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Electron collision studies of cesium using a magneto-optical trap.

J. A. MacAskill

Published Oct 1, 2003• 1 citation
OpenAlex

Abstract

Experiments involving a magneto-optical trap (MOT) as the target in electron impact cross section studies were carried out. The use of a MOT allows the kinematic properties of the target atoms, and their electronic states, to be controlled via operating parameters of the MOT's lasers. A MOT is capable of saturating the excited state of a gaseous target of atoms and provides a unique opportunity to measure exotic collision processes. Measurements use a trap-loss technique to relate the loss rate of atoms from the trap due to electron interactions to a collision cross section. Total scattering cross section measurements are given for the 62S 1/2 ground state and the 62P3/2 excited state of cesium. The excited state measurement is the first measurement of its kind. Total ionization cross sections are also presented for the both the 6 2S1/2 ground state and 62P3/2 excited state. The ionization cross sections are measured by adjusting the radiation force of the MOT lasers to damp the elastic scattering component of the total cross section. The ionization cross section for the excited state is the first experimental determination of its kind. Full details of the experimental apparatus and the analysis procedure are presented. The results of this experiment are combined with other experimental data and compared to the most recent calculations.Dept. of Physics. Paper copy at Leddy Library: Theses & Major Papers - Basement, West Bldg. / Call Number: Thesis2003 .M322. Source: Dissertation Abstracts International, Volume: 64-10, Section: B, page: 4996. Adviser: J. W. McConkey. Thesis (Ph.D.)--University of Windsor (Canada), 2003.

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Measurements of the multiple ionization cross section ratios of Cesium were performed with ion time-of-flight (TOF) spectroscopy with a magneto-optical trap (MOT) apparatus, updating the previous measurement which dates back almost a century. Results are presented for collisions at energies of 50 eV to 120 eV. With a MOT, experiments can be performed with trapped, cold atomic targets which allow for unparalleled accuracy and experiments with signicant excited state target fractions above those achievable in most atomic beam experiments. A basic overview of optical cooling trapping, electron collision and atomic phenomena are presented. Experimental studies of electrons with Argon and Cesium targets were performed, measuring the multiple ionization ratios with ion TOF spectroscopy. The experimental apparatus and analysis methods are described in detail. Results are compared with previous measurements of multiple ionization ratios for both targets. Agreement within experimental error is found with the results of Tate and Smith across the energy range.
Optically trapped atoms are in many ways ideal replacements for atomic beams for scattering experiments. In a typical such experiment, a particle beam ejects atoms from the trap at a rate {Gamma} = {sigma}{phi}, where {phi} is the particle flux density and {sigma} the scattering cross section. {Gamma} is easily measured from trap decay transients. Since the trapped atoms have neV kinetic energies, the method is sensitive to extremely small momentum transfers. Since the beam flux is measured, the absolute target density is not required, removing an important source of error for most atomic beam experiments. At Wisconsin the authors have integrated an electron beam with a magneto-optical trap and performed electron-scattering experiments on Rb atoms in the range of 10-1000 eV. In the first experiment they measured total cross sections for scattering from the 5s state by lowering the capture velocity of the trap below the recoil velocity of the scattered atoms. In the second experiment they isolated the total ionization cross section by operating the trap such that virtually all elastically and inelastically scattered atoms were captured by the trap. These experimental results will test new theoretical methods for calculating total and ionization cross sections. Straightforward extensions ofmore » these methods should allow similar measurements using excited-state atoms (5p, 5d) as well. The techniques should also be readily adaptable to the important case of the metastable noble gases.« less
The key features of the cross sections for electron-impact excitation out of the ground levels of the rare-gas atoms are summarized and application of these cross section data is shown. This is followed by discussions of recent experiments on determination of electron-impact cross sections for excitation out of the metastable levels of He and Ar. The metastiable atom targets are produced by two different methods, i.e., hollow-cathode discharges and charge-exchange collisions of rare-gas ions with Cs atoms. The former method is simple but is limited to low electron energies because of the very small metastable concentration, whereas the latter method yields a fast target beam with predominantly metastable atoms allowing measurements up to the keV range. The measured cross sections show features very different from those of excitation out of the ground levels. Another new development in electron-atom collision is the use of atom traps for measuring ionization cross sections. The experiment is conducted by passing an electron beam pulse through magneto-optically trapped Rb atoms while the trap is momentarily turned off. Turning the trap back on immediately after the electron beam pulse recaptures the unionized atoms allowing only the ions to escape. The ionization cross section is determined bymore » measuring the electron beam current density at the trapped atom region and the fractional loss of trapped atoms due to the electron beam. This method avoids measurements of the absolute number of the target atoms and the overlap between the atomic and electron beams and therefore eliminates the major sources of uncertainty associated with the crossed-beam method for measuring ionization cross section.« less
An atom trapping technique for determining absolute, total ionization cross sections (TICS) out of an excited atom is presented. The unique feature of our method is in utilizing Doppler cooling of neutral atoms to determine ionization cross sections. This fluorescence-monitoring experiment, which is a variant of the ``trap loss'' technique, has enabled us to obtain the experimental electron impact ionization cross sections out of the Cs 62P3∕26\phantom{\rule{0.2em}{0ex}}^{2}{P}_{3∕2}62P3∕2​ state between 7eV7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}7eV and 400eV400\phantom{\rule{0.3em}{0ex}}\mathrm{eV}400eV. CCC, RMPS, and Born theoretical results are also presented for both the ground and excited states of cesium and rubidium. In the low energy region (<11eV)(<11\phantom{\rule{0.3em}{0ex}}\mathrm{eV})(<11eV) where best agreement between these excited state measurements and theory might be expected, a discrepancy of approximately a factor of five is observed. Above this energy there are significant contributions to the TICS from both autoionization and multiple ionization.
This thesis introduces an empirical method for determining and controlling the excited-state fraction of atoms in a magneto-optical trap (MOT), which is essential for the use of cold atoms as a sensor when they are held in a MOT since their interactions with other particles and fields are quantum state dependent. A four-level theoretical atomic model was used to describe the transitions of the atoms in a MOT, and the fluorescence emitted from a fixed number of atoms under different laser conditions were measured to determine the saturation parameters empirically. Two saturation parameters P_sat= 1.15 (0.06) mW and P_r,sat= 2.05 (0.59) mW were successfully extracted from the model, and the excited-state fraction in the four-level model was accurately calculated as a function of the MOT trap parameters, which ranges from 0.045 to 0.415 for the experimental settings currently available. We also observed minor deviations from the four-level model for the photon scattering rate, and a hypothesis of atom pinning under high powers was proposed to explain the problem. We plan to use this simple excited-state fraction determination method to distinguish the ground and excited state collision cross section of Rubidium atoms with species in residual gas of the vacuum. This is the first step to establishing atom loss rates from a MOT as an atomic primary pressure standard.