Alignment of Atoms and Ions with the Beam-Foil Light Source

Abstract

We present conclusive evidence that the passage of fast Ne + ions through a thin foil results in a high degree of alignment of the transmitted excited atoms and ions. We use these observations to measure the mean lives and g values of electronic levels in atoms and ions. Our methods also lend themselves to the measurement of the speeds of fast atoms and ions.One feature of the beam-foil light source is that the time (or place) of excitation is sharply defined. This permits the observation of quantum beats; such beats are a sensitive indicator of the presence of alignment of the excited electronic levels. We used the arrangement of Fig. 1 in order to look for quantum beats. If there is alignment, the detected intensity satisfies the relation 1where t is the time of observation after excitation, a is a constant, y is the gyromagnetic ratio, H is the magnetic field, and r is the damping constant. Equation (1) holds for our geometry when the partial polarization of the light emitted from the beam of fast particles is parallel to the beam axis.In our experiments, 20 Ne + ions with a nominal energy of 425 keV were sent through a carbon LENS LINEAR POLARIZER -MONOCHROMATOR -PHOTOMULTIPLIER TUBE -TO DATA STORAGE ELECTRONICS FIG. 1. Arrangement for observing quantum beats. The spectrometer viewed a 100-/im wide vertical slice of the beam (3 mm diam).foil 5jitg/cm 2 thick. We observed 6402-A photons (2/> 9 -ls 5 ) from Ne I, and 4220-A (4/ 4 Z>°-3d 4 D) fron Ne II. Reasons for choosing neon are that it is a multielectron atom, there is no hyperfine effect, and the fine-structure levels are so widely separated that the interference frequency is too high to be seen.Equation (1) predicts that, for a fixed value of t, or, equivalently, for a fixed point of observation d downstream from the foil, 1(H) should vary sinusoidally with H. Using a linear ramp to vary H, we obtained the result shown in Figs. 2(a) and 2(b) for the Ne I transitions. In our similar work with the Ne II transition, the signal-to-noise ratio was less favorable than in the Ne I case, but the signals, which appear in Fig. 2(c), were still definite.From Fig. 2 we deduce that the magnetic substates which are predominantly populated in the beam-foil interaction have m / = 0, with the axis of quantization parallel to the beam axis. Such alignment has been discussed previously. 2 The effect of applying a magnetic field perpendicular to the beam axis is to create a coherent mixture of +mj and -ntj states. This causes the oscillations displayed in Fig. 2.From the period of oscillation we deduce that the particle speed v is given bywhere H is the mean value of the change in the magnetic field from peak to adjacent peak. Thus, when gj is known, the particle speed can be determined even when the particles are electrically neutral. In the present work, we use the known value 3 of £>= 1.329 for the 2p 9 level of Ne I and the mean of 24 periods to find v = (2.02±0.06) X10 8 cm/sec. This agrees with the speed of the accelerated Ne + ions as determined from the 222

Origins and Reduction of Line Broadening in the Spectra of Fast Ion Beams

“…The magnitude of the line broadening may be estimated by substituting the velocity spread (iXv) for the velocity (v) in equation (1) . These effects may be expected to be largest when energy losses during excitation are a significant fraction of the ion beam energy, and with the use of spectrometers having relatively large acceptance angles (a) .…”

“…Equation (1) was derived assuming that the beam is viewed in directions nearly perpendicular to beam particle velocities . If this restriction is relaxed, and the beam is viewed paraxially (or`end-on '), linewidths may be reduced ; however, a loss in spatial resolution results and large systematic Doppler shifts are produced [6,9] .…”

“…It has been seen in r .f. spectroscopy of foil-excited beams [41], in studies of alignment of such beams [1], and in measurements of widths of laser lines narrowed by saturated absorption [42] .…”

“…Ion beams excited by collisions with foils and/or gases have several advantages over conventional spectroscopic sources : they often can be made isotopically pure ; a wide range of excitation energy is available ; selected high charge states can be obtained ; alignment of the magnetic substates of electronic levels of the beam particles (and target particles if a gas target is used) is often obtained [1] and one can measure mean lives of electronic levels of the beam particles by examining spatial variations of intensities of spectral lines . Reviews of the properties of foil-excited ion beams and techniques for their use have been given by Bashkin [2] and Bickel [3] .…”

New Method for Precision Lifetime Measurements by Laser Excitation of Fast-Moving Atoms

“…Thus the laser excitation of hundreds of levels will be feasible and will therelore allow a rather universal application of our method for precision lifetime measurements. Aside from that it should be stressed that after polarized laser excitation even high-resolution studies can be performed on these levels under favorable alignment conditions with the levelcrossing, 18 rf-resonance, 19 or quantum-beat techniques 20 in fast beams.…”