Observation of optical pumping effects in collinear atomic beam - laser beam(s) interaction

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Observation of optical pumping effects in collinear atomic beam - laser beam(s) interaction

N. Bendali, H.T. Duong, J.M. Saint Jalm, J.L. Vialie

To cite this version:

N. Bendali, H.T. Duong, J.M. Saint Jalm, J.L. Vialie. Observation of optical pumping effects in collinear atomic beam - laser beam(s) interaction. Journal de Physique, 1983, 44 (9), pp.1019-1023.

�10.1051/jphys:019830044090101900�. �jpa-00209686�

Observation of optical pumping ^effects in collinear atomic beam - laser beam(s) interaction

N. Bendali, ^{H. T.} Duong, ^{J. M.} Saint Jalm and J. L. Vialie

Laboratoire Aimé Cotton (*), ^C.N.R.S. II, ^Bâtiment 505, ⁹¹⁴⁰⁵ Orsay ^{Cedex, France}

(Reçu ^{le 9 mai} 1983, accepté ^{le 26 mai} 1983)

Résumé. - La sensibilite de la méthode de détection des résonances optiques par observation de la fluorescence émise par ^un jet rapide

excitation colineaire, ^est limitée par le pompage optique. ^Dans l’ expérience ^décrite ici,

nous tirons parti ^de

pompage optique ^en détectant les résonances optiques à l’aide d’un second laser colinéaire,

réalisant ainsi un gain ^sensible

signal de fluorescence. Les effets de la polarisation ^{et de la} puissance des faisceaux laser ont été étudiés aussi bien dans le cas de l’excitation d’atomes de Na par ^un seul laser que par deux lasers.

Abstract.

The detection of optical resonances by observation of the fluorescence light ^{emitted from a fast atomic}

beam of Na interacting ^with

collinear laser beam is limited in sensitivity by optical pumping processes. In the

experiment reported here, ^{we take} advantage ^{of the} optical pumping by using ^{a second collinear laser} beam, achieving ^then

gain ⁱⁿ fluorescence signal. The effects of the lasers power and polarization ^{on line} shapes ^{have been}

studied in the single laser excitation case as well

in ^{the two laser beams one.}

Classification

Physics ^Abstracts

32.80

1. Introduction.

In recent years, ^a great ^{deal of} experimental ^{effort has}

been devoted to systematic ^{studies of} hyperfine

structure and isotope ^{shifts of} long isotopic sequences

including short lived nuclides far from stability, ⁱⁿ large part because of their importance ^to understanding

nuclear properties [1]. ^{Since most of the on line work}

is performed ^at isotope separator facilities, ^{one has to} deal with ion beams. Since resonance lines of ions ^are

mostly inaccessible to CW single ^mode dye lasers, ^the ions have to be converted into neutral atoms. In the method of laser spectroscopy ^on fast atomic beam [2],

the ion beam is _very efficiently ^converted by charge exchange collisions into atomic beam with unchanged phase space distribution. The narrowing ^{of the} velocity spread resulting from the initial ion acceleration is then conserved and consequently the residual Doppler

width _may be within the natural linewidth. Therefore,

in collinear excitation, ^most of the atoms of the beam may be simultaneously ^resonant with the laser light giving ^{to the method} high resolution and high ^sensi- tivity capabilities. Optical resonances are detected by

observation of the fluorescence light. ^The highest sensitivity is obtained when the fluorescent photon

yield per ^atom is not limited by optical pumping

processes. Resonances have been detected with ^an atomic flux of 1O5/s ^{in the case of the} strong ^resonance line 180-1 P 1 ^{of BaI, for} isotopes having ^no hyperfine

structure [3]. If there is a hyperfine ^{structure in the} ground ^{state, the atoms are removed from the absor-} bing ground ^{state level within a few,} optical pumping cycles, ^thus limiting ^the fluorescence yield ^{of the} experiment ^{to a few} photons ^{per atom. Instead of} bearing ^{with it, we} proposed ^{to take} advantage ^{of the} optical pumping ^{to detect} optical resonances by

means of a second collinear laser beam acting ^{as a}

detector of the pumping process [4]. ^The experiment reported ^{here is intended to check the} gain ⁱⁿ sensitivity

obtained with this method when applied ^{to a sodium}

atomic beam.

2. Experimental conditions.

Figure ¹ presents ^{a schematic} diagram of the appa- ratus. The Na+ ions ^are produced ^{at the oven exit by}

ionization of the Na vapour ^on the inner surface of a

tantalum tube heated by ^d.c. current up ^to about 1200 OC [5]. After acceleration _up to 5 kV and focusing by ^{an einzel} lens, the ion beam is deflected by ^{100 in an}

electrostatic parallel-plate deflector and set collinear with the laser beam. The fast atomic ^{beam is} obtained

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019830044090101900

1020

Fig. ^1.

Experimental set-up.

by charge transfer collisions in a cell of recycled

sodium _vapour [6, 7]; ^the vapour pressure ^at the centre of the cell is around 10 - 3 torr. The geometrical

factors are the length ^of charge exchange ^cell (L1

26 cm), ^the length of fluorescence observation zone

(L2

⁶ cm), the distance between the centres of these two zones (L3

^{35 cm in a first set of} experiments ^and L3

^{60 cm} afterwards). Intensities of ion and neutral beams are monitored by Faraday cups; their shapes

are controlled by ^{two beam scanners. Beam scanners}

are also useful to finely adjust ^the collinearity ^{of the}

fast atomic beam with the laser beam.

The dye ^{lasers are CW ones} single ^mode and tunable

(Coherent Radiation 599 Model). One of these lasers is

frequency piloted by ^a sigmameter [8]. Frequency

sweep of the other one is done by ^the standard Cohe- rent Radiation scanning system. ^With respect ^{to the ion} beam, each laser beam can be sent either co- or counter

propagating. Although ^we have tested Doppler tuning by varying ^the atomic beam velocity by ^{means of a} post acceleration voltage applied ^{to the} charge exchange cell, ^we prefer ^{to scan the laser} frequency ^{with the} help

of the sigmameter. ^{Moreover in the} experiments using ^two different laser beams (see below) ^{the two}

laser frequencies ^{must be} independently ^{varied and} velocity tuning is therefore unadequate. ^{For each laser}

beam a half-wave plate Pl,2 ^{and a} Glan-Thomson

prism G1,2 ^{allow us to adjust the} polarization ^direction

and the intensity of the laser light. ^{The two} laser beams

are superposed ^{on a} dichroic mirror. Atomic fluores-

cence is detected by ^{a cooled} photomultiplier. ^No photon counting technique has been used since the

intensity ^{of the atomic} beam is in the _range of 101 ° _par- ticles/s. ^The background, resulting essentially ^from

laser stray light, ^{is reduced} by ^a carefully designed

blackened observation ^zone.

3. Fluorescence signal observed with one collinear laser beam.

The hyperfine ^structure (hfs) ^{of the} D

line is well known, ^{it is} reproduced in the insert of figure ^2.

Figure 2 also shows a scan of the D1 ^{line obtained}

with a 100 uW linearly polarized laser beam. On the low frequency side of each peak appears ^a small satellite resonance, labelled (a). ^It corresponds ^{to the}

Fig. ^2.

Fluorescence signal

^{function of the} frequency,

in the D1 ^{line, observed with}

collinear laser beam.

inelastic charge ^{transfer via the} 3p ^doublet [9]. ^The frequency difference between one peak and its satellite

(~ 70 MHz) ^{reflects an} energy loss of about 2 eV, corresponding ^{to the} 3p-3s energy difference.

The observed relative intensities of the four hyper-

fine components correspond ^to the theoretical ones.

The linewidth obtainable in collinear laser spectro-

scopy has been discussed in details [10]. ^{In our case we}

measured a frequency ^width (FWHM) ^{of 28 MHz,} comparable ^to the best results reported by ^Anton

et al. [2] for sodium fast beams. This linewidth is mainly

due to the initial kinetic-energy spread of the ions before acceleration. The energy spread corresponding

to a width of 28 MHz is 0,7 eV; ^{it is} larger ^{than the}

thermal spread calculable for a temperature ^of 1 200 OC of the ion source and is probably ^{due to the} heating ^d.c. voltage (~ ² V) applied ^to the tantalum tube. One observes, indeed, that the linewidth of the

resonances increases _very rapidly ^{with the} heating

current intensity. ^{As the} light ^{power is} increased, ^the

resonances broaden (see Fig. 3). ^This broadening ^{is a}

consequence of the hyperfine optical pumping ^pro-

cesses occurring during ^{the atoms} flight ^{from the} charge-exchange ^{cell to} the detection _zone; this

pumping transfers the atoms from the resonant hyper-

fine ground ^{level to the non resonant one. Its} efficiency

is maximum at exact resonance and decreases with laser detuning; therefore, in the detection zone, the

population ^{of the resonant} ground level is minimum at exact resonance. This explains the observed line

broadening. ^{It occurs at low laser power} ( ¹ mW)

for which the usual _power broadening ^is negligible.

At further increase of the light power each fluorescence line exhibits a dip ^{at exact} resonance. For the F

1 -> F’

2 transition the dip ^is easily ^observable

and the signal falls down to zero with increasing ^laser

power. For the three other components, ^F=1

^F’=1 and F = 2 -+ F’= 1, 2, the situation is more complicated

since for each of these resonances atoms in one or two Zeeman sublevels cannot be excited. In order to clearly

observe the dips, ^{one has to} perform ^the experiment ⁱⁿ

zero magnetic ^{field, thus} avoiding ^a mixing ^{of Zeeman}

Fig. ^3.

Effect of an increasing light intensity

^the lineshape ^{in the} D1 ^line (cancelled magnetic field).

states induced by ^{the earth} magnetic field. The curves

shown on figure ^{3 are} recorded with cancelled magnetic

field. Due to the imperfect cancellation of the field,

at very high ^laser power, the dips ^{are less} pronounced

for these three components than for the F = I-+> F’ = 2

one.

Figure ^{7B shows a scan of a} part ^{of the} D2 ^line (low frequency components) ^{at low laser power} (100 uW).

Due to the residual Doppler ^width (28 MHz) ^the

structure of the excited state is not completely ^resolved.

Moreover the presence of the satellite resonances, mentioned above, may blur the structure : in parti- cular, ^for copropagating beams, the satellite line

corresponding ^{to the} largest component F = 2 -+ F’= 3 falls just ⁱⁿ between the two components F = 2-+>

F’ =1,2. ^This difficulty ^{is overcome} by using ^counter propagating beams. The resolved h.f. components ^are observed with the expected intensities.

As the light power is increased, only the transition

F=2-F’=3, for ’which no h. f. optical pumping

occurs, ^can be seen. For this transition one observes the effects of _power broadening ^and optical saturation.

For laser _power higher ^{than 10 mW} the fluorescence does not increase much _more, indicating ^{that the} power

broadening (i.e. ^{the Rabi} frequency) is of the order of the Doppler ^width.

4. Fluorescence observed with two collinear laser beams.

Two laser beams are now aligned ^on the fast atomic beam. The first _one, linearly polarized ^{and at a}

moderate _power, is frequency ^{scanned across the} D1

line while the frequency v2 ^of the second laser is locked to the hyperfine transition F

2 - F’

3 of the D2

line. The fluorescence induced by this second laser

gives ^a measurement of the population ^{of the F}

² hyperfine ground level. When the variable frequency v 1 is tuned to the transitions F

2 -> F’

1, 2, hyper-

fine optical pumping along the beam transfers the atoms to the non resonant ground ^{level F}

^{1 and}

the detected fluorescence light decreases. On the other hand, when the variable frequency v, ^{is tuned to the}

transitions F

1

F’

1, 2, optical pumping along

the beam increases the population ^{of the F}

² hyper-

fine ground level and the detected fluorescence increases. Figure ^{4 shows a} spectrum ^{of the} D 1 ^line

obtained by this method. As expected ^two positive ^and

two negative peaks ^are obtained. This method is _very similar to the « in flight » ^saturated absorption spectro- scopy ^on fast ion beams developed by Beguin-Renier

et al. [11]. ^The frequency width of the observed reso- nances depends ^{on the} power of the two lasers. For moderate _powers ( ~ ^{1 or 2} mW) of both laser beams,

the observed linewidth ( ~ 24 MHz) ^{falls below the} Doppler width of the beam. For a broad longitudinal

beam velocity distribution, ^one expects that the ulti- mate width available with this method would be twice the natural linewidth (2 y = 20 ^{MHz in our} case). ^One

can also notice, ⁱⁿ figure ^4, that the satellite resonances

do not appear ^on the spectrum recorded with this me-

thod, ^since they correspond ^{to slower atoms which are}

off-resonance with the fixed laser. The scan in figure ⁴

1022

is obtained with high power (30 mW) for the fixed laser.

For the negative resonances the system ^{is a three} level system ^{with a common} level in the ground ^state (see ^{insert of} Fig. 4). ^{Due to the} high power of laser 2, the level of the ground ^{state is} split ^{into a doublet} arising ^from dynamic ^{Stark effect} [12,13]. ^{This doublet}

is visible on the recording ^{for each} negative peak.

It disappears ^{when the} light power of laser 2 decreases below 10 mW. This is consistent with the observations

reported ^above (fluorescence ^{of the} D2 ^{line with one} laser). ^{For the} positive resonances, ^one has a four level system ^{without common level} (see ^{insert of} Fig. 4).

Fig. ^4.

Fluorescence signal

function of the frequency

vl, in the D1 ^line, observed with two collinear laser beams.

Therefore, ^no doublet is observed even with 30 mW power for laser 2. Since the laser _power density ^seen by ^{the atoms is not the same} everywhere, and also for

reasons due to Zeeman degeneracy [14], ^{it is} impossible,

with this set-up, ^{to draw more} quantitative conclusions about _power effects. As an other consequence of the three level structure of the system ^{with a common} level in the ground state, the amplitudes ^{and widths}

of the negative peaks depend strongly ^on the relative

polarizations ^{of the two} laser beams. The effect is

Fig. ^5.

Effect of the relative polarizations ^{of the two laser}

beams : @ parallel polarizations. (B) orthogonal pola-

rizations.

obvious on figure ^{5 : one} recording ^is obtained with

parallel polarizations, ^{the other one with} orthogonal polarizations, ^all parameters being unchanged. ^{In the}

same way, the frequency of the scanned laser beam can

be tuned across the D2 line. As for the D1 line, positive

and negative peaks ^{are observed, as shown on} figure ^{6. The two lasers are} oscillating ^{at moderate}

power (- 3 mW). ^{The two} hyperfine component

Fig. ^6.

Fluorescence signal

function of the frequency

V,, in the D2 line, observed with two collinear laser beams :

@ orthogonal polarizations. (B) ^parallel polarizations.

F

2 -> F’

1, 2 appear ^as negative peaks, ^as expected. When the variable frequency ^{is tuned to}

F

2 -> F’ = 3, the atomic transition is excited

by ^{the two} lasers and the fluorescent light ^increases.

For this component, ^the signal ^is comparable (in

width and amplitude) ^{to the one} obtained when using only ^{laser 1.}

All three hyperfine components ^{are now resolved.}

Fig. ^7.

Comparison of the fluorescence intensities, ^{at the}

same

resolution, in the low frequency components ^{of the} D2

line. The geometry ^with L3

³⁵

has been used : @ ^the

atomic beam is excited by ^two collinear laser beams

(- ³ mW). B(B) ^the atomic beam is excited by

^collinear

weak intensity (~ ¹⁰⁰ Jl W) laser beam.

To get ^{the same} resolution with one laser beam, ^one

has to use at most 100 uW light intensity, ^therefore reducing ^the fluorescent light ^seen by ^the photo- multiplier. ^A comparison of vertical scales on figures

7A and 7B demonstrates a gain ^{of at} least 30 for the

signal, by using ^two laser beams. However, ^the

observed gain is obtained with a 30 times more intense laser light, and therefore the noise resulting ^from stray light ^is J3õ ^{more important We intend to check the}

gain ⁱⁿ signal ^to noise ratio obtainable with this method in the case of _very weak atomic beam.

5. Concluding ^remarks.

We have shown that, ^{in the case of} strong optical pumping, gain ⁱⁿ sensitivity ^{is achieved} using ^two

collinear laser beams.

In this scheme, the detection zone can be further downstream from the charge exchange ^{cell. The}

needed light intensity ^{for the} pumping beam may be then consequently ^reduced. Furthermore, ^between

the two zones, ^{one can} perform magnetic ^resonance experiments ^{as it} will be illustrated in a following paper.

References

[1] OTTEN, ^E. W., ^Nucl. Phys. ^{A 354} (1981) 471c;

JACQUINOT, P., KLAPISCH, R., Rep. Prog. Phys. ⁴² (1979) 773.

[2] ANTON, ^K. R., KAUFMAN, S. L., KLEMPT, W., MORUZZI, G., NEUGART, R., OTTEN, ^{E. W. and} SCHINZLER, B., Phys. ^{Rev. Lett. 40} (1978) ^642.

[3] MUELLER, ^A. C., BUCHINGER, F., KLEMPT, W., OTTEN, ^E. W., NEUGART, R., EKSTRÖM, C., HEINEMEIER, J., ^{Submitted to Nucl.} Phys. ^A (1982).

[4] ^{DUONG, H.} T., VIALLE, ^J. L., C.R. Hebd. Sean. Acad.

Sci. 290B (1980) 533 ;

DUONG, H. T., VIALLE, J. L., J. Physique ^{Lett. 41} (1980)

L-407.

[5] RAMSEY, N. F., Molecular Beams (Clarendon Press, Oxford) 1956, p. 379.

[6] ^BACAL, M., REICHELT, W., Rev. Sci. Instrum. 45 N° 6

(1974) 769.

[7] BACAL, M., TRUC, A., DOUCET, ^H. J., LAMAIN, H., CHRÉTIEN, M., Nucl. Instrum. Methods 114 (1974)

407.

[8] JUNCAR, P., PINARD, J., Opt. Commun. 14 (1975) ^438.

[9] ANTON, ^K. R., KAUFMAN, S. L., KLEMPT, W., ^NEU- GART, R., OTTEN, ^E. W., SCHINZLER, B., Hyper- fine Interactions (North ^Holland Publishing ^Com- pany) 41978, p. 87.

[10] ^{KAUFMAN, S.} L., Opt. Commun. 17 (1976) ^309.

[11] BEGUIN-RENIER, F., DESESQUELLES, J., GAILLARD, ^M. L., Phys. ^{Scr. 18} (1978) ^21.

[12] DELSART, C., KELLER, ^J. C., ^J. Phys. ^{B 9} (1976) ^2769.

[13] CHEBOTAEV, ^V. P., Topics ⁱⁿ Applied Physics, ^edited by K. Shimoda (Springer Verlag, Berlin) ¹³ 1976, p. 201.

[14] DELSART, C., KELLER, J. C., ^J. Phys. B ¹³ (1980)

241-252.