VUV Spectroscopy of Atoms, Molecules and Surfaces

50 High-resolution VUV spectroscopy of H −
longitudinal electron-beam temperature is reduced by the acceleration process
to typically kT � ∼1 meV [22] while the transverse temperature, initially
equal to the cathode temperature, kTcath = 0.11 eV, can be reduced by adiabatically
expanding the electron beam [23]. In the present study, the solenoid
magnetic field of the interaction region was reduced by a factor 4.3 with respect
to that of the gun, implying an expansion to a √ 4.3×1 cm=2.1cm
diameter electron beam and a factor 4.3 reduction in the transverse temperature
to kT⊥ = 26 meV. For comparison, the effective ion-beam diameter,
taking betatron oscillations into account, was ∼1 cm and could be considered
constant on the time scale relevant for longitudinal cooling.
The mean longitudinal velocity v and spread δv of the D − beam were derived
from the average revolution frequency ν and spread δν, monitored with
a Schottky pick-up. Due to the large cross section for collisional destruction
of a negative-ion beam, the lifetime of the stored beam was limited to a few
seconds. Stable cooling, characterized by constant mean velocity and a constant,
narrowed velocity spread, must thus be achieved on a similar timescale,
and knowing the exact time when this occurs is important for separating out
the corresponding neutral-atom signal in the data treatment. The frequency
content of the Schottky signal mixed down to ∼10 kHz was sampled by an
analog-to-digital converter and analyzed with a fast Fourier-transform procedure
every 0.16 s, allowing ν and δν to be recorded as a function of time
[24].
In order to obtain stable cooling and the lowest possible velocity spread, it
is important that the electron- and ion beams are well aligned in the electron
cooler [21]. This was achieved by means of a pair of horizontal and vertical
pick-up plates placed in the interaction region as shown in figure 3.3. When
not used for positioning, the latter were modulated with a 250 kHz AC sine
voltage, 6 V peak-to-peak, in order to reach equilibrium within the lifetime
of the ion beam. This procedure has been suggested by Marriner et al. as
a method of reducing the amount of rest-gas ions trapped in the electron
beam [25]. Trapped rest gas ions imply a reduction in the magnitude of
the electrostatic space-charge potential and a corresponding increase in the
electron-beam velocity. Without the modulation voltage the Schottky spectra
exhibited significant drifts and stable cooling could not be obtained within
several lifetimes of D − beam storage.
3.3 Results and discussion
Figure 3.4(a) and (b) show examples of the evolution of the mean longitudinal
velocity and the relative velocity spread (FWHM) with time, respectively,
for the cooled D − beam, as obtained by fitting the Schottky spectra to a