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Review of Scientific Instruments

Velocity analysis of on-axis cesium atoms by the time-of-flight method

Thomas Heindorff, B. Fischer

Published Mar 1, 1984• 7 citations
Publisher

Abstract

Time-of-flight distributions of cesium atoms effusing from the near-ideal orifice of an oven have been measured for Knudsen numbers ranging from 1 to 370. The apparatus consisting of a single-disk chopper and a single-ion counting detector is described. The experimental results are compared with theoretical data computed by using the free-molecule flow model. Experiment and theory are in good agreement. An error analysis is carried out. Deviations from the results of other studies are discussed. The influence of the scattering of cesium atoms by residual gas particles is demonstrated by a distorted distribution curve.

DOI
10.1063/1.1137741

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arXiv:quant-ph/0009111•Sep 27, 2000•5 citations
Time-of-flight distributions of cesium atoms effusing from the near-ideal orifice of an oven have been measured for Knudsen numbers ranging from 1 to 370. The apparatus consisting of a single-disk chopper and a single-ion counting detector is described. The experimental results are compared with theoretical data computed by using the free-molecule flow model. Experiment and theory are in good agreement. An error analysis is carried out. Deviations from the results of other studies are discussed. The influence of the scattering of cesium atoms by residual gas particles is demonstrated by a distorted distribution curve.
A high-resolution, high-intensity, spiral velocity selector has been designed for the study of the velocity distributions of the components of atomic and molecular beams. It has been found possible to design oven slits which closely approximate the ideal aperture of kinetic theory. An analysis has been made of the velocity distributions in beams of potassium and thallium over a range of velocity from 0.3 to 2.5 times the most probable velocity in the oven. The agreement between the observed distribution and that deduced, on the basis of the assumptions that the distribution in the oven is Maxwellian and that the aperture is ideal, is very good.
The free fall of atoms is observed in long molecular beams of potassium and cesium atoms. The measurement of the intensity distribution in a beam deflected by gravity represents the velocity distribution of the beam atoms and permits an accurate determination of this distribution. The results show that the measured values agree in general with those calculated for a modified Maxwellian velocity distribution in the beam. At larger deflections, i.e., for slow atoms, a deficiency of intensity was observed, which increased both with increasing deflection and with increasing pressure in the oven where the beam originates. This deficiency is explained by collisions in the immediate vicinity of the oven slit.
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.
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.
We study the excitation and ionization of cesium Rydberg states in an electric field (∼ 2200 V/cm) near the classical field ionization threshold (∼ −280 cm −1 binding energy at this field) by 3-D ionelectron coincidence spectroscopy. Cesium atoms are produced by an effusive oven and excited with lasers to Stark-shifted Rydberg states or directly ionised. Using a double Time of Flight setup we record 3-D (X, Y, Time of Flight) coincidence imaging of cesium produced electrons and ions. Above-threshold photo-ionisation creates broad images with poor electron/ion-coordinates correlation. Depending on the coupling of particular Rydberg states to the continuum, various images and correlations are observed. Fast ionising states show very good correlations and reveal the electric field lines, creating an efficient electric field mapping of the ionisation region. Slow ionising 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. Spectroscopic LFT agreement is usually very good with better that hundreds of MHz accuracy but on particular states several GHz discrepancy is found. The 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, as well as to provide a detailed analysis of LFT-theory shortcomings.