The ionization potential of O2 was measured by the technique of high resolution photoelectron spectroscopy taking into account the influence of rotational structure on the shape of the vibrational bands. A value of 12.071 + or - .001 eV (1027.1 + or - 0.1 A) was found for the ionization potential. A lowering of the ionization potential caused by a branch-head when delta N = -2 gave an appearance potential for ionization of 12.068 + or - .001 eV (1027.4 + or - 0.1 A)

Gardner, J. L.

Samson, J. A. R.

English

NASA Technical Reports Server

ON THE IONIZATION POTENTIAL OF
MOLECULAR OXYGEN
James A. R. Samson and J. L. Gardner
Behlen Laboratory of Physics
University of Nebraska
Lincoln, NE 68508
ABSTRACT
The ionization potential of 02 has been measured by
the technique of high resolution photoelectron spectroscopy
taking into account the influence of rotational structure
on the shape of the vibrational bands. A value of
0
12.071 + .001 eV (1027.1 ± 0.1 A) was found for the
ionization potential. This value leads to a dissociation
+ 2
energy of D (O 2 X Ig) = 6.661 ± .001 eV. A lowering ofenerg
the ionization potential caused by a branch-head when
AN = -2 gave an appearance potential for ionization of
12.068 + .001 eV (1027.4 ± 0.1 A).
4N75-18349
(NASA-CR- 142294) ON THE IONIZATION
POTENTIAL OF MOLECULAR OXYGEN (Nebraska
Univ.) 22 p HC $3.25 CSCL 07D Unclas
G3/25 12447
https://ntrs.nasa.gov/search.jsp?R=19750010277 2020-03-22T22:15:18+00:00Z
INTRODUCTION
An accurate value of the first ionization potential
of 02 has been sought for many years. Because no
Rydberg series analysis has been successful in identifying
a series limit for the first ionization potential (IP)
other techniques have been used. These methods have
consisted primarily of electron and photon impact
ionization studies. Of these techniques the photoionization
method is the more accurate. A major problem of electron
and photon impact methods is in indentifying the first
appearance of ions. With molecular oxygen ionization
can occur at energies less than the first IP (Samson
and Cairns 1966) because of the process,
02 + hv 
- 02
followed by
* +[1] 02 + 02 -. 02 + 02-
This process is, of course, pressure dependent. The
photo-ionization threshold potential of 02 was first
studied by Inn (1953) who observed ions at photon energies
of 12.049 eV. Watanabe and Marmo (1956) also reported
ions in this region. They measured a photoionization
yield of 0.7% at 12.028 eV. Ionization at these energies
was undoubtedly caused by the high 02 pressure. However,
in the pressure range of a few mTorr process (1) should
not be significant. The many results of photoionization,
with or without mass analysis, and the use of photoelectron
spectroscopy have led to a range of values of the
ionization potential between 12.063 and 12.077 eV.
Samson and Cairns (1966) identified the v' = 1 level
of 02 +(X 2H ) in a photograph of the 02 absorption
bands and this led to a value of 12.063 eV for the IP.
This value was lowered when Asundi (1968) and Bhale and
Rao (1968) revised the vibrational energy separation
between the v' = 0 and v' = 1 levels of the 02 ground
level. However, on re-examination of the absorption
spectrum it is clear that some ambiguity exists in the
identification of the v'-= 1 level because of the presence
of diffuse absorption lines. Further, in light of
high resolution photoelectron spectroscopy giving
consistently higher values of the IP it was decided to
re-investigate the first ionization potential of 02
and in particular to try to understand better the effects
of rotational levels in lowering the IP and their effect
on the photoelectron spectrum. If the IP is lowered
because of rotational transitions this would affect the
1966 results.
I.,s
3Rotational Effects
When a molecular gas is photoionized by a beam of
monoenergetic photons from, say, the ground electronic
state with v" = 0 to the ground ionic state with v' = 0
there is not one single ionizing transition but rather
a large number of individual transitions between
appropriate rotational levels in each state. This is
caused by the population of a number of ground state
rotational levels at room temperature. Figure 1 shows
the percent population of the various rotational levels
for 02 at a temperature of 300 K.
An energy level diagram of 02 and 02 ' in their
vibrationless ground states is shown in Fig. 2. Photo-
ionization of the neutral molecule is possible from any
of the populated rotational levels. Various transitions
are shown by the vertical lines. These transitions are
grouped into several branches, namely, the 0, P, Q, R,
and S branches depending on the value of AN as shown in
the Figure. N is the total angular momentum of the
molecule apart from spin. The levels are actually split
and these are shown beside the J numbering of the levels
(J is the total angular momentum including spin).
-i
However, the spacings are less than 2 cm and will not
be considered further.
No selection rules for AN have been enunciated
4for the photoionization of 02. We assume for the moment
that all AN values are possible.
The ionization potential of a molecule is defined
as the energy between the ground state of the neutral
and the ion both in their zeroeth vibrational and
rotational levels. Let the term value for the transition
between these two states be vo, the band origin, then
the term value for a transition from any rotational
level N" of the neutral to a rotational level N' of the
ion is given by
[2] v = V + F' (N') - F"(N")
where F(N) = B N(N + 1) is the rotational term value0
and B is the rotational constant for zero vibration.
If (F' - F") is negative the transition occurs at an
energy less than that of the ionization potential. Thus,
the true IP is lowered. For 02, because the rotational
constant B' of the ion is greater than B" of the0 0
neutral, a lowering of the IP can only be expected for
the P and O-branches for low values of N. The O-branch
actually forms a band head 22.2 cm-1 below the origin
and thus sets a limit to the lowering of the IP (provided
there are no additional branches with AN = -3, -4 ,...etc.
AN = -3 has a branch head 61.4 cm-1 below the origin).
The appearance potential (A.P.) of ions in any
'V s
5experiment, corrected for instrumental resolution, will
be related to the onset of the lowest energy branch.
If the O-branch is present the A.P. will occur 22.2 cm-1
(2.8 meV) before the true I.P.
The true ionization potential will be determined
when the origin is identified.
A vibrational peak in a differential photoelectron
spectrum is the integral of the intensities of all the
rotational transitions within the vibrational band. The
peak maximum will not necessarily occur at the origin.
The position of the peak maximum depends on the relative
intensities of the various branches within the band.
To a first approximation, the intensity of a branch can
be calculated by assuming that the transition probabilities
of the various rotational transitions are constant.
Thus, the intensities will depend only on the population
of the rotational levels in the ground state (Figure 1).
If the rotational levels, corrected for the Boltzman
distribution, are coAvoluted with the instrument resolution
function of a photoelectron spectrometer the photoelectron
vibrational peak should be reproduced. Conversely, the
shape of a photoelectron peak should yield the position
of the origin when properly de-convoluted. This is
similar to the procedure used by Walker et al. (1973)
in their study of HF.
6Thus, the procedure to find the origin is to obtain
a high resolution photoelectron spectrum of the v' = 0
band of 02 + and to compare the shape of that band with
that obtained from selected branches whose relative
intensities are varied to give the best fit.
RESULTS
The cylindrical mirror analyzer used to obtain
the photoelectron spectrum of 02 has been described
previously (Gardner and Samson 1973). An important
feature of the analyzer for the present work is a
retarding/accelerating lens to allow analysis of the
electrons at a constant pass energy and tl=bby achieve
high resolution. Also the electrons were analyzed at
an angle of 54044 ' to eliminate any effects caused by
varying angular distributions of the photoelectrons
(Samson and Gardner. 1972).
The energy scale was calibrated by mixing trace
quantities of xenon with the 02. The ionization potential
of Xe( P 3/ 2 ) is 12.130 eV, which only lies about 60
meV above that of 02. Further, the photoelectron peak
of Xe provided an excellent instrument function.
A photoelectron spectrum of 02 is shown in Fig. 3.
0
Dispersed NeI radiation at 736 A was used with an analyzer
pass energy of 0.5 eV. The half-width of the Xe(2P3/2)
7peak was 9 meV. This illustrates the use of Xe as a
calibrating gas. The spin-orbit splitting of each
vibrational level into a /2,3/2 doublet, is clearly1/2,3/2
resolved. A similar spectrum, only of the v' = 0 level,
0
was taken with dispersed ArII radiation at 919.78 A to
give better resolution through reduced Doppler broadening
(Fig. 4). The pass energy was 0.3 eV, which gave a
resolution of 7.3 meV (58 cm-l1
The intensity of each rotational line was taken to
be proportional to
It If It
- B N (N + 1) hc/kT
(2N + l)e
Each line was then convoluted with a Gaussian instrument
S-1
function of half-width 58 cm . Figure 5 shows the
resulting distribution of intensity in each of the 0, P,
Q, R, and S branches for the 2 ,,, state. Similar results
were obtained for the 2 T/2 state. The results were
then summed over all rotational levels, for both states,
allowing the intensities of each pair of branches (O,S)
and (P,R), and Q to vary until a best fit to the
experimental data was obtained. The assumption that the
branches have equal intensities within each pair appears
to be justified, to a first approximation, because in
the case of the rotator with A = 0 the line strengths
SN for AN = +1 and -1 are, respectively, SN = N + 1 and
8and SN = N, with slightly more complicated functions of
N for AN = +2 (Herzberg 1950). The relative intensity
2 2
of the /2 and IT states was also allowed to vary.1/2 3/2.
The spectroscopic value of 200 cm-1 for the separation
of the 2 1 /2,3/2 states was used (Krupenie 1972).
The results of this fitting procedure are shown in Fig.
4. The solid line is the calculated photoelectron
distribution and the data points are the experimental
-i
results. With this fit the band origin is located 20 cm-1
(2.5 meV) below the peak maximum. The peak maximum is
measured experimentally at 12.073 ± .001 meV. Thus, the
true ionization potential of 02 is 12.071 ± .001 meV.
The experiment was repeated with a pass energy of
-16.5 eV giving a resolution of 69 cm- 1. Again, an
excellent fit to the experimental data was obtained.
An average of the best fitting parameters gave an intensity
for the 0 and S-branches 0.57 ± .05 times that of the
Q-branch intensity. The P and R branches were found to
have zero intensity. This suggests a selection rule
of AN = 0, + 2. Higher values of AN were considered
but these broadened the peaks more than was observed.
2 2The average ratio of the H3/2 to 21 state was found3/2 1/2
to be 0.93 ± .02. This is of the same magnitude as the
relative intensity of the spin-orbit components found
in the halogens (Frost et.al. 1967).
9Because there is a branch head in the O-branch
-l
the appearance potential of electrons must be 22.2 cm
(2.8 meV) below the ionization potential. That is, the
appearance potential of ionization occurs at an energy
of 12.068 ± .001 eV. This is in agreement with the
completely independent method of simply taking the
energy of the first appearance of electrons (as shown
in Fig. 4) and correcting for the analyzer resolution
(58 cm-l.
Ionization potentials obtained from photoelectron
spectroscopy data cite the energy of the vibrational
peak maximum. As noted earlier this position will not
be, in general, the origin of the band. A comparison
of high resolution photoelectron spectroscopy data shows
close agreement of the energy of the v' = 0 peak maximum.
Similarly, there is close agreement in the published
values of the first appearance potential of ionization.
They lie, on the average, about 6 meV below the photo-
electron spectroscopy data. Table 1 summarizes the
various data. The consistency of the results in the
two categories gives strong support to the present analysis
assigning the origin or true ionization potential to a
value of 12.071 eV.
Table 2 summarizes the relevant findings of this
work.
10
The 2 1/2,3/2 spin-orbit splitting observed in the present
-i
work is 185 cm 1 (23 meV). This must represent a lower
limit of the splitting because the effect of the unresolved
rotational lines is to narrow the separation. This is
demonstrated by the solid curve in Fig. 4 that was
obtained using the spectroscopic value of 200 cm 1
(24.8 meV) for the 2 /2,3/2 splitting. Thus, unless
a vibrational band with spin-orbit splitting is deconvoluted
a partially resolved photoelectron spectrum will always
underestimate the magnitude of the splitting. In the
present case for 02 the spectroscopic value of 200 cm
(24.8 meV) is recommended.
Acknowledgements
This research was supported by the
National Aeronautics and Space Administration
under Grant NGR 28-004-021.
TABLE 1. Ionization potential (eV) of 02 by
various techniques.
Appearance Photoelectron
References potential True IP Spectroscopy
(peak max.)
Present Results 12.068 ± .001 12.071 ± .001 12.073 ± .001
a 12.067 ± .010 -
b 12.063 ± .010 -
c 12.067 ± .001 -
d 12.065 ± .003 -
e 12.072 ± .008 -
f 12.070 ± .005
g 12.071 ± .005
h 12.077
i 12.075
a,b Samson and Cairns (1966). g Edqvist et.al. (1970).
c McNeal and Cook (1966). h Dromey et.al. (1973).
d Nicholson (1963). i Collin and Natalis (1969)
e Dibeler and Walker (1967).
Turner (1968).
Note: The AP were obtained from photoionization studies
using mass spectrometers (refs. a,e), photo-ionization of 02 'A
(refs. a,c) and ion chambers (refs. b,d).
TABLE 2. Summary of data obtained from the
present photoionization study of 02.
Ionization potential 12.071 ± .001 eV
Appearance potential 12.068 ± .001 eV
Dissociation energy 6.661 ± .001 eV
211 2 H 0.93 ± O.01a3/2 21/2
(O0,S) : Q-branch 0.57 ± 0.02a
(P,R) : Q-branch 0
Selection rule AN 0,±2
a Quoted error limits represent the deviations of
two independent sets of data.
REFERENCES
Asundi, R. K. 1968. Current Sci. (India) 37, 160
Bhale, G. L. and Rao, P. Ramakotsewara 1968.
Proc. Indian Academy Sci. 67A, 350
Collin, J. E. and Natalis, P. 1969. J. Mass Spectry.
Ion Phys. 2, 231
Dibeler, V. H. and Walker, J. A. 1967. J. Opt.
Soc. Am. 57, 1007
Dromey, R. G., Morrison, J. D., and Peel, J. B.
1973. Chem. Phys. Letters 23, 30
0
Edqvist, O., Lindholm, E., Selin, L. E., and Asbrink,
L. 1970. Physica Scripta 1, 25
Frost, D. C., McDowell, C. A., and Vroom, D. A.
1967. J. Chem. Phys. 46, 4255
Gardner, J. L. and Samson, J. A. R. 1973. J.
Elect. Spectrosc. 2, 267
Herzberg, G. 1950. Molecular spectra and molecular
structure (Van Nostrand Reinhold Co., New York),
2nd edition, pp. 127 and 207.
Inn, E. C. Y. 1953. Phys. Rev. 91, 1194
Krupenie, P. H. 1972. J. Phys. Chem. Ref. Data
1, 423
McNeal, R. J. and Cook, G. R. 1966. J. Chem. Phys.
45, 3469
References (continued)
Nicholson, A. J. C. 1963. J. Chem. Phys. 39, 954
Samson, J. A. R..and Cairns, R. B. 1966. J. Opt.
Soc. Am. 56, 769
Samson, J. A. R. and Gardner, J. L. 1972. J.
Opt. Soc. Am. 62, 856
Turner, D. W. 1968. Proc. Roy. Soc. A. 307, 15
Walker, T. E. H., Dehmer, P. M., and Berkowitz, J.
1973. J. Chem. Phys. 59, 4292
Watanabe, K. and Marmo, F. F. 1956. J. Chem.
Phys. 25, 965
Figure Captions
1. The fraction of molecules in the various
rotational levels of the ground state of 02 (v"=0)
at a temperature of 300 K.
2. Energy level diagram of 02 and 02+ illustrating
transitions between the various rotational levels.
3. Photoelectron spectrum of 02 photoionized by
the monochromatic Ne I resonance line at 736 A.
The first seven vibrational bands are shown including
2theXe doublet PP used for calibrating the
energy scale. Vibrational bands of the ground state
of 0O continue to v'=20. Analyzer resolution was
9 meV.
4. Photoelectron spectrum of the v'=0 band of 02
photoionized by the Ar II resonance line at 920 A.
The solid curve is the calculated shape of the band
taking into account the rotational structure. The
open circles are the experimental data points.
Analyzer resolution was 7 meV.
5. Relative intensities of the rotational branches
for the transitions AN = 0, ±1, and ±2 assuming a
constant transition probability for all rotational
lines, a thermal distribution of the rotational lines
for T = 300 K, and an instrumental analyzer function
of 58 cm-1 (7 meV). The origin is represented by
zero on the abscissa axis.
8Z 6
0
CL
-
a- 4
2
0 O 20 30
ROTATIONAL LEVEL
1~ 01
J/
T1 (v' o)
-2y2 5
-- * 1/2 A
J 
_ _ . _ -- j
//
a 5
--
2
-- 32
L 1
-- o
O P Q R S
AN= -2 -1 -O' +1 +2
* tl
Xe P Xe P
WI 2 3 4 5
IONIZATION POTENTIAL (eV)
12.06 12.08 2.10 ,
2 2
z 6 TTy2 TT3/2
C6 PEAK 12.073 eV
5 0 0
- 000
I-
4-I
a 2
w
SI BAND ORIGIN0
S12.071 eV
0
0
0 I I I • I
- 100 0 100 200 300
Al) (cm-)
-0
60
i71
-50z ,i I
S40 I
30 --
" I
I
z 20
- I
0 - ,
-100 0 too 200 300
AL\) (cm-')


On the ionization potential of molecular oxygen

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