Hyperfine structure and nuclear moments of...

Hyperfine Structure and Nuclear Moments of Aluminium

By D. A. J ackson and H. K uhn

Clarendon Laboratory, Oxford

(Communicated by F. A . Lindemann, — Received 28 July 1937)

[Plate 1]

Introduction

The structure of the resonance lines of the arc spectrum of aluminium was investigated by the method of absorption in an atomic beam, and the structure of certain other lines of aluminium, emitted by a liquid-air-cooled hollow cathode tube, was also investigated. The nuclear spin of aluminium was thus shown to be f and the magnetic moment about 4-0 nuclear magnetons. A note (Jackson and Kuhn 1937 a) containing these resuJts was published elsewhere.

The light source and the spectroscope

The arc spectrum of aluminium possesses two groups of resonance lines, 3 2P i i -4SJ (3962A, 3944 A)

and 3 2P | r -32Df S(3092-7A, 3092-8 A, 3082 A).

The source of these lines was a fused silica discharge tube, excited by external electrodes connected to a \ kW high-frequency oscillator; the ends of the discharge tube were 8 cm. long and 5 cm. in diameter, and the capillary was 5 cm. long and 6 mm. in internal diameter. The tube was filled with neon at 3 mm. pressure, and the capillary was fitted with a side tube of 5 mm. internal diameter and 10 cm. length containing aluminium tribromide. When this side tube was heated by being immersed in a Dewar vessel containing water at 45° C., the discharge tube on excitation emitted the aluminium resonance lines very strongly, the neon lines being greatly weakened. With this light source the aluminium lines could be photographed in 10 sec. and they were entirely free from self-reversal. In order to obtain these lines equally free from self-reversal in a liquid-air-cooled hollow cathode tube an exposure thirty times longer was needed.

For the atomic beam experiment, the spectroscope ( /= 150 cm.) described in earlier work was used; also the etalon was the same.

[ 48 ]

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Jackson and Kuhn Proc. Roy. SocA, vol. 164, Plate 1

<- AF ig . 2— 3 2Pi — 4Sj

(a) light source ( b)absorption by atomic beam

(Facingp. 48)



Hyper fine Structure and Nuclear Moments of Aluminium 49

The production op the atomic beam

I t was impossible to produce an atomic beam of aluminium by evaporating aluminium in a silica tube in the manner used for silver, for two reasons: first, silica is very rapidly attacked by aluminium; and secondly, it softens a t a temperature below th a t a t which aluminium has a sufficiently high vapour pressure. However, on account of the very great intensity of the resonance lamp, an atomic beam was only needed for a short time, the length of exposure being 10-15 sec. This was first achieved by evaporating aluminium from a helix of tungsten wire of 1 mm. diameter, the helix consisting of 7 turns of 1 cm. diameter and 7 mm. pitch; on each turn was closely wrapped 10 mm. of 1 mm. diameter aluminium wire. I t was possible to evaporate this in about 30 sec., and by placing a slit a t a suitable distance above the helix an atomic beam was made. I t was also possible to observe the structure of the resonance lines by this arrangement, but the supply of aluminium for the atomic beam was of very short duration and rather inadequate.

Fig. 1

A far better source was found to be the evaporation of aluminium from a container made of tantalum of about 0-3 mm. thickness. The arrangement used is shown in fig. 1. A sphere of 10 cm. diameter of pyrex glass was fitted with three side tubes. One of these led to a high-speed mercury pump; through the second was introduced the tantalum container which was carried by the two copper rods R xand R2, and these rods were screwed

Vol. CLXIV—A. 4



50 D. A. Jackson and H. Kuhn

internally into the two copper-glass seals G1 and 6r2; the joint between the side tube and the glass tube with the copper-glass seals was made tight with black wax. The third side tube led to the absorption chamber A. This was a horizontal glass tube (with its axis in the line of sight of the spectrograph) of 3 cm. diameter and 8 cm. length, and at each end a fused silica window was fixed on with sealing wax. The evaporating aluminium reached this chamber through the slit S in a nickel plate which entirely covered the opening into the absorption chamber. This slit varied in width from 14 to 3 mm., according to the degree of collimation required. At the top of the absorption chamber was fitted a tube 0of 3 cm. diameter and 20 cm. length with a window at the upper end, and through this the tantalum container T could be observed, the aluminium deposit here being far less than on the walls of the sphere.

The tantalum container was about 5 cm. in length. Its cross-section was V-shaped, each side being 5 mm. and the opening at the top about 4 mm. The distance from the tantalum container to the slit was about 7 cm. The tantalum container and the slit were parallel to each other, and perpendicular to the line of sight of the spectrograph. The greatest slit width corresponded to a collimation of 7 : 1 and the smallest about 20 : 1. The entire sphere was immersed in water for cooling. The tantalum container held four pieces of aluminium wire 15 mm. long and 2 mm. in diameter. With a current of about 80 amp. these were evaporated in about 90 sec., the rate of evaporation being fairly uniform.

The current required to produce the necessary rate of evaporation was determined experimentally and was accurately reproducible, as the size of the tantalum container and the quantity of aluminium used were always repeated exactly. I t was found tha t by keeping the watts constant, a nearly constant rate of evaporation could be maintained. This was ascertained by taking three or four consecutive photographs of 10 sec. exposure. Two or three of these were found always to have nearly equal absorption. A still better constancy of rate of evaporation was attained by gradually reducing the watts of the heating current throughout evaporation, and in this way it was possible to make four to six photographs of the absorption with one charge of aluminium. The appropriate variation of the current was determined experimentally. The necessity of reducing the current arose from the increased resistance of the tantalum container as the quantity of conducting aluminium decreased.

Though the tantalum containers wrere not much attacked by the aluminium, a new container was used for each evaporation. I f a container was used twice, the conditions of evaporation were not reproducible.



The degree of vacuum obtained in the sphere during evaporation of the aluminium was extremely high. The window at the top of the observation tube was about 23 cm. from the slit S. The “ shadows” of the edge of the slit could, nevertheless, very clearly be seen on the window. The perfection of the vacuum was undoubtedly due to the “ gettering” effect of the evaporating aluminium.


The structure of the line 3 2P j-4S j, A 3944 A

This line was found to possess three components, the structure being observed with etalons of 2 and 3 cm. plate separation and 1 : 12 collimation of the atomic beam.* The plates made with an etalon of 3 cm. separation were measured visually on a micrometer, and the mean result for the positions of the three components was —0-047, 0-000 and +0-047 cm.-1. Those made with an etalon of 2 cm. separation were measured both visually and also by measuring the positions of the minima on the photometer traces. The results obtained by these methods were — 0-048, 0-000 and 0-048 cm.-1, the probable error being between ± 0-001 and + 0-002 cm.-1. The positions of the three components are thus —0-048, 0-000 and + 0-048 cm.-1, and the probable error + 0-001 cm.-1.

The intensity of the three components appeared very nearly equal. A photograph of the line with and without absorption, together with photometer traces, is shown in fig. 2, Plate 1. The absorption lines, in one order are marked by arrows.

The intensity of the components can be determined by photographing first the light source only, and then the light source with absorption due to the atomic beam. An intensity scale is put on the plate, and a photometer trace of the plate is made. The ratio of the absorption coefficients (the difference in the logarithm of the intensity at the position of the absorption in the photograph with absorption and tha t without absorption) for the various components gives a t once the true intensity ratio. This has already been done with a very high degree of accuracy for silver (Jackson and Kuhn 19376). The principal condition for accuracy is constancy of the light source, and this condition was very perfectly fulfilled by the resonance lamp used. Test plates made showed that provided the lamp was run for 2 min. to reach thermal equilibrium, photographs of 15 sec. exposure taken at

* The collimation of the atomic beam is the ratio of half the sum of width of slit and width of aluminium container to the distance between them; this is approxim ately equal to the ratio of the velocity in the line of sight to the total velocity of the atoms.

4 - 2



D. A. Jackson and H. Kuhn

intervals of 5 min. were found to be of equal intensity to within 2 %, any error being due not to variation in light but to uncertainties in length of exposure, which could amount to \ sec.

For the determination of the nuclear spin, the intensity ratio of the central component to the outer components (which are of equal intensity) has to be measured. I t was, however, found impossible to get accurately consistent values. The reason can easily be found in the entirely different conditions of the background for the outer and inner components. The outer absorption lines fall on the edge of the emission line, where the intensity curve is falling very steeply. This leads to only a small error in the measuring of the distance of these lines (about 2 %), but to a very considerable error in the determination of the intensity of the background and therefore of the absorption coefficients; it makes the absorption due to the outer components appear too weak. Moreover, a purely erratic error in determination of the position in the background corresponding to the absorption by the outer components causes the background to appear systematically weaker, for the reduction due to an error outwards is much greater than the increase due to an equal error inwards.

Six plates were measured, three fringes being measured in each. The results varied between 1-04: 1 and 1-23 : 1 for the ratio of the intensity of inner component to that of the outer components. But as these measurements are liable to the above-described systematic error, tending to increase the ratio, it is clear that the value must be substantially less than 1-2 : 1. This indicates a high value for the nuclear spin, the intensity ratio for a spin of § being 1-2 :1 and that for an infinitely great spin 1 : 1. I t would have been possible to eliminate the systematic error by broadening the emission line. But even if this were done, the intensities measured would not have been of much value, on account of the very small change in intensity ratio for a change in spin value.

52

The structure of the line 3 2Pf-4S$, 3962 A

The structure of this line was examined with etalons of plate separations 2, 3 and 4 cm. The atomic beam gave an absorption line with a width of 0-04 cm. 1, which could not be resolved into components even with the greatest collimation (20 : 1) and etalon length. Under these conditions it would have been possible to resolve lines with separations of about 0-012 cm. 1;it appears, therefore, that this line must possess more than four components (probably six, which is the greatest number which a line of this type can possess).



Hyper fine Structure and Nuclear Moments of Aluminium, 53

The structure of the line 3 2P*-3 2D|, A 3082 A

This line was investigated by means of an atomic beam with a 2 cm. etalon and was found to possess two components (fig. 3, Plate 1). The absorption lines are marked by arrows. The separation of these was measured both directly and also by the measurement of photometer traces, the first method giving the value 0-066 + 0-002 cm .-1 and the second 0-067 + 0-002 cm.-1. The structure of the lines was observed with collimations of the atomic beam of 7 :1 and 12: 1. The intensity of the component of longer wave-length was the greater. I t was possible to measure the intensity ratio (ratio of absorption coefficients) of the two components to a very high degree of accuracy. The absorption lines lay well within the emission line and were placed symmetrically on either side of the maximum, so tha t there was no systematic error of the type which made impossible the accurate determination of the intensities of the three components of the line 3 2P j-3Sj . The intensity of the atomic beam required to produce absorption was considerably less than tha t needed for the line 3 2P^-3Sl and the exposure needed was only 10 sec.; consequently it was possible without difficulty to make four successive photographs of the absorption with one charge of aluminium in the tantalum container. A considerable source of error was the purely erratic error inherent to all photographic intensity measurements. In the ultra-violet, where the contrast of plates is relatively low, this introduces a possible error of about 4 %. I t is, however, doubled in the method of determining intensities by absorption, for it is here necessary to determine the ratio of the logarithms of two intensity ratios. Any one determination of the intensity ratio of the two components could therefore be expected to be liable to an erratic error up to about 8 %. In order to reduce this erratic error a large number of intensity measurements was made.

The method was exactly similar to tha t used by the authors in their determination of the intensity ratios in the hyperfine structure of the resonance lines of silver. A cross-wire was fitted to one end of the slit. A photograph of the light source (exposure 10 sec.) was made; then four exposures (10 sec.) of the light source with absorption by the atomic beam; and finally a series of intensity marks was made by giving a succession of exposures (10 sec.) to a hydrogen tube with various slit widths. Photometer traces were made, and by reference to the cross-wire the exact position was found of the absorption lines, in the emission line without absorption. Only those plates were measured in which the degree of absorption in consecutive exposures was constant to within about 30 % (this showing tha t




the density of the atomic beam was fairly constant). In each plate between two and five fringes were measurable, giving two to five intensity determinations. The absorption coefficient of the stronger line ranged between 0-18 and 0-47 (expressed as log10) in the various photographs measured. Two series of measurements were made, the first with a collimation of the atomic beam of 7:1 and the second 12: 1. The results of these are given in

Table I—Collimation of atomic beam 7 : 1Absorption Individual determinationscoefficient of intensity ratio Mean

A 3 0-22 1-28 1-13 1-15 1-19A 2 0-30 1-23 1-18 1-18 1-28 1-22

0-33 1-24 1-15 1-19 1-19<?x 034 1-20 1-21 1-14 1-17 1-18c 2 0-34 1-16 1-17 M 2 1-13 1-15A 1 0-38 1-24 1-17 1-16 115 1-18Bs 0-39 1-20 1-18 1-19Cs 0-41 1-23 1-18 1-18 1-28 1-22c , 0-47 1-19 1-16 1-14 M 2 1-19 1-16

Mean of 33 measurements 1 -184.Corrected for Doppler wing l-206.Mean error 0-033; probable error of mean value 0-006.

Table II—Collimation of atomic beam 12:1Absorption Individual determinationscoefficient of intensity ratio Mean

0-18 1-34 1-21 1-18 1-24 1-24B 2 0-28 1-20 1-21 1-16 1-14 1-18

030 1-24 1-23 1-21 1-24 1-23B x 036 1-20 1-21 1-17 1-17 119

0-35 1-14 1-17 1-20 117C2 0-34 1-19 M 3 1-16 1-22 1-18A x 0-42 1-22 1-25 1-17 1-15 1-23 1-20^3 0-44 1-20 1-24 1-23 1-19 117 1-21

Mean of 33 measurements l-200.Mean error 0-035; probable error of mean value 0-006.

lables I and II. In these tables each row gives the measurements for any one exposure. In the first column is given the mean value of the absorption coefficient for the stronger line and the series to which each exposure belonged (thus A x, A 2, A 3, A 4 are four consecutive exposures during which the atomic beam was running continuously); in the second the values of the intensity ratio of the doublet according to each fringe measured; and in the third the mean of these measurements. To the measurements made with



an atomic beam collimation of 7 :1 a small correction has to be applied on account of the overlapping of the maxima of each component by the Doppler wing of the other. The total half-value width of the absorptionlines is 0-027 cm.-1. The intensity of absorption due to each line is therefore

0066

equal, a t the position of the other line, to ( |) 0014 = 0-05 of its maximum value. From this it follows th a t the observed intensity ratio i is given by

. 7 + 0-05% ~ 1+ 0 -0,57 ’

where 1 is the true intensity ra tio ; and according to this equation an observed intensity ratio 1 • 184 is given by a true intensity ratio T 206. With an atomic beam collimation of 12 : 1 the corresponding correction is negligible.

The possibility of systematic errors must be considered. These can arise in three ways: from unresolved structure due to the term 3 2Df, from insufficient resolving power of the etalon; and from alteration in the intensity of the atomic beam during the course of an exposure.

The separation of the two observed components of the line 3 2Pi-3 2Dj was 0-018 cm .-1 greater than th a t of the two levels of the term 3 2P^. From this it follows (see p. 59) tha t the total splitting of the term 3 2Dj is about 0-03 cm.-1, and th a t each of the observed lines has an unresolved structure of width about 0-02 cm.-1, th a t of the stronger line being somewhat greater. This structure is less than the half-value width with a collimation of 7 : 1 (0-027 cm.-1) and more than th a t with a collimation of 12 : 1 (0-016 cm.-1). The possible influence of this structure would be to make the effective intensity of the stronger line somewhat smaller, on account of its being spread over a slightly greater width. But this effect would be very much stronger with the higher degree of collimation. The perfect agreement of the results obtained with the two collimations shows, therefore, tha t any error must be very small, probably not more than 0-01.

The resolving power of the etalon was a t this wave-length between 0-025 and 0-030 cm.-1. The width of the lines, even with the higher collimation, is rather greater than this, on account of the unresolved structure described above. This width excludes the possibility of loss of apparent intensity due to too much concentration of the absorption compared with the resolution of the etalon.

A further error arising from finite resolving power of the etalon is the intensity, a t the positions of the absorption lines, due to the remaining emission intensity between and on either side of the absorption lines. The absorption maxima have distances from the neighbouring emission peaks

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of about three times the instrumental half-width. Here, the intensity has dropped to less than one-tenth of its maximum value. This effect makes the intensity ratio appear smaller. An approximate calculation shows that this error must be well under 0-01. A similar calculation applied to the authors’ results for silver would indicate a correction of 0-3, the observed value 2-7 :1 becoming 3 : 1, in agreement with the theoretical value.

The last source of systematic error to be considered is the influence of alteration of the intensity of the atomic beam during the course of an exposure. The amount of this can be calculated: a change in absorption coefficient from 0-30 to 0-20 during an exposure results in a loss of rather less than 0-02 in the measured intensity ratio for an intensity ratio of 1-2 : 1, while a change from 0-45 to 0-35 similarly gives rise to a loss of rather more than 0-02. The change in the average value of the absorption coefficient from one exposure to the next of the same series gives a good estimate of the amount of change in intensity of the atomic beam during these two exposures. In this way it can be seen that the average value of the change is 0-04, and from the figures above it follows that a correction of about 0-01 must be applied to the average value of the intensity ratio.

The intensity ratio of the two hyperfine structure components of the line 3 2P^-3 2Dg is thus shown to be 1-21 : 1; all possible sources of systematic error were considered and allowed for; the erratic error is probably about ± 0-01 and certainly less than + 0-02, on account of the great number of measurements made. The value 1*21 : 1 can therefore be relied on to be accurate to ± 0-02.

The structure of the line 3 2P f- 3D4, A 3092*7 A

The above line could not be resolved into components. Its width was about 0-04 c m r1 and the resolving power of the etalon (2 cm. plate separation) with which it was examined was 0-03 cm.-1, the half value width of the line in the atomic beam (collimation 12 : 1) being 0-016 cm.-1. From this it can be assumed that this line possesses more than two components, for under these conditions a doublet structure would have been resolved.

On account of its small intensity and closeness to the line 3092-7 the line ̂2kf—3 2D| (3092-8) was not examined by means of absorption in an atomic

beam.

The structure of the lines 4Sj-5 2Pf, A 6696 A and 4Sj-5 2P4, A 6699 A

These lines were emitted by the resonance lamp, their intensity being about equal to that of the neighbouring neon lines. Their structure was




investigated with an etalon with plates silvered so th a t each plate transmitted 1^ % ,the etalon being used in conjunction with the 1| m. spectrograph and a very dense glass prism.

The separation of these two lines by the dispersion of the prism was only about y mm. I t was therefore necessary to use a plate separation of the etalon of such length th a t the order of interference of the two lines would differ by n + \orders. This condition was satisfied by using a plate separation of T5 cm. In order to make the Doppler width of the lines as small as possible the capillary was cooled with water a t 50° C.

Both of the lines were found to consist of doublets only just resolved, the separation being 0-05 cm.-1. The half-value width of the lines is 0-04 cm.-1. This doublet structure wras observed by Ritschl, but he gave no intensity ratio.

The resolution of the lines was just enough for the photometer to show a definite minimum between them. Accordingly four plates of the line were made, each being given intensity scales by means of a V-shaped slit. The exposure of the lines was 1 hr., and three exposures of 1 hr. of the V-shaped slit with different intensities of the light were made. Photometer traces v7ere made of the plates, and it v7as found th a t in two or three fringes in each plate the maxima and minima were just sufficiently well defined in the stronger line, 4Si-5 2P |, for a comparison of the intensities of the two components. In all, ten measurements were made, the values found being 1T7, M 2, 1*19, T 20, 1-17, 1-18, M 3, 1-19, 1-18, 114. The component of longer wave-length was the stronger, the intensity ratio from the figures above being 1*16 + 0*04. This value requires the usual correction for the overlapping of the Doppler wings. The intensity of these wings a t the

0-05maxima of the other components is equal to ( |) 0'02 = 0*18, so the true intensity ratio I is given by

1-16 7 + 0*18 1 + 0*18x7*

This gives 7 = 1*23 ± 0*06. No great accuracy is claimed for this value, as the maxima and minima were only just visible in the photometer traces. Also the correction applied for overlapping is very great.

The structure of the line A1II, 3S0-3 3PX, A 2669 A

A hollow cathode tube, in which the cathode was made entirely of aluminium, was used as a light source for the above line. The tube was cooled with liquid air and the current used was 200 mA. The intensity of the line




was investigated with the tube filled with neon, helium and argon. I t was found to be the strongest with neon, and the most favourable pressure was about 1 mm.

The structure of the line was investigated with a reflexion echelon mounted in the manner previously described by Jackson (1930). The possible resolving power of the grating was about 1,300,000 (0-03 cm.-1) a t this wave-length, for it possessed 25 plates of thickness 7 mm. However, as the half-value width of the lines of aluminium at liquid-air temperature is about 0*06 cm . -1 the slit width of the spectrograph was made equal to 0-06 mm., corresponding to a resolving power of about 600,000 (0-06 cm .-1). In order to reduce still more the necessary length of exposure the plate was set normal, instead of at the appropriate tilt of 55°. This increases the intensity two or three times, but only the line under investigation is sharply focused. Under these conditions a rather underexposed photograph of the line was obtained with a 2 hr. exposure, and a good one with 4| hr. exposure.

0 cm

Fig. 4—A1II, 2669 A.

The photographs showed that the structure consists of a fine line of width about 0T cm.-1 with a much broader line, width about 0-2 cm.-1, of longer wave-length, the separation of the estimated centres of the two lines being 0T7 cm.-1. A photometer trace of this line is shown in fig. 4.

This observed structure can be satisfactorily explained as a triplet not completely resolved, the broad line being two lines separated by about 0*1 cm.-1, but not quite resolved, and the narrow line a single line. The intensity distribution in the echelon spectrum makes the central component appear stronger, being nearest to the centre of the spectrum.

The structures of the lines 3s 3 p24Pf- 3s 3 4s 4P4, A 3057 and 3s 3 p 24Pf-3s 3 4s 4P5, A 3050

The above lines were emitted by the liquid-air-cooled hollow cathode tube, and their structure was examined with the echelon grating referred



to above (resolving power about 0-66 cm.-1) and also a small Lummer plate (resolving power about 0*03 cm.-1). The line 3057 appeared to possess two components separated by about 0T 2 cm.-1, the width of each being about 0* 1 cm.-1; a reduction in the current of the hollow cathode tube did not cause any reduction in the width of the components. Under these conditions the Doppler width of the lines is about 0-05 cm.-1; it is therefore clear that the observed doublet structure is not a complete resolution; there must be finer unresolvable structure. The line 3050 also appeared to possess two components; these were even wider than those of 3057, their width being about 0T4 cm .-1 and the separation of their centres about 0*16 cm.-1; this observed doublet structure is therefore also an incomplete resolution.


Theoretical discussion of results

From the observed triplet structure ( — 0-048, 0-000 and +0-048 cm.-1) of the line 3 2Pj-4S^ together with the doublet structure (0-00 and 0-05 cm.-1) of the line 4Sj-5 2Pj, it can be seen th a t the levels 3 2P 4 and 4S ̂ are both double with a splitting of 0-048 cm.-1. The structure of the line 3 2P^-3 2D| shows that the term 3 2D | possesses an inverted structure the total width of which is about 0-03 cm.-1. This structure is probably due to perturbation by the term 3 s3 p22D|. The splitting of the terms 4S ̂ and 3 2P* can be explained as a normal hyperfine structure splitting. The value of the nuclear spin can be calculated from the intensity ratios of the three components of the line 3 2P j-4S j and from the intensity ratio of the two components of the lines 3 2P^-3 2D | and 3Sj-5 2P^. The first of these was not measured to a very high degree of accuracy; it was found tha t the ratio of the intensity of the central component to tha t of either of the outer components was certainly less than 1-2 : 1. From this it follows th a t the nuclear spin must be greater than f (the ratios for spins of \ and § being respectively 2 : 1 and 1-2 : 1).

The value of the intensity ratio of the two components of the line 3 2P j-3 2D | was determined with very great accuracy. After allowing for all possible systematic errors it was found to be 1-21 : 1. From this it follows tha t the value of the nuclear spin is §; the intensity ratios corresponding to spins of j ,§ and -j-being respectively 1-29 :1, 1-22 :1 and 1-18 :1. The values corresponding to spins of f- and ^ are outside the probable limit of error.

The intensity ratio of the components of the line 4Sj—5 2P | was approximately measured. The value was found to be 1-23 : 1, but the accuracy of




this was not very high, an error of ± 0-06 being possible; the results of this measurement are however of some interest as they afford a certain confirmation of the value of the nuclear spin found from the intensities of the components of the line 3 2P^-3 2D|.

In a preliminary note Ritschl (1933) suggested that the nuclear spin of aluminium was the reason for this being an observed doublet structure in the lines 3057 and 3050 of the arc spectrum and the line 3S0-3 3P l5 2669, of the spark spectrum. However, a careful investigation of the structure of these lines showed that the doublet structure is only a partial resolution, the width of the components of the lines 3057 and 3050 being about twice as great as the normal width of a simple line under the conditions used. The spark line 2669 was found to consist of one narrow line and another line of width twice as great. This again clearly shows tha t the resolution into two components is incomplete, the observed structure being perfectly explained by assuming a triplet structure.

The nuclear spin of § found from the accurate measurement of the intensity ratio of the hyperfine structure components of the line 3 2P^-3 2D, is thus seen to be supported by the less accurate determinations of the intensity ratios of the components of the lines 4Sj-5 2P | and 3 2P^-4Sj, and the apparent discrepancy of the doublet structures observed by Ritschl (which would indicate a spin value of is explained by incomplete resolution.

Assuming the value f for the mechanical moment of the nucleus, the magnetic moment can be calculated from the width of the splittings of the levels 4Sj and 3 2P^. Using the formulae given by Goudsmit, it is 4T nuclear magnetons according to the splitting of the level 4S ̂ and 3-6 nuclear magnetons according to the splitting of the level 3 2Pi . The sign is positive, for in the lines 3 2P ^-3 2D | and 4S^—5 2P^ the components of longer wavelength are the stronger. The discrepancy of 0-5 between the values of the magnetic moment calculated from the splittings of the levels 4S$ and 3 2P V is much outside the limits of experimental error; however, Goudsmit states that the formula for P levels gives rather too low values of the magnetic moment with light elements.

The authors take this opportunity of thanking Professor Plaskett for placing at their disposal his excellent photometer, and Professor Lindemann for his continued interest in the research; and also Queen’s College and St John’s College for the stipends granted to one of them.



Hyperfine Structure and Nuclear Moments of Aluminium 61

Summary

The hyperfine structure of the resonance lines of aluminium was investigated by means of the absorption in an atomic beam of aluminium. The line 32Pi-4Si was found to possess three components at —0-048, 0-000 and + 0-048 cm .-1 of nearly equal intensity. The line 32P i-3 2Dj possessed two components of separation 0-066 cm.-1, the intensity ratio of which was measured to a high degree of accuracy; the value found was 1-21 : 1, the component of longer wave-length being the stronger.

From the structure of the line 3 2P|-4Sj it follows th a t the levels 4Sj and 32Pj are both split into two levels, of separation 0-048 cm .-1 (the greater splitting of the line 3 2Pj-3 2Df is due to a small unresolved, inverted structure of the level 3 2DS which is probably caused by perturbation by the term 3«s 3p22D|). The observed intensity ratio of the components of the line 3 2Pj-3 2D | gives a value § for the nuclear spin, and the splittings of the levels 4Sj and 32P^ give values 4-1 and 3-6 nuclear magnetons respectively for the magnetic moment of the nucleus according to the formulae of Goudsmit.

The lines 4S^-52P | and 4Si- 5 2P^ were observed in emission and were found to be doublets, only just resolvable on account of their Doppler width, of separation about 0-05 cm .-1 and of intensity ratio (when corrected for overlapping) about 1-23:1, the component of longer wave-length being the stronger. This structure is in complete agreement with the interpretation of th a t observed in the resonance lines.

This result is in disagreement with the contents of a preliminary note by Ritschl in which, from an observed doublet structure of the arc lines 3057 and 3050 and the spark line 2669, he suggested a nuclear spin of an investigation of the structure of these lines showed that the observed doublet structures do not represent a complete resolution.

R eferences

Jackson, D. A. 1930 Proc. Roy. Soc. A, 128, 508-22.Jackson, D. A. and Kuhn, H. 1937a Nature, Lond., 140, 110.

— — 1937 bProc. Roy. Soc. A, 158, 372-83.Ritschl, R. 1933 Nature, Lond., 131, 58-9.



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