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Physical Science International Journal

Potential of the Penning Ionization Gauge (PIG) Ion Source in Attaining High Energy Rydberg States

Pennan Chinnasamy

Published Jan 10, 2015• 0 citations
Publisher

Abstract

Aims: The Morgan’s Lab, at Wesleyan University – Connecticut, experimentally probed the dynamics of an atom/molecule using Semi-Classical approach. The experimental setup at Morgan’s Lab is designed to generate a neutral atomic or molecular beam, which is then excited with a finely tuned laser to Rydberg states; the highest quantized energy states the electron can be in before ionization. Study Design: In this current study, the potential of the Penning Ionization Gauge (PIG) as an ion source was evaluated for the Hydrogen atom. Place and Duration of Study: Physics Department, Wesleyan University, Middletown, Connecticut, USA, between June 2007 and August 2009. Methodology: The experimental setup is designed to generate a neutral atomic or molecular beam, which is then excited with a finely tuned laser to Rydberg states; the highest quantized energy states the electron can be in before ionization. The setup consists of an ion beam system consisting of a fast- metastable machine, vacuum pumping systems, an oven, a laser system consisting of a master YAG laser and a slave tunable dye laser, a second harmonic generator for doubling the frequency of the laser beam, Stark plate assembly, an ion detection system and a computer based data acquisition system with LABVIEW software. Results: The PIG found to populate the Hydrogen Rydberg states at 300% times more than the conventional ion source (8 µ Amp Vs. 2 µ Amp). Such high populations of ions lead to a high population of Rydberg states which in turn aided the overall experiment. Conclusion: A better understanding of the dynamics of an electron can aid experiments in condensed matter physics and molecular physics. Obtaining high population of Rydberg states can aid in increasing electron dynamic experiments by increasing the resolution of the scaled-energy absorption spectra. Penning Ionization Gauge (PIG) showed to have a better potential in producing high energy Rydberg states for the hydrogen gas.

DOI
10.9734/psij/2015/20646

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A thermal-energy beam of helium atoms in the metastable state is used to coherently excite ions of Sr, Ca, and Ba in a Penning collision. The anisotropic excited state distributions appears as Polarized line emission from the excited ions. The degree of ion alignment may be used to determine the populations of the quasi-molecular states of the ion-atom pair. The coherent emission is also utilized to obtain Hanle signals from which the radiative decay rate of the excited ion is determined. This is the first case in which coherence is produced in a collision process involving Penning ionization. The alignment of the Penning ions is an important new parameter in the description of these reactions. In an effort to extend such observations to molecular systems a nitrogen molecular metastable beam source was demonstrated. The use of metal vapor targets and the excitation rate of the excited states of Cd and Zn in energy transfer collisions with the N2/sub (m)/ enable us to determine the vibrational distribution of the A /sup 3/SIGMA state of N/sub 2/ in the beam. Energy transfer cross sections and some Penning cross sections were measured in the beam system.

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Rydberg atoms are increasingly being used as a means of introducing long-range, tunable interactions into atomic systems through their strong dipole-dipole interactions. When creating such interactions, however, there are a variety of potential effects that can occur, such as state (n and/or L) changing or Penning ionization, which can complicate the dynamics of the system. In the following work, such interactions are investigated for Rydberg-Rydberg collisions using very high-n, n ~300, strontium Rydberg atoms in an atomic beam configuration. By exploiting Rydberg blockade and the inherent distribution of atomic velocities present in an atomic beam, it is possible to create a 1D array of moving Rydberg atoms which will naturally overtake each other over time. Measurements of both Penning ionization and state changing for such collisions are made and compared with classical trajectory Monte-Carlo (CTMC) simulations, with both indicating large collisional cross sections on the order of 1 x 10-6 cm2 for Penning ionization and nearly 8 x 10-5 cm2 for state changing interactions. In a second series of experiments, previous work involving doubly-excited Rydberg states is expanded upon by investigating autoionization with high-n, L > 3, Rydberg states. Past research involving autoionization has shown that the effect is strongly L-dependent, with high-L states having significantly smaller autoionizing rates. To create well-defined, L > 3 states, the use of microwave fields to drive various high-n, n ~ 150, n1F3 → (n+1)1G4, (n+2)1H5 transitions is investigated, with spectroscopic measurements of these transitions providing quantum defect values for n1G4 and n1H5 states. These measurements provide a basis for future studies into the L-dependence of high-n, n ~ 300, autoionizing Rydberg atoms, with the eventual goal of creating a long-lived, doubly-excited “planetary” Rydberg state.

•Apr 1, 2021•0 citations
Aims: The Morgan’s Lab, at Wesleyan University – Connecticut, experimentally probed the dynamics of an atom/molecule using Semi-Classical approach. The experimental setup at Morgan’s Lab is designed to generate a neutral atomic or molecular beam, which is then excited with a finely tuned laser to Rydberg states; the highest quantized energy states the electron can be in before ionization. Study Design: In this current study, the potential of the Penning Ionization Gauge (PIG) as an ion source was evaluated for the Hydrogen atom. Place and Duration of Study: Physics Department, Wesleyan University, Middletown, Connecticut, USA, between June 2007 and August 2009. Methodology: The experimental setup is designed to generate a neutral atomic or molecular beam, which is then excited with a finely tuned laser to Rydberg states; the highest quantized energy states the electron can be in before ionization. The setup consists of an ion beam system consisting of a fast- metastable machine, vacuum pumping systems, an oven, a laser system consisting of a master YAG laser and a slave tunable dye laser, a second harmonic generator for doubling the frequency of the laser beam, Stark plate assembly, an ion detection system and a computer based data acquisition system with LABVIEW software. Results: The PIG found to populate the Hydrogen Rydberg states at 300% times more than the conventional ion source (8 µ Amp Vs. 2 µ Amp). Such high populations of ions lead to a high population of Rydberg states which in turn aided the overall experiment. Conclusion: A better understanding of the dynamics of an electron can aid experiments in condensed matter physics and molecular physics. Obtaining high population of Rydberg states can aid in increasing electron dynamic experiments by increasing the resolution of the scaled-energy absorption spectra. Penning Ionization Gauge (PIG) showed to have a better potential in producing high energy Rydberg states for the hydrogen gas.
We study the excitation and ionization of cesium Rydberg states in an electric field ($\ensuremath{\approx}\phantom{\rule{0.16em}{0ex}}2200\phantom{\rule{4pt}{0ex}}\mathrm{V}/\mathrm{cm}$) near the classical field-ionization threshold ($\ensuremath{\approx}\ensuremath{-}280\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}1}$ binding energy) by three-dimensional (3D) ion-electron coincidence spectroscopy. Cesium atoms are produced by an effusive oven and excited with lasers to Stark-shifted Rydberg states or directly ionized. Using a double time-of-flight setup we record 3D (X,YX, YX,Y, time of flight) coincidence imaging of electrons and ions. Above-threshold photoionization creates broad images with poor electron-ion spatial correlation. Fast ionizing states produce very good correlations and the images reveal the electric-field map of the ionization region. Slow ionizing states show that the relatively high atomic velocity is detrimental to the correlations. Experimental data are accurately reproduced by detailed Monte Carlo excitation and ionization simulations based on Stark maps obtained with local-frame transformation (LFT) theory. Agreement between our spectroscopic experiment and LFT theory is very good, with better that hundreds of megahertz accuracy. But, on rare particular states, several gigahertz discrepancy is found. This study can be used to select appropriate states for the creation of ion and electron beams with high brightness, good correlation, and low energy dispersion.
Research in optically induced transitions between dierent atomic levels has a long history. For transitions between states driven by a coherent optical eld, the theoretical eciency could be ideally high as 100% but there could be many factors preventing this. In the three state helium atom excitation process, i.e. 23S! 33P! nL , the stimulated emission from intermediate state makes it hard to achieve ecient population transfer to the nal state through an intuitive excitation order. One technique to achieve a higher eciency is Stimulated Raman Adiabatic Passage (STIRAP) which is being studied and under research in our lab. Unlike traditional three level excitation processes, STIRAP actually uses a counter intuitive pulsed laser beams timing arrangement. The excitation objects are metastable helium atoms traveling in a vacuum system with a longitudinal velocity of 1070 m/s. We are using a 389 nm UV laser to connect the 23S and the 33P state and a frequency tunable 790 nm iii IR laser to connect the 33P state and the dierent Rydberg states. A third 1083 nm wavelength laser beam drives the 23S ! 23P transition to transversely separate the residual metastable atoms and the Rydberg atoms for eciency measurements. The data is taken by a stainless steel detector in the vacuum system. As the Rydberg atoms will get ionized by blackbody radiation under room temperature, we can utilize this for their detection. An ion detector sitting on the eld plate is capable to collect the ion signals of the Rydberg atoms for detection. So far the whole system has not been ready for data collection and measurement, so here we are using data and results from previous theses for discussions. The highest transition frequency that has ever been achieved in our lab is around 70% after corrections. | 144 pages
The emission of fast ions in Penning ionization of cold Rydberg atoms is experimentally observed, and the underlying ion acceleration mechanisms are identified. In the experiment, the Rydberg atoms in nS/nP/nD (n = 20-50) states were prepared by laser excitation of cold atoms in a magneto-optical trap, and the Penning ions were measured using velocity-map imaging technology. The influence of Penning process on the lifetime of Rydberg gas and the ion velocity distribution was investigated. The ion originating from conventional Penning ionization gains a velocity significantly higher than the thermal velocity of cold atom. It is accelerated by Coulomb repulsion from the nearby de-excited Rydberg atom. In addition, a small fraction of ions with velocities exceeding 40 m/s has been observed. These ions are interpreted as arising from double-ionization process of Rydberg atom-pair. The strong Coulomb repulsion between two ions within a close distance gives them considerable acceleration, resulting in a large velocity of the ions.
Driving atoms from an initial to a final state of the same parity via an intermediate state of opposite parity is most efficiently done using STIRAP, because it does not populate the intermediate state. For optical transitions this requires appropriate pulses of light in the counter-intuitive order - first coupling the intermediate and final states. We populate Rydberg states of helium (n = 12 ~ 30) in a beam of average velocity 1070 m/s by having the atoms cross two laser beams in a tunable dc electric field. The &ldquo red &rdquo light near &lambda = 790 ~ 830 nm connects the 33P states to the Rydberg states and the &ldquo blue &rdquo beam of &lambda = 389 nm connects the metastable 23S state atoms emitted by our source to the 33P states. By varying the relative position of these beams we can vary both the order and the overlap encountered by the atoms. We vary either the dc electric field and fix the &ldquo red &rdquo laser frequency or vary the &ldquo red &rdquo laser frequency and fix the dc electric field to sweep across Stark states of the Rydberg manifolds. Several mm downstream of the interaction region we apply the very strong bichromatic force on the 23S &rarr 23P transition at &lambda = 1083 nm. It deflects the remaining 23S atoms out of the beam and the ratio of this signal measured with STIRAP beam on and off provides an absolute measure of the fraction of the atoms remaining in the 23S state. Simple three-level models of STIRAP all predict 100% excitation probability, but our raw measurements are typically around half of this, and vary with both n and l of the Rydberg states selected for excitation by the laser frequency and electric field tuning on our Stark maps. For states with high enough Rabi frequency, after correction for the decay back to the metastable state before the deflection, the highest efficiencies are around 70%. An ion detector readily detects the presence of Rydberg atoms. We believe that the observed signals are produced by black-body ionization at a very low rate, but sufficient to ionize about 0.5 ~ 1.0 % of the atoms in a region viewed by our detector. Many measurements provide support for this hypothesis.
A thermal-energy beam of helium atoms in the metastable state is used to coherently excite ions of Sr, Ca, and Ba in a Penning collision. The anisotropic excited state distributions appears as Polarized line emission from the excited ions. The degree of ion alignment may be used to determine the populations of the quasi-molecular states of the ion-atom pair. The coherent emission is also utilized to obtain Hanle signals from which the radiative decay rate of the excited ion is determined. This is the first case in which coherence is produced in a collision process involving Penning ionization. The alignment of the Penning ions is an important new parameter in the description of these reactions. In an effort to extend such observations to molecular systems a nitrogen molecular metastable beam source was demonstrated. The use of metal vapor targets and the excitation rate of the excited states of Cd and Zn in energy transfer collisions with the N2/sub (m)/ enable us to determine the vibrational distribution of the A /sup 3/SIGMA state of N/sub 2/ in the beam. Energy transfer cross sections and some Penning cross sections were measured in the beam system.
Rydberg atoms are increasingly being used as a means of introducing long-range, tunable interactions into atomic systems through their strong dipole-dipole interactions. When creating such interactions, however, there are a variety of potential effects that can occur, such as state (n and/or L) changing or Penning ionization, which can complicate the dynamics of the system. In the following work, such interactions are investigated for Rydberg-Rydberg collisions using very high-n, n ~300, strontium Rydberg atoms in an atomic beam configuration. By exploiting Rydberg blockade and the inherent distribution of atomic velocities present in an atomic beam, it is possible to create a 1D array of moving Rydberg atoms which will naturally overtake each other over time. Measurements of both Penning ionization and state changing for such collisions are made and compared with classical trajectory Monte-Carlo (CTMC) simulations, with both indicating large collisional cross sections on the order of 1 x 10-6 cm2 for Penning ionization and nearly 8 x 10-5 cm2 for state changing interactions. In a second series of experiments, previous work involving doubly-excited Rydberg states is expanded upon by investigating autoionization with high-n, L > 3, Rydberg states. Past research involving autoionization has shown that the effect is strongly L-dependent, with high-L states having significantly smaller autoionizing rates. To create well-defined, L > 3 states, the use of microwave fields to drive various high-n, n ~ 150, n1F3 → (n+1)1G4, (n+2)1H5 transitions is investigated, with spectroscopic measurements of these transitions providing quantum defect values for n1G4 and n1H5 states. These measurements provide a basis for future studies into the L-dependence of high-n, n ~ 300, autoionizing Rydberg atoms, with the eventual goal of creating a long-lived, doubly-excited “planetary” Rydberg state.