrahigh vacuum environment for trapping atoms and ions, and comprises a scientific cavity (11), a three-stage sputtering ion pump (12), a mechanical pump, a molecular pump and an atom source (13), wherein the three-stage sputtering ion pump (12) is communicated with a cavity of the scientific cavity (11) through a main pipeline; the main pipeline is provided with a plurality of branch pipelines, the mechanical pump and the molecular pump are communicated with the main pipeline through a molecular pump valve (15) arranged at the tail end of the second branch pipeline and used for pre-pumping the vacuum system to a set vacuum degree, and then the vacuum degree of the system is maintained through the three-stage sputtering ion pump (12); the atom source (13) is arranged at the tail end of the first branch pipeline, is communicated with the main pipeline through an atom source valve (14) arranged on the first branch pipeline and provides neutral atoms for preparing cold molecular ions;
the scientific cavity (11) is of a polyhedral flat metal cavity structure, the side face of the scientific cavity is provided with at least six vacuum observation windows, and the upper end face and the lower end face of the scientific cavity are respectively provided with one vacuum observation window;
the hybrid trap (2) is arranged in the center of the scientific cavity and comprises a magneto-optical trap (21) for cooling and confining neutral atoms and a linear Paul trap (22) for confining ions, wherein the magneto-optical trap center coincides with the linear Paul trap center, and the hybrid trap comprises:
the magneto-optical trap (21) is composed of an optical field formed by three pairs of laser beams (211) and a gradient magnetic field formed by a pair of reverse Helmholtz coils (212), the laser beams (211) are combined beams formed by cooling light and re-pumping light, and the three pairs of laser beams (211) are respectively emitted along three coordinate axis directions of a three-dimensional Cartesian coordinate system and respectively converged at the center of the scientific cavity (11) through four vacuum observation windows and an upper vacuum observation window and a lower vacuum observation window on the side surface of the scientific cavity (11); a pair of reverse Helmholtz coils (212) is symmetrically arranged above and below the scientific cavity (11) respectively and provides gradient magnetic fields required by the magneto-optical trap (21); the magneto-optical trap (21) cools and traps neutral atoms to form a cold atom cloud in the center of the magneto-optical trap (21);
ionizing light is emitted to the center of the scientific cavity (11) through a vacuum observation window on the side surface of the scientific cavity (11), acts on the cold atom cloud, and generates ions through two-step photoionization together with the cooling light;
the linear Paul trap (22) comprises four mutually parallel radio frequency electrode rods and two end electrodes, two pairs of diagonals of the radio frequency electrode rods are loaded with radio frequency signals in opposite phases, the two end electrodes are loaded with the same positive voltage, and the ions are trapped in the center of the linear Paul trap (22) to form an ion cloud;
the ions react with cold atoms and collide to generate cold molecular ions, and the cold molecular ions are trapped in the center by the linear Paul trap (22), so that a large amount of cold molecular ions are obtained;
the optical path system comprises a laser system and a laser frequency stabilization and shift system, wherein the laser system is used for providing cooling light and re-pumping light required by the magneto-optical trap (21) and ionizing light for ionizing cold atoms to generate ions; the laser frequency stabilizing and shifting system is used for regulating and controlling the frequency and stability of laser;
the time sequence control system is used for controlling experiment time sequence and experiment parameters, and the controlled experiment parameters comprise magneto-optical trap gradient magnetic field coil current, cooling light and re-pumping light switch and frequency shift quantity, ionizing light switch, ion trap radio frequency switch and terminal voltage switch.
2. The apparatus for preparing a mass of cold molecular ions according to claim 1, wherein the ultra-high vacuum system (1) further comprises an ionization gauge (16) for measuring a vacuum degree of the system, the ionization gauge (16) being installed in the third branch pipe, the vacuum degree of the ultra-high vacuum system being better than 10-7Pa。
3. The apparatus for preparing a large number of cold molecular ions according to claim 2, wherein the scientific cavity (11) is made of 316L non-magnetic stainless steel, twelve vacuum observation windows with the type of CF35 are arranged on the side surface, and vacuum observation windows with the type of CF200 are respectively arranged on the upper end surface and the lower end surface, and the vacuum observation windows and the scientific cavity body are sealed by a full knife edge, so that the sealing performance of the scientific cavity (11) is ensured.
4. The apparatus for preparing a plurality of cold molecular ions according to any one of claims 1 to 3, wherein the RF electrode rods have a diameter of 3mm, a length of 10cm and a rod spacing of 15 mm; the end electrodes are circular rings, the inner diameter of each end electrode is 10mm, 1-degree notches are axially formed to break the circular rings so as to avoid generation of eddy current electric fields, and the distance between the axial centers of the two end electrodes is 10 cm.
5. The apparatus according to claim 1, wherein the RF electrode rod is at an angle of 45 ° to the horizontal laser beam.
6. Device for preparing a multitude of cold molecular ions according to claim 1, characterized in that said laser beam is circularly polarized, said laser beam being of equal intensity for each pair of opposite rays and having a polarization direction σ+And σ-。
7. The apparatus according to claim 6, wherein the total power of the cooling light is 95mW, the total power of the re-pumping light is 10mW, and the diameter of the spot of the cooling light after being expanded with the re-pumping light is 18 mm; the magnetic field gradient produced by the reverse Helmholtz coil (212) is 10G/cm.
8. The apparatus for preparing a plurality of cold molecular ions according to claim 1, wherein the signal detection system (3) comprises a microchannel plate (31) disposed at one end of the linear Paul trap (22), a photodetector (32) and two vertically disposed CCD cameras (33), and an ion time-of-flight spectrum is obtained through the microchannel plate (31); and collecting cold atom fluorescence through the photoelectric detector (32) and the CCD camera (33) to obtain related parameters of the cold atom cloud.
9. Method for producing a multitude of cold molecular ions according to any of claims 1-8, comprising the steps of:
step S1, starting the magneto-optical trap, continuously working for a first preset time, and cooling and trapping atoms in the center of the magneto-optical trap under the action of the optical field and the gradient magnetic field;
step S2, after the magneto-optical trap works for a first preset time, the ionization light and the linear Paul trap are simultaneously opened, the magneto-optical trap works for a second preset time, the ionization light continuously ionizes cold atoms to generate cold ions, and the cold ions are imprisoned in the center of the linear Paul trap; the cold ions react and collide with atoms, so that a large amount of cold molecular ions are generated;
step S3, after the ionized light and the linear Paul trap continuously work for a second preset time, the ionized light and the end electrode close to the microchannel plate are closed at the same time, the radio frequency electrode rod is kept continuously work for a third preset time, and ions are prevented from radially diffusing; keeping the end electrode far away from the microchannel plate to continuously work for a third preset time, and guiding the ions to the microchannel plate along the axial direction under the action of an electric field pointing to the microchannel plate; and (3) the ions are driven to the microchannel plate, and an ion flight time mass spectrum signal is obtained through an electron multiplication effect.
10. The method of claim 9, wherein the number of cold molecular ions is directly obtained by calibrating the voltage signal of the microchannel plate to the number of ions and then obtaining the number of cold molecular ions from the intensity of the ion time-of-flight mass spectrum signal.
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