Beam Source for Electrons and Ions:
Sharp tungsten tip
The tip is prepared in a two-step process:
Electro-chemical etching of a single crystal tungsten wire (111)
in a 2n NaOH-lye.
Sputtering: In a 0.0001 mbar atmosphere of Ne a negative voltage
(typically 600 V) is applied to the tip so that an electron current of
0.01 mA is emitted. Ne-atoms are ionised by the electrons and
accelerated towards the tip. Because the tip is sharpened by the
ion-bombardment less and less voltage is needed to maintain an electron
current of 0.01 mA. When the voltage applied to the tip decreases
to about 350 V, the tip has reached a structure with just a few
atoms on the top. If we observe the structure of such a tip by
field-ion-microscopy, we can see the foremost atoms as bright spots.
Field-ion-microscopic image of a few-atom tip. (JPG, 3k)
Such a tip serves as a point-source for charged particles in two modes:
Field-emission of electrons:
In the field emission mode a negative voltage is
applied to the tip. Due to the high electric field near the tip
electrons may tunnel from the metal surface into the vacuum.
In our experiments we use voltages in the range from 200 to 1500 V.
The emission is stable only under UHV (ultra high vacuum)
conditions (0.1 pbar).
Field-ionization of atoms:
In the field ionisation mode the tip is operated in a gas atmosphere with
a positive voltage. Gas atoms are ionised in the high electric field
near the tip. The positive ions are then repelled from the tip.
To get reasonable intensities the gas pressure has to be in the range
of 0.0001 mbar and the voltage applied to the tip has to be high enough
(about 1000 V).
A modified mounting for the tip has been constructed in such a way, that the emission direction can be
adjusted by means of a simple wire pull mechanism. A gif-animated 3d-picture (200 k) of this
mounting is available here.
