Absorption spectrum of shock heated thallium vapo

Garton, W. R. S.

Parkinson, W. H.

Reeves, E. M.

English

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ON THE 6s6p' UVEL AND * 
THE CONFIGURATION-MIXING OF 
THE 6s6pa 'P LEVEL IN T 1  I Q 
S c i e n t i f i c  R e p o r t  #11 
W, R, S, Garton 
Imperial College, London 
and 
W, H,  Parkinson and E. M ,  Reeves 
Harvard College observatory 
Cambridge, Massachusetts 
May 1966 
N o t  i n  f i n a l  form for publication 
"Reproduction i n  whole or i n  part is permitted by the 
U.S,  Government and distribution is unlimited," 
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Experiments on the absorption spectrum of shock-heated 
- 6s6pa T1 vapour have revealed a new T1 I doublet (6 a #  
$8# 
The profile of  the T1 I (6 "9 - 6s6p3 ' 3)  autoioni- 
zat ion feature has been obtained by experiments with a 
furnace and scanning monochromator8 and the r e s u l t  is com- 
pared with the theory of "Beutler-Fano" resonances. 
i) 
i 
. 
Ident i f ica t ions  of the level-posit ions of the  terms 
a a a  P, D, P, S) , due t o  the sp'-conf igurat ion of T1 I, and 
o u r  understanding of the combinations of these l eve l s  with 
those of the  series spectrum have been less s e c u r e  than i n  
the cases of the analogous spectra A ~ I ,  through I n  I, An 
important contribution on the T 1  I spectrum was m a d e  
by B e u t l e r  and D e m e t e r  (1934) who studied the absorption 
spectrum of thallium vapour - produced with an absorption 
t u b e  heated i n  a furnace - in  the quartz-ul t raviolet  and 
Schumann regions, a t  a dispersion of 17b/mmr 
t o  ident i fy  t r ans i t i ons  from the ground-level 6 a #  t o  four 
- of the possible  e igh t  of 
' 5  - 6 'Pb) be ing  nearly spar  The ground-term i n t e r v a l  (6 
7800 an-', the  upper l e v e l  6 a $  was negl igibly populated 
a t  the  l o w  furnace t e m p e r a t u r e s  used, so t h a t  the J = 5/2 
l eve l s  of spa w e r e  not accessible. A l s o ,  B e u t l e r  and me te r  
did not report discovery of 6s6p a a  S and, i n  f a c t ,  presented 
an argument, based on intensi ty  expectations associated w i t h  
approach t o  jj-coupling, predict ing absence of t h e  l i n e  
They w e r e  able 
b 
4 a 
l eve l s  - namely, 's,t# D B I  a 
* 
# 
. The argument is, however, s u s p e c t  s ince 
s# 
6 'p"s - 6 ~ 6 ~ '  a 
. 2 
similar reasoning led  t o  the f a l s e  iden t i f i ca t ion  of the 
I n  I 17571 l i n e  (Garton, 1954)- 
case has since been proved fa l lac ious  (Marr and Reppinstall,  
1966a; Garton, Parkinson and Reeves, 1966)- 
The argument in  the la t ter  
Other relevant work on t h i s  port ion of the T1 I spec- 
trum was reported by Clearman (1952), who examined the l i n e s  
e m i t t e d  by a thallium arc i n  a nitrogen atmosphere, C l e a r -  
man obtained the four l i n e s  of B e u t l e r  and Demeter and 
added three new combinations of the ground doublet, one of 
1685.41 confirming the sp  a a  Pthese a t  Xvac l eve l ,  one . i? 
1847A confirming spa 'Q, and a l i n e  a t  2209,7A 
at Avac, 8 
being interpreted as 6 '4 - 6s6pa '% a 
W e  have recently obtained absorption spectra of thallium 
vapour by means of the shock t u b e  method, The temperature 
of the (argon-thallium) plasma being 5000 t o  7 0 0 @ K ,  the 
6 ag l eve l  was populated su f f i c i en t ly  f o r  absorption l i n e s  
a 
s t a r t i n g  on it t o  be recorded. Consequently, we have found 
the  l i n e  6 ap - 6s6p' a Q % 8 and also i ts  companion 6 'p"s - 
8 which has proved to  be reasonably strong, i n  ss 6s6p' a 
contradiction t o  the expectation of B e u t l e r  and Demeter, 
The l a t t e r  workers f a i l e d  to  observe the "p"S combination 
because they used the many-line spectrum of the & molecule 
i 
I 
3 I 
I 
as  a background: the  I& spectrum is poor i n  l i n e s  i n  the  
region concerned. 
The l i n e  found by B e u t l e r  and Demeter a t  about 20071, 
and ascribed to  6 'PO - 6s6pa ' 3 ,  has the asymmetrical 
type of p r o f i l e  recent ly  explained by the work of Fano and 
col laborators  (Fano, 1961; Fano and Cooper, 1965), and now 
described as a "Beutler-Fano resonance", These workers 
have shown tha t ,  i f  an autoionizing leve l  has only one 
avai lable  continuum for the rad ia t ion less  t rans i t ion ,  then, 
on one s ide of the absorption maximum, there should be a 
minimum of zero-absorption cross sect ion,  a predict ion t h a t  
found its f i r s t  ver i f ica t ion  i n  the work of Madden and 
Codling (1965) on the H e  I resonance 1s' 'so - 2s2p '9" 
near 200A , Since achievement of su f f i c i en t  wave-number 
dispersion fo r  adequate resolut ion of a l i n e  p r o f i l e  a t  
such short  wavelength is d i f f i c u l t ,  we  have thought it worth- 
while t o  examine the p ro f i l e  of the T 1  I 20071 l i n e  because 
here  again the autoionizing leve l ,  6s6pa Pa 8 has avai lable  
only a s ingle  continuum - in t h i s  case, of even p a r i t y  and 
J = 3/2, viz ,  , 6s'E (d) '&. The l i n e  p r o f i l e  proved wide 
enough for f u l l  resolut ion a t  the dispersion (8,3A/m) of 
a McPherson one-meter scanning vacuum-spectrometer, 
* 
a 
a 
A 
furnace was used to  provide a column of thallium vapour 
50 a n  long, a t  a b o u t  8 5 @ ~ ,  a hydrogen positive-column dis- 
charge provided a background continuum, and an EM1 6256 
quartz-window photamultiplier served as detector. 
The p l a t e  shows a set of spectra obtained with the 
shock t u b e  apparatus for varying dens i t i e s  of T1 vapour. 
The four l i n e s  discovered by B u t l e r  and D e m e t e r  are pres- 
e n t ,  and a new doublet w i t h  components a t  1 3 0 d  and 1449A, 
exhibi t ing large autoionization broadening, can be seen, 
So f a r  a s  the LS-designation is meaningful, the  upper l eve l  
, which is therefore placed a a  here concerned w i l l  be 6s6p 
76804 cm-' above 6 'P The shock p ic tures  a l so  contain 
Clearman's l i n e  a t  16851, viz., 6 'Pi - 6s6pa a 
been unable  t o  confirm h i s  l i n e  6 
a 
t h i s  would show a s  an extremely d i f fuse  absorption feature ,  
and i ts  absence is not surprising, from the  in tens i ty  r u l e s  
f o r  a aPo - 'D mult iplet ,  
sa 
b- 
pr we have 
- spa a t  1847A; 
O u r  plates show an extremely 
- weak d i f fuse  absorption l i n e  a t  a b o u t  the pos i t i on  kvac, 
3 -  A con- 
siderably stronger l i n e  present on our exposures ,  a t  Xvac - 
a ' - 6s6pa * 2210,4A given by Clearman for 6 
p: 
. 
2231,511, seems def in i t e ly  ascribable t o  T1 I. The possi- 
b i l i t y  e x i s t s  t h a t  t h i s  is the  'p combination, though it 9 
5 
a - GaFO, ps could equally w e l l  be the forbidden t r a n s i t i o n  6 
s ince  several m e m b e r s  of 6 a 0  Pa - naFi (n = 5,6,7) are pres- 
a 
e n t  i n  absorption a t  high vapour dens i t ies ,  
The l i n e  lair 2671.0 mentioned by B u t l e r  and m e t e r  
(1934) as being reported by former workers, is present,  weak 
and q u i t e  sharp, on o u r  spectra,  It can be safely ascribed 
, and t h e  measurements reported recent- t o  6 aPo - 6s6pa ' 
a o  l y  by Reeves, Garton and B a s s  (1965) on i ts  companion 6 
(a close doublet unresolved i n  the  present  work) 6s6pa ' 
indicate  the improved wavelength X = 2671.84, a ir 
p8 Q 
p* - 
p8 
The profile of the "resonance" fea ture  near 20071, now 
0 0  in te rpre ted  as a t r a n s i t i o n  f r o m  6 P t o  the m i x t u r e  of 
' 3 ,  is shown reduced t o  absorp- states 6s6p P and 6saE(d) 
t i o n  cross sect ion by t he  f u l l  l i n e  i n  Fig, I, The profile 
was obtained, as  mentioned above, w i t h  a furnace and w i t h  
t he  scanning monochromator resolving 0,lSd , The scale of 
8 
a 4  
8 
absolute cross sect ion has been taken f r o m  the recent  re- 
v is ion  by M a r r  and Heppinstall (1966b) of M a r r ' s  (1954) 
measurements of t he  A2007 p ro f i l e ,  made w i t h  a f l u o r i t e  prism 
spectrograph and photographic photometry, 
Fano (1961) and collaborators (Fano and Cooper, 1965) 
have shown tha t  for  autoionization i n t o  a s ing le  continuum, 
. 6 
c 
the  absorption cross-section p r o f i l e  should follow the 
formula 
(9 + e ) =  
1 + ca K(v) = const. * 
2 (v-vo 1 
r where q is a " l ine  p r o f i l e  index" and c = is a di- 
mensionless 'sreduced" energy var iable  , which denotes the dis- 
tance of the point  a t  frequency V i n  the p r o f i l e  from the 
idealized "resonance1' frequency vo corresponding t o  the auto- 
ionizing state when expressed as a multiple of the resonance 
half-width r/2. 
The absorption cross section (1) takes a maximum (1 + 8 )  
a t  8 = l/q, and a zero minimum a t  C = -q. Consequently, q 
can be obtained from the height of the maximum i n  Fig. I, 
r e l a t i v e  t o  the l eve l  of the unperturbed continuum absorption, 
and r can then be found from the  distance c - c  - 
q + l/q along the horizontal  axis.  
 
max m i n  
From the f u l l  curve of Fig. I we thus f ind  q = 2.30 ,  
r = 1.36 x lea sec-'. Points corresponding t o  the experi- 
mental data of M a r r  (1954) a re  marked and, while these gen- 
e r a l l y  conform, it is clear t h a t  the instrumentation avail-  
able  t o  him did not permit f u l l  resolut ion of the maximum 
of the l i n e  prof i le .  With the values of q and r derived as 
above, w e  have shown i n  Fig. I the course of equation (l), 
normalized t o  the experimental data a t  the posi t ion of the 
maximum, The agreement between theory and experiment is 
excel lent ,  save fo r  the departure a t  the short-wavelength 
s ide ,  which can be reasonably ascribed t o  departure from 
the assumption contained i n  (1) t h a t  the absorption cross  
sect ion of the unperturbed continuum does not change appre- 
c iably over the width of the resonance. 
The portion of the work reported here,  performed with 
the shock t u b e  apparatus, has added one important new leve l  
of T1 I and provided de f in i t e  confirmation of another pre- 
viously ident i f ied  by the work of B u t l e r  and Clearman. 
work with the furnace has provided the best quant i ta t ive  
check of a Beutler-Fano resonance t o  date,  However, several  
features  of t h i s  absorption spectrum s t i l l  require under- 
standing. 
determined with any cer ta inty,  
be expected t o  give a strong combination with the upper level  
of the ground-term, has not been revealed, Likewise, the 
spa a P  
LS-symbols has l i t t le  significance i n  t h i s  configuration of 
~1 I, and f a i r l y  extensive ca lcu la t ion  w i l l  be necessary t o  
account fo r  peculiari t ies outstanding. 
The 
The sp a '% level  cannot be sa id  t o  have been 
The '5 level ,  which could 
l e v e l  has not been found, Probably the u s e  of the Q 
ACKNOWLEDGEMENTS 
W e  a r e  indebted t o  D r .  G. V. M a r r  and M r ,  R, Heppins- 
t a l l  of Reading University for a s igh t  of oscillator st rength 
determinations ahead of publication, 
M r .  W. Bingham f o r  h i s  technical ass is tance i n  performing 
the shock tube experiments, and Messrs, G,  Grasdalen and K, 
Nicholas for assis tance with the furnace experiments. 
W e  also wish to  thank 
This work has been supported by NASA Grant NsG-438 and 
Contract NONr-1866(48) of the Office of Naval Research, 
. I 
I 
I .  
REFERENCES 
Clearman, €3, E,  1952, J, Opt, SOC, Amer, - 42, 373, 
Fano, U, 1961, Phys, Rev, 141, 1866. 
and Cooper, J. W. 1965, Phys, Rev, - 137, A1364, 
Garton, W, R, S, 1954, Proc. Phys, SOC, 67, 864, 
,parkinson, W. H , ,  and Reeves, E ,  M ,  1965, 
Ap. J, 140, 1269, 
1966, 
Can. J, Phys, ( i n  press), 
Madden, R, M, and Codling, K, 1965, Ap, J. 1 4 1 8  364, 
Marr, G ,  V, 1954, Proc, R o y ,  SOC, (London), Ser, A, - 224, 83, 
and Hepphsta l l ,  R,  1 9 6 6 3  Proc, Phys, SOC, -4 87 
547. 
Reeves, E ,  M , ,  Garton, W, R. So, and Bass, A, 1965, PrOC, 
PhyS, SOC, 86, 10770 
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2. REPORT S E C U R I T Y  C LASSIFICATION 
Z b  CROUP 
- 
I. R E P O R T  D A T E  
May 1966 
la. CONTRACT OR CRANT UO. 
NONr-1866 (48) and 
NASA-NSG-438 A P R O J E C f ? d O .  
7b. NO. OV REFS 7 r .  T O T A L  UO. O F  P A G E S  
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8.. ORIQIUATOR'S REPORT UUMDER@> 
Sc ien t i f i c  Report #11 
1. SUPPL EYaYTARV MOTES 
3. ABSTRACT 
$1. SPaYIc I I I IHB YILITARV ACTIVITV 
Experiments on the absorption spectrum of shock-heated 
T1  vapour have revealed a new T 1  I doublet (6 '$4 - 6s6pa 
'Pi - 6s6p' 'P 1 auto- The p r o f i l e  of the T1 I (6 
ionization feature  has been obtained by experiments with 
a furnace and scanning monochromator, and the r e s u l t  is 
compared with the theory of iiBeutler-Fano'i resonances . 
3 S+) . 


On the 6s6p2 2S1/2 Level and the Configuration-mixing of the 6s6p2 4P3/2 Level in T1I Scientific Report No. 11

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