Beam foil measurements of multiplet mean lives in nitrogen deca

Bashkin, S.

Berry, H. G.

Bickel, W. S.

Desesquelles, J.

English

NASA Technical Reports Server

* 
The Mean Xves of Sone Excited Levels in N I 
- 
t W.'S;sBickeS, Jean Desesquelles, H. G, B e r r y  and S. Basbkin 
/ 
Department of Physics, University of Arizona, Tucson, Arizona 85721 
f l /  G- 0 3 r c r a - z - 0 1 3  
Abstract 
We measwed the mean lives of t v o  lov quartet and 
t w o  low doublet excited l eve l s  of N I using the beam- 
foil technique, 
- 
The t rans i t ions  observed, their wave-$ 
lenkhs in 8, and the mean lives of the upper Levels in 
nanoseconds are: 
2s2zP3 "so - 2s22p2 ( 3 ~ ) 3 s  "P E O d ?  2.4 f 0,1 
149@ 2*2 f 0.1 2s22p3 2 ~ o  . 2S2zp2' ( " ~ ) 3 s  2 P 
2s22p3 2Do - 2s22p2 ('D>3s 2 I) 
2s%p3 *s" - 2s22p4 - 'P 33-342 7.0 f 0.2 
E438 2*6 f 0-1 
These experimental results d i f f e r  from theory by fac tors  
as  large as 4, 
I 
https://ntrs.nasa.gov/search.jsp?R=19700002243 2020-03-12T04:48:46+00:00Z
2 
INTRODUCTION 
There have been s u b s t m t i a l  d i f f i c u l t i e s  i n  t h e  way of obtaining 
r e l i ab le  t r ans i t i on  probabi l i t i es  for electronic t rans i t ions ,  especial ly  
where t h e  radiat ion l i e s  i n  the vacuum ul t rav io le t .  A good example of 
the discordant values given by e q e r i x e n t  a d  t h e o q  is  offered by t h e  
E I t r ans i t i on  2p3 'So - 3s 4P (Xl2(X$). 
approximation ca lcaa t ion '  gave a t o t a l  o s c i u z t o r  s t rength of 0.12; a 
measurement, 
value of Id" e 
confinned the  o r ig ina l  t heo re t i ca l  o sc i l l a to r  strength, and subsequent 
experinent~~'~ agreed with the  theory, 
An early Bates-Damgaad Coulomb 
2 based on absorption i n  a gas cell., gave the confl ic t ing 
3 Recalc&zLion , using the  self-consistent field approach, 
The ultimate concordance of the foregoing theore t ica l  and experimental 
6 resultis was not definitive, hove-;cr, 
shrift technique to measure the mean l i f e  of t he  3s 
Lamence and Savage, i.;g a phase- 
P multiplet ,  deiluced a, 4 
t r ans i t i on  probabi l i ty  l a rge r  by a factor  of 2.3 t h a  that found i n  Refs .  
3 and 4, while s t i l l  mother  experi;aent, based on the emission of l i g h t  
from a s t zb i l i zed  arc,' gave an. even larger  value of A (2,9 times t h a t  of 
Refs, 3 and 41, It is  worth noting that a different  s e t  of calculations,  8 
on other N I mult iplets  yielded t rans i t ion  probzbi l i t i es  severa l  times 
la rger  even than those measured by La-mence and Savage. .. 
It is, therefore ,  abundantly c lear  that there  is need for additional. 
work on e lec t ronic  t r z x s i t i o n  r a t e s ,  The theoretics calculat ions nust 
take account of configuration interaction: while froin the  standpoint of 
experiment a consistent and re l i ab le  method which has widesDread apylica- 
b i l i t y  i s  required. 
3 
. -  
The experimental dexands can be s a t i s f i e d  to ~ m e  extent by t h e  
lo techniques of be=-foil spectroscoFjr, 
pendent means of detem2nine; t r ans i t i on  probabi l i t i es  and a re  w e l l  su i ted  
t o  cases in which the  observed radizt ion l i e s  in the  vacum ultraviolet, 
i n  which the l eve ls  of i n t e r e s t  a r e  in mlt iply- ionized en i t t e r s ,  and in 
which the  decays do not necesstrrily connect t o  the ground state. 
present pzper describes beam-foil measurements of t he  mean lives of a number 
These techntques of fe r  an inde- 
The 
of mult iplets  in N I; t h e  N I mult iplet  discussed above is  one of those 
studied i n  this  paper. 
la + Eitrogea atoms were obtained by parb ia l  neutral izat ion of N ions 
. .  
i n  a thin carbon f o i l  (10 pg cm-”) a f t e r  acceleration ‘GO 2 MeV with a 
Van de G r a a f f  accelerator,  The radiat ion emitted by the excited l eve l s  was 
observed with a one-zieter NcPherson sgectrameter (600 l i n e s / m  grat ing 
blazed a t  15008 i n  first order),  
p l i e r .  
The detector was m E4R 51k2-G photclmulti- 
The D.C. output was amplified by z Keithley p i c o a e t e r  and recorded 
on a, strip-chax2, the  analyzed spectrum being obtained by rotatiorz of the 
grating. In order t o  measure mean l i v e s  the  spectrometer was set t o  pass 
the required wavelengkh while view5ng a 5 m len@h of t he  bezm. 
l i ves  were determined by measuring the photomultiplier signal obtained by 
The mean 
observing the  beam a t  different  points  downs-tPean from the  f o i l ,  the l i n e  
of s igh t  being perpendiculzs to the  b e m  direct ion,  
current was rrleasured with a?? unshielded Faaday  cup to assure 5 t s  constzncy. 
The beam p a r t i c l e  
frorn the  f o i l ,  corresponding to about 4 cm of the  beaa by photon counting, 
The energy of the  pa r t i c l e s  before passing through the f o i l  i s  known i n  
- .  
4 
most of the uncer'zaihty-being i n  the  measureaent of i t s  thickness, and thus 
the final speed is  known t o  f 3$* Its variat ion during a mean l i f e  measure- 
ment is Less than 1%. 
Sgectra 
' The pazt ic les  Leaving the f o i l  are in many di f fe ren t  charge states, 
Although previously the cmbon f a i l  has been used f o r  "stripping" electrons 
from singly-ionized atoms passing through it, a cer ta in  f r ac t ioa  of the 
ions gain an electron, leaving them i n  neut ra l  states as they leave the 
foil. 
that a r e  neut ra l  increases,  
A s  t h e  energy decreases the f rac t ion  of the t o t a l  number of atom 
Although t h i s  has been known ?E z m e  time, 
f n i t i a l  spectroscopic s tud ies  of t he  foi l -exci ted bean gave l i t t l e  indication 
Hovever, most of the  previous . t h a t  neut ra l  pa r t i c l e s  were i n  excited s ta tes .  
experiments were concerned &th the wavelength range above h350& 
research, where the vacum u l t r av io l e t  region w a s  exmined down t o  l05Q!?, 
In t t i s  
many lines were seen which could be a t t r i bu ted  t o  neut ra l  nitrogen. Thus 
the eaz l i e r  d i f f i c u l t i e s  i n  detecting excited neutral emitters may have been 
due to s m a l l  o sc i l l a to r  strengths f o r  the t r aas i t i ons  i n  the wavelength 
regions observed, 
Figure 3. illxstrztes pzrt of the observed in tens i ty  d is t r ibu t ion  
f for the radiat ion ex i t t ed  from 250 key M ions a f t e r  passing tbrough the 
fo i l ,  The wavelengths were cal ibrated from l ines  of unzxbiguous origin, 
&- G', f o r  a m p i e ,  giving a wavelength uncertainty less than 3.8, In the 
spec t ra l  region betwcen l-O5Q? and lT5Q?, the most intense l i n e s  (Fig. 1) 
m e  at h1035a, h l l 3 @ ,  h ~ 8 4 E ,  h120Q8, h12h3f?, zqd hlkj$.. These a r e  
5 
_. . -  
i den t i f i ed  as the  resonance l i n e  of N I1 at kl08j8, and l i nes  of N I at  
AEOd,' ?4l34.., h149$?, and Xl249. 
i den t i f i ed  w i t h  the  de-excitation of neut ra l  nitrogen atoms t o  the  meta- 
s t ab le  leve ls  2p P : k13&, h14l2.8, and h174$?. Other weak l ines ,  
just =bo-v-e the  noise signs1 coincided with unclassif ied t r a s i t i o n s  in 
D I~ f o r  e x a q l e ,  1156~8, X1653a. 
much higher b e m  energies, but with higher resolution, it could be 
decomposed i n t o  two l i n e s  at  1238.$ and 1242.$?, which we a t t r i b u t e  t o  
a doublet i n  N V. A t  250 key the  l i n e  at 1238,5x was absent and we assumed 
Ot'ner l e s s  intense l i n e s  were 
3 2 0  
 fie l i n e  a t  1.~49 appears strong a t  
t h a t  t he  l i n e  at l24$ was caused en t i r e ly  by the  N I t r ans i t i on  
2p23s 'D -J 2p 3 2 0  D e 
, 
Mean Lives 
Figure 2 shows the  r e s u l t s  obtained fron measuring about 20 points.  
The mean l i f e  7.0 +- 0.1 nanoseconds wits determined wit'n both a computer 
least-squares fit, and graphically. 
distance corresponding t o  three  t h e s  the  l /e  decay length and were negligible.  
This mean l i f e  was measured at two diff'erent incident pxct ic le  energies, 
No cascade e f fec ts  were seen over a 
250 keV znd 590 key. 
obtained the saxe value €or t he  mean l i f e .  
Taking in to  account t he  energy loss  i n  t h e  f a i l ,  we 
ALthoush we expected a contribution 
.from t he  t rans i t ion  2s3s 3S - 2p3s "Po of N IV a t  this higher energy, t'he 
equality of the  two m l u e s  f o r  the  ae8n l i f e  suggests that  there  w a s  a 
negl igible  contribution fron t h e  N I Y  t ransi t ion.  
The in t ens i ty  deczy is  not exponential but a good least-squares f i t  
was obtained using two exponentizls, one corresponding to t he  .zpproximtely 
6 
exponential decay of t'ae t a i l  of the curve, m d  a shorter  decay which \re 
a t t r i bu ted  t o  the mean l i f e  of the  3s *P leve l ,  The mean l i f e  associated 
with the l e v e l  wits 2,4 f 0-1  nanoseconds, and that for the  long-lived 
component 37 f110 naoseconds. 
more cascades from the  leve ls  3p 
other decay nodes. 
The latter corresponds perhaps t o  one o r  
*Do, since these levels have no 
(Transitions t o  t he  2sPp4 'P l e v e l  require a two-electron 
t rans i t ion ,  and a r e  unlikely i n  the f r ee ly  decaying beam). 
of these levels  a r e  knotin from Richter'' t o  be 28.7, 43-8, 52.6 nsec re- 
spectively, consistent with our obse&ations on thTs cascade lifetime a 
The mean l i v e s  
2p3 "Do - 3s 2D, A3249 
The decay curve wcs f i t t e d  with two eqonen t i a l s ,  one a short-l ived 
D level, 2.6 f 0.1 nanoseconds, 2 decay a t t r i bu ted  to t he  mean l i f e  of t h e  3s 
and a long-lived decay of 51 & L O  nanoseconds, a t t r i bu ted  t o  cascading i n t o  
the  upper l e v e l  of t he  t r aas i t i on .  
bution from B V. 
A s  noted earlier, we exclude my contri-  
The correction t o  the mean l i f e  by accounting f o r  the  cascade term 
was 3 6  even though its i n i t i a l  i n t m s i t y  was only one-tenth t h a t  of t he  
fast decay. This i s  a good exzmple of the c m e  t h a t  nust be taken to correct  
for cascad.es e 
2p3 "Do - 3s 2P, A14932 
This l i n e  i s  affected by cascades (Fig. 2 ) .  The e x t e n t  of the 
cascade contribution was determined frorn points  fur ther  than 1,5 cm down- 
stream from the  f o i l ,  
3s 'P l e v e l  as 2,3 
three levels 3p "Do9 3p "Po, and 3p "So have allowed t r ans i t i ons  only t o  
A least-squares analysis gave t h e  mean l i f e  of t h e  
0.1 nsec aad the cascade term as 34 * 5 nsec, The 
the  3s ’P leve l ,  
with t he  observed cascade 
Their mean  l i v e s  a re  around. 50 nsec, i n  good agreemept 
.DISCUSSIO?T - TRANSITION PROEVBILITIES 
The results are reproduced i n  columrq 3 of Table I, The ccnparison 
FTith the resu l t s  of Lamence and Samge“ is  satisfactory.  
multiplets were resolved. 
l i fe t ime ‘I and the  t o t a l  t rans i t ion  probabili ty A = 1/T of the upper t ern ,  
and the  l i fe t ime is the  same f o r  a l l  levels  i n  t h i s  term. There is some 
experimental evidence 
None of the  4 
In  L-S coupling the experimental decay gives the  
13 that the mean l ives  are independent of t h e  par t icu lar  
level in the upper term. 
The t rans i t ion  probzbi l i t i es  (Table 11) 2s22p2 (3P)3s “P - 2s’2p3 “So 
3 4 0  and 2s2p4 ‘P - 2s22p S havz t h e  s u e  value. There are  m a n y  calculations 
for the  resonance line at PO&. 
Coulorab approximation by Ba5es a d  Dmzaard’” has been adjusted to 1.39 by 
K e l l y , 3  who also used a Hatree-Fock appoximation t o  o b t a h  a value 1.53 x 
lo“ sec-’. 
factors of 3 to h. 
The m l u e  1.85 x 10” sec-’ determined i n  the  
These theore t ica l  results differ f r o m  o w  experimental r e s u l t  by 
This factor  was considered possible by Kelly. 
On the other hand, the  value calculated by Kelly3 fo r  t he  t r ans i t i on  
probabili ty of 2s2p4 4P - 2.5’2~ S i s  apparently overestiroated, Varsavsky’ 
using a screened Coulonb potent ia l  produces an even higher result. Labuhnrsa 
experiment gave a t r a m i t i o n  proba3ili ty l c 5  times greater than this experi- 
ment 
3 4 0  
3 2 0  The 3s ’P l e v e l  can depopulate to the 2p D l e v e l  giving the  
3 2 0  t rans i t ion  studied, but it e a  also go to t he  2p. P leve l ,  The branching 
ratio i s  given by l2~buh.n. It i s  i n  agreerient with our observed in tens i ty  
ratio of about 0.4 fo r  t he  l i nes  a t  1743.a and 149.6x. However, t h i s  
8 
. -  - 
r a t i o  w i l l  be dependent on a slight vakiatiori of instrumental eff ic iency 
with wavelerigtlli which we have not  included in  t h i s  coqxrison,  
differences i n  background, t h i s  kind. of detemroimtioi of t he  r e l a t i v e  
t i-ansit ion probabilities of two or more decay modes is essen t i a l ly  inde- 
pendent of the  point along t h e  beam at which t h e  per t inent  l i n e  i n t e n s i t i e s  
Negiecting 
a r e  measured and of cascading into the upper s ta tee ,  In the present instance 
t h e  backgromd in t ens i ty  was t h e  same. 
- 
The 3s 2D, 4s 'P leve ls  have many possible depopulating transitions, 
3 2 0  but  the pr inc ipa l  deczys a re  those t o  the metas-table leve ls  2p D and 
3 2 0  
?P 
J. D. wishes t o  express h i s  appreciation fo r  hai6ng enjoyed the  
Bospita3it-y of the  University of Arizona V a n  de G r a a f f  Laboratory, 
9 
3- 
Work supported 5n p z r t  by EWA and OXR.  
t Present address: Universite de Lyons, Frmce, .. 
3, H, Armstrong, J, Sokoloff, R. 11, Micholls, D, R, Holland and R. E. 1 
w e r o t t ,  J, Quaat. Spectrosc, RadZzL. Transf, - 1, 143 (lsl), 
G, E. Fairchi ld  and K, C, Clzrk, Phys. Rev, Let ters  9, 100 (1962)a a -
p, S, K e l l y  and B. H. Armstrong, Phys. Rev. Let ters  426 (15621, 3 
4 A, B, Prag, C .  E, Fa i rch i ld  md ic'. C. Clark, Phys. Rev, I 137, A1358 (195).  
F, A. Morse a-d F. KaJufmm, J. Chern. Pbys. 42, 1785 ( lS5) .  
G. M, bwrence and D. 3, Savage, Phys. Rev. 141, 67 (1966). 
5 -
6 -
7 
' .F .  Labuh, Z. Natwrforsc& - 2Oa, (lS5). 
C. M, Varsa~sky, Astrophys. J, SuppL. Ser. 53, 75 (lsl). a 
'We I,, IXese, M. W, Smith, R. M e  Glennora, NS_Sf)S-lTEG k (lW61. 
'*S. Bashkin, ed., Beam-Foil Spectroscopy, Proceedings of the  Beam-Foil 
Spectroscopy Conf,, Tucson, Arizona 1967, (Gordon & Breach, New York 19581, 
I1 
. . Le C, Morthcliffe, Ann. Rev. Nucl. Sc. I?, 89 (1953). -
J, Tichter, Z. Astrophys. 51, 3.77 (1$1). 
W, S, Bickel, H. G, Berry, J, Desesquelles, S, Bashkin, 3. Qualit. Spectrosc. 
12 -
13 
and Radiat. T r a n s f  e ( to  be published). 
D, R. Bates and A, Dzagmxa, Phil. Trans ,  Roy SOC. (London) P21:2, 101 (lg@) E 14 
__I 
FIGURE cm1oi\!s 
-. . 
I 
Fig, lo The bean-fo'll spectrum of nitrogen at  250 keV, hX80 - 175c?Q, 
Some ident i f ied  trmsitions a r e  ind2cated. 
Fig. 2, Decay curves for t w o  PI I transitions. 
Table I. Lifetimes of IT I 
Wzvelength This Experiment Lawrence & S a ~ z g e  
Levels a nsec nsec 
1200 2.4 f 0.1, 2.5 f 0.3 
2s22p2 ("P)3s "P 14% 2.2 0.1 1.9 f 0-3 
2 ~ 2 ~ ~  4~ Ur: 7.0 0,2 7.2 f 0-7 
2s22g2 ('D)3s 2D 32-43 2,6 f 0.1 2-2 f 0,3 
2s22p2 C3P)3S . 4 P 
Table 11, Trami t ion  Probabi l i t ies  
Aik (measured) As (calculated) 
lo” sec-l 18 see-= 
Trans it ion A.8 us Uwrence ~ . b m -  Kelly Others 
zP3 - 3s ’P I200 4*3 4.0 5-4 1.53 
1494 3.3 3 09 505 2.4 
1743 1.2 1.4 2 e o  I*l 
3 2 0  2p D - 3s 2P 
3 2 0  
2p P ” 3s 2P 
2p3 *s* - 2 . ~ 2 ~ ~  ‘P ~ 3 4  1.45 1.39 2 43 a.w+ 
2p3 ”Do - 3s 2D 1243 3.7 4 ,I 4-6 2 *68 
18 .le 
,’ 
R. Bates m d  A, Daragaazd, P’rail. Trans, Roy. SOC. (London) A242, 101 (Sg45), -
bP, S ,  K e l l y  and B, E, Amstrong, PhyS. Rev. Letters 9, 426 (1952). 
c“ Ywsavsliy, Astrophys, J, Suppl, 53, 75 (1$l), 
- 
I 
, 
t 
- . - .  - .. - . . ... . . . - . . " _  . . ~ .  . 
ul
 
r
 
52
 
.I 
z
 
w
 
-
. 
...
. 
. 
.
~
 
.
 .
 .
 -
 .
_
 - 
..
. 
. .
.
 
. 
.
 
- 
~ 
. .
 .-
_
 . 
. .
 .
 .
 .
 
.. 
.
 .. 
..
. 


The mean lives of some excited levels in nitrogen 1

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server