Electron transitions in optically pumped mercury vapor emissio

Cunningham, D. E.

Helm, F. A.

English

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Adelphi Universiti 
A REPORT FROM THE 
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https://ntrs.nasa.gov/search.jsp?R=19660014488 2020-03-16T19:44:56+00:00Z
ADELPHD U N I V E R S I T Y  
GARDEN C I T Y ,  L. I . ,  N.Y. 
STUD I ES ON EM! SS B ON PROCESSES 
IN 
OPT I CALLY PUMPED MERCURY VAPOR 
BY 
0 ,  E. Cunningham 
and 
F. A. Helm 
JANUARY 1966 
Research supported by the National Aeronautics and Space Administrat ion 
NASA Grant NsG-394 
. 
I 
. 1. INTRODUCTION 
- 1 -  
When o p t i c a l l y  pumped mercury vapor reemits rad ia t ion ,  
the  character o f  the rad ia t i on  from the 6 3 P, e lec t ron  t r a n s i t i o n  
- 1  t o  6 So (i.e. the 2537A rad ia t i on )  changes markedly as a 
func t ion  o f  numbers o f  atoms per m3 i n  the rad ia ted  and 
pumped mercury gas sample. Because o f  the importance o f  
t h i s  e f fec t  on the  measurement o f  l i f e t i m e s  o f  exc i ted  s ta tes  
i n  atoms (using the Hanl/e method), and the use o f  the t r a n s i t i o n  
as a t o o l  f o r  studying atomic c o l l i s i o n  cross sections, -1- a 
complete i nves t i ga t i on  o f  t h e  absorpt ion and emission process 
5 
was undertaken. E a r l i e r  work reported by J. P. Bar ra t  was 
extended t o  lower pressure ranges and the agreement between 
tha t  work and t h i s  i n  regions where the data could be compared, 
was reasonably good. The ' investigations reported on here were 
1 
2,3?4 
c a r r i e d  on i n  the n a t u r a l l y  occurr ing i so top ic  mix tu re  o f  
mercury, and l a t e r  w i t h  the mercury 198 isotope alone. 
The ana lys is  o f  the data obtained does i nd i ca te  t h a t  
t he  process which occurs i n  the  re-emission i s  one that  can 
be descr ibed i n  terms o f  an apparent increase o f  the exc i ted  
s t a t e  l i f e t i m e  as the  pressure i s  increased. This f a c t  can 
be determined s ince  data taken a t  f i x e d  pressures (o r  what 
is equiva lent ,  temperatures) as a funct ion o f  app l ied  magnetic 
f i e l d s  can be i n te rp re ted  i n  terms o f  the l i f e t i m e  o f  the 
exc i ted  s ta te .  A t  d i f f e r e n t  pressures, a d i f f e r e n t  l i f e t i m e  
. 
r 
- 2 -  
o the r  than the llnaturaBIB l i f e t i m e  QCCUU-S. The l i f e t i m e  data, 
however, i s  sfnglle valued a t  a given pressure (or temperature). 
Typica l ly ,  these experimentaluy determined 1 Ofetimes may vary 
8 8  from 1.10 seconds a t  pressures o f  80 atoms/m3 t o  
several times t h a t  value a t  80 12 atoms/cm 3 . This behavior 
1 .  
I 
(H). The resonance c e l l  Os surrounded by two p a i r s  o f  
i s  noted both i n  the na tu ra l  mercury i s o t o p i c  mix tu re  and 
i n  the  Hg '98 sample. The e a r l i e r  resu l t s  w i t h  na tu ra l  
mercury have been reported in (6). This repor t  w i l l  deal 
more ex tens ive ly  w i t h  the H9'98 data and the resu l t s  obtained. 
2. EXPERIMENTAL TECHNIQUE 
The geometry o f  the experiment 'is indicated i n  
F igure I .  L igh t  f r o m  a Bow pressure mercury a r c  (A)  passes 
through wol las ton  pr ism (C). The component o f  po la r i zed  
r a d i a t i o n  perpendicular t o  the plane of the  diagram then 
4 f a l l s  on the resonance c e l l  (O),  an evacuated quar tz  c e l l  
i n  which a drop o f  mercury i s  contained i n  a t i p  which i s  
i n  t u r n  imnersed i n  a dewar. The temperature i n  the dewar 
controls the vapor pressurs  o f  the mercury and hmce the 
number o f  atoms per cm3 s f  mercury i n  the  pa th  o f  the 
rad9at!sn. The 2537A D O  ne of the Inc ident  beam Os s t rong ly  
absorbed by the mercury vapor In the c e l l ,  and re-emissions 
a t  r i g h t  angles t o  the inc ident  beam a re  s tud ied  by usOng 
the po la r izer -ane lyzer  prisms ( E  & F) and the pho tomu l t i p l i e r  
- 3 -  
Helmholtz co i l s ,  one p a i r  annuling the ea r th  f i e l d  and the 
o ther  used t o  apply a f i e l d  p a r a l l e l  t o  the d i r e c t i o n  o f  
observation, Pre l iminary pump-down and outgassing assured 
e l im ina t i on  of impur i ty  atoms i n  the system. 
Measurements were made o f  the p o l a r i z a t i o n  P o f  the 
emi t ted rad ia t ion ,  where P i s  here def ined as 
p=H 
and l y  and Ix  are the ' in tens i t ies o f  emi t ted r a d i a t i o n  
perpendicular t o  and p a r a l l e l  to, respect ive ly ,  the plane 
of  the diagram o f  Figure I .  The p o l a r i z a t i o n  was measured 
f i r s t  as a func t i on  o f  the dewar temperature and then as a 
func t ion  o f  a magnetic f i e l d  appl ied p a r a l l e l  t o  the v iewing 
d i r e c t i o n  wh i l e  ho ld ing the dewar temperature constant, 
These measurements were taken a t  var ious temperatures 
ranging from -4OOC t o  +2OoC and i n  app l ied  magnetic f i e l d s  
from 0.0 t o  1.0 gauss. 
I n  the method o f  analys is  used ( the  Cornu method) 
the p o l a r i z a t i o n  observed i s  given by 
P = 'COS 28 (2) 
where 8 i s  the  angle between the aces o f  the  t w ~  #0!1astm 
prisms used as p o l a r i z e r  and analyzer. The appropr ia te 
i n t e n s i t y  match was performed by use o f  a pho tomu l t i p l i e r .  
I n  some of the data t o  be presented l a te r ,  p l o t s  a re  made 
w i t h  t h e  a n g l e 8  as one o f  the var iab les.  
- 4 -  
As the temperatures were decreased,, the number o f  
' atoms/m3 i n  the  resonance bu lb  decreased. This, o f  course, 
I lowered the i n t e n s i t y  o f  the re-emit ted s igna l  which i s  
analyzed. The s igna l  s i ze  set  a l i m i t , ,  us ing present 
equipment (a IP21 pho tomu l t i p l i e r  tube) a t  -4OOC f o r  
I reasonable i n t e n s i t y  readings, 
3. RESULTS 
a) Na tu ra l l y  Occurring Mercury 
Data f o r  the  angle 0 (and Po la r i za t i on )  vs. 
Temperature i s  presented i n  Figure 2. The d r a s t i c  
decrease i n  p o l a r i z a t i o n  as a func t ion  o f  dewar temperature 
i s  ev ident .  
-2OOC) t o  l . ~ ( l ~ ' ~ ) a t o m s / c m ~  (approximately 4OC) the observed 
p o l a r i z a t i o n  decreases from approximately 80% to approximately 
50% as shown i n  the F igure The steep increase i n  €3 once 
a temperature corresponding t o  -1ToC i s  reached i s  a l so  evident.  
I n  the range between l ( l 0  112 )atoms/cm 3 (approximately 
Measurements were taken t o  determine the  l i f e t i m e s  o f  
the exc i ted  s ta te .  I n  the Haaale method f o r  f i n d i n g  the l i f e t i m e ,  
a magnetic f i e l d  i s  app l ied  along the d i r e c t i o n  o f  the l i n e  o f  
observat ion.  This magnetic f i e l d  depolar izes the emi t ted 
r a d i a t i o n  i n  a t h e o r e t i c a l l y  predic ted fashion, The p o l a r i z a t i o n  
i n  a g iven magnetic f i e l d  i s :  
- 5 -  
where the subscript  r indicated the r t h  isot~pe, and s denotes 
the  sth spec t ra l  Pine, 
A and B the components sf rad ia t i on  8n the  y and x d i rec t i on ,  
61, is  t he  f r a c t i o n  o f  isotope P, 
i-espectlveny, e the e l e c t r o n i c  charge, H t h e  magnetic f i e l d ,  
m the e lect tow mass, c the speed o f  Bight, and g the Bande' 
fac to r  %OR" the appropr iate isotopic t r a n s i t i o n ,  
The typical P-essmUt sf applying a magnetic f i e l d  t o  
the  na tu ra l  mercury vapor is shown i n  Figure 3, The l i f e t i m e  
of the exc i ted  s t a t e  'is determined by cornparin? t h e o r e t i c a l l y  
p red ic ted  po la r i za t i ons  (which are a function o f  the l i f e t i m e ) ,  
and observed po la r i za t i ons .  The best  fOt is taken and the 
appropr ia te  l i f e t i m e  Os obtained, 
b) 
EssentiaP By the same data has been taken us'i ng the 
mercury 898 isotope as sample I n  the resonance bulb.  As is 
i nd i ca ted  'in F igure  4, the angle e) and p o l a r i z a t i o n  vs. 
temperature behavior i s  essentlalny ob the same f o r m  as t h a t  
o f  n a t u r a l l y  occurring mercury wlth the d i f f e rence  t h a t  the 
low temperature p o l a r i z a t i o n  vaUue i s  apprsximately BOOS. 
T h i s  .- aa L- w h w e  eApbbLuu slfiari-crrl since t he  198 !s~p+c?pe has a70 nuclear 
sp in  and so there 'is 030 s g i ' i t t i n g  of the e lec t ron  energy 
Bevels due t o  nuclear sp in  e f f e c t s ,  This means, of course, 
t h a t  On Equa(Osn 3 there  is no isstape to be considered o ther  
than Hg ' g 8  with t h e  r e s u l t i n g  simplification sf  analysis.  
l m  Figure 5 typical p o l a r i z a t i o n  v s ,  resonance c e l l  
- 6 -  
I 
temperature i s  p lo t ted .  The zero magnet ic f i e l d  and 0.3 gauss 
. data are shown. The 'Jffetimes deterrnSsJed from data simOUar 
to that shown 8 w  Figure 5 Ps presented loa Figure 6, 
These ii!fe=times are determined i n  the fo89cwfng way. 
I The values o f  polarization as a function o f  W at various 
temperatures are plotted. A tkeoretlca' l l  p l o t  o f  p o l a r i z a t i o n  
as a function o f  Z where 2 = "7'J 
the excited state) is made. Using t h i s  definition of 8 ,  
being the Uifetime o f  
equat8orn l reduces to (2) 
Typical polarization v s ,  2;Jata is shaww i n  Figure 7. bowing , .  
the various values of W ,  fu-orn a Ueast squares f i t  o f  observed 
and theoretical poUarSzat9aw vs, Z date i t  8s possibUe to 
determine "t' ., %h is  is the procedure used to caUcuUate the 
lifetime values presented In Figure 6 ,  This p l o t  is an 
average of t h e  various data. 
4 . CQNCLUS B QNS 
I t  appears tha t  there is goad agreement between 
this extended w o r k  and t h e  work reported by Barrat. The 
var ied  magnetic f i e l d  values used hcre, and the temperature 
ranges iwvo"uved aUUowed extensive analysis o f  data. 
The behavior of emitted irad3atlsm indicates that a 
sirague vaUue of Uifetlme o f  t h e  excited s t a t e  can be assigned 
a t  var ious pressures o f  mercury vapor. The  vanone so assigned 
increases w i t h  pressure as shown i n  Figure 6., 
values does appear to increase as more elevated pressures 
The spread o f  
a r e  studied, but t h l s  can be explained as priwcipa89y due 
t o  the e f f e c t s  o f  walls and walU col8is%osas on the character 
of the emit ted radiation. The f ac t  t h a t  a general f l a t t e n i n g  
o f  resu l t s  below temperatures s f  -18°C occurs, Ondicates 
t h a t  there the  %aturaB'* I l i f e t lme  I s  the appropr ia te one. 
Photons emi t ted by exc i ted  atoms a t  pressures Bower than 
10 12 a toms lm 3 a r e  not reabsorbed by other  atoms before 
they escape from the  resonance bulb.  A t  pressures higher 
than t h i s  photons are  absorbed, bu t  the subsequently 
remi t ted  r a d i a t i o n  is apparently coherent ly absorbed and 
can be described by the increased Iliifetlme. 
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FIG. 4 Angle (e) vs. Temperature ( T )  for  Hg 198 OC 
Pp 
1 ooo / 
8Oo/c 
7O0/c 
6Oo/c 
3O0/c 
-5OOC -4OOC -3OOC -2OOC - 1 ooc ooc +looc 
C 198 0 FIG. 5 Polarization ( P )  vs.  Temperature ( T )  f o r  Hg 
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REFERENCES 
1 .  A .  C.  G o  M i t c h e l l  & M. W. Zemansky, Resonance Radiat ion 
and Excited Atoms, Cambridge Univers i ty  Press, Chapter I V .  
2, L. 0. Olsen, Phys. Rev, E, 739, 1948. 
3. L. 0. OBsen and G, P. Kerr ,  Jr . ,  Phys. Rev, E,  115, 1947. 
4. 0 ,  E. Cunningham & L. 0. Olsew, Phys. Rev., m, 691, 1960. 
5. J. P. Barrat ,  Le Journal de Physique e t  Le Radium, T. 20, 
No. 7 ,  July 1959. 
6. Adelphi Un ivers i ty  NASA Semi-Annual Progress Report, 
Grant NsG-394, Ju ly  1965, pgs. 1 & 2. 


Studies on Emission Processes in Optically Pumped Mercury Vapor

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