Optical pumping of the metastable (2p5 3s 3P0) state of 21Ne

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Optical pumping of the metastable (2p5 3s 3P0) state of 21Ne

J.-P. Lemoigne, F. Sage, D. Lecler

To cite this version:

J.-P. Lemoigne, F. Sage, D. Lecler. Optical pumping of the metastable (2p5 3s 3P0) state of 21Ne.

Journal de Physique, 1978, 39 (2), pp.125-128. �10.1051/jphys:01978003902012500�. �jpa-00208745�

OPTICAL PUMPING OF THE METASTABLE (2p5 ^3s 3P0)

STATE OF 21Ne

J.-P.

LEMOIGNE,

^{F. SAGE and D. LECLER}

Laboratoire de

Spectroscopie Atomique (*),

Université de

Caen, 14032

^Caen

Cedex,

^France

(Reçu

^{le 4 octobre}

1977, accepté

^{le 8 novembre}

1977)

Résumé. ²⁰¹⁴ Le pompage

optique

^{dans une}

décharge

H.F. faible du niveau métastable

2p5

^3s

3P0

( 1S3 ^en notation de Paschen) ^a été obtenu à l’aide d’un laser continu à colorant. On observe un signal

de résonance

magnétique

correspondant ^{à un} facteur de Landé

g(3P0) =

(3,02 ± 0,30) ^{x 10-4}

supérieur

^au facteur de Landé de l’état fondamental

g(1S0) =

(2,402 ⁷⁵ ± 0,000 05) ^{10-4 et en}

bon accord avec les prévisions

théoriques

(3,08 ^x

10-4)

^tenant compte ^du mélange ^{des états}

3P1

et

1P1

^{de la} configuration

2p5

^{3s avec l’état}

3P0

^{sous l’effet} de l’interaction hyperfine. ^La précision

médiocre du résultat est due au temps de relaxation relativement court de l’orientation des méta- stables dans les conditions expérimentales ^actuelles.

Abstract. ²⁰¹⁴ Metastable

2p5

^3s

3P0 (1S3

in Paschen notation) ^atoms of 21Ne are optically

pumped

in a weak

discharge

by ^{means of a C.W.}

dye

laser. The magnetic ^resonance is observed for a g-factor

g(3P0) = (3.02

± 0.30) ^{x 10-4}

larger

^{than the} ground ^state g-factor

g(1S0) =

^{(2.402 75 ± 0.000} 05) ^{x 10-4}

and in

_the

_1P1

^good _and

_3P1

^agreement levels of the ^with

the theory 2p5

^3s configuration through ^{(3.08 x}

^{10-4) taking}

^{the into}

hyperfine

^account interaction. The ^the

^mixing

^{of the} précision

^3P0

^withof

the result is _poor because of a

relatively

short relaxation time of the orientation of the

3P0

^{state in the}

present

experimental

conditions.

Classification

Physics ^Abstracts

32 . 80B

1. Introduction. ^- For odd

isotopes,

^{the J = 0}

levels of atoms may have

magnetic

^moments

signi- ficantly

different from the _pure nuclear values. The effect is due to the

hyperfine coupling

which mixes

non zero J contributions into the wave functions

describing

^{J = 0 states and} therefore introduces some

electronic

magnetism

into these states.

Experimental

evidence of such effects has

already been reported [1, 2].

Similar behaviour is

expected

^{in the J = 0 metastable}

states of rare gases. On the other

hand, optical pumping

^of metastable

3P2

^{states of rare gases has}

been achieved

by

several authors

[3, 4],

the metastable states

being initially populated by

^{a weak H.F. dis-}

charge.

The orientation of the

2p5

^3s

3Po

^state

of

2 ’Ne

has also been observed

[4] but,

^{to our know-}

ledge,

^we

give

below the first

report

^{on a}

magnetic

resonance

experiment leading

^to the determination of the Lande factor of the

3Po

^state

^{of 2’Ne.}

2.

Experiment.

^{- The}

geometrical arrangement

^of the set up is shown

schematically

ⁱⁿ

figure

^1.

The cubic cell

(3

^cm

edge length)

^is filled with a

mixture of 0.5 torr of He and 0.015 torr of Ne enriched at

50 % ^{in 21 Ne.}

^{A weak}

capacitive discharge

^{at 7 MHz}

is maintained in the cell. A

simplified

energy level

diagram

of the Ne levels involved in our

experiment

FIG. 1. ^- Optical pumping ^{and detection.}

is

given

ⁱⁿ

figure

^{2. One can} think of several

optical pumping

^{schemes to} achieve the orientation of the F ⁼

3/2, ’Po

^{state. With our}

experimental

arrange- ment we found that the most efficient _way was to tune the

dye

^{laser to 5 882}

^À (lss-2p2) (Í).

^The

absorption

of the

circularly polarized light propagating

^{in the}

magnetic

field direction Oz and the

spontaneous

(’) Pumping ^schemes starting ^{from the} 3Po ^{state need the laser to}

be tuned on a particular hyperfine component, ^{which was not} possible ^{with our « broad} band » and free running dye ^laser.

(*) ^{Associé au C.N.R.S.}

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:01978003902012500

126

FIG. 2. ^- First excited levels of Ne.

emission at 6 164

A (ls3-2p2)

^ensure the desired orientation. The laser has a

long symmetric cavity (1.5 m)

^{of the Z} type ^so that the modes are

separated by only

^{100 MHz.} The line width of the laser is about 2

GHz, slightly

smaller than the

Doppler

^{width. We}

found that a small

detuning

of the laser to the blue from the maximum of fluorescence

gives

^{a better}

signal, probably

because the

hyperfine

components

(F

⁼

5/2 -

^{F’ =}

5/2) (F

⁼

5/2 -

^{F’ =}

7/2)

^{of the}

21Ne

5 882

À

line are

preferentially

^{used for}

optical pumping.

^{The total power at 5 882}

^A

^is

typically

^{30 mW}

and the beam is

expanded

in the cell to have a section of about 1

cm2.

The

steady magnetic

^field

Ho

^is

produced by

Helmholtz coils and has been calibrated

using

proton

magnetic

^resonance.

The

rotating

field for

magnetic

^{resonance is} pro- duced

by

^two

pairs

^of coils whose axes are in the Ox and

w

directions

(perpendicular

^to

Ho). They

^{are fed}

with R.F.

signals

^{90° out of}

phase

with each other

by

^{means of two}

high

power

amplifiers.

^{With such a}

rotating

^{field we can avoid}

Block-Siegert

effects that could have occurred with a conventional

oscillating

field.

The detection uses the crossed beam

technique (see

^e.g.

[6]).

^{The source is an} electrodeless

discharge lamp

filled with 2 torr of natural Ne. The

light

^is

circularly polarized

^{and we} monitor the modulation of

absorption

^{at the R.F.}

frequency

^{on the 6 267}

A

line

(lS3-2p.).

^The

signal

^is detected with a lock-in

amplifier

^whose

phase

^is

adjusted

^to get ^{either Lorent-} zian

shaped

^or

dispersion shaped signals.

^A

signal

averager is used to

improve

^the

signal

^to noise ratio

further ;

^{its memories are}

explored synchronously

with the

magnetic

^field sweep.

3. Results. ^- We have observed

magnetic

^reso-

nuances at

43.2, 46.7

and 52.3 kHz.

Averaging

^{times of}

about two hours are necessary ^to obtain curves with a

signal

^to noise ratio of about 20. The

experimental

curves are fitted to a sum of Lorentzian and

dispersion shaped

curves, centred at the same location and with the same width. This

procedure

^is necessary because it is difficult to set the

phase

of the lock-in

precisely.

^For

example

^{if we set the}

phase

^to

get

^a Lorentzian

shaped

curve,

typical experimental

^{curve shows a} deformation

corresponding

^{to a} mixture of 20

%

^{to 25}

%

^{of dis-}

persion shaped

^curve.

Figure

³

represents

^{the mean} values obtained for the

position

^{of the resonances}

FIG. 3. ^- Position of the resonances versus frequency. ^{The errors} quoted ^{in the} graph takes into account the standard error of the measurements and the systematic difference of the position ^of

symmetric ^and antisymmetric ^curves.

FIG. 4. ^- Statistics of the g-factor measurements : e with anti-

symmetric dispersion shaped curves ; + with symmetric ^Lorentzian shaped ^curves.

versus

frequency.

^The

slope

^{of the}

straight

line fitted to the three

experimental points

^of

figure

³

gives

^a

g-factor significantly

different from the _pure nuclear

one of the

ground

^state

lso (dotted

^{line on}

figure 3).

Figure

⁴ represents the statistics of our

g-factor

measurements. It can be seen from this

figure

^{that a}

slightly

^{different mean} value is obtained for the measurements made with a Lorentzian

shaped

symme- tric curve

(S)

^{or with a}

dispersion shaped antisymmetric

curve

(AS).

^{In our}

present experimental conditions,

the relaxation time of the orientation of the

3Po state

^is

relatively

short. If we assume for the

3Po

^{state a}

metastability exchange

^{cross section}

equal

^{to the one}

measured

by

^M. Pinard and M. Leduc

[5]

^{for the}

3P2

state, ^{we calculate a} relaxation time of the orientation of about 12 _us

(in

^our pressure

conditions)

^{that is to}

say ^a full width at half

height (FWHH)

^{of 63} gauss ;

experimentally

our curves have a FWHH of about 70 _gauss. The

magnetic

^{field is} swept ^{from a few} gauss to about 200 _gauss. With such field variation the

discharge

in the cell

depends

^on

Ho,

^{and the}

optical pumping

may also _vary with it. The

resulting

^{effect is a}

background

^{which can} have different effects on the

position

^of

symmetric

^or

antisymmetric

^resonance

curves

(Fig. 4). Taking

^{into account the}

dispersion

^of

the measurements and

possible systematic

^{errors we}

obtain the

g-factor

but we

hope

^to

improve

this result in a near future.

4.

Theory.

^{- The}

mixing

^of

3P1

^and

iPl

^{states in}

the

3Po

wavefunction is

responsible

for the difference between the

g-factor

of the metastable state and the pure nuclear

g-factor

^{of the}

ground

^state. Calculation of these effects have

already

^been

published

^{in the case}

of cadmium

[1]

^and mercury

[2].

The relations esta-

blished for an sp

configuration

^can be used for the

p’s configuration

of Neon and the

hyperfine coupling

constants for the s electron and the _p hole are deduced from the

hyperfine

structures of the

1 P 1 (ls2), 3P1(ls4)

and

3p 2(lss)

^levels

using

the Breit and Wills

theory [7].

In the case

of Neon,

^{it is not} _necessary to use the modi- fication of the Breit and Wills

theory

introduced

by

Lurio

[8],

^{and as Neon is a}

light

element the relativistic parameters ⁰

and ç

^used

by

^Lurio

[8]

and several references therein can be taken

equal

^{to 1.}

Finally,

^we

need the intermediate

coupling

coefficients introduced

by expanding

^the

1 PI

^and

3P1

^{real wave functions}

1 ’Pl > and 13pl >

^on the basis of _pure LS

coupling

functions : ^,

The table

gives

the different theoretical values of the Lande factor

g(3Po)

^of

^21Ne

^{that can be}

^computed

using

the different

possible

^{sets of A} values and also different values of the intermediate

coupling

^cons-

tant

fi.

Theoretical values

of

^the

Landé factor

The

following A

values have been used in the

computation

and

for the

ground

^state

g-factor.

As can be seen from the

table,

^the

theoretical g

^value

does not

depend strongly

^on the intermediate

coupling

coefficient

fi.

^{In the} frame of the Breit and Wills

theory,

^{we use}

only

^two individual

hyperfine coupling

constants a3, and a2p ^{for the s} electron and the _p hole

respectively.

^{Such a}

simplified theory

^{does not}

give

very

good agreement

between the

experimental

^{set of A}

values and the

computed

^one

[ 11 ] .

^A

discrepancy

^of

the same

type

^{is to be seen} between the different columns of the table where ^we have used the différent

possible

^{sets of A values to determine a3s and} a2p.

5. Conclusion. ^- In

spite

^{of the} poor

precision

^of

our present

experimental

determination of the

g-factor

of the

3Po(ls3)

^{level of odd}

isotope 21Ne

^the

agreement

between

experiment

^and

theory

^is

fairly good

^and

sufficient to

distinguish clearly

^the

3Po g-factor

^from

the

groundstate

^{one. An}

experiment

is in progress ^to obtain narrower resonance curves

especially by lowering

^{the Ne} pressure in the cell. An atomic beam of metastable rare gases ^atoms is also in construction in the

laboratory

^to enhance the

precision

^{of our}

present

measurement and to allow us to extend it to J ⁼ 0 metastable states of other rare gases.

We are very

grateful

^{to F. Laloe and M. Leduc from}

the Laboratoire de

Spectroscopie

Hertzienne

(ENS Paris)

^for

helpful

discussions

throughout

^{this work.}

(2) ^C. Delsart, ^{J. C.} Keller, Private communication.

128

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Phys. ^{Lett. 28A} (1969) ^660.

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Sci. Paris 271B (1970) ^342.

[5] PINARD, M., LEDUC, M., J. Physique ³⁸ (1977) ^609.

[6] LALOE, F., Thesis Paris (1970).

[7] BREIT, G., WILLS, ^M. A., Phys. ^{Rev. 44} (1933) ^470.

[8] LURIO, A., Phys. ^{Rev. 142} (1966) ^46.

[9] DELSART, C., KELLER, ^J. C., Opt. ^{Comm. 16} (1976) 388.

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