ALIGNMENT STUDIES IN THE HYDROGEN 2P-STATE AFTER BEAM-FOIL EXCITATION

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ALIGNMENT STUDIES IN THE HYDROGEN 2P-STATE AFTER BEAM-FOIL EXCITATION

H. Winter

To cite this version:

H. Winter. ALIGNMENT STUDIES IN THE HYDROGEN 2P-STATE AFTER BEAM- FOIL EXCITATION. Journal de Physique Colloques, 1979, 40 (C1), pp.C1-307-C1-314.

�10.1051/jphyscol:1979165�. �jpa-00218445�

ALIGNMENT STUDIES IN THE HYDROGEN 2P-STATE AFTER REAM-FOIL EXCITATION

H. Winter

Institut ffir Atom- und Festkdrperphysik der Freien Universitst Berlin 1000 Berlin-West, Boltzmannstr. 20

Abstract The alignment of the 2P-state of hydrogen after beam-foil excitation was measured by the Lyman-6polarization a t energies between 13 keV and 1.6 MeV.

The dependence of the alignment on energy shows changes of sign at about 17 keV and 50 keV and a minimum at 6 0 0 keV. In a time dependent polarization measurement the time resolved transfer of alignment 3 D j 2 P is observed.

Resum& Des atomes d'hydrogene de 13 keV d 1,6 MeV sont excit6s par feuille de car- bone et la polarisation linhaire des raies Lyman-a est mesuree avec un ana- lyseur d triple reflection avec des miroirs A1+MgF2. Nous avons troyve deux croisements de l'alignement dans la gamme d'bnergie observee pour 17 et 47 keV. Entre ces deux Qnergies, l'alignement est positif, ailleurs il est nega- ti£.

I. Introduction

The production of a nonstatistical magnetic (4,5,6) and hydrogenlike helium ( 6 ) and sublevel population by the beam-foil inter- oxygen-states (7) is opposite to the obser- action is the base of high resolution stu- vations o n other systems.

dies and an important aspect in understand- ing of the beam-foil interaction itself. In case of a foil perpendicular to the beam- axis from arguments of symmetry follows that in this geometry the sublevel-popula- tion is independent from the sign of the magnetic quantum numbers; this results in a quadrupole sublevel polarization (alignment) so that pure fractions of linear polariza- tionare detected. The quantum-beat techni- que enables the beam-foil light source to high resolution spectroscopy (1,2). As an alignment is necessary for the observation of beats, alignment studies were carried out in order to investigate the range of applicability of this technique and to get further informations in understanding the interaction process itself.

From the quantum-beat and polarization mea- surements undertaken so far it is difficult to extract general dependences of the align- ment on parameters like energy, quantum numbers, or foil conditions. One evident ob- servation, however, refers to the different sign in polarization of the emitted light of hydrogenlike in contrast to all other

In a preceding publication (5) we reported on the alignment of the hydrogen 2P-state in a limited range of energy. The concept of this paper is the extension of the former measurements over the whole range of energy one Can cope from experimental reasons. The choice of a simple level system

-

the hydro- gen 2P-state

-

should ease theoretical ap- proaches.

11. Alignment

An appropriate definition for an alignment is the alignment parameter which is iden- tical to ACol from the work of Fano and Ma-

cek ( 8 ) . The quadrupole-polarization with

respect to the z-axis (beam-axis) is norma- lized to the total sublevel ponulation and the angular momentum L

(1) The probability for the excitation of pure

states

is given by

'-Q-HL

= < L H ~ \ ~ \ LML)

( 2 )

where Q is the density operator of the syS-

-

tem.For a P-state one gets from eq.(l)

systems (1,3). The preferential population there is no descrip.

of sublevels with larger magnetic quantum tion of an alignment in literature. For a numbers in foil excited hydrogen states P-alignment one often finds the definition

( 9 , 4 )

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1979165

CI-308 JOURNAL DE _PHYSIQUE

(*re - ^{/ (}

^{Pr, t ( 4 )}

which i s related to the deqree of polariza- tion of radiation emitted in the IS- P-tran- 1 sition.

The experimental methods for determininq the alignment of a state a r e the measure- ment of the angular distribution, the li- ne3r polarization of the emitted light, and the resolution of quantum beats. Because of the exponential spatial density distribu- tion of foil excited atoms, the angular di- stribution method is unpracticable from the e x ~ e r i m e n t a l point of view.

The development of the density operator of tie system under study in the base of the irreducible tensorial operator set Tk is

-a

where the components L ~ : are related to the excitation crass-sections according

L$,

= _u,. Z ,

(-A)~-*'

-- ⁽ Qnt9)

^{LrHL ( 6 )}

Lso is connected to the population, Ls4 to an orientation, and Lga to an alignment.

As discussed by Ellis ( 1 0 ) one g e t s in case of a foil perpendicular with respect to the beam axis from rotational symmetry around and reflection symmetry t o a nlane contain- ing the beam-axis the restrictions: k even

and q = O ; i.e. an orientation and a result-

inn contribution of circular polarization does not appear.

T h e denree o f polarization o f radiation emitted in transitions between singlet sta- tes wlth anqular momentums L , Lo is given

( 7 ) shows that the linear polarization is

directly connected with the alignment. Fur- thermore follows fron the evaluation of the 6 j-symbol's that for given L ~ : and

LQ1

^the

amount o f polarization is larger for L > L

0 SO that it is wise to observe an alignment via transitions between states of decreas- ing angular momentums. For the IS-2P Lyman-

<

transition one gets from ( 7 ) the well known result

p a

- ^?u.

'f =

P r ' o r

?r.,

^{( 8 )}

which is identical with the definition ( 4 ) .

In the H-Lyman-& transition, however, one has to consider the fine-structure couplin.g, since the natural linewidth o f about 1 0 0 M H z is considerably smaller than the 2P-fine structure splitting o f 1 1 GHz, whereas the hyperfine structure is neqligible ( 9 . 1 1 ) . In the case of fine structure ( 7 ) is modified according to

from what follows for the Lvman-A transition

and for the P-alignment

(Pg,-p%)/(Pp,+9c4)

= ^9'P /(3+2'9)

^{( 1 1 )}

Due to the short interaction time durinq the excitation process fine structure le- vels are coherently excited and zero field quantum beats are observable. From the for- mula for the time dependent intensity ( 1 0 )

I (c) = c iN [I

:I+ Z/G

A; -5 ^{A$ as}

^(038-

^{0 ~ 4 *I}

with CnJt.1) u3'+4)

b.11

A:'

C' 26+4

and the photon density operator ( 1 3

' 4 = Z

^{P V}

^{e )}

^{( 1 4 )}

in case of an alignment and a sufficient time resolution a modulation of the expo- nential decay will be observed.

The advantage of the quantum beat technique in comparision to a polarization measure- ment t o determine an alignment is the fact that in principle no analyzer for linear polarization is necessary. This is o f im- portance for wavelength regions where high quality analyzers are not available. As pointed out in the subsequent chapter the analyzer is the crucial point in the ~ o l a - rization measurement o n the As1216 line reported here.

On the other hand at fine structure fre- quencies l&,-QJ8with a quantum beat wavelen- 9th

hB=2yr~/(WJ-WJ)

Iv-velocity o f atoms) in the submillimeter domaine the resolution o f the oscillatory part o f the decay curve a n d

especially the determination of the align- ment from the beat amplitude gets critical.

For instance the velocity o f 1 0 0 keV hydro- gen atoms is v = 4 . 4 mm/ns and the 2P-fine structure splitting is (i$-UJl = 2 2 1 . 1 0 , 9 7 G H z

o f

h e =

0.40 mm. This means that the spatial resolution of the experimental setup should be about a tenth of a millimeter and s m a a r a t lower energies, respectively. Further- more the reliability of measurements under those conditions is reduced by energy and angular stragglings and corrections by un- certain detection functions. These inherent difficulties in the quantum beat tec3nique caused us to determine the alignment of the hydrogen 2P-state by polarization measure- ments.

T o measure the alignment by the polarization one has to consider the influence of quan- tum beats o n those experiments. From eq.(12) one qets for the Lyman-a intensities pola- rized parallel and perpendicular with re- spect to the beam axis

1 , =

C

i P r t

^{( 5}

)re

^t

Y - t

t

Y(%-&~)(DL(%-uJ~)

rr = C i Y ( 2 ' ~ ~

r.t

+'q- ~ ( ' r . - ~ ~ ) r a r ( ~ - ~ ~ t )

(15,16) The observation of the emitted light in the t i m ~ interval A t = t - t l leaAs to the ratio

2

of the exponential to the quantum beat in- tensity contribution ( @ * m y p - W V t )

and yields for the alignment

For a sufficient averaging over the oscilla- tory part 'a' gets large and (1%) is iden- tical with (117. In all other cases one has to take into account the influence of the quantum beats on the measurement.

111. Experiment

The energy range which is reasonable for alignment studies after foil excitation is limited by the angular straggling of the ions at low energies and by the small exci- tation cross-seetion in the high energy re- gion. For the 2P-state of h y d r o g ~ n this re- sults in an interval of energy between lo keV and about 2 MeV. Since no single accele- rator is able to match this energy range, two different accelerators have been used:

a 4 0 0 keV-SAMES- and a 2.5 MeV van d e Graaff

After mass separation a H+-beam of about 1 ,

A and 10-100 nA/mm2 is sent through a car- bon foil 1OPg/cm2 thick which was control- led during the measurements by means of a telescope. T o avoid effects resulting from tilted foils, the perpendicular position is controlled within a few deqrees.

The detector is a BENDIX-Channeltron B X 7600 4413 with a MgF2-window and NaJ-cathode. As the cutoff-wavelength of MgF2 is about 1140

8

and the sensitivity of the photocathode ends a t about 1700

8 ,

the wavelength f sola- tion of the ~ y m a n - A l i n e is easily achieved without producinq an additional pclarization a s by using a grating spectrometer. A fur- ther advantage of this detector is the low dark count rate (-1 cts/min) which makes possible the measurement of polarization even at high ion energies where coutinq ra- tes are very low.

The analyzer is from the triple-reflection type and consists of three mirrors coated with a double layer of aluminum and MgF,

L

( 1 2 ) . Usinq an interference effect between

the two layers this system produces both a high defjree of polarization an8 transmission.

The evaluation o f the analyzer data with the help of a hydrogen discharge and a spec- trometer of the Seya-Namioka type at 1216

%

yields an degree of polarization

'aria

⁼

(97.522.0)s and (7.2+0.5)$ for the effec- tive transmission.

The analyzer data are important for this kind of experiment, since from former mea- surements (4) degrees of polarization o f a few percent can be deduced. In addition the beam-foil light source is of low inten- sity

-

esp. for hydrogen at hiqh ion ener- gies. The diaphrams of the analyzer system have diameters of 3 mm, resulting in a de- tection function o n the beam axis of 3.3 mm which is sufficient to averaqe at all measured energies over several Lyman-#

quantum beats

.

With the help of a photo- graphic procedure the observation zone was determined to lie 1.7 mm and 5.0 mm behind the foil so that the detection of light e- mitted from the surface of the foil is ex-

C1-310 JOURNAL DE PHYSIQUE

cluded. The vacuum produced by a turbo-pump and a liquid N2-cool trap was during all mea- surements between 3.10-' and l ~ Torr. The - ~ experimental setup is shown in fig.1.

Fig.1 Section through experimental setup along the beam-axis

IV. Measurements and results

Because of mechanical asymmetries while ro- tating the analyzer areund its axis the ap- plication of the technique of a polarization measurement by recording the intensities in parallel and perpendicular position of the analyzer with respect to the beam-axis is critizal. Due to small deviations between the optical and mechanical axis of the ana- lyzer system, imperfections in the adjust- ment of the analyzer-mirrors, and a Local dependence of sensitivity of'the detector one qets during, the rotation of the analyzer an intensity modulation in addition to the polarization signal which is in good appro- qimation described by a sinusoidal function (13). Therefore the polarization of Lyman- liqht emitted by the atoms were measured by recording the dependence of the intensity while rotating the analyzer around its axis.

?his rotation was carried out automatically in steps of 30° resulting in 12 steps per full rotation and the data were stored in subsequent channels in a MCA. The measure- ment at each position was normalized to a

preset charge accumulated by the Faraday-cap.

In fig.2 the anguL lar dependence of the Lyman-8 intensi- ty is shown for three different en- ergies which repre- sent the energy ran- 5 11"

Z ge under study: 3 0

keV, 200 keV, and

00 1.1 MeV. Especially

in the 3 0 keV case

E m

8 the influence of the

asymmetry is evident

0 69 lo 129 W I W 2 ( 0 Z W 3 2 9 3 0 TO extract from the-

n o w

curves the polariza- Fig. 2 tion signal, the data were fitted with the function

I(&\

^{a ((14} C O S ' ( ~ ~ Q L ) ~ ~ ~ ) - ( Q ~ C.0s(dt4)trl) ( 1 9 ) where the first factor describes the polari- zation signal and the second one the asymme- try: the solid lines in fig.2 are best fits to the data with eq.(19). The measured de- gree of polarization follows from Pm = a l /

(a1+2a ) and has to be corrected for the in- 3

trinsic polarization of the analyzer P ana yielding P = Pm/Pana.

The parameter a4 describes the asymmetry and should not exceed the degree of polarization.

As can easily be understood the asymmetry de- pends on the intensity distribution in the detection zone and therefore on the focussing of the beam. This implies that often several measurements at the same energy with slightly changed focussing conditions have to be car- ried out to get I ( g ) with a small contribu- tion of asymmetry. In the measurements re- ported here a4 is about 1 % or less. A good test of the reliability of a measurement is the phase a 2 in eq.(19) since maximum inten- sity has to be detected at a position paral- lel or pendicular with respect to the beam- axis.

The resulting alignment as function of ener- gy (corrected for energy loss) is shown on top of the next page in fig.3 together with data from Dobberstein ( A ) and Andra

( m

)

(4).

I I I I I I I I I I I I I I 1 I I l l I J

-

-

-

-

-

:I _I ^b+

I \ ^{a@ $8 i8Bt ik4}

- P n i n ~ ~

-

i- 1 I I 1 1 I I I I I I 1 I I I I I I I I I

-

energy I keV

Fig. 3

T h e m e a s u r e m e n t b e t w e e n 2 5 k e V and 1 9 0 k e V w a s a l r e a d y s u b j e c t o f a p r e v i o u s p u b l i c a - t i o n ( 5 1 , d e m o n s t r a t i n g a c h a n g e o f s i g n o f t h e a l i g n m e n t a t a b o u t 5 0 keV. T h e exten- s i o n o f t h e s e d a t a t o l o w e r e n e r g i e s r e s u l t s in a f u r t h e r c h a n g e o f s i g n a t a b o u t 17 kev.

T h e d i f f i c u l t i e s i n m a n a g i n g a s t a b l e and s u f f i c i e n t i o n beam a t t h o s e e n e r g i e s w i t h a 4 0 0 k e V - a c c e l e r a t o r l e a d t o t h e r e l a t i v e - ly l a r g e e r r o r bars.

At e n e r g i e s a b o v e 3 0 0 k e V t h e a m o u n t o f a l i g n m e n t i n c r e a s e s a s c o u l d b e e x p e c t e d f r o m f o r m e r p o l a r i z a t i o n m e a s u r e m e n t s w i t h a L i F - s t a c k p o l a r i z e r by A n d r a (4). At a b o u t 600 k e v , h o w e v e r , t h e a l i g n m e n t h a s a m a x i m u m and t h e a m o u n t d e c r e a s e s w h i l e s t e p p i n g t o h i g h e r e n e r g i e s . T h i s b e h a v i o u r i s o p p o s i t e t o c o n c l u s i o n s o n e m i g h t d r a w f r o m a m e a s u r e m e n t a t 1 MeV o f ref.(4) w h i c h i m p l y a f u r t h e r i n c r e a s e o f t h e a - m o u n t o f alignment. S m a l l e x c i t a t i o n pro- b a b i l i t i e s in t h e MeV-energy r a n g e c a u s e e x t r e m e l o w c o u n t i n g r a t e s of a b o u t 1 t o 5 cts/sec a n d a r e t h e r e a s o n for t h e l a r g e e r r o r b a r s i n t h i s p a r t o f t h e e n e r g y scale.

T h e d a r k c o u n t i n g r a t e o f t h e d e t e c t o r o f a b o u t 1 c t s / m i n i s t h e b a s e f o r L y m a n - &

p o l a r i z a t i o n m e a s u r e m e n t s in t h i s e n e r g y region. A s t o t h e s e m e a s u r e m e n t s o n e h a s t o p o i n t o u t t h a t d u e t o t h e l o w c o u n t i n g r a t e s t h i s p a r t o f t h e e x p e r i m e n t is r a t h e r t i m e c o n s u m i n q . E a c h d a t a p o i n t a t t h e h i g Q e n e r g y end r e p r e s e n t s a p u r e b e a m t i m e b e t - w e e n 4 and a b o u t I 0 hours.

I n e v a l u a t i n q t h e m e a s u r e m e n t s s o m e s o u r c e q o f s y s t e m a t i c e r r o r s a r e t o b e d i s c u s s e d . T h e d e g r e e o f p o l a r i z a t i o n o f t h e e m i t t e d r a d i a t i o n f o l l o w s f r o m P = P /P

.

A c h a n -

m a n a

g e o f t h e a n a l y z e r c h a r a c t e r i s t i c s c a u s e d by a m i s a l i g n m e n t o r c o n t a m i n a t i o n o f t h e m i r r o r s c a n c a u s e a s y s t e m a t i c s h i f t i n P.

T w o i n d e p e n d e n t e v a l u a t i o n p r o c e d u r e s h a l f a y e a r a p a r t w e r e c o n s i s t e n t w i t h t h e a n a - l y z e r d a t a g i v e n a b o v e s o t h a t t h i s in- f l u e n c e i s n e g l i g i b l e .

T h e t r a n s f e r o f a l i q n m e n t f r o m u p p e r s t a t e s may i n f l u e n c e t h e 2P-alignment. T h e i n v e s t i - g a t i o n o f t h i s e f f e c t by p e r f o r m i n g a p o l a - r i z a t i o n m e a s u r e m e n t a s f u n c t i o n o f t h e d i s t a n c e f r o m t h e f o i l w i l l b e d i s c u s s e d in a s e p a r a t e c h a p t e r and s h o w s t h a t at t h e p o s i t i o n o f t h e d e t e c t i o n z o n e t h e c o n t r i - b u t i o n o f a n a l i g n m e n t - t r a n s f e r i s small.

C1-312 JOURNAL DE PHYSIQUE

The influence of quantum beats on the polari- VI. Transfer of aliqnment zation measurement was already discussed.

Secause o f the 'broad-band' excitation by From the geometry of the detection zone one

the foil uDDer levels are populated notice-

- - -

- calculates an energy dependence of the para-

I I ably and via cascades will transfer part of

meter a from eq. (17) a s shown in fig.4. At

their populations and aliqnments to the

'energy I keV

Fig. 4

eneraies lower than 200 keV the influence of an imperfect averaging over quantum beats is negligible (a.20), while for energies above 300 kev a shift in alignment up to

2

15% is possible. The alignment of corresponding data points is corrected and the uncertainty in this procedure gives a major contribution to the error-bars.

V . Molecules as primary particles

The use of H;-molecules as primary particles shows an energy dependence which is quite similar in comparison to measurements with A+-projectiles (fig.5). This result implies

Fig. 5

that the sublevel population is independent from the structure of the incoming ion while the initial populations with respect to dif- ferent L-skates show a pronounced 'molecu- lar effect' (14).

state under study. The transfer of popula- tion is evident in beam-foil lifetime mea- surements and limits the precision of this technique, while a time resolved transfer of alignment has not been observed. In 1972 Berry et al. (15) found that the transfer of alignment in some helium and lithium states is negligible. They proposed to eli- minate the influence of cascades on a life- time m-easurement by recording the polari- zation of light a s a function of distance.

To find out the favourable conditions for observing a transfer of alignment we con- sider the cascade with states of angular momentums L o, L 1 , and L2. Lo belongs to the final state, L1 to the state under study, and L 2 to the upper state. Modifying former discussions with respect to this subject

(15,16,17) two questions have to be inve- stigated: (i) what are the conditions a s to L 1 , L2 for an effective transfer of alignment and (ii) how can this tranfer be detected under optimum conditions. The se- cond question has already been answered in chapter 1 1 , where it was pointed out that at a given alignment the degree of polari- zation is maximum for L < L 1 . To discuss the actual transfer of alignment from the level with angular momentum L2 to L 1 , we take the alignment-parameter o f eq.(l) and get for the ratio of the transferred to the orisinal alimnment parameter

of alignment is almost independent from the choice of L1 and L ~ . In conclusion one should detect a transfer of alignment by a transition of decreasing angular momentum.

For the hydrogen 2P-state one gets the essential cascading contributions from the 3s- and 3D-state. Since the 3s-state has no alignment only the transfer of alignment from the 30- to the 2P-state i s observable.

between fine fine structure states J J ' 2 2 and J I J i is based on the relation between the density o ~ e r a t o r components

and yields for the incoherent transfer

For the 3D-2P cascade one gets

The experimental setup for the transfer- measurement is identical with the one de-

scribed above with the exception that the foil is moved along the beam-axis by step- motor drive. The energy of the incoming H+- ions was 100 k e V which is a compromise on a sufficient alignment of the 2?-state and intensity of radiation.

distance I rnrn

Fig. 6

For evaluation of the transfer of alignment from the time-dependent Lyman-6 polarization one has to know the initial populations of the levels of interest. Therefore

-

^{as shown}

in the upper part of fig.6

-

a lifetime- measurement with a fixed analyzer a t the magic angle' was carried out. The qolid line represents a best fit with three exponen- tials describing the 2P-, 3 5 - , and ln-decays and a detection function g(z,z') a s given in fig.6 resulting from the diaphrams of the analyzer system.

tudes a 2p, a3s, a3D, and the lifetimes, the relative initial populations are derived.

The agreement with the work of Tielert et

al. ( 1 4 ) is satisfactory.

The degree of polarization as a function of distance from the foil is displayed in the lower part of fig.6. Defining the polariza- tion ratio Q = I /I and noticing that from

P S

the intensity I = I + Is it follows I = P Q e I / (Q+1) and Is = I/ (Q+l ) ; the degree of polarizatior, p(z) a t a distance z is given

where

4 3 ~ 4 ~%qs);+

Q,+l

Q=+d

Q S B ~ W

- % < & ) ~ i + . ~ .

⁽²⁶⁾

+-

B l o t 4

-

^{+ Qllt.4 Qua4}

The solid line in fig.6 is a best fit of eq.

(25) to the data where all parameters with the exception of Q 2P,

Q3D, and a. were known from the precedinq lifetime measurement. As can be seen from the figure, a distinct transfer of alignment takes place which ex- ceeds the initial alignment of the 2P-state.

At the moment of excitation the polarization is completely determined hy the initial alignment of the 2P-state. After several lifetimes of this level

(T

= 1.6 ns) the

2 P

transferred population from the 3E-state (7 = 15.5 ns) has the same magnitude as the

3 D

original one, thus the effect of the pola- rization on the transferred alignment in- creases. The fact that the 3s-state does not transfer alignment reduces the polari- zation towards t h e end of the measured curve. The dashed line in fig.6 shows the ZP-polarization without transfer of align- ment and demonstrates the observed effect.

From the fit one obtains for the initial 2~-alignment(%,-6~/(%~+~0~) = -0.028+0.005 and for the transferred alignment

(&-:r4)/

(:qs:T..) = -0.082~0.012 or :A = 0.053~0.008.

Application of eq.(23) yields for the ini- tial 3D-alignment D~ = 0.160+0.022. The 3D- alignment determined by quantum beat mea- surements is in good agreement with this value. Denis et al. (6) reported D~ = 0.186

C1-314 JOURNAL DE PHYSIQUE

a t 2 5 0 keV, and Burns and H a n c o c k ( 1 8 ) R e f e r e n c e s D~ = 0.234 at 1 3 0 kev. Regarding the limi-

( 1 ) H.J. A n d r Z , P h y s . S c r i p t a 2(1974)257 ted accuracy inherent in t h i s kind o f ex-

( 2 ) J. Macek, Phys.Rev.Letters Z ( 1 9 6 9 ) l p e r i m e n t s t h e v a l u e for D~ g i v e n here agrees

( 3 ) H.G. B e r r y , Rep.Prog.Phys. g ( 1 9 7 7 ) 1 5 5 w i t h t h o s e d a t a and s u p p o r t s t h e interpre-

( 4 ) P. Dobberstein, H.J. Andrd, W. Wittmann, t a t i o n of the measurement a s a t i m e resol- H.H. B u k o w , z.Physik ~ ( 1 9 7 2 1 2 7 2 ved t r a n s f e r of alignment. (5) H. Winter and H.H. ~ u k o w , ~ . P h y s i k A=

( 1 9 7 6 ) 27

VII. C o n c l u s i o n (6) A. D e n i s , J. Desesov-lles, M. Druetta

and M. Dufay, in 'beam-foil spectroscopy' pronounced d e p e n d e n c e of t h e hydrogen 2P- Vo1.2, ed. I.A. S e l l i n and D.J. Pegg,

Plenum Press, New York 1976 alignment o n energy i s evident from t h e

(7) L.J. Curtis, R. H a l l i n , J. Lindskog, J.

p r e s e n t measurements. T h e alignment shows P h i l and H.G. Berry Phys.Letters =A t w o c h a n g e s in sign a t a b o u t 17 keV and (1977)297

5 0 k e v and i s in t h i s energy inter- ( 8 )

''

F a n o a n d J.H. M a c e k , Rev.MOd.Phys.

45(1973)553 val. A minimum i s found t o be a t about 6 0 0

-

(9) I.C. Percival and M.J. Seaton, Phil.

keV, for h i g h e r e n e r g i e s t h e alignment Trans.Roy.Soc.London A251(1958)113 s e e m s t o a p p r o a c h z e r o or even a positive (lO)D.G. Ellis, J.Opt.Soc.Am. =(1973)1232 value. T h i s d e p e n d e n c e o n energy i s r a t h e r (l1)H. Kleinpoppen, in 'Physics o f the o n e - d i f f e r e n t in c o m p a r i s o n t o the a l i g n m e n t and two-electron atoms', North Holland

(1°'9)612 in non-hydrogenlike s y s t e m s where it i s in

(12)H. , . ~ n t e r , H.H. B u k o w , and P.H. Heckmanq g e n e r a l less structured a n d a positive Opt.Com. 1 1 (1974)299

d e q r e e o f polarization i s found. S i n c e t h i s (13)H'

d i f f e r e n c e in s i g n i s hard t o understand (14)R. T i e l e r t , K.H. Bukow, P.H. H e c k m a n n , R. Woodruff and H . v . B u t t l a r , 2 - P h y s i k from collision physics, o n e has t o t a k e

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264(1973) 129

into account o t h e r influences which may c a u s e t h i s effect.

(15)H.G. Berry, L.J. Curtis, and J.L. S u b t i l J.Opt.Soc.Am. g ( 1 9 7 2 ) 7 7 1

(16)M. D u f a y , Nucl .Instr .Meth. %(1973)79 An electric field at the foil-surface

-

(17)O. Nedelec, thesis, G r e n o b l e 1966 postulated earlier td d e s c r i b e t h e produc-

(18)D.J. Burns and W.H. Hancock, Phys.Rev.

t i o n o f circular polarization in tilted Letters =(I971 1 3 7 0

foil experiments ( 1 9 )

-

l e a d s via S t a r k - (19)T.G. Eck, Phys.Rev.Letters 2 ( 1 9 7 3 ) 2 7 0 effect t o d i f f e r e n t m o d i f i c a t i o n s of the M. Lombardi, Phys.Rev.Letters g ( 1 9 7 5 )

11 7 2 sublevels populations in hydrogenlike and

(20)E. Kupfer and H. Winter, Z.Physik A=

o t h e r atomic systems. We have shown t h a t ( 1 9 7 8 ) 3 a model of a s u r f a c e electric field which

m o d i f i e s the alignment of hydrogenlike a t o m s i s a b l e t o explain t h e discrepancy between hydrogen and o t h e r a t o m s (20).

The experiments were performed at t h e Ruhr- Universitwdt Bochum and a t the Labora-toire d e Spectrometrie Ionique e t , M o l e c u l a i r e in Lyon. The s u p p o r t in v a r i o u s k i n d s o f Prof.

B u k o w ( B o c h u m ) , Prof.M.Dufay, Dr.M.Druetta, Dr.M.L.Gaillard (Lyon), and Prof.R.J.AndrB,

~ . K u p f e r (Berlin) i s g r a t e f u l l y acknow-