Hyperfine structure of 14NIII 2p 2P3/2 by quantum beats after ion surface interaction at grazing incidence

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Hyperfine structure of 14NIII 2p 2P3/2 by quantum beats after ion surface interaction at grazing incidence

A. Schirmacher, H. Winter, H.J. Andrä, Y. Ouerdane, J. Désesquelles, G.

Docao, A. Denis

To cite this version:

A. Schirmacher, H. Winter, H.J. Andrä, Y. Ouerdane, J. Désesquelles, et al.. Hyperfine structure of 14NIII 2p 2P3/2 by quantum beats after ion surface interaction at grazing incidence. Journal de Physique, 1987, 48 (6), pp.905-909. �10.1051/jphys:01987004806090500�. �jpa-00210519�

HYPERFINE STRUCTURE OF 14NIII _2p

2P3/2

^{BY QUANTUM} BEATS AFTER ION

SURFACE INTERACTION AT GRAZING INCIDENCE

A.

Schirmacher,

^H.

Winter,

^H.J. Andrä, ^Y.

Ouerdane+,

^J.

Désesquelles+,

^{G. DoCao+ and A. Denis+}

Institut für

Kernphysik

der Universitât

Münster, Domagkstr.

^{71, D-4400} Münster, ^F.R.G.

+Laboratoire de Spectrométrie Ionique ^et

Moléculaire,

43 Bd. du 11 novembre 1918, ⁶⁹⁶²²

Villeurbanne,

^France

(Reçu

^{le 12} fivrier 198?’, accepte ^le !4 ^mars

1987)

Résumé.- Nous avons mesuré, par ^une éthode de battements quantiques, ^{les écarts de structure} hyperfine

de 14NIII 2p

2P3/203BD3/2,5/2

⁼

(252,0 ± 1, 1)

^{MHz et} 03BD1/2,3/2 ⁼

(126,2 ± 0,4)

^{MHz. De ces fréquences}

on déduit le moment quadrupolaire électrique ^du noyau Q

(14N) = (19,4 ± 0,9)

^{mb et la constante de}

couplage

quadrupolaire

pour l’électron 2p ^de 14NIII

1s22s22p 2P3/2 :

^{e Q qat =}

-(16,6 ± 0,6)

^MHz.

Abstract.- Applying ^{a modified} quantum ^beat method, ^the hyperfine splittings ^of 14NIII 2p

2P3/2

^are

determined to 03BD3/2,5/2 ⁼

(252.0 ± 1.1)

^{MHz and} 03BD1/2,3/2 ⁼

(126.2

^±

0.4)

^{MHz. From the} frequencies ^we

deduce the nuclear electric quadrupole ^{moment of}

14N Q (14N) = (19.4 ± 0.9)

mb and the

quadrupole

coupling ^{constant for the} 2p-electron ^{in 14NIII}

1s22s22p 2P3/2 :

^{e Q}

qat = - (16-6 ± 0.6)

^MHz.

Classification

Physics ^Abstracts

34.50 H ,- 35.20 S- 21.10 K

Measurements of hyperfine

(hf) -

splittings ^{in sta-}

ble and excited terms in ionized 14N allow to deduce the electric nuclear

quadrupole

^{moment of}

14 N [1]

^and

the

quadrupole coupling

^{constant of the 14NI}

2pS -con-

figuration ^{which is a relevant} quantity ^{in mol-ecular} spectroscopy. ^{Since the} quadrupole interaction is _neg-

ligible ^{in the}

14NI-atom,

^the coupling ^{constant has}

been deduced from molecular spectra ^{with some un-} certainty

[2].

^{We pursue the concept to deduce this}

constant by ^an extrapolation ^from coupling ^constants

for the 14 NII

2p2-

^{and the 14 NIII} 2p-configurations.

In this _paper we will concentrate our description ^on

measurements in the 14NIII 2p

2P3/2-term

^{to get the}

quadrupole coupling ^{constant for this} configuration ^and

the nuclear electric quadrupole ^{moment of}

14 N Q (14N) .

For determining ^the hf-splittings ^{of 14NIII} 2p

2P3/2

we apply ^the ground ^term hf-quantum ^beat technique

[3]

^after the interaction of fast ions with a solid sur-

face at grazing incidence. These collisions result in a

large orientation in the distribution of electronic or-

bital angular ^momenta

[4]

^{which is} partly transferred to the nuclear spins ^via hf-interaction. Since ground

and metastable terms in 14N ^are dominantly popu- lated after ion-surface scattering, the transient transfer of anisotropy between electronic shell and nucleus re-

flects the hf-frequencies ^{of those terms.}

To apply ^{the fast beam} quantum ^beat technique

in ground and metastable terms, ^we observe after ion- surface scattering the transient nuclear spin ^orienta-

tion. This is accomplished by ^the interaction of the scattered ions with a thin foil which rearranges

(and excites)

^the electronic shell, ^but preserves the nuclear orientation. The observation of the circular polariza-

tion of foil excited light given by ^{the Stoke’s} parameter

S/1 = (1 ((1-) - 1 ((1+)) / (I (a-) + I (a+)) (I ((7:1:) =

intensity ^of light ^with negative ^and positive helicity,

respectively)

^{is a direct measure of the nuclear orien-}

tation

[5].

The

quantum-beat

technique ^{for stable terms} by

Andri and Winter

[3]

^is successfully applied by ^Schir-

macher

[7]

^{in most} of the stable terms in 14NI and

14 NIL At the high energy end of the accelerator used in the studies of reference

[7] (Emax=

³⁵⁰

keV)

^{the quan-}

tum beat spectra

yield

^an indication of ^a frequency component which is estimated to stem from a hf-split- ting ⁱⁿ 14NIII 2p

2 P3/2.

^The experiments reported ^here

are motivated by ^{the fact that an} increase in _energy of the projectiles will also increase the fraction of the

N2+-component

in the surface scattered beam so that hf-measurements in 14NIII ^are feasible.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01987004806090500

906

Experiment

A sketch of our

experimental

setup ^{is shown in} fig-

ure 1. The experiments ^are performed ^{at the 2.5 MeV}

van de Graaff accelerator of the Institut de

Physique

Nucl6aire

(Lyon, France). Nt-ions (0.8 -

^1.8

MeV)

are used instead of N+-ions because of reasons of beam intensity. ^The ion-beam is bent by ^{electric field} plates ^{onto a} highly

polished

silicon surface of lenghts ranging ^{from 2.5 to 5 mm at a} scattering angle ^of

about 0.5°. In some runs the surface scattered ions

are selected with respect ^to charge ^states by ^electric

field

plates

^{and slit} systems close behind the target.

All these components ^{are mounted} on a common block which is moved

by

stepmotor drive with respect ^{to a} carbon foil about 5

jig/CM 2

thick and its surface nor-

mal

parallel

^{to the beam.}

The circular polarization ^{of the} light emitted from foil excited atoms is detected by ^{a lens} system, quarter

wave plate, ^beam splitter polarizer, ^{and two} indepen-

dent

photomultiplier

channels. This detection scheme

provides ^a simultaneous measurement of

I(a-)

^and

I(a+)

which cancels the effects due to fluctuations of the beam intensity ^{on the data.}

For the optical ^{detection of} nuclear orientation transitions from terms with large ^transfer factors be- tween nuclear and electronic orientation should _spec-

troscopically ^{be selected}

[5].

^{In these} experiments, however, ^the

intensity

^of

light

is rather low so that we

relinquish ^an interference line filter and simultaneously

observe all transitions within the spectral response of the detector

("white light detection").

^{This causes}

some reduction in the detected polarization

(and

^quan-

tum beat

amplitudes)

ⁱⁿ comparison ^{to the selection of}

favourable lines, ^{but this} drawback is overcompensated by ^the significant ^{win in} intensity. ^{The same} argu- ment holds with respect ^{to the use of}

N+-molecular

ions instead of

N+ -ions,

^{where some reduction in col-}

lision induced orientation after ion-surface scattering

and

dissociation

of molecules in comparison to pure ions is observed

[6].

Data sets are taken by varying ^{the distance be-}

tween solid surface and foil via microcomputer-control

in equidistant steps ^and recording

1(0’-)

^and

1(0’+).

Due to the mechanical

design

^the minimum in _ap-

proach of surface and foil is about ^{65 mm.}

Results

In a number of experiments ^{with 14}

N+

-projecti-

les at energies ^{between 0.8 and 1.8 MeV we resolve} quantum ^beat pattern ^{as shown in}

figure

^{2 at an en-}

ergy of 870 keV. Figure ³ shows in the _upper part ^a Fourier-transform of the data in

figure

^{2 and in the}

lower part ^{of data at an energy of 1.57} MeV. Whereas the transform of the data in figure ^{2 shows} components of terms in NI-III

(for

detailed discussion see

Ref.[7]),

the transform of the data at 1.57 MeV

(lower curve)

shows two prominent

frequencies

^{which dominate the}

spectrum ^{and can be ascribed to the}

hf-splitting

ⁱⁿ 14NIII 2p

2P3/2.

^{Figure 4}

^displays

the data where the

N2+-component

of the reflected beam at an energy of 1.1 MeV is selected

by

electric field

plates

and slits be- hind the target. The data show a simple ^{beat structure}

which is

generated

by ^two

frequencies

^{of the}

splittings

in 14 NIII 2p

2P3/2 ^(see

Fourier transform in

Fig. 5).

The quantitative analysis ^{of the} hf-splittings ^of

Fig.l.-Experimental

setup

Fig.2.-Quantum ^beat pattern after the interaction of 870 keV -

N2 -

molecular ions with a solid Si-surface at

an angle ^{of incidence of about} o.3° . The plot shows the variation of the circular polarization ^fraction

8/1 of

^foil

emitted light ⁱⁿ dependence ^on the distance between surface and foil.

Fig.3.-Fourier-transform of the data in figure ²

(upper curve)

^{and of data at 1.57 MeV}

(lower curve).

^Details

with respect ^{to the} identification of charge ^{states in}

the Fourier-spectrum ^{will be} given ^{in reference}

[7].

14 NIII

2p2P3/2

^{is based on known} frequencies ^{in 14NI}

[8]

^14NII

[7]

^{which are} simultaneously ^{observed as}

shown in the Fourier spectra ⁱⁿ figure ^3. By ^{this cal-}

ibration

procedure

^{of the} frequency ^{scale we find for} 14 NIII

2p 2pS/2 :

From these frequencies ^we derive the A-factor

A3/2 - (96.6

^z

0.4)

^{MHz and the B-factor}

r83/2

⁼

(8.32 ±0.30)

MHz, respectively.

In previous investigations of hf-structures in ion- ized 14N

[1]

good agreement ^{of the} experimental ^data

with single configuration Hartree-Fock calculation

[9]

is found. For 14NIII 2p

ZP3/2

^{we compute}

(r-S)2P

⁼

4.677

ao 3 (see

^{also Ref.}

[10])

^{and with pi= 0.4037}

JJB

me jmp [11]

^{where 3B} is the Bohr magneton, me

and mp ^are the masses of electron and proton, respec-

tively, ^{we have for the} 2p-electron :

and

A3/2= 8/15

a2p= 96.05 MHz. This is in excellent agreement ^{with our} experiment ^{and confirms earlier} findings ^that single configuration Hartree-Fock calcu- lations provide

(r- 3)

-integrals ^{for an} adequate ^de- scription ^of hf-splittings ^{in ionized 14N.}

From the B-factor we deduce the electric

quadru-

pole ^{moment of}

14 N Q (14N)

^{and the} quadrupole ^cou- pling ^{constant e} Q gat with the electric field gradi-

ent

qat= a2v /az2

^{at the} nucleus. From measured

A3/2

^{we get}

(r-3)2P

⁼

(4.707 :f:: 0.015) uo 3

^{and by}

neglecting magnetic shielding ^{one deduces from}

b2p=

e2 Q (r -3 )2p, ^b2p

⁼

5/2 B3/2

⁼

(20.B:f:: 08)

^{MHz and}

for the "experimental quadrupole moment"

(18.8 ± 0.7)

mb. By

applying

^the Sternheimer correction

[12]

^with

(1-R)-1 =

^{1.03 from} calculations by Sen and Weiss

908

Fig.4.-Quantum ^beat pattern ^after the interaction of 1.1 MeV -

N2+

-molecular ions with a solid Si-surface. The

N2+-component

of the surface scattered ions is selected by ^means of electric field plates ^{and slits.}

[10],

^we finally obtain Q

(14N) = (19.4 ± 0.9) ^mb.

For the 14NIII 2p

ZP3/2-term

^{we get for the coupling}

constant of the single

2p-electron

^e Q qat= -2

Bs/2=

- (16.6 ± 0.6)

^MHz.

Fig.S.-Fourier-transform

^of the data in figure ^{4. The}

two

frequencies

^{are ascribed to the}

hf-splittings

^{of 14NIII} 2p

2P3/2-

Conclusion

In this _paper we report ^{on the} application ^{of the} ground ^term quantum ^beat technique ^{for the measure-}

ment of the hf-splittings ^{in 14NIII}

2p 2P3/2.

^{The exper-}

imental A-factor is well reproduced by ^a theoretical _ap-

proach ^{based on} single configuration Hartree-Fock cal- culations. From the B-factor we deduce an electric nu-

clear quadrupole ^moment Q

( 14 N) = (19.4 ± 0.9)

^mb.

This result supports the results of a former experiment

[1]

^{where Q}

(14N) = (19.3 ± 0.8)

^{mb is found, and can}

be considered as an independent ^{test for the evaluation}

of this quantity.

In addition we determined the electric quadrupole coupling ^{constant for the 14NIII} 2p-configuration. ^In

molecular spectroscopy ^the coupling ^{constant for the} single 2p-electron ⁱⁿ the neutral 14NI

2p3-configura-

tion is of importance ^{to deduce} structures and orbital

populations ^{in molecules} containing ^{14N from mea-}

sured hf-splittings. ^{Since the} quadrupole interaction in 14NI is too small, ^{the constant has} been estimated from molecular data with a high degree ^of uncertainty

[2].

^{There is good hope that an} extrapolation ^{based on}

experimental

^{data in 14NII and 14NIII}

(reported here)

will provide ^{a more accurate} determination of this con-

stant. Details of corresponding measurements in 14 NII and the analysis ^to determine the coupling ^{constant in} 14NI will be given ^elsewhere

[7].

Acknowledgements

The

hospitality during

^{the runs at} the Labora-

toire de Spectrometrie Ionique ^et Moléculaire is grate- fully acknowledged. We thank M. Champlovier

(Lyon)

for his assistance in running ^the accelerator, ^{U. Linke}

(KFA J31ich)

and Fa. Wacker-Chemie

(Burghausen)

for providing ^{us with the} Si-target, ^{and B.}

Hippert (Bochum)

and H. Baumeister

(Mfnster)

^{for the pro-}

duction of the carbon foils. We also acknowledge ^the support ^{of H.W.}

Ortjohann

during ^the

preparations

^of

the experiments. This work is supported by ^{the Son-}

derforschungsbereich

²¹⁶

Bielefeld/ Miins ter.

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