DISTORTION IN ATOMIC BEAM ABSORPTION LINESHAPES

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Submitted on 1 Jan 1981

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DISTORTION IN ATOMIC BEAM ABSORPTION LINESHAPES

B. Peuse, M. Prentiss, S. Ezekiel

To cite this version:

B. Peuse, M. Prentiss, S. Ezekiel. DISTORTION IN ATOMIC BEAM ABSORPTION LINESHAPES.

Journal de Physique Colloques, 1981, 42 (C8), pp.C8-53-C8-57. �10.1051/jphyscol:1981807�. �jpa-

00221702�

JOURNAL DE PHYSIQUE

CoZZoque C8, suppZe'ment au n o 12, Tome

de'cembre 1981 page

DISTORTION IN ATOMIC BEAM ABSORPTION LINESHAPES

B.W. Peuse, M.G. Prentiss and S. Ezekiel

Research Laboratory o f EZectronics, Massachusetts I n s t i t u t e o f TeehnoZogy, Cambridge, Massachusetts 021 39. U.

We have performed laser absorption spectroscopy using an atomic beam of sodium and have observed distortions in the absorption line- shapes. The ability to achieve high signal to noise, approaching the shot noise limit, in the recorded data was made possible by recently developed modulation techniques.

The experimental setup (1) consists of a frequency stabilized ring dye laser and an atomic beam of sodium that is collimated by two 0.5 mm diameter pinholes separated by 40 cm and the interaction region is 20 cm downstream. The probe laser beam intersects the atomic beam at right angles at the interaction region and absorption out of the beam is detected by a photodiode. In order to improve the signal to noise, two different modulation techniques were used. The first technique, method A, shown in figure 1, consisted of phase modulating the probe beam with an electro-optic crystal and demodulating the transmitted light to yield a dispersive-like lineshape (2). By choosing a modulation frequency outside the intensity noise spectrum

PHASE MOD m 7

LASER BEAM

BEAM

PSD

Fig. 1

Experimental setup

of the laser, the signal to noise ratio becomes close to that predic-

2 2

ted by shot noise. When the 3 SlI2 (F=2, mF=2)c*3 P3/2 (F=3, m =3) F transition of sodium is probed by this method, the absorption line- shape (figure 2a) exhibited asymmetry.

t

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1981807

C8-54 JOURNAL

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Fig. 2: Experimental

. -

lineshape for the 3 S 2 2 1/2

(F=2)

P3/2 (F=2,3) -transitions in sodium

(a) direct absorption lineshape obtained by method B, (b) dispersive- .like absorption lineshape (obtained by method A,

(c) fluorescence lineshape.

The second technique, method B, involves modulating the atomic population of the mF

2 magnetic sublevel in the 3 SlI2(F=2) state 2 which yields a direct absorption lineshape as shown in figure 2b.

Once again the lineshape is asymmetric. In contrast, the lineshapr obtained when the fluorescence from the interaction region is monitored with a photomultiplier (figure 2c) does not exhibit an asymmetry. The disparity between the fluorescence and absorption lineshapes lead us to investigate the spatial dependence of the ab- sorption.

A small pinhole was therefore mounted in front of the photodiode which was placed in the probe beam, 43 cm behind the interaction region. The laser frequency was held at a fixed detuning from line center by a system of acousto-optic frequency shifters and a servo system that locks the laser frequency to that of the sodium transition of interest.

Figure 3a shows the laser intensity profile as the pinhole detec- tor assembly was translated across the laser beam and along a line mutually perpendicular to both laser and atomic beams. Figures 3b-d show the direct absorption profiles for several detunings & obtained by method B and figures 3c-d show corresponding data obtained using method A.

The structure in the data may be explained if one considers an optical wavefront scattered by a thin nearly transparent atomic beam.

The interference of the scattered light with the laser light produces the observed spatial intensity variations. Calculations are in progress to explain in detail the features observed in the data.

This study points out the existence of serious lineshape distor- tion problems that must be considered when performing precision ab- sorption measurements in atomic beams. In order to minimize these problems the detector area must be large enough to collect the full laser field and the laser field must be uniform and accurately centered with respect to the atomic beam. Furthermore, the atomic beam must have a symmetric population distribution.

This work was supported by the National Science Foundation and

by the Joint Services Electronics Program.

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Fig. 3: (a) Spatial intensity profile of laser beam;

(b) Experimental spatial scans of the absorption

signal for a detuning 6 SO Mhz obtained by method

B; (c-d) Same as (b) but for detunings 6

-10 Mhz

and 6 = 10 Mhz; (e) Experimental spatial scans of

the absorption signal for a detuning 6

Mhz ob-

tained by method A: (f-g) Same as (e) but for de-

tunings 6 = -10 Mhz and 6

10 Mhz.

1. The experimental setup is similar to that described in:

HEMMER, P.R., PEUSE, B.W., WU, F.Y., THOMAS, J.E., and EZEKIEL,S., Opt. Lett. 6 (1981) 531-533.

2. POUND, R.V., Rev. Sci. Instrum. 17 (1946) 490; DREVER, R.W.P. and HALL, J.L., Jount Institute for Laboratory Astrophysics, Boulder, Colo. 80309 (personal communication); BJORKLUND, G.C., Opt. Lett.

5 (1980) 15; PEUSE, B.W., PRENTISS, M.G. and EZEKIEL, S., Re-

search Laboratory of Electronics, Progress Report $123, January

1981 (Massachusetts Institute of Technology,' Cambridge, Massa-

chusetts).