The Rydberg states 3d' (3Po)2,1,0 and 3s'' (3Po)2,1,0 and the inner shell transition 2s 2p5 (3Po)2,1,0, which are forbidden to autoionize on the basis of LS coupling, were observed in emission spectroscopy and in autoionization spectra produced in the photoelectron spectrum of atomic oxygen

Petrosky, V. E.

Samson, J. A. R.

English

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PARTIALLY AUTOIONIZING STATES OF ATOMIC OXYGEN
JAMES A. R. SAMSON and V. E. PETROSKY
Behlen Laboratory of Physics
University of Nebraska, Lincoln, Neb. 68508
ABSTRACT
The Rydberg states 3d' 3P2,1,0 and 3 o2,1,0
2,1,0 2 ,1 ,0
the inter-shell transition 2s2p P 2 ,1 ,0 1 which are forbidden
to autoionize on the basis of LS coupling, have been observed
in emission spectroscopy and in autoionization spectra
produced in the photoelectron spectrum of atomic oxygen.
CQ 30313
.74-20332
AsAc_13 7 30 PARTIALL AUTOIOIzI G
(HASAC F$I0 0 CSCL 07D Unclas
STAT PESC .00 G3/ 2 4 1680 3
https://ntrs.nasa.gov/search.jsp?R=19740012219 2020-03-23T11:14:00+00:00Z
2 Preceding page blank
Previously unidentified structure has been observed in
the photoelectron spectrum of atomic oxygen when it is produced
by the 584 A HeI line from an undispersed discharge in helium.
This structure was also observed in the work of Jonathan
et al.
In the present work it is shown that.the structure is
caused by impurity OI resonance lines from the light source.
These resonance lines populate Rydberg states lying above the
first ionization potential of atomic oxygen. The states
responsible for the structure are the Rydberg levels
3d' P and 3s" 3po and the inter-shell transition2,1,0 2,1,0
2s2p5 3 0 The states subsequently de-excite via the two2 ,1 ,0.
competing paths of emission and autoionization. When autoion-
ization takes place an electron is ejected with an energy
equal to the difference between the energy of the state and
the ionization potential of the continuum interacting.with the..
state (in this case the 4S continuum). The interesting point
is that these states are forbidden to autoionize on the basis
of LS coupling. However, in the present work the states are
observed both in emission spectroscopy and in photoelectron
spectroscopy.
The emission spectrum of atomic oxygen was taken over the
wavelength range 770-920 A by adding a trace of 02 (about %)
into a helium glow discharge. The radiation was dispersed
3with a m Seya monochromater operating with a 1 A band pass.
The detector was a photomultiplier sensitized to vacuum uv
radiation with a coating of sodium salicylate. The resulting
spectrum is shown in Fig. 1. The top spectrum was run with
the He + 02 mixture whereas the bottom spectrum was taken with
helium purified by passing through a zeolite trap cooled with
liquid nitrogen. With pure helium in the discharge no emission
lines were observed in the wavelength range of interest. When
a trace of 02 was added to the discharge radiation was
observed from the autoionization-forbidden states with wave-
o o
lengths 791.98 A, 811.08 A, and 877.89 A. Actually, each state
0 2
emits six lines covering about a 2 A spread. The wavelengths
quoted above correspond to the J' = 2 + J" = 2 transitions.
In Fig. 1 the 3s" 3po multiplet (877.89 A) is partially
resolved. If more than a few percent of 02 was added to the
helium discharge the lines were quenched. No radiation was
observed from states which were fully allowed to autoionize
on the basis of the selection rules, namely AS = 0, AJ = 0,
AL = 0, and no change in parity. Many other oxygen lines are
produced at longer wavelengths (e.g. the 1304 OI triplet).
The helium lamp was used undispersed (with and without
the trace of 02) as an ionizing source to produce the photo-
electron spectrum of atomic oxygen. The lamp is still essen-
tially a monochromatic source of 584 A radiation even with the
trace of 02 because the intensity of the 791.98 A OI line
5 3 2 4 3 o(2s2p P + 2s 2p P) is about 200 less than the 584 A line
under optimum conditions for the OI lines. Thus, no appreciable
contribution to the PE spectrum would be expected.
Atomic oxygen was produced by flowing 02 through a micro-
wave discharge. The main products emerging from the discharge
were O(3 P), 02 (X 3E ), and 02 (1A). These products have been
identified in previous photoionization studies.3-6 The experi-
mental arrangement for flowing the atomic oxygen into the
cylindrical mirror analyzer is shown in Fig. 2. The stream
of atomic oxygen was directed to intersect the photon beam and
the electron trajectory leading into the analyzing region.
The original energy of the photoelectrons was modified as they
passed through a retarding/accelerating lens, L. This allows
the electrons to be analyzed at a fixed voltage. A pass energy
of 2 eV was used which gave a constant energy resolution of
30 meV. The analyzer is described in more detail elsewhere.7
Photoelectrons will be produced from each product in the gas
stream. Figure 3 shows the photoelectron energy spectrum from
0 to 5 eV. Figure 3(a) was taken with pure He in the light
source while Fig. 3(b) was taken with a trace of 02 added to
the light source. Both spectra show identical peaks caused by
the direct photoionization of the discharged products by the
o 7
HeI 584 A line. However, Fig. 3(b) shows the additional peaks-:.
produced by the OI resonance transitions at 877.89 A (3s),
0 0
811.08 A (3d'), and 791.98 A (2p ). Althouh the emission lines
are about 200 times less intense than the 584 A line.they pro-
duce comparable photoelectron signals. Thus, the photoionization
cross section of atomic oxygen at these lines is roughly-'200
50
times greater than at 584 A. Figure 4 shows, schematically,
a term diagram of the above transitions autoionizing into the
4S continuum. The predicted energies of the ejected photoelec-
trons are shown in the diagram. These energies correspond to
the autoionizing structure in the photoelectron spectrum.
Further confirmation of the identity of the structure comes
from the photoionization - mass spectrometry studies of Dehmer
et al. 8 By the use of the Hopfied continuum they studied the
photoionization of atomic oxygen as a function of wavelength
0
over the range 912 to 660 A. They observed autoionizing struc-
ture at all the allowed transitions including the LS forbidden
states discussed here.
Thus, it is firmly established that the 3d' 3p0, 3s" 3po
and 2s2p 5 3po states partially auto-ionize and that emission
processes compete with autoionization. It is just this com-
bination that produces the spurious peaks in the 584 R
undispersed photoelectron spectrum of atomic oxygen when a small
air leak exists in the light source.
.ACKNOWLEDGEMENT
This work was supported in part by the National Science
Foundation and the National Aeronautics and Space Administration
under grants GA-36718 and NGR 28-008-021.
6REFERENCES
1. N. Jonathan, A. Morris, M. Okuda, K. J. Ross, and D. J. Smith,
to be published.
2. C. E. Moore, "Atomic Energy Levels," Nat'1 Bur. Std. Circ.
No. 467, Vol. I (1949).
3. R. B. Cairns and J. A. R. Samson, Phys. Rev. 139, (1965)
1403.
4. R. J. McNeal and G. R. Cook, J. Chem. Phys. 45, (1966) 3469..
5. R. E. Huffman, J. C. Larrabee, and Y. Tanaka, J. Chem.
Phys. 46, (1967) 2213.
6. N. Jonathan and D. J. Smith, J. Chem. Phys. 53, (1970) 3758;
and N. Jonathan, A. Morris, K. J. Ross, and D. J. Smith,
J. Chem. Phys. 54, (1971) 4954.
7. .J. A. R. Samson and V. E. Petrosky, Phys. Rev. (to be
published).
8. P. J. Dehmer, J. Berkowitz, and W. A. Chupka, J. Chem.
Phys. (to be published).
7FIGURES
1. Emission spectrum of atomic oxygen from a helium dc glow
discharge with a trace of 02 added (~- % 02). The OI
emission lines are produced from partially autoionizing
states. The bottom trace illustrates that no observable
emission could be seen from the light source in this wave-
length region when no 02 was present.
2. Cylindrical mirror electron energy analyzer. L is the
retarding/accelerating lens and C is the channeltron
detector. The microwave discharge for the production of
atomic oxygen is shown in relation to the analyzer.
3.. (a) Photoelectron spectrum of the discharged products
produced by a microwave discharge in 02. Pure helium was
used in the light source.
(b) Photoelectron spectrum of the discharged products as
in Fig. 3(a) but with a trace of 02 added to the helium
discharge. The autoionizing states are clearly observed
at 0.51, 1.67, and 2.037 .eV.
4. Simplified term energy diagram of atomic oxygen illustrating
the Rydberg levels nd' 3  3 and ns"the 2s2p 5 3po
level. The resonance wavelengths corresponding to the
lowest level of each state is given in Angstrom units.
Autoionizing transitions into the 4S continuum are indicated
along with the predicted energy of the ejected electron.
OI
879.0
OI
877.89.
OO (833)
OI
791.98
OI
811.08
He +0Oz
He
, 0 MICROWAVE CAVITY0 Op
-V
hv 15.84 IA) I
O+ 2D
0+ 2
(a)
O D 0+ 2p 3s"
2p"(b)
3d'
2 1.
5 4. 3 2 0
ELECTRON ENERGY (eV)
18.6 eV 0+ 2 P ns" 3PO
16.9 eV 0+ 2D nd' 3p
2p5  P
3d'
0 3s" cv
13.6 eV 0 + 4S o'-----
oQ o<[ o<
OD ) CD
0o 3
OeV O"0 ' DI __ _


Partially autoionizing states of atomic oxygen

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