The k inem atics of th e (e,2e) ex p erim en t is schem atically show n in figure 5.8. An electron beam w ith energy E 0 = 605 eV is incident along th e y-direction. The sodium atom ic beam is in th e x-direction, an d th e o + - pum ping beam p ro p ag ates along th e z-direction. F ig u re 5.8 also shows th e charge cloud distrib u tio n of th e sodium 3 p x and 3 p y orbitals in th e x-y plane. The sc atterin g plane is th e y-z p lan e an d both outgoing electrons m ake a polar angle 6X = 02 = 45° w ith resp ect to th e incident (y) direction. They are detected a t th e sam e energy E 1 = E% = 300 ± 3 eV. E lectron A is fixed w ith in th e scatterin g plane, w hile electron B moves w ith th e out-of- p lan e a z im u th a l angle 0. T he m o m en tu m s tr u c tu r e s of bo th sodium 3 S y an d 3 P ^ ionisation sta te s, w ith th e respective binding energies of 5.1 eV and 3 eV, are probed.
S O O I U M
B E A M
B E A M ( 2 m m )
L A S E R
B E A M
3 mm
2 mm
F ig u r e 5.7 The in teractio n of th ree crossed beams.
La s e r
b e a m
Na
beam
Incident
e l e c t r o n
beam
Re a c t i o n
p l a n e
F ig u r e 5.8 Schem atic diagram of th e experim ent and th e electron charge and m om entum densities of the excited 3p ( ML = +1) sodium atom s.
The geom etry of th is non-coplanar (e,2e) ex p erim en t is show n in figure 5.9. A tu n g s te n filam en t electron gun is m ounted on th e bottom flange of th e s c a tte rin g cham ber. The electron beam p a sse s v ertically along th e centre of th e scatterin g cham ber a t th e in teractio n region, and th e beam c u rre n t is m onitored by a dual F arad ay cup about 80 mm above th e in teractio n region. Two electron analysers are located on th e first and second tu rn -ta b le s w hich can be moved by stepping m otors. The top tu r n tab le is u sed to su p p o rt th e sodium beam source. The h o rizo n tal la s e r beam p asses th ro u g h one vacuum window and in te rse cts w ith th e o th er two beam a t rig h t angle in the in teractio n region. The in te rac tio n region, form ed by th e th re e crossed beam s (i.e., laser beam , sodium atom ic beam , an d electron beam ) is show n in figure 5.7. It is very im p o rta n t to align th ese th re e beam s to m eet a t th e sam e point, and th is point m u st be viewed by both electron analysers.
■FaRAOAV c u p INTERACTION \ I REGION \ RETARDING LENS SYSTEM LASER LIGHT IN VACUUM ---- STEPPIN G MOTOR TO PUMP BASEPLATE
F ig u r e 5.9 S ch em atic view of th e m u ltip a ra m e te r (e,2e) coincidence sp e ctro m eter. T he electro n beam , la s e r beam , a n d th e sodium beam in te rs e c t a t r ig h t an g les. The sodium oven (not show n) is m o u n ted perp en d icu lar to th e plane of the diagram .
A sim ilar experim ent w as done a t th e in cid en t energy of 805 eV, both the outgoing electrons being detected a t 400 ± 3 eV (Zheng et al. 1990). The sam e geom etry was used. The only difference in th e equipm ent is th a t th e recircu latin g oven replaces th e previous short-life oven, an d hence d a ta collection occurred for a m uch longer tim e th a n before. The reason for u sin g 600 eV incident electrons in ste a d of 800 eV is to expand th e d istrib u tio n in m om entum space. B oth th e PW IA calcu latio n an d th e previous m om entum profile m easu rem en ts show th a t th e sodium 3p sta te h a s a very compact m om entum d istrib u tio n . The peak position w as found a t 0 - 2.5° for th e previous k in em atics given by Z heng et al. (1990). L ow ering th e in cid en t an d d etected en erg ies expands th e m om entum space. For instance, th e 3p peak in th e p re se n t k inem atics is expected a t 0 - 3 .5 ° . W ith th e sam e experim ental a n g u la r resolution, th e m om entum reso lu tio n is th erefo re im proved. A n o th er a d v an tag e is to in crease th e (e,2e) cross section by lowering th e incident energy to 600 eV, which would be still acceptable for EMS probing.