- Email: [email protected]
Experimental indication of a nuclear volume contribution to the isotope shift of atomic oxygen
Volume 78. number 2
OPTICS COMMUNICATIONS
15 August 19q~l
Experimental indication of a nuclear volume contribution to the isotope shift of atomic oxygen L i v i o G i a n f r a n i , A n t o n i o Sasso, G u g l i e l m o M a r i a T i n o a n d M a s s i m o l n g u s c i o l)iparttmento di Scwnze l"tswlu" dell'Untver~tta dt Napoli. 1 ,~'0125 Napoh. Ita[l
Recmved 2 Januar3 It~9{~
We investigate the isotope shift for several optical transiuons of the three stable 2solopes of the oxygen atom ('"(). ~ ), '~{) using laser polarization spectroscopy. The analysis of the experimental results b~. means of King plots suggests a nuclea( volume contribution to the isotope shift. This rcsull, at first surprising tier such a light element, must be related to the doubb magtc structure oflhe k"t) nucleus.
It is well known, that isotope shift in atomic spcct,a is caused by mass shift (heavily d o m i n a t i n g in light elements with Z~< 30) and by field or volume shift ( usually d o m i n a t i n g in heavy elements ) [ 1-3 ]. For m e d i u m weight elements the isotope shift is usually difficult to be accurately measured. However. the precision of laser spectroscopy has reached the point where it is becoming necessary to be aware of the volume effect in the optical spectra [4] even for a light element. The lighter the element lhe more challenging is the measurement, also because of the large Doppler broadening of the spectral lines. This paper deals with the experimental evidence we are obtaining of a nuclear volume contribution to the isotope shift in the spectrum of oxygen ( Z = 8 ) . The small effect, otherwise negligible in such a light element, originates from the completely spherical s.vmmetry. of '60 nucleus (doubly magic), which is altered by the addition of nucleons outside the closed shell. Up to now. in fact. the lightest element where deviations from the pure mass effect had been unambiguously detected was sodium [5]. The occurrence of three stable isotopes - ~'O. ' : O (0.04%), '~() (0.2%) - is thvourable since at least three isotopes arc necessary. to distinguish the volume effect from the p r e d o m i n a n t mass one. l lowever, the difficulties for the investigation of oxygen start with the production of the atom from 158
the molecule, in e n v i r o n m e n t a l conditions suitable for high resolution spectroscopy. Furthermore. no allowed optical transitions are accessible from the ground state and the onl.~ laser spectroscopy investigation of isotope shill had been performed on the P , - P : fine structure at far infrared frequency [6]. ()nly rccently our group made atomic oxygen accessible to high resolution optical investigations by means of intermodulated optogalvanic spectroscopy [ 7 ]. Oxygen atoms could be produced in the excited states from trace a m o u n t s of (), m a neon or argon sustained radiofrequenc.,, discharge (50 W at 60 MHz). Rather complicated collisional processes make the production strongly dependent on the experimental parameters [ 8 ], nevertheless allowing fine structure [ 7 ] and preliminary ' " ( ) - ~ O isotope shift measurements [9]. In these measurements, performed by means of intermodulaled oplogalvanic spectroscop.~ ( I M O G S ) . velocity-changing collisions affect the sub-Doppler lineshape with the presence of a strong and broad pedestal, which reduces both the sensitivity and the resolution of this l)oppier-free technique. In order to improve sensitivity and resolution and to extend investigations to enriched samples, in the present work we used laser polarization spectroscop.~ [10]: the experimental apparatus is schematically shown in fig. 1. Radiation from a tunable narrow band t A r / t , - 1 0 - " ) ring dye-laser, operating with
0030-40181901503.50 ~) 1990 - Elsevier Science Publishers B.V. (Norlh-Holland)
Volume 78, number 2
OPTICSCOMMUNICATIONS
I Ar ÷
% EP.
.
~
"
~
,
~
re[. ret
ygen atoms were produced in a radiofrequency discharge; we exploit the higher sensitivity offered by polarization technique to reduce buffer gas pressure to lower values. The measurements reported here have been performed using 1 Torr of argon as buffer gas and 0.05 Torr from a 50% '70-~sO enriched sample. The improvements come not only from the reduction in the homogeneous width but, even more important, from the complete elimination of the collisional Doppler background. A scheme of oxygen energy levels and investigated transitions is shown in fig. 2. A typical recording, using an enriched sample, is shown in fig. 3 for the
3S
65 discharge ~
15 August 1990
3P
aD
(13.05eV)5
5S
~D
5
~ "~ ~./4 cube
•
cellref.
5p
~
I11
NI'N
,,,, 3
' (ltev) 3
Fig. I. Experimental apparatus for a sensitive Doppler-free investigation of the atomic oxygenspectrum by means of polarization spectroscopy.Atomicoxygenis produced by dissociation of O~ present at trace level in an Ar radiofrequencydischarge. Rhodamine 6G or DCM, was split into two beams of different intensities, counterpropagating through the sample. The stronger beam is circularly polarized by means of a mechanically stressed quartz cube, in order to create the orientation of a velocity selected group of atoms. The induced birefringence is probed by the weaker linearly polarized beam. High sensitivity is achieved detecting the transmitted intensity after a crossed linear polarizer with high extinction coefficient ( ~ 1 0 - 7 ) . Measurements were performed with a power of about 100 mW for the pump beam and 10 mW for the probe beam. A small portion of the laser light was sent to a stable Fabry-Perot interferometer (FSR = 75 ( 1 ) MHz) for calibration of the frequency scan, while the laser wavelength was determined by recording the absorption from molecular iodine. As in the previous works [7-9], ox-
3~ 2 --
'~
(lO.7eV)
metastable
ground
Fig. 2. Partial energy level scheme of the oxygenatom. Transitions investigated in the present work, searching for a nuclear size contribution to the isotopeshift, are indicated by arrows.
160
m
75 MHz 11111111111111111111111111111 Fig. 3. Laser polarization spectroscopyrecording of the isotope shift of the 3 5 P3-45 D4 transition at 615.8 nm, using an enriched sample. 159
Volume 78, number 2
()PI'I('S COMMt!NI('A'II()NS
3p ~P~-4d s[)4..~.z transition at 615.8 nm: isotope shift values can be obtained with an accuracy, better than 1%. Both the levels involved in the transition are radiative with a lifetime of the order of" hundred nanoseconds, leading to a natural broadening of tens of MHz, The observed width ( ~ 150 M H z ) is then essentially caused by pressure and saturation broadening. It must be noticed that for each isotope only one of the three fine structure components, namely the ~P~->D4. is observed. This is due to the different relative intensities of the lines [ 11 ]. but also to the fact that. when using circularl+~, polarized p u m p beam, transitions corresponding to ~.6J=0 ha,,e a smaller cross-section w'ith respect to transition with * I = + I. This induces a simplification in the spectra avoiding overlapping of fine structure c o m p o n e n t s of dill t~'rent isotopes. To show the power of the polarization technique, an IMOGS recording of the Sl>~-s[)+ ~ oxygen transition is shown in fig. 4. for a natural a b u n d a n c e sample. From a comparison with fig. 3 it is evident that fine structure c o m p o n e n t s and crossover peaks, affected by residual Doppler pedestal, would mask completely the isotope shifts.
3-4
!i ,i
,/ / \
15 .\ugusl 1990
Unlike ~"() and J'~(). whose nuclear spins are zero. ~() has a nuclear spin l = 5/2 and both a magnelic dipole ( - I . 8 9 4 /~,,I and an electric quadrupole --2.578 ~' fro-') arise [12]. The unresolved hyperfine structure is responsible for the broader linewidlh of ~:() peak. and therefore hi" separation can De estimated to be comparable with the homogeneous width ( ~ 150 M H z l . In ladle 1 we summarize the isotope shill xalues measured for the investigated transitions. 1o demonstrate the better accurac} provided b,, laser polarization spectroscop.\, v+c report also the ' " ( ) - ' " t ) values available from intermodulated spectroscop.~ [9]. In the last column previous Doppler limiled measurements of the ~"()-~'() isotope shift are reported for comparison: they dated back to more lhan 35 years ago [ 13 ]. In order Io evidence the contribution of the size of the nucleus to the observed shift, it is nccessar\ lo sublracl the p r e d o m i n a n t mass effect. According to a commonl~ used model, the mass shift is the resuh of a "'normal" mass shift ( N M S ) . AVN~4S. which is equivalent to that for hydrogenlike systems, and a "'specific" mass shift (SMS). which originates from the influence of correlations in the motion of the electrons on the recoil energy. Following this division the two terms are treated differentl\: the NMS is easily computed, while more difficult is the SMS calculations requiring accurate knowledge of the electronic wavefunctions. Following a generally used semiempirical approach, both the SMS and field shift (FS) are taken as the product of a purely nuclear fhctor and a factor depending on the wavefunctions of the electronic states involved in the observed optical transitions [1 ]. By means of this t'actorization, the residual isotope shift, i.e. the sum of SMS and FS. on a transition "'a'" tot a couple of" isotopes ~ and fl with mass n u m b e r s ..I,, and .-Ii¢ is Ao-~',/~= k". t,,/, + I:'"( ,,/,.
~1 )
300 MHz
Illlllill
I I
Fig. 4. Intermodulated optogalvanic spectroscopy of the 3p SP~4d '~[),.4oxygen transition obtained with a natural enriched ox',gen sample; the isotope shift would be completely masked by the wide Doppler pedestal affecting both line structure components ¢3-4 and 3-3) and cross-over peak (('(.)).
160
In the first (mass) term. K" is the factor depending on the electronic states of the transition and ..1,,~= (.-Is~-..t,, )/..Is~.l,,; in the second ( field ) term, ( ],/s is proportional to the difference of the mean square radii of the two isotopes and I:TM is proportional to the change in electronic density at the nucleus during the transition.
Volume 78, number 2
15 August1990
OPTICS COMMUNICATIONS
Table I Measured isotope shifts ( IS ) values on oxygen atom. Transition
Wavelength (nm)
160-170 (MHz)
160-IsO (MHz)
160-JsO [9] (MHz)
'60-IsO [13] (MHz)
3p~P~-4dSD4 3p~P~-5sSS2 3p~P~-6s~S~
615.8 645.7 604.6
686 (10) 646 (10) 723 (10)
1320 (10) 1163 (10) 1278 (10)
1310 (40) 1160 (20) 1300 (40)
< 900 < 600 -
35P3-55S~ ( 2 = 6 4 5 . 7 n m ) transition is plotted against the residual modified shift in the 3 3P2-6 3Si ( 2 = 6 0 4 . 6 n m ) (fig. 5a), and 3 5P3-4 5Da ( 2 = 6 1 5 . 8 n m ) (fig. 5b) transitions. The error bars indicate the standard deviation derivated from a set o f at least five Doppler-free recordings. It must be noticed that for these transitions the residual shifts are negative and in the figure absolute values are reported. Since the points for the two isotope couples do not c o i n c i d e , we can conclude to be in presence o f a non negligible volume contribution. Unfortunately, from our investigation it is not possible to isolate the absolute contribution o f the field shift; however, some quantitative informations can be obtained from the slopes o f the King diagrams whose evaluation is, in our case, limited to the only two available isotope couples. In particular from the King graphs it can be
It is useful to plot ( K i n g plot [2] ) the "'residual modified isotope shift" ( R M S ) a
lt./~
Ao,B/A,~
--
(2 )
a
-
for all the possible pairs o f values A,, A~ against the corresponding expression/~b for another transition "'b". by means o f the following relation easily obtained by ( 1 ) and ( 2 )
lt,~ " -(E"/Eb)H~+
[K"-Kb( E"/Eb) ]
(3)
In absence o f a volume effect for both the transitions ( E " = E b = 0 ) all the points o f such a plot overlap. On the contrary, in presence o f a volume contribution, the plotted points do not overlap and should lie on a straight line, with a slope given by the ratio Ea/ E b o f the field shifts o f the two different transitions. In fig. 5 the residual modified isotope shift in the
e.
E 95
....
l ....
~-''
.... all
' I . . . .
I . . . .
I . . . .
I ....
--
1_ b
-
N
~-~ 9o ~D
~ 8s It3
L9
--
--~
i
~
90
(16-t8)
+
L-
(16-t8)
Fr
Zso
80
I
(16-17)
I
n
,
75
7o
a_. 70
.
. 75
.
. 80
L .... 85
I .... 90
J 95
RMS 33Pz - 6~Sz (604.6 nm) [GHz ° mp]
70
....
70
1 .... 75
Ill, 80
, I,,,,F 85
.... 90
I| 95
RklS 3SPs - 45D4 (615.8 nm) [GHz ° rnF]
Fig. 5. Analysis of the isotope shift asing "King plots". A nuclear size contribution is evidenced as explained in the text. On the axes the absolute vahue of the residual modified shift ( RMS ) are plotted for different optical transitions: units adopted are MHz times the proton mass rap. 161
Volume 78, number 2 estimated that
OPTICS COMMUNICATIONS
E~3e-sS~/EI3P-6s~=0.7(±0.2) and
EI3P-ss)/E(3P-41)I= - 2 . 5 ( _+ 1.0). These values are consistent with the expected behaviour of the field displacement of spectral lines. Indeed, for transitions involving s states (fig. 5a ) the nuclear size c o n t r i b u t i o n is shown to be comparable: on the contrary, the p - d transition exhibits a field shift less pronounced than the p - s one, as shown in fig. 5b. The volume effect, usually negligible in light nucleus, is enhanced in the case of oxygen by the closed shell lpl/2 structure of 160 nucleus (doubly magic ): the addition of one or two neutrons in the outside d~/~ orbital introduces a significative difference of the nuclear charge distribution. It is worth noting that oxygen would be by far the lightest element for which a field shift is now observed, apart for the particular case of hydrogen for which there is no specific mass effect and the normal one can be computed exactly. A theoretical estimate of the field shift requires the accurate knowledge of nuclear parameters, such as the difference of the mean square nuclear radii, and electronic density at the nucleus, the latter obtainable, for instance, from the analysis of the hyperfine structure. However only few theoretical calculations for the isotope shift of oxygen have been performed for the 3s sS-3p 5p and other transitions outside the spectral region investigated in this work, taking into account only the specific mass shift c o n t r i b u t i o n [14]. The values of such calculations agree within cxperimental errors with Doppler-limited measurements [ 13 ] whose accuracy is at least one order of magnitude lower than our Doppler-free measurements. A deeper confirmation of the field shift requires a further refinement in the performed measurements and the investigation of other transitions at present not accessible by our laser source. In conclusion, in this work we have demonstrated the possibility to perform high resolution measurements for the study of isotope shift of the three stable oxygen isotopes. This investigation has evidenced for
162
15 August 1990
the first time a nuclear volume contribution to the isotope shift in optical transitions of a light clement: work is in progress in our laboratory for a full confirmation and more complete understanding of the p h e n o m e n o n . The high accurac~ of our measurements is also stimulating tbr more detailed theoretical calculations of the isotope shift for such an interesting atom. One of the authors, A.S., gratefully thanks Prof. W. Demtroedcr tbr a fruitful period spent in his group at the University of Kaiscrslautern which has helped to use polarization spectroscopy in this experiment.
References [I]J. Bauchc and R.J. ('hampcau, Adv. &t. Mol. Phys. 12 (1976) 39. [2 ] W.I-I.King, Isotope shifts in atomic spectra ( Plenum. New. York, 1984). [ 3 ] P. Aufmuth, K. tteilig and A. Steudel: At. Data Nucl. Data Tables 37 (1987) 455. [4] For the recent progresses in atomic spectroscop} see for instance: The hydrogen atom, eds. F. Bassani, M. lnguscio and T.W. H~insch ( Springer, Berlin, 1989). [5] F. Touchard, J.M. Serre, S. Bungenbach, R. Klapisch. M. De Saint Simon, C. Thibault, H.T. Duong, P. Juncar. S. Liberman, J. Pinard and J.L Vialle, Phys. Rev. (25 ( 1982 ) 2756. [6] PB. Davies, B.J. Hand}, F.K. Murray Lloyd and I).R Smith, ('hem. Ph~,s.68 ( 1978~ 1135. 17 I M. lnguscio, P. Minutolo, A. Sasso and G. Tino, Phys. Re', A37 (1988) 4056. [8] A. Sasso, P. Minutoto, M.I. Schisano, (i.M. Tino and M, lnguscio, J. Opt. Soc. Am. B5 ( 1988 ) 2417; A. Sasso, M. Inguscio,G.M. Tino and L.R. Zink, in: Nonequilibrium processes in partially ionized gases, eds. M. ('apitelli and J.It. Bardsle>,NATO.,LSI.Series ( Plenum, New York, 1989). [ 9 ] K. Ernst, P. Minutolo, A. Sasso. (;.M. Tino and M. Ingusclo, Optics Left. 14 (1989) 554.
[ 10] C. Wieman and T.W. H~insch, Phys. Rev. Lcn. 36 ( 1976 I 170.
[ I 1 ] ('.E. Moore. Natl. Stand. Re|. Data Set. Nat. Bur. Stand. 35, vols. 1, II. and I11 ( 1971 ) {12} F. Ajzenberg-Selove. Nucl. Phys. A375 (1982) I. [ 13} I,.W. Parker and J.R. Holmes, J. Opt. Soc. Am. 43 { 1953 )
103. {I 4] J.P. Nlckols and C.E. l'reanor, Phys. Rev. I10 (I 957~ 370
OPTICS COMMUNICATIONS
15 August 19q~l
Experimental indication of a nuclear volume contribution to the isotope shift of atomic oxygen L i v i o G i a n f r a n i , A n t o n i o Sasso, G u g l i e l m o M a r i a T i n o a n d M a s s i m o l n g u s c i o l)iparttmento di Scwnze l"tswlu" dell'Untver~tta dt Napoli. 1 ,~'0125 Napoh. Ita[l
Recmved 2 Januar3 It~9{~
We investigate the isotope shift for several optical transiuons of the three stable 2solopes of the oxygen atom ('"(). ~ ), '~{) using laser polarization spectroscopy. The analysis of the experimental results b~. means of King plots suggests a nuclea( volume contribution to the isotope shift. This rcsull, at first surprising tier such a light element, must be related to the doubb magtc structure oflhe k"t) nucleus.
It is well known, that isotope shift in atomic spcct,a is caused by mass shift (heavily d o m i n a t i n g in light elements with Z~< 30) and by field or volume shift ( usually d o m i n a t i n g in heavy elements ) [ 1-3 ]. For m e d i u m weight elements the isotope shift is usually difficult to be accurately measured. However. the precision of laser spectroscopy has reached the point where it is becoming necessary to be aware of the volume effect in the optical spectra [4] even for a light element. The lighter the element lhe more challenging is the measurement, also because of the large Doppler broadening of the spectral lines. This paper deals with the experimental evidence we are obtaining of a nuclear volume contribution to the isotope shift in the spectrum of oxygen ( Z = 8 ) . The small effect, otherwise negligible in such a light element, originates from the completely spherical s.vmmetry. of '60 nucleus (doubly magic), which is altered by the addition of nucleons outside the closed shell. Up to now. in fact. the lightest element where deviations from the pure mass effect had been unambiguously detected was sodium [5]. The occurrence of three stable isotopes - ~'O. ' : O (0.04%), '~() (0.2%) - is thvourable since at least three isotopes arc necessary. to distinguish the volume effect from the p r e d o m i n a n t mass one. l lowever, the difficulties for the investigation of oxygen start with the production of the atom from 158
the molecule, in e n v i r o n m e n t a l conditions suitable for high resolution spectroscopy. Furthermore. no allowed optical transitions are accessible from the ground state and the onl.~ laser spectroscopy investigation of isotope shill had been performed on the P , - P : fine structure at far infrared frequency [6]. ()nly rccently our group made atomic oxygen accessible to high resolution optical investigations by means of intermodulated optogalvanic spectroscopy [ 7 ]. Oxygen atoms could be produced in the excited states from trace a m o u n t s of (), m a neon or argon sustained radiofrequenc.,, discharge (50 W at 60 MHz). Rather complicated collisional processes make the production strongly dependent on the experimental parameters [ 8 ], nevertheless allowing fine structure [ 7 ] and preliminary ' " ( ) - ~ O isotope shift measurements [9]. In these measurements, performed by means of intermodulaled oplogalvanic spectroscop.~ ( I M O G S ) . velocity-changing collisions affect the sub-Doppler lineshape with the presence of a strong and broad pedestal, which reduces both the sensitivity and the resolution of this l)oppier-free technique. In order to improve sensitivity and resolution and to extend investigations to enriched samples, in the present work we used laser polarization spectroscop.~ [10]: the experimental apparatus is schematically shown in fig. 1. Radiation from a tunable narrow band t A r / t , - 1 0 - " ) ring dye-laser, operating with
0030-40181901503.50 ~) 1990 - Elsevier Science Publishers B.V. (Norlh-Holland)
Volume 78, number 2
OPTICSCOMMUNICATIONS
I Ar ÷
% EP.
.
~
"
~
,
~
re[. ret
ygen atoms were produced in a radiofrequency discharge; we exploit the higher sensitivity offered by polarization technique to reduce buffer gas pressure to lower values. The measurements reported here have been performed using 1 Torr of argon as buffer gas and 0.05 Torr from a 50% '70-~sO enriched sample. The improvements come not only from the reduction in the homogeneous width but, even more important, from the complete elimination of the collisional Doppler background. A scheme of oxygen energy levels and investigated transitions is shown in fig. 2. A typical recording, using an enriched sample, is shown in fig. 3 for the
3S
65 discharge ~
15 August 1990
3P
aD
(13.05eV)5
5S
~D
5
~ "~ ~./4 cube
•
cellref.
5p
~
I11
NI'N
,,,, 3
' (ltev) 3
Fig. I. Experimental apparatus for a sensitive Doppler-free investigation of the atomic oxygenspectrum by means of polarization spectroscopy.Atomicoxygenis produced by dissociation of O~ present at trace level in an Ar radiofrequencydischarge. Rhodamine 6G or DCM, was split into two beams of different intensities, counterpropagating through the sample. The stronger beam is circularly polarized by means of a mechanically stressed quartz cube, in order to create the orientation of a velocity selected group of atoms. The induced birefringence is probed by the weaker linearly polarized beam. High sensitivity is achieved detecting the transmitted intensity after a crossed linear polarizer with high extinction coefficient ( ~ 1 0 - 7 ) . Measurements were performed with a power of about 100 mW for the pump beam and 10 mW for the probe beam. A small portion of the laser light was sent to a stable Fabry-Perot interferometer (FSR = 75 ( 1 ) MHz) for calibration of the frequency scan, while the laser wavelength was determined by recording the absorption from molecular iodine. As in the previous works [7-9], ox-
3~ 2 --
'~
(lO.7eV)
metastable
ground
Fig. 2. Partial energy level scheme of the oxygenatom. Transitions investigated in the present work, searching for a nuclear size contribution to the isotopeshift, are indicated by arrows.
160
m
75 MHz 11111111111111111111111111111 Fig. 3. Laser polarization spectroscopyrecording of the isotope shift of the 3 5 P3-45 D4 transition at 615.8 nm, using an enriched sample. 159
Volume 78, number 2
()PI'I('S COMMt!NI('A'II()NS
3p ~P~-4d s[)4..~.z transition at 615.8 nm: isotope shift values can be obtained with an accuracy, better than 1%. Both the levels involved in the transition are radiative with a lifetime of the order of" hundred nanoseconds, leading to a natural broadening of tens of MHz, The observed width ( ~ 150 M H z ) is then essentially caused by pressure and saturation broadening. It must be noticed that for each isotope only one of the three fine structure components, namely the ~P~->D4. is observed. This is due to the different relative intensities of the lines [ 11 ]. but also to the fact that. when using circularl+~, polarized p u m p beam, transitions corresponding to ~.6J=0 ha,,e a smaller cross-section w'ith respect to transition with * I = + I. This induces a simplification in the spectra avoiding overlapping of fine structure c o m p o n e n t s of dill t~'rent isotopes. To show the power of the polarization technique, an IMOGS recording of the Sl>~-s[)+ ~ oxygen transition is shown in fig. 4. for a natural a b u n d a n c e sample. From a comparison with fig. 3 it is evident that fine structure c o m p o n e n t s and crossover peaks, affected by residual Doppler pedestal, would mask completely the isotope shifts.
3-4
!i ,i
,/ / \
15 .\ugusl 1990
Unlike ~"() and J'~(). whose nuclear spins are zero. ~() has a nuclear spin l = 5/2 and both a magnelic dipole ( - I . 8 9 4 /~,,I and an electric quadrupole --2.578 ~' fro-') arise [12]. The unresolved hyperfine structure is responsible for the broader linewidlh of ~:() peak. and therefore hi" separation can De estimated to be comparable with the homogeneous width ( ~ 150 M H z l . In ladle 1 we summarize the isotope shill xalues measured for the investigated transitions. 1o demonstrate the better accurac} provided b,, laser polarization spectroscop.\, v+c report also the ' " ( ) - ' " t ) values available from intermodulated spectroscop.~ [9]. In the last column previous Doppler limiled measurements of the ~"()-~'() isotope shift are reported for comparison: they dated back to more lhan 35 years ago [ 13 ]. In order Io evidence the contribution of the size of the nucleus to the observed shift, it is nccessar\ lo sublracl the p r e d o m i n a n t mass effect. According to a commonl~ used model, the mass shift is the resuh of a "'normal" mass shift ( N M S ) . AVN~4S. which is equivalent to that for hydrogenlike systems, and a "'specific" mass shift (SMS). which originates from the influence of correlations in the motion of the electrons on the recoil energy. Following this division the two terms are treated differentl\: the NMS is easily computed, while more difficult is the SMS calculations requiring accurate knowledge of the electronic wavefunctions. Following a generally used semiempirical approach, both the SMS and field shift (FS) are taken as the product of a purely nuclear fhctor and a factor depending on the wavefunctions of the electronic states involved in the observed optical transitions [1 ]. By means of this t'actorization, the residual isotope shift, i.e. the sum of SMS and FS. on a transition "'a'" tot a couple of" isotopes ~ and fl with mass n u m b e r s ..I,, and .-Ii¢ is Ao-~',/~= k". t,,/, + I:'"( ,,/,.
~1 )
300 MHz
Illlllill
I I
Fig. 4. Intermodulated optogalvanic spectroscopy of the 3p SP~4d '~[),.4oxygen transition obtained with a natural enriched ox',gen sample; the isotope shift would be completely masked by the wide Doppler pedestal affecting both line structure components ¢3-4 and 3-3) and cross-over peak (('(.)).
160
In the first (mass) term. K" is the factor depending on the electronic states of the transition and ..1,,~= (.-Is~-..t,, )/..Is~.l,,; in the second ( field ) term, ( ],/s is proportional to the difference of the mean square radii of the two isotopes and I:TM is proportional to the change in electronic density at the nucleus during the transition.
Volume 78, number 2
15 August1990
OPTICS COMMUNICATIONS
Table I Measured isotope shifts ( IS ) values on oxygen atom. Transition
Wavelength (nm)
160-170 (MHz)
160-IsO (MHz)
160-JsO [9] (MHz)
'60-IsO [13] (MHz)
3p~P~-4dSD4 3p~P~-5sSS2 3p~P~-6s~S~
615.8 645.7 604.6
686 (10) 646 (10) 723 (10)
1320 (10) 1163 (10) 1278 (10)
1310 (40) 1160 (20) 1300 (40)
< 900 < 600 -
35P3-55S~ ( 2 = 6 4 5 . 7 n m ) transition is plotted against the residual modified shift in the 3 3P2-6 3Si ( 2 = 6 0 4 . 6 n m ) (fig. 5a), and 3 5P3-4 5Da ( 2 = 6 1 5 . 8 n m ) (fig. 5b) transitions. The error bars indicate the standard deviation derivated from a set o f at least five Doppler-free recordings. It must be noticed that for these transitions the residual shifts are negative and in the figure absolute values are reported. Since the points for the two isotope couples do not c o i n c i d e , we can conclude to be in presence o f a non negligible volume contribution. Unfortunately, from our investigation it is not possible to isolate the absolute contribution o f the field shift; however, some quantitative informations can be obtained from the slopes o f the King diagrams whose evaluation is, in our case, limited to the only two available isotope couples. In particular from the King graphs it can be
It is useful to plot ( K i n g plot [2] ) the "'residual modified isotope shift" ( R M S ) a
lt./~
Ao,B/A,~
--
(2 )
a
-
for all the possible pairs o f values A,, A~ against the corresponding expression/~b for another transition "'b". by means o f the following relation easily obtained by ( 1 ) and ( 2 )
lt,~ " -(E"/Eb)H~+
[K"-Kb( E"/Eb) ]
(3)
In absence o f a volume effect for both the transitions ( E " = E b = 0 ) all the points o f such a plot overlap. On the contrary, in presence o f a volume contribution, the plotted points do not overlap and should lie on a straight line, with a slope given by the ratio Ea/ E b o f the field shifts o f the two different transitions. In fig. 5 the residual modified isotope shift in the
e.
E 95
....
l ....
~-''
.... all
' I . . . .
I . . . .
I . . . .
I ....
--
1_ b
-
N
~-~ 9o ~D
~ 8s It3
L9
--
--~
i
~
90
(16-t8)
+
L-
(16-t8)
Fr
Zso
80
I
(16-17)
I
n
,
75
7o
a_. 70
.
. 75
.
. 80
L .... 85
I .... 90
J 95
RMS 33Pz - 6~Sz (604.6 nm) [GHz ° mp]
70
....
70
1 .... 75
Ill, 80
, I,,,,F 85
.... 90
I| 95
RklS 3SPs - 45D4 (615.8 nm) [GHz ° rnF]
Fig. 5. Analysis of the isotope shift asing "King plots". A nuclear size contribution is evidenced as explained in the text. On the axes the absolute vahue of the residual modified shift ( RMS ) are plotted for different optical transitions: units adopted are MHz times the proton mass rap. 161
Volume 78, number 2 estimated that
OPTICS COMMUNICATIONS
E~3e-sS~/EI3P-6s~=0.7(±0.2) and
EI3P-ss)/E(3P-41)I= - 2 . 5 ( _+ 1.0). These values are consistent with the expected behaviour of the field displacement of spectral lines. Indeed, for transitions involving s states (fig. 5a ) the nuclear size c o n t r i b u t i o n is shown to be comparable: on the contrary, the p - d transition exhibits a field shift less pronounced than the p - s one, as shown in fig. 5b. The volume effect, usually negligible in light nucleus, is enhanced in the case of oxygen by the closed shell lpl/2 structure of 160 nucleus (doubly magic ): the addition of one or two neutrons in the outside d~/~ orbital introduces a significative difference of the nuclear charge distribution. It is worth noting that oxygen would be by far the lightest element for which a field shift is now observed, apart for the particular case of hydrogen for which there is no specific mass effect and the normal one can be computed exactly. A theoretical estimate of the field shift requires the accurate knowledge of nuclear parameters, such as the difference of the mean square nuclear radii, and electronic density at the nucleus, the latter obtainable, for instance, from the analysis of the hyperfine structure. However only few theoretical calculations for the isotope shift of oxygen have been performed for the 3s sS-3p 5p and other transitions outside the spectral region investigated in this work, taking into account only the specific mass shift c o n t r i b u t i o n [14]. The values of such calculations agree within cxperimental errors with Doppler-limited measurements [ 13 ] whose accuracy is at least one order of magnitude lower than our Doppler-free measurements. A deeper confirmation of the field shift requires a further refinement in the performed measurements and the investigation of other transitions at present not accessible by our laser source. In conclusion, in this work we have demonstrated the possibility to perform high resolution measurements for the study of isotope shift of the three stable oxygen isotopes. This investigation has evidenced for
162
15 August 1990
the first time a nuclear volume contribution to the isotope shift in optical transitions of a light clement: work is in progress in our laboratory for a full confirmation and more complete understanding of the p h e n o m e n o n . The high accurac~ of our measurements is also stimulating tbr more detailed theoretical calculations of the isotope shift for such an interesting atom. One of the authors, A.S., gratefully thanks Prof. W. Demtroedcr tbr a fruitful period spent in his group at the University of Kaiscrslautern which has helped to use polarization spectroscopy in this experiment.
References [I]J. Bauchc and R.J. ('hampcau, Adv. &t. Mol. Phys. 12 (1976) 39. [2 ] W.I-I.King, Isotope shifts in atomic spectra ( Plenum. New. York, 1984). [ 3 ] P. Aufmuth, K. tteilig and A. Steudel: At. Data Nucl. Data Tables 37 (1987) 455. [4] For the recent progresses in atomic spectroscop} see for instance: The hydrogen atom, eds. F. Bassani, M. lnguscio and T.W. H~insch ( Springer, Berlin, 1989). [5] F. Touchard, J.M. Serre, S. Bungenbach, R. Klapisch. M. De Saint Simon, C. Thibault, H.T. Duong, P. Juncar. S. Liberman, J. Pinard and J.L Vialle, Phys. Rev. (25 ( 1982 ) 2756. [6] PB. Davies, B.J. Hand}, F.K. Murray Lloyd and I).R Smith, ('hem. Ph~,s.68 ( 1978~ 1135. 17 I M. lnguscio, P. Minutolo, A. Sasso and G. Tino, Phys. Re', A37 (1988) 4056. [8] A. Sasso, P. Minutoto, M.I. Schisano, (i.M. Tino and M, lnguscio, J. Opt. Soc. Am. B5 ( 1988 ) 2417; A. Sasso, M. Inguscio,G.M. Tino and L.R. Zink, in: Nonequilibrium processes in partially ionized gases, eds. M. ('apitelli and J.It. Bardsle>,NATO.,LSI.Series ( Plenum, New York, 1989). [ 9 ] K. Ernst, P. Minutolo, A. Sasso. (;.M. Tino and M. Ingusclo, Optics Left. 14 (1989) 554.
[ 10] C. Wieman and T.W. H~insch, Phys. Rev. Lcn. 36 ( 1976 I 170.
[ I 1 ] ('.E. Moore. Natl. Stand. Re|. Data Set. Nat. Bur. Stand. 35, vols. 1, II. and I11 ( 1971 ) {12} F. Ajzenberg-Selove. Nucl. Phys. A375 (1982) I. [ 13} I,.W. Parker and J.R. Holmes, J. Opt. Soc. Am. 43 { 1953 )
103. {I 4] J.P. Nlckols and C.E. l'reanor, Phys. Rev. I10 (I 957~ 370