Journal ArticleDOI

# Quasi-metastable quartet levels in alkalilike atoms and ions.

01 May 1984-Optics Letters (Optical Society of America)-Vol. 9, Iss: 5, pp 168-170

TL;DR: The properties of a subclass of quartet levels of alkalilike atoms and ions that often retain metastability against autoionization and may have large radiative yields are described.

AbstractWe describe the properties of a subclass of quartet levels of alkalilike atoms and ions that often retain metastability against autoionization and may have large radiative yields. Gain cross sections for XUV lasers with wavelengths between 20 and 100 nm are given.

Topics: Metastability (57%), Autoionization (54%)

## Summary (1 min read)

### Summary

• Even in heavier elements, often retain relative metastability against autoionization and are radiatively allowed.the authors.
• In elements such as Cs and Ba these levels are candidates for the upper level of XUV lasers.
• The distinguishing property of, these quasi-metastable levels is that the selection rules on the spin-orbit matrix elements allow nonzero matrix elements only to doublet basis levels, which are themselves prohibited from autoionizing.
• In second order, through the diagonalization, these levels do develop components of autoionizing doublet levels and therefore do autoionize, but often sufficiently slowly that the branching ratio for XUV radiation remains large.
• Therefore, of the possible quartet levels, those that couple only to the nonautoionizing 2PI/2,3/ 2 levels are 4S3/2 and 4DI/2.
• Table 1 lists the quartet levels of the 2p53s3p configuration and their autoionizing rates as predicted by the atomic-physics code RCN/RCG,3 with only this configuration included.
• Single and double stars in the left-hand column denote predicted quasi-metastability and metastability, respectively.
• The nonzero autoionizing rates of the quasi-metastable levels result from small components of autoionizing doublet levels, which are acquired in second order through the diagonalization.

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168
OPTICS
LETTERS
/ Vol.
9, No.
5
/ May
1984
Quasi-metastable
quartet
levels
in
alkalilike
atoms
and
ions
S.
E.
Harris,
D.
J. Walker,
R.
G.
Caro,
and
A.
J. Mendelsohn
Edward
L. Cinzton
Laboratory,
Stanford
University,
Stanford,
California
94305
R.
D.
Cowan
Los
Alamos
National
Laboratory,
Los
Alamos,
New
Mexico
87545
January
5,
1984;
accepted
February
14,
1984
We
describe
the
properties
of
a subclass
of
quartet
levels
of
alkalilike
atoms
and
ions
that
often
retain
metastability
against
autoionization
and
may
have
large
yields.
Gain
cross
sections
for
XUV
lasers
with
wavelengths
between
20
and
100
nm are
given.
It is
well
known
that,
in
many
cases,
the
quartet
level
of
highest
J
of
a given
configuration
of
an
alkalilike
atom
or
ion
is
metastable
against
both
autoionization
and
against
in the
extreme
ultraviolet
(XUV).
This
occurs
sincu,
irrespective
of the
extent
to
which
LS
coupling
holds,
there
is
no
doublet
level
in the
config-
uration
to
which
the
quartet
level
may
couple.
1
In
this
Letter
we
note
the
existence
of
a
subclass
of
quartet
levels
that,
even
in
heavier
elements,
often
re-
tain
relative
metastability
against
autoionization
and
are
allowed.
In elements
such
as
Cs
and
Ba
these
levels
are
candidates
for
the
upper
level
of XUV
lasers.
In
lighter
alkalilike
atoms
and
ions
they
provide
storage
levels
for
store
and
transfer
lasers,
which
require
orders-of-magnitude
less
transfer
laser
power
than
an
earlier
proposed
Li
system.
2
The
distinguishing
property
of, these
quasi-meta-
stable
levels
is that
the
selection
rules
on
the
spin-orbit
matrix
elements
allow
nonzero
matrix
elements
only
to
doublet
basis
levels,
which
are
themselves
prohibited
from
autoionizing.
In
second
order,
through
the
di-
agonalization,
these
levels
do
develop
components
of
autoionizing
doublet
levels
and
therefore
do
autoionize,
but
often
sufficiently
slowly
that
the
branching
ratio
for
XUV
remains
large.
To
illustrate
this
idea,
consider
the
2
p
53
s3p
config-
uration
of
Na.
First,
the
pure
doublet
levels
that
may
not
autoionize
are
2
p
5
3s3p
2
P
1
/
2
,
3
/
2
; their
doing
so
would
not
allow
simultaneous
conservation
of parity
and
an-
gular
momentum.
One
then
tabulates
the
possible
quartet
and
doublet
levels
of
this
configuration:
4S3/2,
4P
1
/2,32,5/
2
)
4
D1/
2
,
3
/
2
,
5
J
2
,
7
/
2
, 2S1/2,
2
P
1
/
2
,
3
/
2
,
2
D
3
/
2
,
5
/2.
The
pertinent
selection
rule
on
the
spin-orbit
element
is
AL
-0,
+
1.
Therefore,
of
the
possible
quartet
levels,
those
that
couple
only
to
the
nonautoionizing
2PI/2,3/
2
levels
are
4S3/2
and
4DI/
2
.
We
term
these
levels
quasi-meta-
stable.
Table
1 lists
the
quartet
levels
of
the
2p
5
3s3p
con-
figuration
and
their
autoionizing
rates
as predicted
by
the
atomic-physics
code
RCN/RCG,
3
with
only
this
configuration
included.
Single
and
double
stars
in the
left-hand
column
denote
predicted
quasi-metastability
and
metastability,
respectively.
The
nonzero
au-
toionizing
rates
of
the
quasi-metastable
levels
result
from
small
components
of autoionizing
doublet
levels,
which
are
acquired
in
second
order
through
the
diago-
nalization.
The
code
expansion
of
the
quasi-metastable
2p
5
3s3p
4S3/2
level
of
Na
is
4S3/2
=
0.98
4S3/2
-
0.15
4P
3
/
2
+
0.07(lPj2P
31 2
+
0.04(
3
P)2P
31 2
-
0.OO1(lP)
2
D
3
/
2
-
.O-OI(3P)
2
D
3
/
2
-
(1)
The
(0.001)2
components
of
2
D
3
/
2
result
in
the
micro-
second-time-scale
autoionizing
rate.
The
much
larger
(0.07)2
component
of
(1P)2P3/
2
causes
no
autoionization
and
to
a
rate
of
7.7
X
106
on
the
transi-
tion
2p
5
3s3p
4S
3
/
2
-
2p
6
3p
2
P
31 2
at
41.5
nm.
The
quasi-metastable
4D
1 1
2
level
lies
higher
in
the
configuration
and
couples
more
closely
to
nearby
dou-
blet
levels.
In
second
order
it
acquires
a
(-0.014)2
component
of (
1
P)
2
8
11 2
, which
in
turn
to
its
au-
toionizing
rate
of
2
X
109
sec-'.
Table
2
tabulates
the
quasi-metastable
quartet
levels
of
all
sp,d
configurations
of
the
alkalilike
atoms
and
ions.
The
requirement
for
quasi-metastability
is
summarized
by
I J-L
I
=
3/2;
parity
and
angular
mo-
mentum
must
be
both
even
or
both
odd.
Table
3 tabulates
the
calculated
single
configura-
tional
autoionizing
rate
and
dominant
XUV
rate
for
the
lowest-energy
quasi-metastable
level
for
the
elements
of
columns
I and
II.
In
general,
the
selection
Table
1.
Single
Configurational
Autoionizing
Rates
of
Quartet
Levels
in
the
Sodium
2p
5
3s3p
Configuration
Autoionizing
Level,
Rate
(sec-')
*4S312
6.06
X 105
4
P
1
/
2
10.1
X
1010
4p,1/2
4.5
x
1010
4P
5/2
2.67
x
1010
*4D
12
0.181
X
1010
4Ds/2
1.25
x 10"0
4Ds,52
0.99
x 1010
**4D7/2
0
a
Single
and
double
stars
indicate
predicted
quasi-metastability
and
metastability,
respectively.
0146-9592/84/050168-03$2.00/0 © 1984, Optical Society of America May 1984 / Vol. 9, No. 5 / OPTICS LETTERS 169 rule on quasi-metastability holds better and is less af- fected by the inclusion of other configurations for col- umn I than for column II elements. As expected, it also tends to hold better in lighter elements and for low-lying levels. We have made multiconfigurational code runs on several elements. In sodium, the inclusion of the 2p 53 s3p, 2p 5 3s4p, 2p 5 3p4s, and 2p 5 3p3d configura- tions changes the single configurational (Table 1) 2p 5 3s3p 4S3/2 autoionizing rate to 4.1 X 10i sec-' and the 2p 5 3s3p 4Dl/2 rate to 1.1 X 109 sec- 1 . In Ca+ the inclusion of the 3p 5 3d4s, 3p 5 4d4s, 3p 5 5d4s, and 3p 5 3d 2 configurations changes the 3p 5 3d4s 4P 5 / 2 autoionizing rate (Table 3) to 1.03 X 106 sec-'. Bat does particularly poorly. The 5p 5 5d6s and 5p 5 5d 2 configurations are strongly mixed, and the autoionizing rate of the 4P 5 / 2 level can be made to vary between 1012 and 107, de- pending on the choice of the relative average energy of the two configurations.4 On the other hand, Cs is rel- atively insensitive to the relative energy position and has a multiconfigurational autoionizing rate of 7.46 X 107 sec-'. Radiation in the XUV has been observed and at- tributed to the 4S3/2 and 4P5/2 levels in several column I and column II metals by Aleksakhin et al. 5 and by Table 2. Quasi-metastable Quartet Levels of Alkalilike Configurations Metastable Configuration Quasi-metastable Levels Levels p 5 sp 4S3/2, 4D1/2 4D7/2 p 5 sd 4P 5 1 2 , "F 3 / 2 4F 9 / 2 p6pd 4S3/2, 4DD/ 2 , 4D 7 / 2 , 4G5/ 2 "G11 12 p5p2 4P5/ 2 p 5 d 2 4P 512 . 4F 31 2 , "F 9 / 2 - 138798 //S //~''CS+ 5p56S [I I 0 107219 5p 5 5d6S 4 P 5 / 2 107.5 nm 3 1 307 1/_// / e,//Z CS+ 5p 6 1r 3 14597 . - 5p 6 5d 2 D 5 12 0 5p 6 6S 2 S 1 /2 Fig. 1. Energy-level diagram for 107.5-nm Cs laser. Zhmenyah et al. 6 Also, a single configurational analysis by McGuire7 predicts slow autoionizing rates for the 4S3/2 levels of Na and Mg t . Table 4 gives the transition wavelength, Doppler width, and gain cross section a for XUV laser transitions that originate from the lowest quasi-metastable level of each element. The Doppler width is calculated at a Table 3. Single Configurational Autoionizing and Radiative Rates for Quasi-metastable Column I and Column II Elements Element Upper Level Autoionizing Rate Radiative Rate X (nm) Lower Level Na 2p 5 3s3p 4S3/2 6.1 X 105 7.7 X 106 41.5 3p 2 P 3 / 2 K 3p 5 3d4s 4Ps/ 2 4.6)X 102 3.0 X 106 71.1 3d 2 D 3 / 2 Rb 4p 5 5s5p 4S3/2 8.6 X 107 2.6 X 107 82.1 5p 2 P 3 /2 Cs 5p 5 5d6s 4P 5 / 2 5.1 X 107 4.1 X 107 107.5 5d 2 D 5 /2 Mg+ 2p 5 3s3p 4S3/2 9.3 X 104 1.9 X 107 25.6 3p 2 P 3 / 2 Ca+ 3p 5 3d4s 4P 5 / 2 9.3 X 105 4.1 X 106 52.9 3d 2D152 Sr+ 4p 5 4d5s 4P 51 2 1.4)X 103 5.3 X 107 62.0 4d 2D5/2 Ba4 5p 5 5d6s 4P 512 2.7 X 109 8.8 X 107 77.1 5d 2D5/2 Table 4. Gain Cross Sections of Transitions from Quasi-metastable Levels Doppler Element Transition X (nm) Width (cm-') a (cm 2 ) Na 2p 5 3s3p 4S312 - 2p63p 2 P 5 / 2 41.5 1.04 1.6 X 10-16 K 3p 5 3d4s 4P 5 /2 - 3p63d 2P5/ 2 71.1 0.43 4.4 X 10-16 Rb 4p 5 5s5p 4S3/2 - 4p65p 2P 3 / 2 82.1 0.24 9.1 X 10-15 Cs 5p 5 5d6s 4P 5 /2 - 5p 6 5d 2D 51 2 107.5 0.15 3.9 X 10-14 Mg+ 2p 5 3 s3p 4S3/2 - 2p63p 2P 31 2 25.6 1.78 8.71 X 10-17 Ca+ 3p 5 3d4s 4P 51 2 - 3p 6 3d 2D 5 / 2 52.9 0.83 1.72 X 10-16 Sr+ 4p 5 4d5s 4P 5 /2 - 4p64d 2D5/2 62.0 0.45 5.6 X 10-15 Bat 5p 5 Sd6s 4P 5 / 2 - 5p 6 5d 2 D 512 77.1 0.27 2.4 X 10-14 170 OPTICS LETTERS / Vol. 9, No. 5 / May 1984 temperature corresponding to a vapor pressure of 10 Torr. Figure 1 is an energy-level diagram for a 107.5-nm Cs laser. The 4PP 5 2 level will be populated by electron collisions at a peak excitation cross section that Alek- sakhin et al. 5 estimate as 5.8 X 10-17 cm 2 and also by charge transfer from the Cs+ 5p 5 6s ion. The lower laser level 5p65d 2D5/2 will also be excited by electron colli- sions but can probably be emptied by laser excitation to a Rydberg level or to the Cs+ Sp" continuum. Population in the quasi-metastable levels of the col- umn II elements of Table 4 can be produced by soft- x-ray photoionization8' 9 of populated valence levels. For example, photoionization of atoms in the 5p 6 5d6s 3D level of Ba will produce 5p55d6s 4P 5 / 2 ions with a statistical branching ratio of about 7%. A possible ad- vantage of using column II metals is that the lower laser level is an excited level of the ion rather than of the neutral. The authors gratefully acknowledge helpful discus- sions with T. Lucatorto, A. W. Weiss, and P. J. K. Wi- soff. The research described here was supported by the U.S. Air Force Office of Scientific Research and the U.S. Army Research Office. References 1. P. Feldman and R. Novick, "Autoionizing states in the alkali atoms with microsecond lifetimes," Phys. Rev. 160, 143 (1967). 2. S. E. Harris, "Proposal for a 207-A laser in lithium," Opt. Lett. 5, 1 (1980); and J. E. Rothenberg and S. E. Harris, "XUV lasers by quartet to doublet energy transfer in alkali atoms," IEEE J. Quantum Electron. QE-17, 418 (1981). 3. R. D. Cowan, The Theory of Atomic Structure and Spectra (U. California Press, Berkeley, Calif., 1981), Secs. 8-1, 16-1, and 18-7. 4. J. P. Connerade, M. W. D. Mansfield, G. H. Newsom, D. H. Tracy, M. A. Baig, and K. Thimm, "A study of 5p ex- citation in atomic barium. 1. The 5p absorption spectra of Ba I, Cs I and related elements," Phil. Trans. R. Soc. London Ser. A 290,327 (1979); S. J. Rose, I. P. Grant, and J. P. Connerade, "A study of 5p excitation in atomic bari- um. II. A fully relativistic analysis of 6p excitation in atomic barium," Phil. Trans. R. Soc. London Ser. A 296, 41 (1980). 5. I. S. Aleksakhin, G. G. Bogachev, I. P. Zapesochnyl, and S. Yu. Ugrin, "Experimental investigations of radiative decay of autoionizing states of alkali and alkaline earth elements," Sov. Phys. JETP 53,1140 (1981). 6. Yu. V. Zhmenyak, V. S. Vukstich, and 1. P. Zapesochnyl, "Radiative decay of Na I autoionization states excited in electron-atom collisions," Pis'ma Zh. Eksp. Teor. Fiz. 35, 321 (1982). 7. E. J. McGuire, "The L-MM Auger spectra of Na and Mg," Phys. Rev. A 14,1402 (1976). 8. R. G. Caro, J. C. Wang, R. W. Falcone, J. F. Young, and S. E. Harris, "Soft x-ray pumping of metastable levels of Lit," Appl. Phys. Lett. 42, 9 (1983). 9. W. T. Silfvast, J. J. Macklin, and 0. R. Wood II, "High-gain inner-shell photoionization laser in Cd vapor pumped by soft-x-ray radiation from a laser-produced plasma source," Opt. Lett. 8, 551 (1983). ##### Citations More filters Journal ArticleDOI A. J. 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