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ALIGNMENT OF P- AND D-STATES OF
THREE-ELECTRON CARBON IONS PRODUCED IN COLLISIONS OF C4+ (1s2 ) WITH H2
R. Hoekstra, M. Suraud, F. de Heer, R. Morgenstern
To cite this version:
R. Hoekstra, M. Suraud, F. de Heer, R. Morgenstern. ALIGNMENT OF P- AND D-STATES OF THREE-ELECTRON CARBON IONS PRODUCED IN COLLISIONS OF C4+ (1s2 ) WITH H2.
Journal de Physique Colloques, 1989, 50 (C1), pp.C1-387-C1-392. �10.1051/jphyscol:1989145�. �jpa-
00229343�
JOURNAL DE PHYSIQUE
Colloque C1, supplement au n 0 l , Tome 50, janvier 1989
ALIGNMENT OF P- AND D-STATES OF THREE-ELECTRON CARBON IONS PRODUCED IN COLLISIONS OF
c4
+ ( Is2 ) WITH H2R. HOEKSTRA* * *
,
M. G. SURAUD* * * * , F.
J. de HEER" and R.
MORGENSTERN' '~ernfysisch Versneller Instituut, Zernikelaan 25, NL-9747 AA Groningen, The Netherlands* * F O M - ~ n s t i t u t e for Atomic and Molecular Physics, P.O.Box 41887, NL-1009 DB Amsterdam. The Netherlands
Rlsuml.- Nous avons dCterminC, par spectromCtrie, U.V. la polarisation du rayonnement Cmis lors de transitions de type (31--1211) d'ions LithiumoPdes de Carbone, produits par Cchange de charge lors de la collision B basse Cnergie (0.2 (v< 0.5 u.a), entre un ion projectile de
c4+
et une cible neutre de gaz dthydrog&ne molCculaire. Les fractions de polarisation mesurCes correspondant aux Ctats 3p et 3d sont positives et croissent tr&s sensiblement avec lrCnergie des ions incidents. Ceci indique que le sous-niveau magnetique m -0 de llCtat 3p est prif6rentfellement peuplC parz-
capture electronique et que ce phCnom&ne augmente avec l'energie des ions incidents.
Abstract.- We have determined by means of V.U.V. spectroscopy the polarization of the (3 2 - 4 2 1') transit ions of the
c3+*
ion prcltlrlr-erl icec4+-
Hz collisions, in the 0.2<v<0.5 a.u velocity range. Thr polarization fraction, quantitatively discussed in terms of alignment of the orbital momentum substates, shows that the m l substates, in the n=3 shell ofc3+*
are not populated equally by the electron capture process. For the 3p- and the -3d-states the polarization increases with the energy, and indicates, for capture to the 3p-state, a dominant population of the ml=O substate.Since a few years, collisions of multicharged ions with atoms, at low impact velocities (v(1 a.u), are in the focus of attention of both theorists and experimentalists. Electron capture in such collision systems leaves the projectile ion in a specific excited state, and considerable efforts have been devoted to the determination of the n, 1 and m distributions /I/, /2/.
In particular, the knowledge of the angular momentum di'stribution is important for several theoretical and practical aspects. The magnetic substate cross sections provide even more detailed information on the excitation mechanism. Theoretically, it has been shown by Salin /3/ that the m distribution is completely governed by the primary charge exchange process. While theoretical nl partial cross sections are now available, calculating the capture rate into well-defined magnetic-substates m remains
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989145
JOURNAL DE PHYSIQUE
a challenging problem, which calls for sophisticated theories. Thus the study of the state-alignment yields relevant information on the collision process, which should help in proper theoretical modelling.
Due to different populations of magnetic sublevels within a certain (nl) subshell, the radiation may be polarized and, consequentely, anisotropic /4/,/5/. Hence, from an experimental point of view the knowledge of the polarization is important for the determination of accurate absolute and relative, photon emission cross sections. In the present study, polarization measurements were performed for .(31+2lt) transitions of Li-like Carbon ions. These c3+*ions were produced by electron capture in
(c4+-
H2) collisions for incident velocities of O.Z(v(0.5 a.u..
In theconsidered incident velocity range the charge exchange process populates predominantly the n=3 state. It is important to note that in this Ease, there is hence no strong influence from cascading contributions and we have direct access to the alignment of the states produced in the collision.
Polarization effects in this collision system have earlier been investigated by Baptist et al. /6/.
The beam of
c4+
ions has been provided by the E.C.R. source installed at the K.V.I. In the collision chamber, typical beam currents (electric) of c4+ ions varied between 0.02 FA (at 1.3 KeV amu-' projectil$ energy) and 1.5 pA (at 6 KeV amu-' projectile energy). The target is a molecular hydrogen beam, as described in detail by Cirid et al. /7/. The post-captureKVI 8134
vuv -
monochromator,--.
, . . . /
-
'
\..,---
/..-<
K _--.
! $\.
.../---- Ir':
\ T- i4!,\,
]
---I.,\!.
_.,. j_-c- Nod!
-
1-\.L./-/---
H, beam
Fig.1 Schematical view of the set up.
radiation resulting from the decay of c'+'-states is observed by a grazing-incidence spectrometer for the V.U.V. light (10 - 80 nm). The
experimental set-up has been described extensively in previous articles, see e.g. / 8 / . However to make polarization measurements possible, the collision chamber itself has been replaced by one, which has not only observation ports perpendicular to the beam, but also at the %nagic angle" ( 5 4 . 7 O ) , where there is no influence of polarization. Figure 1 shows schematically
the major components of our experimental set-up, including the various
0
positions of the spectrometer. The monochromator is tilted by 4 5 with respect to the plane defined by the beam axis and the djrection of observation, such that the sensitivity of the monochromater is independent of the polarization. For i,llustration, fig 2 shows a typical spectrum obtained for the (c4+- Hz) collision at 32 KeV impact energy in the 3 0 - 4 5 nm wavelength range. The full curve in the spectrum corresponds to a least mean square gaussian fit of the peak and the broken line defines the background. The uncertainties in the fitting lead to a maximum error in the intensities of the order of 2%
.
Fig.2 Typical photon emission spectrum for
ci'-
H, collisions.The procedure for evaluating the polarization of the ( 3 p - + 2 s ) and
( 3 k 2 p ) transitions, is to measure the relative change in the intensity of
0 0
each peak at two spectrometer orientations, namely 9 0 and 5 4 . 7
,
with respect to the ion beam. The measured intensities are normalized by relating them to the ( 3 s - 2 p ) line intensity, which is isotropic. For the case of constant target gas pressure in the whole observation area of theJOURNAL DE PHYSIQUE
spectrometer the normalization factor is given by:
I (90')
s-P = sin 54.7' r 0.82 I (54.7O)
s-P
where I (8) is the measured intensity of the non-polarized ( 3 ~ 4 2 ~ )
s-P
transition at the angle 8. Experimentally the ratio eq. (1) is reproduced within 20%
.
The intensity of light emitted at an angle 8 is related to the
0
polarization at 90 by the relation /9/:
0
where I and II are the intensities of radiation emitted at 90 with respect
I I
to the projectile beam with electric vector parallel and perpendicular to the beam respectively. Using the normalisation factor (eq. ( I ) ) , we can obtain the polarization of the (3p -+ 2s) and (3d + 2 p ) transitions from the measured signals by applying the following formula :
Before discussing our data, let us shortly recall the expression for the theoretical polarization. For the (np-ns) and the (nd-np) doublet lines the cross section for production of the ml=O (a,) the Imll=l (a,) and the
l m l l = 2 (a ) magnetic substates of the np and the nd states are related to
0
the linear polarization fraction at 90 by :
and
The experimental results of the polarization of the light emitted from the the 3p- and the 3d-states, respectively, are given in figure 3 and 4. The error bars are obtained as a combination of a statistical error and the error resulting from the fitting procedure. The polarization fraction is found to be positive and to increase strongly with collision energy.
Fig.3 Polarization U of the 3p state in CIV, produced in collisions of
c4'
on molecular hydrogen. Results: ElBaptist et a1./6/, this work.
Fig.4 Polarization U of the 3d state in CIV, produced in collisions of
c4'
on molecular hydrogen. Results: this work.
For electron capture into the 3p-state, the polarisation varies from ll =0.05 at ~ ~ 0 . 2 2 a.u to U =0.3 at v=0.5 a.u, which clearly indicates that this state is highly aligned at large velocities, as the theoretical maximum is U =0.43 (eq.(6) for al=O). Although our data cannot be directly compared to the theoretical predictions of Salin /3/, vho performed calculations for completely stripped projectiles, it is interesting to note that both his prediction and our experimental results show a predominant population of the
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m -0 substate. Population of this substate represents between 1- 56% (at v=0.22 a.u) and 91% (at v= 0.5 a.u) of the total 3p-state population. On the other hand these results are not in agreement with the data of Baptist et al. /6/ who find an increasing population of the Imll-l substate with the velocity.
In the case of the 3d-state a definite conclusion is more difficult to reach. We can nevertheless state that the lmtl=2 substate population cannot be strong and is certainly less than the average population of the ml=O and Imll=l substates. This remark also supports the results of Salin's calculation /3/ which shows a decreasing population for sublevels with m)2.
We cansummarize our present conclusions in the following way. Firstly the polarization of radiation detected in the n=3 to n=2 transitions in Li-like carbon ion, shows that the m distribution strongly varies with the velocity of the incident ions. In addition, the excited states formed in the collision of few-electron ions with Hz are aligned, with a fraction of the 3p-state with m 1- -0 exceeding 90% at vz0.5 a.u.
The study of the alignment is a very delicate subject both for the experimentalists and the theorists but represents a challenge for a more detailed understanding of the charge exchange processes.
Acknowledgement: The authors gratefully acknowledge the excellent technical assistance of J. Sybring and J. Eilander. This work is part of the research program of the Stichting voor Fundamenteel Onderzoek der Materie (FOM) with financiel support from the Nederlandse organisatie voor Wetenschappelijk Onderzoek (NWO).
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