Experimental aspects

The new theories about laser-assisted (e,2e) m easu rem en ts are very en co u rag in g for f u rth e r e x p erim e n tal in v e stig a tio n . T h ere a re a few p ro b lem s in th e p re s e n t e x p e rim e n t. O ne p ro b lem is th e sodium b a c k g ro u n d in th e s c a tte r in g c h a m b e r. S odium v a p o u r from th e recircu lato r, collim ator, and along th e beam line can difuse, contributing to th e background. A lthough th e oven h a s been shielded an d cooled to reduce such background, th e b ackground still in creases a lot a fte r th e re c irc u la tin g sodium oven ru n s for a few day s. T h is b a c k g ro u n d c o n ta m in a te s th e electron gun, electron a n a ly se rs, an d F a ra d a y cage. The h e a t an d th e contam ination from th e sodium vap o u r also affect th e perform ance of the MCPs in th e spectrom eters, especially the one close to th e sodium oven (i.e., th e scan n in g an aly ser). T his “side effect” of th e r e c ir c u la tin g oven r e s tr ic ts c o n tin u o u s lo n g te rm m e a s u re m e n ts . A n o th er problem in th e m ea su re m e n ts a rise s from th e d en sity of th e sodium beam . Since th e density of th e beam is about th ree orders lower th a n th a t of th e u su al gas beam , th e coincidence co u n trate is extrem ely

low. T he co u n tra te from the 3p excited s ta te is fu rth e r reduced because th e m axim um population in th e 3 P 3/2( F / = 3;M ^/ = 3 ) su b sta te can only reach 15% (theoretical estim ation) by th e p resen t optical pum ping system . To reduce th e sodium background in th e sc atterin g cham ber, a se p ara te oven cham ber h as been built. To increase th e sodium beam d en sity an d th e population in th e excited sta te , th e tech n iq u es of optical collim ation an d d u a l-laser pum ping are m entioned.

Separate oven chamber

Oven chamber Connection valve to the

main chamber

Diffusion pump Roughing pump

valve

Back line

Diffusion pump valve

F ig u r e 5.17 The diagram of th e sep arate sodium oven cham ber.

The se p ara ted sodium oven cham ber is a cylindrical cham ber w ith a d iam eter of about 77 mm and a length of about 130 mm (see figure 5.17). I t is connected to th e sc a tte rin g cham ber from a side p o rt a t th e sam e horizontal level as th e in teractio n region. A gate valve is used in betw een th ese two cham bers so as to operate th em se p ara tely in th e p rep a ratio n stage. A diffusion pum p is used for th e oven cham ber, sh a rin g th e sam e

backing line w ith th e scatterin g cham ber. The sam e recircu latin g sodium oven is used, m ounting on the top flange of th e cham ber. T here are th ree ports on the top flange for feedthroughs an d cooling w ater. The te s t of the vacuum system and th e oven system is in progress.

Optical collimation

W hen th e sodium oven is in th e s e p a ra te cham ber, it is expected t h a t th e b ack g ro u n d in th e s c a tte rin g ch am b er will be s u b s ta n tia lly reduced. However, th e distance betw een th e nozzle an d th e in te rac tio n region is in creased by an e x tra 400 mm. To keep a 2-m m -diam sodium beam in th e in teractio n region, a divergence angle of a < 0.3° is required. The re s t of th e beam h as to be cut off by depositing th ese atom s into a tra d itio n a l geom etrical collim ator. B esides th e reduction of beam density in th e in te r a c tio n reg io n , th e r e s id u a l a to m s m ay also p ro d u ce background in th e sc atterin g cham ber or, possibly block th e a p e rtu re of th e collim ator. A new collim ation tech n iq u e, optical collim ation, is now feasible to “sq u eeze” th e atom ic beam to w ard s th e cen tre so th a t th e divergence angle is reduced and th e d en sity of th e beam is in creased significantly (Hoogerland et al. 1995).

In ste ad of using m etal a p ertu res to collim ate the beam , a CW laser is used to in te ra c t w ith the atom ic beam w ith a frequency tu n e d h ig h er th a n th e m ean resonance frequency v0 of th e atom ic beam. Because of the divergence of th e sodium beam , th e ato m s m oving to w ard s th e la s e r p ro p ag atio n d irection absorb th is frequency an d are excited. As th ese ex cited ato m s decay, th e sp o n ta n e o u s e m issio n is iso tro p ic. T h ese resonance atom s th erefo re feel a n e t force from the" laser, to w ard s th e centre of th e beam . Deflecting th e laser light by a p air of m irrors along the direction of th e atom ic beam , th e la se r can trav e l, m any tim es, th ro u g h th e beam from both directions so th a t th e atom ic beam is forced tow ards th e centre from both sides. By sp littin g th e la s e r to form two la se r beam s perp en d icu lar to th e atomic beam direction, th e atom ic beam can th en be co llim ated in tw o dim ensions. I t is ex p ected to o b tain a v ery well collim ated sodium beam by th is kind of optical collimator.

Dual-laser optical pumping

As one can see, the population of th e 3p sta te of sodium is very low, an d an y te c h n iq u e to in crease th e u p p e r s ta te p o p u latio n w ould be d esirab le. T h ere is a optical p u m p in g schem e w hich can in crease th e population of th is excited state: u sin g two la se r beam w ith th e frequency

differen ce of 1712 M Hz, one for th e e x c ita tio n of 32S y 2(F = 2) —>

32P3/2(F' =3) as before, and the other for th e excitation of 32S y 2(F = 1) — >

32P3/2(F' =2). Because th e excited s ta te 32P3/2(F' = 2) can decay back to e ith e r 3 S y 2(F = l) or 3 S y 2(F = 2) ground sta te s , a fte r a few cycles of p u m p in g th e 3 jSy2(-^ = 1) s ta te w ill be em p ty . A g ain , w ith < 7 polarisation for both laser beam s, th e sodium atom s will be populated only in two hyperfine states: 32S y 2(F = 2;M F = 2) an d 32P3/2(F' = 3\ M F> = 3).

The population w hich w as trap p ed in th e 3 S y 2(F = 1) sta te in th e single­ la s e r pum ping system is now pum ped out so th a t th e population of th e excited s ta te will be in creased significantly. M orever, th e ground s ta te w ill be in a p u re o rien ted s ta te as w ell. Since th e o rb ita l a n g u la r m o m en tu m of an s s ta te is alw ays zero an d th e n u clear sp in I is not coupled in the electron collision, th is s s ta te orientation corresponds to the p u re o rie n ta tio n of electro n sp in S . T he sodium atom is th e re fo re polarised not only in th e excited sta te , b u t th e ground s ta te as well. This optical p u m p in g m eth o d is called th e d u a l-la s e r p u m p in g tech n iq u e, w hich is w idely used to p rep are lith iu m and cesium atom s (Baum et al. 1980, 1991). A ctually it is not n ecessary to use two dye la se rs for th is technique, as a frequency m odulator shifting th e frequency by 1712 MHz also works (Lorentz et al. 1993).

Atomic beam (Na)

Scattering

h v , <y± -excitation Laser beam

F ig u r e 5.18 The k in em atics of th e expected coplanar asym m etric (e,2e) e x p e rim e n ts w ith p o larise d sodium ato m s an d /o r p o larise d in cid en t electrons.

W ith the ex p erim en tal im p ro v em en ts, in ten siv e in v estig atio n s of (e,2e) collisions can be carried out w ith th e guidance of new theories. If th e coplanar asym m etric geom etry is used, th e -excitation la s e r will e n te r th e collision cham ber from u n d e rn e a th , as in figure 5.18. The s c a tte rin g plane is th e re fo re p e rp e n d ic u la r to th e sodium 3p s ta te o rien tatio n in order to m easure th e dichroism by flipping th e polarisation of th e laser beam . E xtrem ely in te re s tin g experim ents can be carried out w hen th e polarised electron beam is used. B oth th e incident electron and th e ta rg e t will be spin-resolved and th e ir sp in directions are p arallel or an tip arallel. As the spin degeneracy of both collision particles is removed, th e stu d y of (e,2e) collisions is th e n a t a n o th e r stage, and spin-dependent in te r a c tio n s , as w ell as th e d y n a m ic s of th e collision, can be u n am biguously revealed. T his is a n o th e r ste p to w ard s th e s p irit of a “perfect experim ent” for the study of ionisation collisions.

Chapter 6

Summary

An (e,2e) detection system , to g eth er w ith a polarised electron source an d a M ott p o larim eter provides a very pow erful tool for exploring th e sp in -d e p en d e n ce in th e e le ctro n im p a c t io n is a tio n p ro cesses. T h is a p p a ra tu s is described in d etail in c h a p te r 3. A GaAs photocathode is used as th e polarised electron source. It is pum ped by a GaAlAs la s e r diode w ith th e w avelength of 780 nm . By u sin g ^ - p o la r is e d p um ping lig h t and Cs-02 activation of th e cathode, an em ission c u rre n t of up to 30 pA w as o b ta in e d . T he e le c tro n s a re d e fle c te d th r o u g h a 90° h e m isp h e ric al deflector an d 1 keV beam tra n s p o rt to th e in te ra c tio n region w ith tran sv erse spin polarisation. The direction of th e polarisation can be easily reversed by reversing th e polarisation of the laser. At 147 eV incident energy a beam c u rre n t of up to 4 pA w as obtained in th e in n er F a rad a y cup.

The spin polarisation is m easured by a compact spherical reta rd in g p o larim e ter. M ott s c a tte rin g h a p p e n s from a gold foil of 100 nm in th ic k n e s s a n d th e beam en erg y of 60 keV is u sed . T he d etectio n asym m etries can be elim inated by rev ersin g th e spin direction of incident electrons. W ith th e energy loss window ex trap o latio n , th e p o larisatio n of 0.24 ± 0.01 w as d etected , w here ± 0 .0 1 is th e s ta tis tic a l e rro r of th e m e a s u re m e n ts .

The (e,2e) s c a tte rin g system consists of two 180° h e m isp h e ric a l analysers. D ual-layer m u ltich an n el p late an d a G ear-type resistiv e anode are employed in each an aly ser to detect th e energy and arriv in g tim e of e lectro n s. T he (e,2e) sig n a ls a re a n a ly s e d by th e m u ltip a r a m e te r coincidence circu it w hich d isc rim in ates a g a in s t back g ro u n d events on th e basis of both energy and tim ing criteria.

W ith th is spin-resolved in c id e n t ele ctro n beam an d th e (e,2e) co in cid en ce sp e c tro m e te r, th e in v e s tig a tio n of th e sp in -d e p e n d e n t p ro cesses in electro n im p act io n isa tio n w ith xenon atom s h a s been carried out. D etails of th e experim ent are given in ch ap ter 4. A coplanar asy m m etric (e,2e) geom etry is used. The in cid en t energy is E0= 147 eV, a n d th e spin d irectio n is p e rp e n d ic u la r to th e sc a tte rin g p lan e. The sc a tte re d electrons are detected a t an energy of Es = 100± 3 eV an d a n angle of 9S = 28° a t the left side of the incident beam . The ejected an aly ser is k ep t a t an energy of Ee = 35 ± 3 eV w ith a v a ria tio n in a n g u la r ran g e of

31° to 119°, a t th e rig h t side of th e in c id e n t beam . The tim e-of-flight v a ria tio n s w ithin th e analysers are corrected in the d a ta collection circuit a n d hence th e s ta tis tic s of d a ta is sig n ifican tly im proved. W ith such a r r a n g e m e n ts , -two ion s ta te s 5 P y 2 a n d 5 P 3/2 of x e n o n a re sim u ltan eo u sly detected. T hese two fin e-stru c tu re s ta te s w ith an energy difference of 1.3 eV are well resolved in th e energy loss spectrum .

To confirm th e d irec tio n a n d th e m a g n itu d e of th e electro n p o larisatio n which is analysed in th e M ott detector, an elastic sc atterin g ex p erim en t for xenon h as been done a t an energy of 50 eV. The spin-up-