Electron beam experiments

The expertise in electron spectroscopy in the group was acquired only in late 90's. We were eager to build home-made instruments which just suited our needs. This involves design and production of the following equipment:

  • An electron gun which delivers up to 10 µA current with 0.5 eV resolution in the range 0.1 - 5 keV (COIN station) . Another low energy electron gun was made for studies of dissociative attachment of hydrogen (VEVOF station).
  • plan-parallel electrostatic analyser with 3-element lens and single channel detection. On strong lines we can reach resolving power of 0.0002 in the energy range from 0.1 to 2 keV. A word of warning: we failed to adapt this spectrometer for parallel energy detection. The problem is that there is no clear exit focal plane in this kind of geometry!
  • Time-of-flight spectrometer. This is 80 cm long tube with MCP detector and adjustable flight path to move easily between the different regimes of Wiley-McLaren focusing (first order, second order, time lag).
TOF spectrometer graph

Fig. 1) A mass resolution of 1/130 was reached with our TOF ion spectrometer.

recently, a hemispherical electrostatic analyser with 66 mm mean radius and 3-element lens was kindly put to our use by dr. Lorenzo Avaldi. Currently this is being adapted to multichannel detection. This development relies on measuring time delays of the current pulse from which the position (and therefore the energy) of the electron is decoded in the focal plane of the analyser. A successful accomplishment of this goal would greatly enhance the speed to acquire the data in pulsed electron-ion coincidence measurement as described below.

Pulsed electron-ion coincidence measurements

K.Bučar and M. Žitnik, "Auger electron-ion coincidence spectrometry after electron excitation of L-shell in argon", Rad. Phys. Chem. 76, 487-491 (2007).

We have successfully done the first coincidence measurements of Auger electron-ion pairs where the electron energy resolution is comparable to the natural width of the Auger transitions. The electron L--MM spectrum in argon was decomposed into contributions related to the specific final ionic charge states Ar2+ to Ar5+. The data allowed to verify and to determine more accurately decay schemes whose signals are summed up in the non-coincidence Auger spectrum.

Various decay paths

Fig. from the left: 2) Various decay paths of multi-hole states in Ar. 3) There is a small difference in ensemble of Ar ions when Auger electron is produced (F) or not (R). Target is excited by short electron pulse.

Argon was excited with short pulses of 1 keV electrons. The final ionic charge states were detected by the time-of-flight spectrometer. The energy of electrons ejected from the target at right angle to the incoming beam was analyzed by the plan-parallel electrostatic spectrometer in the range from 170 to 225 eV. Both spectrometers and the electron gun were coupled into the coincidence measuring loop running at frequency 1 MHz. At each repetition a short excitation pulse was generated and the presence of an electron was checked. Only when the electron was detected the ions were analyzed, otherwise they were removed from the target.

Various decay paths

Fig. 4) Note the difference in distribution of Ar ions when the collection of TOF spectrum is triggered by detection of L3-M23M23 Auger electron (red) or when TOF spectrum is acquired by quasi random triggering (blue line).

The final ionic multi-hit distributions detected by the spectrometer in the coincidence and non-coincidence measurement were described by the statistical model. From both measurements we have extracted true ion charge state distributions in the limit of weak excitation: the Auger spectrum was decomposed into different final ionic charge states.

non-coincidence Auger spectrum Ar2+

Fig. 5) From non-coincidence Auger spectrum (black line) a part is extracted which pertains to Ar2+ final ion states (red). Previous best measurement is given by the green line.

non-coincidence Auger spectrum Ar3+

Fig. 6) From non-coincidence Auger spectrum (black line) a part is extracted which pertains to Ar3+ final ion states (red). Previous best measurement is given by the green line.

Contact person:

Dr. Matjaž Žitnik
Dr. Klemen Bučar

Last updated: 01/22/2014