N I and N II fluorescent radiation was observed when N2 was irradiated by undispersed synchrotron radiation with an upper energy limit of approximately 200 eV. The excited fragments originate from dissociation of a band of excited ionic states of N2 lying between 34.7 and 44 eV

Chung, Y.

Lee, E.-M.

Samson, James A. R.

English

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EXCITED IONIC AND NEUTRAL FRAGMENTS PRODUCED
BY DISSOCIATION OF THE N2 +* H BAND
James A.R. Samson, Y. Chung, and E-M Lee
Behlen Laboratory of Physics, University of Nebraska, Lincoln, NE 68588-0111
ABSTRACT
NI and NII fluorescent radiation has been observed when N 2 was irradi-
ated by undispersed synchrotron radiation with an upper energy limit of approx-
imately 200 eV. The excited fragments originate from dissociation of a band of
excited ionic states of N2 lying between 34.7 and 44 eV.
INTRODUCTION
Photoelectrons produced by ionization of inner shell valence electrons can
interact with the remaining orbital electrons to create excited states of the result-
ing ion. Such two electron transitions, or satellite states, have been observed in
the photoelectron spectrum (PES) of N 2 by many groups who used photon energies
ranging from 41 to 1487 eV. 1-8 The binding energies of the satellite states lie be-
tween 23 and 45 eV. These states appear to play a significant and interesting role
in the photoionization of N 2. For example, Krummacher et al 8 have shown that
the total cross section for producing these states amounts to roughly 30% of the to-
tal photoionization cross section between 43 and 65 eV. It also appears that these
https://ntrs.nasa.gov/search.jsp?R=19910009478 2020-03-19T19:40:56+00:00Z
states are responsible for most of the dissociative ionization that occurs in N 2
above the threshold at 24.3 eV. 9-11 Although considerable theoretical attention
has been directed to understanding the electron configuration of these states 12-21
there is little information on the products of dissociation. The H band is particu-
larly interesting because, if it predissociates, it is the only band capable of produc-
ing atomic fluorescence. A composite of the PES data, taken from Krun_nacher
et al. 8 and Nyholm et al. 7 is shown in Fig. 1 along with the various spectroscopic
dissociation limits. Krummacher et al. 8 note that the absence of any observable
Auger decay in their PES data of the H band indicates that this band does predis-
sociate. As can be seen from Fig. I the lowest dissociation limit that can produce
fluorescence is N+(3P) + N 3s(4p) at 34.7 eV. The majority of the remaining lim-
its lie between 34.7 eV and 38.9 eV, the (N + + N +) limit. All lie within the bounds
of the H band. Langhoffet a119 note that their calculations predict that the H
2+2 +
band is comprised of a large number of _:g, _.u, and 2H u ionic states converging
on the N22+ limit (~ 43 eV). Sambe and Ramaker 21 attribute the sharp structure
observed in the H band as belonging to the lower members of the Rydberg series
converging to N22+.
It is the purpose of this paper to present evidence that the H band does pre-
dissociate into excited neutral and ionic atoms. The method chosen for this study
was to observe and identify atomic fluorescence produced by dissociative photoion-
ization of N 2.
EXPERIMENTAL
Fluorescent radiation was produced by irradiating N 2 with undispersed
synchrotron radiation. The band-pass of the radiation, after reflection from the
focussing mirrors, ranged from the visible to about 60/_. The radiation passed
through a glass capillary (part of a differential pumping system) so that it would
intercept a N2 gas jet located about 2ram in front of the entrance slit of a 0.5 m
Seya monochromator. The interaction chamber and monochromator were both
pumped with turbomolecular pumps.
was maintained at about 10 -4 Torr.
and was scanned from 645 to 1250/_.
The pressure in the interaction chamber
The monochromator had a 3/_ band-pass
The spectrum was detected with a multi-
channel plate electron multiplier. The fluorescent radiation observed originated
mainly from excited NI and NII states with a small amount from excited molecu-
lar states.
RESULTS AND DISCUSSION
All NI and NII emission lines observed between 645 and 1250/_ are listed in
Tables I and II, respectively. The threshold energies for the dissociation limits
are given along with the relative intensity of each line observed. These intensities
were obtained by dividing the measured signal with the ring current in the stor-
age ring and dividing by a typical detection efficiency for the microchannel plate
detector. 22 Minor corrections were made for possible absorption of the fluores-
cent radiation by the N 2 background.
We have assumed that the florescence originates from the excited products
of dissociative photoionization and this must be true for N +* fluorescence.
However, excited neutrals can be produced by simple photodissociation. The
minimum threshold for producing N(4S) + N* (1200/_ fluorescence) is 20.1 eV.
Fluorescent radiation has been observed in this region (20 to 27 eV) by Beyer and
Welge. 23 However, they measured the efficiency for fluorescence to be less than
1%. This is a narrow wavelength band where the photoionization yield is slightly
less than 100% 24 and where the Codling Rydberg bands 25 are observed. These
3
states, in which autoionization and predissociation presumably compete, can be
produced only through resonance absorption of the incident radiation. Thus,
only an extremely narrow band of the undispersed synchrotron radiation could
excite these states. On the other hand, N2+* can be excited continuously from its
threshold to the energy limit of the incident radiation (- 60 ]%). Fluorescence from
higher excited states such as (N +* + N*) or (N +* + N +) may be possible but three
electron transitions are about a factor of 10 to 100 times less probable than two
electron transitions. We conclude, therefore, that the fluorescence observed in
the present work originates from N2+* states and that the products of dissociation
are mainly those given in Tables I and II. It is true that cascading will take
place from most of the excited neutrals given in Table I. Thus, Table I reflects
what is observed and not the relative probability for producing a specific product.
However, it is clear that the main products of dissociation are,
N + (3p) + N (3p) ns (2,4p)
and N + (3p) + N (3p) nd (2F, 4p)
To identify N2+* we note that from Table I we have a minimum threshold of
34.7 eV for producing the observed fluorescence. An upper limit can be deter-
mined by measuring the kinetic energies of the N + fragments. Measurements
made with the 40.8 eV HeII line 26 yield N + energies ranging from zero to about 5
eV. Similar results were obtained by use of undispersed synchrotron radiation
with the same band-pass as used in the present work. 11 In this case the high en-
ergy tail extended to about 6 eV. This results in a total fragment energy ranging
from 0 to < 12 eV. Thus, the major amount of fluorescence observed must origi-
4
nate from N2 +* states in the energy range from 34.5 eV to < 51 eV (taking 2 N + as
an upper limit). The only states observed within this energy range are those that
make up the H band including the small peaks between 40 and 44 eV. No dis-
cernable structure above 44 eV has been observed in VUV or X-ray PES studies.
The kinetic energy measurements of N + made by Wu et al. 27 are particularly sig-
nificant. They studied the kinetic energy distribution of the fragments by mea-
suring the N + signal in coincidence with any 1200/_ fluorescence emitted by N*
Their results show str_cture in the kinetic energy spectrum with ion energies
ranging from 0.18 to 2.58 eV. Thus, the excited states of N2+ responsible for the
1200/_ fluorescence must lie between 35 and 40.8 eV. In fact the structure ob-
served by Wu et al appears to correlate with the structure observed by
Krummacher etal.
We feel that the evidence supports the conclusion that the observed N2+*
states lying between 34.7 and 44 eV dissociate and are responsible for producing
the fluorescent radiation tabulated in Tables I and II.
ACKNOWLEDGMENTS
This material is based upon work supported by the National Science
Foundation under Grant No. ATM-8617670 and by the National Aeronautics and
Space Administrator under Grant No. NAGW-1751. It is with pleasure that we
acknowledge the help of the personnel at the Stoughton Synchrotron Radiation
center supported by NSF Grant No. DM:R-8821625.
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TABLE I. Observed fluorescence from excited neutral atoms produced from dissociative
photoionization of N 2. The core state of the atom is 3p unless otherwise indicated.
Fluorescence Products Dissociation Relative
(/_) limits (eV) Intensity (%)
1200 N+(3P) + N 38 (4p) 34.7 34.2
1493 3s (2p) 35.0 11.4
1135 2s2p4(4p) 35.3 5.2
1243 38'(2D) 36.7 4.1
965 4s (4p) 37.2 3.5
1177 48 (2p) 37.3 0.9
1168 3d (2F) 37.3 1.7
954 3d (4p) 37.4 3.1
910 5s (4p) 38.0 1.3
905-907 4d (4p) 38.0 1.7
1100.4 58 (2p) 38.0 0.6
1097.2 4d (2F) 38.0 1.0
887-888 68 (2, 4p) 38.3 0.5
1067.6 5d (2F) 38.4 0.8
1054.3 7s (2, 4p) 38.5 0.3
875-877 78 (4p), 6d (2F, 4p) 38.5 0.5
869.4 88 (4p), 7d (2F, 4p) 38.6 0.3
1043-1044 88 (2p), 7d (2F, 2, 4p) 38.6 0.3
862.1 9d (4p) 38.7 0.2
865.95 9s (2p) 38.7
865.65 2s2p 3 (5S°) 3p (4p) 42.2
TABLE II Observed fluorescence from excited atomic ions produced by dissociative
photoionization ofN 2
Fluorescence Products Dissociation Relative
(_) limits (eV) Intensity (%)
1085.7 N(4S) + N + 2s2p 3 (3D°) 35.8 18.3
916.7 2s2p3 (3po) 37.9 4.2
776 2s2p 3 (1D°) 42.2 0.8
747 2s22p (2p_) 3s (1, 3po) 42.8 1.4
670-672 2s22p (2po) 3s (1, 3po) 42.8 1.9
645 2s2p 3 (3S°) 43.6 0.4
660 2s2p 3 (11_) 45.0 0.3
FIGURES
Photoelectron spectrum of N 2 compiled from the data of Kmm_nacher et
al. (ref. 8) and Nyholm et al. (ref. 7) showing the E, G, and H bands of
N2 +* as a function of their binding energies. The dissociation limits re-
lating to the observed NI and NII fluorescence is shown on the same en-
ergy scale.
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Excited ionic and neutral fragments produced by dissociation of the N2(+)* H band

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