The kinetic energies of ions produced by dissociative photoionization of NO have been measured at the discrete resonance lines of He (584A) and Ne (736A), and with undispersed synchrotron radiation. O sup + ions were identified with energies from 0 to approximately 0.5 eV and two groups of N sup + ions one with energy of 0.36 eV and another with energies between 0.9 and 1.5 eV, apparently produced by predissociation of the C sup 3 P 1 and B'1 sigma states respectively

Angel, G. C.

Rstgi, O. P.

Samson, J. A. R.

English

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KINETIC ENERGIES OF FRAGMENT IONS PRODUCED
^V	 :1	 BY DISSOCIATIVE PHOTOIONIZATION OF NOQ
QLy^}J^ti,
James A.R. Samson, G.C. Angel, and O.P. Rustgi*
Behlen Laboratory of Physics, University of Nebraska
Lincoln, NE 68588, U.S.A.
ABSTRACT
The kinetic energies of ions produced by dissociative photoionization
O
of NO ha' :. -, been measured at the discrete resonance lines of He (584A) and
Ne (136A), and with undispersed syri-hrotron radiation. 0 + ions were iden-
tified with energies from 0 to approximately 0.5 eV and two groups of N+
ions one with energy of 0.36 eV and another with energies between 0.9 and
1.5 eV, apparently produced by predissociation of the c 3 1 and V I E states,
respectively.
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*Present address: Department of Geoscience/Physics, SUNY College at
Buffalo, 1300 Elmwood Ave, Buffalo, NY 14221.
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Introduction
A limited number of studies have been made of the kinetic energies of
ion fragments produced by the process of dissociative photoionization,
especially near the ionization threshold for these processes [1-4]. The
information derived from such experiments is very important in developing
potential energy curves or locating repulsive curves that perturb stable
electronic states causing predissociation.
Studies have been made of the unimolecular dissociation of (NO+)*
where the excited NO + states were produced in ion sources by electron bom-
bardment [5-7]. Energy resolved spectra of the 0 + fragments showed ion
i
peaks at energie ,  between 0.037 and 0.354 eV [5]. The peak energies were
in good agreement with the extrapolated vibrational data of Edqvist et al.
[8] for the w3 ,1, W 1 -1 , and A' 1 -7 - states of NO*, implying predissociation of	 +
some of these states.	
i
The present work descr4bes the first measurement of the kinetic ener-
gies of ion fragments produced directly by dissociative photoionization of
NO.
Experimental
O
Photoionization of NO was produced by the 584A resonance line of He
o
and by the 736A resonance line of Ne in order to study low energy 0 + ions
created near the threshold for dissociative photoionization. The He and Ne
light sources were used with a normal incidence monochromator to insure
spectral purity. More energetic ions particularly N + were produced from
higher lying excited states of NO by use of synchrotron radiation from the
Fi
i
it
13
Stoughton storage ring in Wisconsin. Because of the weak signals encoun-
tered preliminary measurements were made by using the zero order spectrum
from a grazing incidence monochromator. The integrated spectrum produced
radiation of all energies up to about 100 eV.
The gas jet and photon beam crossed at the center of a cylindrical ion
chamber, producing the majority of the ions in a small central region. The
directed motion of the molecules in the jet effectively prevented NO + ions
and low energy fragment ions (< 0.1 eV)	 from escaping through the exit
aperture of the ion chamber. The NO + ions that were observed came from the
low general background pressure of NO that permeated the ion chamber. This
provided excellent suppression of the NO + peak, thereby, allowing the low
intensity ion fragments to be more readily resolved.	 The ions were formed
in a field-free region and allowed to drift through an exit aperture.
They were then accelerated or decelerated to match the fixed pass energy of
the analyzer and subsequently were detected with a channel electron multi-
plier. The lens action produced by the acceleration/deceleration field had
a large effect on the transmission efficiency of ions of different ener-
gies. Essentially, the greater the acceleration field the better the
transmission of the higher energy ions.
The energies of the ions were measured with a 90' sector cylindrical
energy analyzer, which had an energy resolution of about 0.5"'D. The contri-
bution of the doppler effect, caused ty the random motion of the parent
ion, was minimized somewhat by using a molecular jet.
Although a mass spectrometer was not used with these exploratory meas-
urements the identification of the major ions are not too ambiguous. Cer-
tainly, future work should use mass analysis for finer understanding of the
ion energy results.	 \
47-1
4
Results and Discussion
A potential energy diagram of NO + is shown in fig. 1. Only a few of
the electronic states are shown for clarity in the discussion to follow.
The energies of the electrc,iic and vibrational levels are taken from spec-
troscopic data and the photoelectron data of Edqvist et al.[8]. There has
been some uncertainty in the assignment of the states at 21.72 eV and 22.73
eV. In fig. 1 they are identified as c 3 a and/or B l a and B' 17+ and/or 81a,
respectively. The and/or assignment is given in bracke`s in ff3. 1. In
the present discussion these states will be described simply as the c 3q and
B' 1
.
r+ states.
The first dissociative ionization limit of NO occurs at 20.124 eV
producing 0+ ( 4S) and N( 4 S), whereas the second limit occurs at 21.04 eV
producing N+ ( 3p ) and 0( 3p ). Both thresholds can be reached by the 584A
line (21.21 eV) but not by the 736A line (16.85 W. We know `.hat both 0+
and N+ ions are produced immediately their respective thresholds are
reached [9-11] and we have measured the absolute cross sections for produc-
ing NO + , 0+ , and N+ at 584A [9,10].	 These are, respectively, 25.2 Mb, 0.37
Mb, and 0.11 Mb (1 Mb = 10- 18 cm2).
Figures 2 (a) and (b) show the ion kinetic energy spectra produced by
the 736A and 584A lines. The ions were accelerated to 37 eV, which gave
an energy resolution of about 230 meV (this includes the effect of the
Maxwell-Boltzmann distribution). With this resolution the zero of the
energy scale is effectively located at the NO + maximum. fhe counting time
for each peak was normalized to the same value of the p roduct of the light
la tensity and the ionization cross section at each wavelength. The high
energy tail on the 736A NO + data (fig. 2a) is a characteristic of our en-
ergy analyzer and is caused by ions scatterd inside the analyzer. For
5•	 example, identical structure can be seen on Ar* ion peaks, However, in
i
fig. 2(b) the 584A data shows a definite fragment ion superimposed on the
instrumental tail. Thus, by subtracting the two spectra we obtain the true
spectrum of the fragment ion. This result is shown in fig. 2(c). We can
identify this ion as primarily 0 + because the N+ ions are produced with
energies between 0 and 90 meV and are thus effectively lost because of the
jet-action of the gas beam. This would also apply to that portion of the
0
0 + signal. Furthermore, the cross section for N + production at 584A is
0
about a factor of three less than 0+ . The true ratio of NO + : 0+ at 584A
is 68:1. From figs. 2(a) and 2(c) the observed areas give a ratio of only
1.6:1. This shows the effectiveness of the jet-action in discriminating
against the parent NO + ions. From the potential energy diagram in fig. 1
we would expect the 0 + energy to range between 0 and 0.5 eV. Allowing for
the instrumental resolution and the high energy tail that must accompany
each peak the observed spread between 0 and 1.0 eV is reasonable.
A direct transition into the 5y+ state is spin forbidden. However, a
large number of bound vibrational levels of electronic states of NO+ (for
example, w3 A, W 1 -, and A l i states) lie between 20.1 and 21.0 eV. These
states could be perturfed by the presence of the repulsive portion of the
57+ state causin g predissociation [12]. From the peak position of the data
shown in fig 2(c) the 5;+ state should cross the center of the Franck-
Condon region at about 20.6 eV or less. Note, that the dashed portion of
the 5L+ curve is hypothetical, although there are theoretical studies that
indicate that it has a bound potential [12]. The positioning of the re-
pulsive portion of the 5:+ state agrees with the continuous production of
0+ ions that are observed in the mass spectrum of NO at all photon energies
greater than 20.124 eV [9-11].
	
There are also several ^ y dberg series of
neutral states in this energy region that converge to the c 3 1 (or B11)
--	
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•rte	
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_
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b
1
state of NO' and produce numerous absorption lines between 20 and 21 eV
[13,14]. From the work of Hertz et al.[11] we know that resonance transi-
tions into these states readily autoionize and predissociate into 0+(4S).
For higher photon energies, using synchrotron radiation, a more com-
plex energy spectrum is produced. A typical spectrum is shown in fig. 3.
Because the zero order photon spectrum was used photons of all energies
less th,.i	 100 eV were present. Again, the ions were accelerated to 37
eV. Thus, the resolution and focusing conditions were similar to that used
for the data in fig. 2. We would expect, therefore, that the main NO + peak
and the low energy 0+ ( 4 S) continuum would be similar to that produced by
the 584A line in fig. 2(b). However, because the undispersed radiation
would have a high percentage of low energy photons it is reasonable to
expect a somewhat higher ratio of n0 + to 0+ ( 4 S) than observed at 584A. The
1	 dashed line in fig. 3 is an estimate of the contribution of NO + and 0+(4S)
1
	
ions to the energy spectrum. Subtracting this contribution leaves a single
c^Prriy peak at 0.36 eV. The dotted line indicates the high energy instru-
mental tz.il of this peak (see discussion of fig. 2(a)). The 0.36 eV frag-
ment ion can be identified as N + ( 3 P) originating from predissociation of
the c3 Z state. This electronic state has been ouserved in photoelectron
I	 spectroscopy of NO [8] as a single peak (v = o) with a small unresolved
tail (tentatively assigned by Edqvist et al. as the B I n state). Thus,
predissociation of this state should also yield an ion fragment with a
narrow energy range. This is in agreement with the present results. The
observed ion energy is in excellent agreement with the predicted value of
0.362 eV if predissociation occurs from the C3- (v = o) value into N+ ( 3P) +
0( 3 P) as indicated in fig. 1.
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The last remaining feature is a broad continuum that can be correlated
with predissociation of the B' I :: state into N + ( 3p ) + 0( 3 P).	 In the photo-
electron spectrum of NO, Edgvist et al. have completely resolved the vibra-
tional structure of the 	 state from v - 0 to v = 13 lying between
22.727 eV and 23.756 aV. If these states are perturbed and predissociation
leads to N + ( 3 P) + 0( 3p ) the N+ fragment ions would have energies lying
between U.9 and 1.5 eV, as indicated in Fig. 1. Allowing for the resolu-
tion of our instrument there is excellent agreement with the observed ener-
gies as shown in fig. 3.
Evidence for predissociation of the c 3 a and B' 1 E+ states comes from
studies of the dissociative photoionization of NO [9-11]. The increase in
the number of N+ ions at the thresholds of the above states is very dra-
matic. No corresponding increase in 0 + occurs at these thresholds.
Although other ion groups of N+ and 0+ may be present the primary
energetic ions appear to come from three groups, namely, 0+ ( 4S) ions with
energies from 0 to 0.5 eV from predissociation via the 5 z+ state, N+(3p)
ions with 0.36 eV energy, and an N+ ( 3 P) group with energies from 0.9 to 1.5
eV produced by predissociation of the c 3 :L and B' 1 z+ states, respectively.
Further work is in progress to obtain mass analysis of the ionic frag-
ments as a function of photon energy and in tandem with energy analysis.
Acknowledgment
This material is based upon work supported by the National Aeronautics
and Space Administration under ;rant No. NGR 28-002-01 and by the National
Science Foundation under Grant No. ATM-8412820. It is with pleasure that
we acknowledge the help of the personnel at the Stoughton Synchrotron Ra-
diation Center supported by NSF Grant No. OMR 8313523.
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REFERENCES
1. J.L. Gardner and J.A.R. Samson, J. Chem. Phys. 62 (1975) 1447 and
4460:	 Phys. Rev. Al2 (1975) 1404.
2. P.H. Doolittle, R.I. Schoen, and K.E. Schubert, J. Chem. Phys. 49
(1.968) 5108.
3. J.H.D. Eland, Int. J. Mass Spectrom. Ion Phys. 9 (1972) 397 and 12
(1973) 389.
4. B. Brehm, J.H.D. Eland, R. Frey, and A. Kustler, In*_. J. Mass
Spectrom. Ion Phys. 12 (1973) 197 and 213.
5. T.R. Govers and J. Schopmar, Chem. Phys. Letters 12 (1971) 414.
6. A.S. Newton and A.F. Sciamanna, J. Chem. Phys. 50 (1969) 4868.
7. P. Dong and M. Bizot, Int. J. Mass Spectrom. Ion Phys. 10 (1972/73)
227.
8. 0. Edqvist, L. Asbrink, and E. Lindholm, Z. Naturforsch. 26a (1971)
1407
9. J.A.R. Samson, T. Masuoka, and P.N. Pareek, J. Chem. Phys. (to be
published).
10. Y. Iida, S.R. Daviel, S. Carovale, and C.E. Brion, (to be published).
11. H. Hertz, H.W. Jochims, H. Schenk, and W. Sroka, Chem. Phys. Letters,
29 (1974) 572.
12. D.L. Albritton, A.L. Schmeltekopf, and R.N. Zare, J. Chem. Phys. 71
(1979) 32ii).
13. B. Narayama and W.C. Price, J. Phys. B5 (1972) 1984.
14. M. Sasanuma, Y. Morioka, E. Ishiguro, and M. Nakamura, J. Chem. Phys.
60 (1974) 327.
—
1:
9
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FIGURE CAPTIONS
1.	 Potential energy diagram for N0 + illustrating probable predissociation
path-ways and suggested location of the repulsive portion of the 5F+
states (the depth of the potential well is purely schematic). The
vertical lines represent the Franck Condon region.
2.	 Ion kinetic energy spectrum from NO (a) produced
O
(b) produced by 584A radiation and (c) energy of
O
produced by subtracting the 736A energy spectrum
trum. Note, the high energy tail in (a) is inst
O
by 736A radiation,
the 0 + fragment ion
O
from the 584A spec-
rumental. I
11
3.	 Ion kinetic energy spectrum produced by dissociative photoionization
of NO by undispersed synchrotron radiation. *rhe dashed line extending
from the main NO +
 peak is an estimate of the contribution of NO + and
0+ ( 4 S) ions to the energy spectrum. The dotted line indicates an
estimate of the high energy instrumental tail for the 0.36 eV peak.
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Kinetic energies of fragment ions produced by dissociative photoionization of NO

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