An extension of work on wall-coated cells was made to include observation by a triple resonance technique of the 0-0 hyperfine transitions in 87Rb and 133Cs. Conventional RF excited lamps were used. Interest in such cells is for possible application in atomic clocks. The Rb cell would appear to remain especially promising in this respect

Johnson, C. E.

Robinson, H. G.

English

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NARROW 87Rb and 133Cs HYPERFINE TRANSITIONS I N  EVACUATED WALL-COATED CELLS 
H.G. Robinson 
Physics Department, Duke University 
Durham, North Carolina 27706 
C.E. Johnson 
Physics Department, North Carolina S t a t e  University 
Raleigh, North Carolina 27650 
ABSTRACT 
An extension of our work on wall-coated cells has been made t o  
include observation by a t r i p l e  resonance technique of t he  0-0 
hyperfine t r ans i t i ons  i n  87Rb and 133Cs. 
lamps were used. 
cation i n  atomic clocks. 
especial ly  promising i n  t h i s  respect. 
Conventional rf excited 
In t e re s t  i n  such cells i s  fo r  possible  appli-  
The Rb cell  would appear t o  remain 
INTRODUCTION 
We have previous1 RbIAmFI = 1 hyperfine t ransi-  
t i ons  i n  a 200 cm5, evaucated wall-coated c e l l .  The narrow Lorentzian 
component of t he  lineshape has a width of n., 11 Hz, F W ,  giving a 
Q n., 0 . 6 5 ~ 1 0 ~  fo r  t he  resonance. In t e re s t  i n  the  (F,mF) = (2,0)*(1,0) hy- 
perf ine t r ans i t i on  f o r  po ten t ia l  use i n  Rb frequency standards has focused 
our a t ten t ion  toward exploring the  charac te r i s t ics  of t h i s  llclockll t ran-  
s i t i o n .  
87 reported’ observation of 
EXPERIMENT AND RESULTS 
The apparatus avai lable  uses a conventional rf plasma excited Rb lamp which, 
a f t e r  f i l t e r i n g ,  produces c i r cu la r ly  polarized D 1  l i gh t .  The c e l l  i s  placed 
i n  a shielded solenoid producing a 1.5G magnetic f i e ld .  
a(+ or  -) radiat ion permits observation of IAmFI = 1 hyperfine t r ans i t i ons  
by monitoring the  in t ens i ty  of  t he  transmitted l i gh t .  
scheme r e l i e s  on a change i n  (Sz) , the  z-component of the  electron spin.  
Where the population is  pumped toward the  (2,2) level ,  t he  la rges t  s igna ls  
correspond t o  t h e  (2,2)*(2,1) Zeeman t r ans i t i on  and the  (2,2]+1,1) hyper- 
f i n e  t rans i t ion .  However, s ince the  change i n  ( S )  is zero f o r  t h e  0-0 hf  
t r ans i t i on ,  t he  method i s  not su i t ed  for  d i r e c t  oke rva t ion  of t h i s  reso- 
nance. Nevertheless, by exci t ing the  Zeeman t rans i t ions  ( 2 , 2 ) 4 2 , 1 )  and 
(2,1)-(2,0), w e  can detect a change i n  the  ( 2 , O )  population caused by a 
0-0 t rans i t ion .  This t r i p l e  resonance scheme was employed t o  obtain i n i -  
t i a l  da ta  on t h e  clock t r ans i t i on  i n  the sealed,  evacuated wall-coated 
Rb cell.  
The use of  t he  
This detection 
307 
https://ntrs.nasa.gov/search.jsp?R=19830027095 2020-03-21T01:14:13+00:00Z
Linewidths as narrow as 9 Hz FWHM are observed f o r  the  0-0 re ce a 
Figure 1 displays such an observation having a 13 Hz linewidth with a 
Lorentzian lineshape function f i t t e d  t o  t h e  data .  
sonance is  a sens i t i ve  function of the tuning of the two Zeeman resonance 
drives.  The observed wall s h i f t  of the hyperfine frequency due t o  atom-wall 
in te rac t ion  is  - 52 Hz a t  26 OC. Both width and wall s h i f t  are consis tent  
with our previously reported1 observations made on the (2,IZ l)++(l, 11 I )  
hf t rans i t ions .  
A 100 cm sealed, evacuated wall-coated cell  was a l so  avai lable  f o r  133Cs. 
We repeated the  above procedure with the C s  c e l l  using a conventional rf 
driven C s  lamp as the  pumping source. The linewidth extrapolated t o  zero 
l i g h t  and rf i n t e n s i t i e s  is  'L 101 Hz.  The wall s h i f t  observed is  - 180 Hz 
a t  26 OC. 
w a l l  t o  50 OC w e  obtained a w a l l  s h i f t  of - 160 Hz for  t he  (4 ,14( )43 ,131)  
t r ans i t i ons .  
The symmetry of t h e  re- 
3 
Upon cooling the C s  reservoir  t o  5.5 OC and heating the ce l l  
These i n i t i a l  r e s u l t s  leave unanswered questions of aging, r e t r aceab i l i t y ,  
and whether subs tan t ia l ly  b e t t e r  evacuated wall-coated cells can be f ab r i -  
cated. 
a sealed,  evacuated wall-coated c e l l  shows a poten t ia l  f o r  use of t h i s  type 
of ce l l  i n  an atomic frequency standard. We are  pursuing pumping/detection 
schemes permitting e f f i c i en t  d i r e c t  observation of the 0-0 t r ans i t i on  using 
laser diodes. 
However, t he  observation of 87Rb hf  resonance with Q 'L 6 . 5 ~ 1 0 ~  i n  
REFERENCES 
1. H.G. Robinson and C.E. Johnson, Appl. Phys. Lett .  - 40, 771 (1982). 
Figure 1. 
i n  87Rb with a linewidth of 
13 Hz. The s o l i d  curve i s  
a Lorentzian function f i t t e d  
t o  the  data  points .  Observation 
is by a t r i p l e  resonance tech- 
nique. 
A 0-0 hf  t r ans i t i on  
308 
QUESTIONS AND ANSWERS 
PROFESSOR JACQUES VANIER, Quebec, Canada 
What kind of a coating material do you use in your cell? 
MR. ROBINSON, Duke University 
The material that's used here is, we believe, is TE IN. These 
cells were built at a very chaotic time and the record keeping on these 
cells is not all it should be. But, we could tell, we could test that 
by taking the cell up to where we would melt the surface wax, find out 
unequivocably, but, we'd rather not do that, until we have played with 
the cells a little longer. 
MR. DEHMELT: 
Do you have an estimate of the optic frequency Zeeman for the transition 
in the rubidium cases? It varies -- more than I know. 
MR. ROBINSON: 
The line width of the zero-zero transition is very insensitive to the 
amount of power put on the Zeeman. The Zeeman line width is considerably 
in excess by broadening of RF, due to the transition due to the Zeeman 
transition power. 
In other words, I would say the Rabi frequency is larger than 9 hertz 
for the Zeeman transition. 
not worked with understanding it either, the experimental fact is that 
the zero-zero transition line is very insensitive to the amount of Zeeman 
power, you put on. The amplitude is sensitive. The amplitude is very sen- 
sitive, but the line -- it may be an Autler (? )  -Townes situation. 
We haven't understood that yet, but we have 
AUDIENCE : 
Yes. What do you think is the dominant contribution to the line width in 
these transitions you're seeing? 
MR. ROB1 NSON : 
The dominant contribution of the line width, we were able to, essentially, 
write a line width budget for the case of the Delta M-1 transition, and the 
dominant contribution, by far, is the effect on the wall. While it sits on 
the wall, there is a dispersion in the phase shift that you get. Each at- 
tack of the wall of the atom does not give you exactly the same phase shift 
as the random walk in phase. And it's that dispersion that is the dominant 
cause. 
309 
O u t  of t ha t  ten cycles for instance, a t  leas 
andom nature of this phase shift o 
So, the wall i s  the dominant cause of the line w i  . The l i g h t  i n -  
tensity t h a t  we've used t o  take these resonances i s  on the order of,  1 1 /2  
microamps. T h a t  issue came up yesterday -- and I d o n ' t  know how many 
microwatts per square centimeter tha t  is, b u t  t h a t ' s  the transmitted 
l i g h t  intensity. I t ' s  a very, very low l i g h t  intensity, so we d o n ' t  get 
very much 1 i g h t  broadening. 
Now, we could go down a whole l i s t  of potential sources of line width,  
and they a l l  reasonably add up. So, we have looked a t  t h a t  budget, b u t  the 
dominant one by f a r  i s  the wall itself. 
DR. KELLOGG: 
Do you have any plans t o  go the other way i n  the temperature spectrum? 
Maybe cooling the wall before you risk melting the wax? 
MR. ROBINSON: 
Well, theoretically, i t  looks like it 's better to  heat the wax. The l ife- 
time, the time the atom s i t s  on the wall, is related t o  the absorption 
energy. 
What you want t o  do i s  t o  heat the wall up and, i n  fact, for the 
cesium case, we heated the wall up t o  about 5 5 O C ,  and the line width nar- 
rows as you do this. 
The wall shift  decreases as you do this, or rather, the magnitude of 
the wall shift  decreases. So, things get better as you heat the wall ,  un- 
t i l  you reach a p o i n t  where, perhaps, the wax begins t o  melt or change 
character, and then i t  turns around and goes the other way. 
B u t ,  i n  fact, i t  looks like i t ' s  better t o  heat the wall i n  this parti- 
cular case than, than t o  cool it. 
These cells aren't expensive t o  produce. I d o n ' t  know wha t  -- how 
you count production costs, and whatnot, b u t  there's just a tr ivial  
amount of wax invovled i n  them, and glassblowing time and so for th ,  so 
the cost i s  not really significant, I t h i n k .  
AUDIENCE: 
I would like t o  ask one question, please. About the insensitivity to mag- 
netic field gradient. I believe tha t  i s  true if  you use a h i g h  magnetic 
fie1 d,  bu t  1 i ke i n  a hydrogen maser, when you go t o  lower fields , t h a t  i s  
when used for frequency standards, you start t o  have some trouble w i t h  
the Zeeman frequency. I f  i t  i s  of the magnitude o f  the collision fre- 
quency of the atoms, then you start t o  loose signal and you have a shift  
i n  the frequency of the line. So the insensitivity to magnetic gradient 
i s  true only for h i g h  magnetic field and not for low field. 
310 
R. ROBINSON: 
I'm not sure understood a 1 t h a t ,  b u t  you're commenting on the fact t h a t  
we have 1 1 /2  gauss field and we d i d  t ha t  i n  particular t o  resolve a l l  the 
Zeeman transitions so tha t  we could indiv idua l ly  diagnose things, see what 
was going on. 
The Zeeman line width i s  about 2 1/2 cycles i n  these cells. There's 
no reason why we couldn't  reduce t h a t  field as far  as we see. 
we understand how the averaging proc i t y  occurs, and there 
should be no problem a t  all i n  reducing the field w i t h  respect t o  tha t  par- 
ticular parameter. So I d o n ' t  see any, just offhand, any problem i n  going 
down t o  very small fields. There's no problem w i t h  optical pumping as I 
understand i t  and going t o  essentially zero fields as long as we have some 
direction for the Z direction for the system. 
I t ' s  -- 
AU D I ENC E : 
What I mean is ,  i s  i t  possible t o  go t o  a lower field? Also w h a t  about 
the uniformity of the RF field? 
(QESTION NOT TRANSLATABLE IN THE RECORDING) 
MR. ROBINSON: 
The wall shif t  for the Delta M-1 transition was a much cleaner situation. 
And t h a t  was also -52 Hz. And we expect theoretically t h a t  the two wall 
shifts should be the same, to ,  w i t h i n  some small discrepancy. 
fact tha t  we confirmed t h a t  by this very obtuse triple resonance techni- 
que I t h i n k  i s  a good sign. I t  just means t h a t  we believe t h a t  result. 
So the 
AUDIENCE : 
Is there any plan t o  repeat the experiment w i t h  fresh cells? 
MR. ROBINSON: 
Yes, we need t o  do tha t .  I t ' s  clear t h a t  there's some critical questions 
tha t  arise as t o  how t o  get the cell t o  stabilize, will the wall shift 
itself dr i f t  forever, will i t  stabilize i n  a week or  a month o r  two days, 
o r  just wha t .  Retraceability i f  you heat 
and cool these things . What happens? Same t h i n g  f o r  the line width.  
We'd like t o  do some of these things. I t  just takes research time 
t h a t  we haven't managed t o  get. 
31 1 
as there any data over en year period as t o  how much the wa 
changed? 
MR. ROBINSON: 
No, our i n i t i a l  experimental use o f  these c e l l s  was i n  shimming magnetic 
f i e l d s  t o  look a t  magnetic moments. We have a h is to ry  of looking a t  G 
factors i n  atoms, and i n  fact, we were j u s t  not  interested i n  the hyper- 
f i n e  t rans i t ions i n  these ce l l s .  So we never even looked a t  the hyper- 
f i n e  t rans i t i on  when they were o r i g i n a l l y  made. That was very unfor- 
tunate but t h a t  was the case. 
They're extremely useful i n  tuning away magnetic f i e l d  gradients, 
because the averaging process gives you a measure o f  the magnetic f i e l d  
a t  a point, mainly the very center o f  the sphere, so i t ' s  actua l ly  mea- 
suring the magnetic f i e l d  l i t e r a l l y  over a volume i n  one sense, but the 
average i s  leg i t imate ly  mathematically t ha t  of a po int  i n  the center of 
the ce l l .  
And not only that, but i t  produces essent ia l ly  a Lorentzian l i n e  
shape, so t h a t  if you have magnetic f i e l d  inhomogeneity shims which we 
have, you can tune each one and i t  uncouples them. So tha t  essent ia l ly  
there's only one pass. 
i s  our use f o r  such c e l l s  and we j u s t  d idn ' t  have the foresignt t o  look 
a t  these things. 
It makes a l l  o f  these knobs orthogonal. So t h i s  
I t ' s  c lear  tha t  tha t  needs t o  be done now. That i s  one o f  the main 
things i f  you look a t  the long-term d r i f t  o f  the wall s h i f t .  
AUDIENCE: 
Do you know why the l i n e  width o f  the cesium was approximately ten times 
tha t  f o r  rubidium? 
MR. ROB I NSON : 
There -- l e t ' s  see, I think i n  pr incipal  from a theoret ical  point  of view, 
you expect the cesium t o  be much worse because i t  has a higher po lar iz-  
a b i l i t y .  I t ' s  a much sof ter  atom and we haven't put numbers i n to  that .  
That would be a nice th ing t o  do, t o  see i f  we could actual ly  give a back 
o f  the envelope ca lcu lat ion t o  show that.  Whether tha t ' s  the factor t ha t  
one would expect, I ' m  not certain. But t ha t ' s  cer ta in ly  one reason. 
coatings t h a t  we get  are reproducible. 
These are one of a k ind ce l l s ,  and i t  may be tha t  the cesium c e l l  j u s t  
simply i s n ' t  as wel l  coated. 
A second th ing  i s  t ha t  we don't  have enough c e l l s  t o  know whether the 
That i s ,  from one th ing t o  another 
312 
so tha t  i t  l e f t  i t s  o r ig ina l  reservoir  and migrated through the c e l l  and 
out  i n t o  some o f  the other arms tha t  we have on t h i s  c e l l .  
cesium s i t t i n g  on it. We actual ly  noticed an aging, or de-aging if you 
l i ke ,  process over a period o f  maybe ten hours when we cooled the ceslum 
c e l l .  We d idn ' t  notice t h a t  f o r  the rubidium. 
And, it could be tha t  t ha t  wall was damaged, o r  actua l ly  had some 
But, we j u s t  don't know the answer t o  the question and there's some 
good theoret ical  reasons why i t  could have happened. And I w i l l  f i r s t  go 
t o  th is ,  t o  the polarized s t a b i l i t y  o f  the atom, I th ink  tha t ' s  the most 
1 i k e l y  thing. 
313 


Narrow 87Rb and 133Cs hyperfine transitions in evacuated wall-coated cells

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