Journal of Electron Spectroscopy and Related Phenomena 114–116 (2001) 85–92
www.elsevier.nl / locate / elspec
High resolution K-edge spectroscopy of oxygen transient species:
the metastable O 2 a 1 D g molecule and the O ( 3 P) atom
a
a
b
c,
Michele Alagia , Marcello Coreno , Monica de Simone , Robert Richter *,
Stefano Stranges d
a
Laboratorio TASC-INFM, Area Science Park, I-34012 Basovizza ( TS), Italy
Universita di Roma Tre, Dip. Fisica ‘ E. Amaldi’ e Unita’ INFM di Roma Tre, Rome, Italy
c
Sincrotrone Trieste, Area Science Park, I-34012 Basovizza ( TS), Italy
d
Dipartimento di Chimica, Universita di Roma La Sapienza, Unita INFM, I-00185 Rome, Italy
b
Received 8 August 2000; received in revised form 13 September 2000; accepted 18 September 2000
Abstract
The K-edge photoionization spectra of metastable O 2 (a 1 D g ) molecules and O ( 3 P) atoms have been investigated at high
resolution. Both species were prepared on-line using a microwave discharge. For the molecule the transition to the p* 1 P
state has been identified below the ionization threshold, using total-ion-yield spectroscopy. Above the edge the previously
unobserved 2 D state of the O 1
2 core-hole ion has been located by photoelectron spectroscopy. The results provide a link
between the singlet and triplet manifolds of the core excited states of the oxygen molecule. The 1s excited states of the
oxygen atom were also studied. The high resolution allowed the characterization of higher members of the Rydberg series
and first linewidth measurements for those transitions.  2001 Elsevier Science B.V. All rights reserved.
Keywords: Inner shell spectroscopy; Synchrotron radiation; Free atoms and molecules
1. Introduction
Due to advances in synchrotron radiation sources
and soft X-ray monochromators, core spectroscopy
of small molecules has recently received much
attention. The higher intensity and resolution now
available allows the observation of more details in
the spectra, leading to a better understanding of the
core excitation and decay processes. The higher
sensitivity allows also the study of some elusive
*Corresponding author. Tel.: 139-40-3758-692; fax: 139-409380-902.
E-mail address: [email protected] (R. Richter).
molecules such as reactive intermediates [1], metastable molecules [2] and open shell atoms [3–5].
The spectrum of atomic and molecular oxygen
below their 1s ionization thresholds have been the
subject of several recent experimental [3,4,6–8] and
theoretical [9–11] investigations. The O 2 spectrum
near the K-edge has been known for a long time, but
the assignment is still controversial. The lowest
energy feature is the excitation to the p* 3 P state,
followed higher up by the core-to-valence s* resonance and core-to-Rydberg excitations. The location
of the s* resonance in particular is still subject to
debate [12,13]. Both atomic and molecular oxygen
are model cases for open-shell systems, for which
0368-2048 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved.
PII: S0368-2048( 00 )00242-5
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M. Alagia et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 85 – 92
the theoretical description is difficult and still being
developed. The practical interest in the K-shell
spectra of the species derives mainly from astrophysical work ([9], and references therein).
The electronic configuration of the O 2 molecule is
1s 2g 1s 2u 2s 2g 2s 2u 3s 2g 1p 4u 1p 2g .
This configuration gives rise to three electronic
states, the X 3 S g2 , a 1 D g and b 1 S g1 [14]. The two
metastable singlet states are important atmospheric
constituents, both being products of many photolysis
reactions. The a–X and b–X emissions of molecular
oxygen contribute strongly to the atmospheric airglow and are used to verify predictions from atmospheric chemistry models [15]. The spectroscopy of
the two singlet species has therefore been the subject
of many studies, concentrating mainly on the low
energy range. The low energy photoelectron spectrum of the a 1 D g state has also been studied using
HeI [16,17] and, more recently, synchrotron radiation [2]. In those investigations ionizing transitions
have been identified leading to states of the O 21 ion
which are inaccessible from ground state O 2 due to
selection rules. Similarly for the O atom, states of the
O 1 ion can be reached from the same configuration
metastable singlet states ( 1 D and 1 S).
Above the 1s ionization threshold a similar situation exists, namely that (in the one particle, dipole
approximation) low lying states of the O 1 (1s 21 )
core hole ion can only be produced from the
metastable states of the atom [18], and states of the
molecular ion only from the singlet metastable states
(a 1 D and b 1 S 1 ). Below threshold the singlet manifold of molecular states becomes accessible due to
spin conservation selection rules. As the metastable
species are produced here using a discharge source
the spectra are dominated by transitions originating
from the precursor–the O 2 X 3 S g2 ground state
molecule, making the analysis difficult.
2. Experimental
The experiments described here were carried out
at the Gas Phase Photoemission beam-line at Elettra.
As the general performance of the beamline at the
oxygen K-edge was described elsewhere [19], only
the relevant modifications to the ARPES (Angular
Resolved PhotoElectron Spectroscopy) endstation to
allow the production and detection of chemically
unstable species are described here.
The species of interest were produced directly in
the gas inlet line, which for these experiments
consisted of a quartz tube (10 mm ID) leading
directly into the ionization region. The singlet oxygen molecules were prepared in a microwave discharge (2.45 GHz, power variable 30–100 W) in a
sidearm of the tube. The yield of their production
was varied by changing discharge conditions. Most
experiments were performed using pure oxygen,
although–as has been reported previously [20]–the
relative yield of O ( 3 P) atoms could be increased by
adding other gases (e.g. N 2 ). The relative concentration of the species of interest was monitored in
situ using photoelectron spectroscopy. From photoelectron spectra recorded at low photon energies a
relative yield of approximately 15% for the production of metastable O 2 a 1 D molecules was estimated (Fig. 1). For most of the experiments a
hemispherical photoelectron energy analyzer (VSW
Ltd., 50 mm mean radius) and a time-of-flight mass
spectrometer were mounted facing each other. The
photoelectron spectrometer was used also for diagnostics. When recording the ion yield spectra the
entrance lens of the PES spectrometer was used as a
repeller. While recording photoelectron spectra the
mass spectrometer lens was kept at ground. Due to
the construction of the gas inlet, spectra were
recorded parallel to the photon polarization vector.
The photoelectron spectra recorded above the 1s
ionization threshold were recorded using the hemispherical analyzer operated in constant pass energy
mode with the pass energy set to 10 eV. The
resolution of the analyzer under these conditions was
measured directly by recording valence photoelectron spectra of the discharge products using a photon
energy of 25.2 eV and found to be |130 meV (Fig.
1). The core photoelectron spectra were recorded at a
photon energy of 565 eV with an estimated monochromator resolution of |250 meV. The resulting
overall energy bandwidth in the photoelectron experiments is therefore estimated as |280 meV.
Energy calibration of the photoelectron peaks has
been carried out using the 2 S 2 and 4 S 2 bands of
M. Alagia et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 85 – 92
87
Fig. 1. Part of the outer valence photoelectron spectrum recorded with the microwave discharge ON at 25.2 eV photon energy. Transitions
originating from ground state O 2 , the metastable a 1 D state and O 3 P atoms are indicated. The peak marked (*) is due to ionization of CO 2
produced in the reaction of oxygen with the graphite-coated surfaces near the ionization region.
molecular oxygen and photoelectron spectra recorded
at low photon energy.
The total-ion-yield spectra of O 2 below threshold
were recorded using a high extraction voltage in the
ionization region of the time-of-flight spectrometer to
minimize the discrimination against the detection of
energetic fragments. The beamline was operated at
high resolution (50–60 meV band pass), which was
estimated by recording NEXAFS spectra of pure
oxygen in an absorption cell located behind the
experimental chamber [19]. Those spectra were also
used for ‘on line’ photon energy calibration using the
energies of oxygen molecular states reported by Ma
et al. [21].
3. Results and discussion
3.1. The photoelectron spectrum
The photoelectron spectrum recorded above the 1s
ionization threshold is shown in Fig. 2, uncorrected
for the analyzer transmission. The photon energy
was set to 565 eV to reduce post-collision interaction
effects and the influence of resonances near threshold. The spectrum is dominated by the ionization
from the ground state oxygen molecule, but other
peaks are clearly distinguishable. Two features result
from the ionization of atomic oxygen, which is
invariably present among the discharge products,
another from the ionization of the O 12 D metastable
molecule. The location and shape of the peaks due to
ground state oxygen molecule ionization ( 4 S 2 and
2 2
S ) can be determined independently by recording
the photoelectron spectrum with the microwave
discharge off. The relative intensity of the other
peaks in the spectrum can be varied by changing the
discharge conditions. The observed intensity ratio
between the 4 S and 2 S states is similar to the ratio
reported previously at much higher photon energies
[22]. This is an indication that differences in the
asymmetry parameter and partial ionization cross
section for the two channels are negligible for the
ionization of the ground state molecule at the chosen
photon energy. The linewidths and lineshapes reflect
the molecular structure, as the combined analyzer
and photon energy bandpass is less than the observed
peak width. The geometry of the two lowest states of
88
M. Alagia et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 85 – 92
Fig. 2. The K-edge photoelectron spectrum of oxygen recorded
using various discharge conditions. The top trace includes a
simulation of the 4 S 2 band using spectroscopic parameters
derived from the equivalent-core OF 1 ion (see text). The solid line
through the low O-atom yield spectrum is the sum of the
components shown at the bottom. The ionization energies are
summarized in Table 1.
21
the O 1
) core hole ion has been investigated
2 (1s
theoretically by ab initio configuration interaction
calculations [22,12]. The predicted equilibrium bond
distance for the 2 S 2 state of O 21 is significantly
larger than for the 4 S 2 state. Therefore, the vibrational Franck–Condon envelopes are very different, as can be seen in the photoelectron spectrum.
The observed widths and shape agree with the highphoton-energy photoelectron spectrum reported by
Larsson et al. [22]. Uncertainties in the photon
bandwidth and possible influence of post-collision
interaction on the peak shape preclude the extraction
of accurate molecular parameters from the data. It
should be noted however, that using the published
internuclear equilibrium distance and spectroscopic
parameters derived from the equivalent-core molecule (OF 1 ) [23] constants the 4 S 2 band can be
simulated with reasonable accuracy assuming a
Morse potential. The result of the simulation, convoluted with a Voigt line shape (280 meV gaussian and
150 meV lorentzian contribution) is also included in
Fig. 2.
After removal of a 1s electron from the oxygen
molecule the remaining orbital configuration is
1s 21 . . . p 2 . This leads to four electronic states, the
4 2 2 2 2
S , S , D and 2 S 1 , only the first two of which
are accessible following ionization from the ground
state 3 S g2 molecule. Due to selection rules in the one
particle dipole picture the other two states are
accessible from the a 1 D and b 1 S 1 metastable states
of the molecule. As verified by the valence photoelectron spectra the microwave discharge used in this
study does not produce a significant concentration of
b 1 S 1 metastable molecules or metastable states of
the oxygen atom. The impurities produced by the
discharge (mainly CO, CO 2 and NO) were present in
much lower concentrations than the oxygen transients. Features due to the two carbon containing
molecules are well separated from the oxygen spectrum (1s ionization potentials of 542.54 eV and
541.25 eV for CO [24] and CO 2 [25], respectively)
and have not been observed in the core photoelectron
spectrum. The strongest NO band is located at 543.3
4 2
eV [24], i.e. underneath the O 1
band (depending
2 S
on the energy calibration – see below), and was also
not detected. The remaining peaks in the spectrum
are therefore assigned to the ionization of O 3 P
atoms and O 2 a 1 D molecules. The relative intensity
of the peaks could be varied by changing the
discharge conditions. This allows a further distinction between the species. The atomic and molecular
ionization potentials derived in the present study are
summarized in Table 1. The measured separation of
the 2 P and 4 P states agrees well with the previously
determined value [26], providing a further confirmation of the assignment. The derived value for the 2 D
state takes into account the difference in the initial
state of the ionization process. This places the 2 D
state of the core hole ion 0.54 eV above the 2 S 2 state
and around 1.78 eV above the 4 S 2 ground state. To
our knowledge, excited states of the O 1
2 core hole
ion have not been studied theoretically. In the
equivalent core model their spectroscopic parameters
M. Alagia et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 85 – 92
Table 1
Summary of the 1s ionization potentials for molecular and atomic
oxygen derived in the present study a
O1
2 State
4
2
S
S2
2
D
2
Energy / eV
O 1 State
Energy / eV
543.37
544.61
545.15
4
544.48
549.35
2
P
P
a
2
3 2
The energy of the O 1
2 D state is given relative to the X S g
ground state of O 2 . The relative energies are believed to be
accurate within 100 meV.
can be derived from the constants of the OF 1 radical
ion. The low-lying electronic states of that species–
X 3 S 2 , a 1 D and b 1 S 1 –have been studied by photoelectron spectroscopy [23] and found to be very
similar. Thus, the vibrational envelope of the 2 D
state should be similar to the 4 S 2 spectrum.
It has to be noted that in the literature data there is
a small disagreement between the ionization potentials of the oxygen molecule determined by X-ray
photoelectron spectroscopy [22] and the ZEKE X-ray
coincidence measurements of Rubensson et al. [7].
The ionization potentials given in Table 1. are
calibrated using the value given by Larsson et al.
[22] for the 4 S 2 state of O 21 (1s 21 ). Using this value
the ionization potentials obtained for the 4 P and 2 P
states of O 1 are higher than the values given by
Caldwell and Krause [26]. They are also higher then
the energies obtained from the quantum defect
analysis of Rydberg states recorded in this study (see
below). Using the energy of the 2 P ionization limit
from that analysis as a reference point all ionization
potentials in Table 1 need to be lowered by |0.5 eV.
The energies obtained in this way for the 4 S 2 and
2 2
S states of the molecular core hole ion are in
better agreement with the ZEKE-fluorescence coincidence measurements [7].
The intensity ratio of the two atomic peaks in the
spectrum is not statistical. Although part of that
difference could be attributed to analyzer transmission and the deconvolution procedure from which the
intensity of the 4 P peak was obtained, such a large
deviation cannot be explained by invoking experimental effects only. Two other factors can
dramatically change the peak ratio: the partial crosssection for the two channels ( 4 P and 2 P) and
differences in the angular distribution. The partial
cross-section ratio for the two channels has been
89
found to be nearly statistical at a photon energy of
590 eV [26]. The photon energy used in this study
(565 eV) is closer to the respective ionization
thresholds, so large differences in the energy dependence of the relative cross section for the two
channels could lead to the observed intensity ratio. In
contrast theoretical and experimental studies of the O
atom above its 2s ionization threshold [27,28] reported a relatively smooth variation of the relative
cross section with energy. However large variations
in the asymmetry parameter for ionization from s
sub-shells of open shell atoms have been predicted
theoretically [29,30] and observed in experiments
[5]. From our measurements, we cannot separate the
two contributions.
3.2. The total ion yield spectrum
An overview of the O 2 total-ion-yield spectrum
below the K-edge is shown in Fig. 3a. The main
feature of the spectrum corresponds to the core
excitation to the p* orbital (|530.7 eV), while
farther up in energy a complicated pattern of
Rydberg states is observed. The same spectrum
recorded with the microwave discharge ON is shown
in Fig. 3b. It is dominated by lines resulting from the
excitation of oxygen 3 P atoms produced in the
discharge.
The strongest peak in the 1s oxygen atom spectrum results from the 1s–2p valence excitation.
Although it is well known that this transition is
composed of six multiplet transitions [4] no clear
structure is visible in the observed line profile, and
so in the first step a Voigt line shape was assumed.
The best fit yielded a lorentzian linewidth of 159
meV with a gaussian contribution of 48 meV. In a
second stage a fit of the six multiplet transitions was
performed using the theoretical relative intensities
and positions given by Menzel et al. [4]. The lower
state 3 PJ population is not known exactly in this
experiment, but can be assumed thermal, as the path
from the discharge to the ionization region is long.
The resulting line profile contains a lorentzian width
of 154(10) meV. It should be noted that, although
more correct, this procedure did not give a significantly better description of the experimental data.
The lorentzian width is larger than the previously
90
M. Alagia et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 85 – 92
determined value of 140 meV [4], but the combined
error limits overlap.
Two atomic Rydberg series are clearly distinguishable in the spectrum below 545 eV. Both have been
observed previously [4,20], but here we are able to
study them at high resolution for the first time. The
series have been assigned to np states converging to
the 4 P and 2 P ionization limits. The 1s 1 2s 2 2p 4 np
configuration gives rise to several states ( 3 P, 3 D and
3
S states split by spin–orbit interaction and their
singlet counterparts), allowing many transitions from
the 3 P ground state. As has been noted previously the
width of the observed lines is therefore determined
not only by the lifetime of the states, but also the
splitting of the lines [4]. In this study we have
concentrated mainly on the series converging to the
2
P ionization threshold as this region of the spectrum
does not overlap molecular resonances. An expanded
view of the ( 2 P)np Rydberg series is shown as trace
(c) in Fig. 3. The solid line is a fit to the data using a
sum of Voigt functions sharing a common gaussian
contribution of 60 meV placed on an arctangent
step-shaped background [31]. Several peaks in the
spectrum could not be assigned, but are also included
in the fit to minimize their influence on the parameters derived for the Rydberg states. Although the
Rydberg resonances consist of several transitions, no
clear structure is visible with the current resolution
and signal-to-noise levels. The Auger decay widths
of the Rydberg states have been calculated previously and predicted to be very narrow [9]. The apparent
widths of the resonances (see Table 2.) are also
significantly larger than the values obtained theoretically for the two series limits [11] and therefore
Table 2
Rydberg states of atomic oxygen a
State
5 3
Fig. 3. Total-ion-yield spectrum recorded with microwave discharge source OFF (a) and ON (b). The spectrum shown is a
composite of several scans, so the relative intensity of the
structures can not be compared. Trace (c) shows the region of the
( 2 P)np Rydberg series recorded at high resolution. The peaks
marked (*) are unassigned and probably due to impurities
produced by the discharge.
Energy / eV
2p P
( 2 P)3p
( 2 P)4p
( 2 P)5p
( 2 P)6p
( 2 P)7p
( 2 P)8p
527.03
545.90
547.49
548.04
548.30
548.49
548.60
O 1 ( 2 P)
548.84
a
Width / meV
State
Energy / eV
230 (10)
140 (10)
117 (18)
( 4 P)3p
( 4 P)4p
( 4 P)5p
( 4 P)6p
541.29
542.81
543.36
543.61
O 1 ( 4 P)
544.13
Energies within each Rydberg series are accurate within 50
meV. The relative accuracy is approximately 100 meV.
M. Alagia et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 85 – 92
most likely reflect the separation of the levels
involved. The energies of the two Rydberg series are
in good agreement with the values reported previously by Stolte et al. [20]. The ( 2 P)np series has been
extended to include higher members. A quantum
defect analysis of the data gives yields 544.13 eV and
548.84 eV for the two ionization thresholds, 4 P and
2
P, respectively. The corresponding average quantum
defects are 0.81( 4 P np) and 0.85 ( 2 P np), typical for
p orbitals.
The only feature in the spectrum which could be
assigned to a transition originating from the a 1 D state
of O 2 is located at |530 eV. It has been noted
previously [4] that in this region of the spectrum a
peak remains after subtraction of discharge ON / OFF
spectra. It partially overlaps the strong p* band of
ground state O 2 making a detailed analysis impossible at the present stage. Its location in the
spectrum suggests an assignment to the corresponding p* singlet transition. To estimate the
spectroscopic parameters of the upper state we have
fitted the envelope of the whole band (singlet and
triplet) using the spectroscopic constants for the
triplet state derived previously [6] also for the singlet
state (assuming a Morse potential). The constants for
the X 3 S g2 and a 1 D g state were taken from Huber and
Herzberg [32]. The only adjustable parameters were
the relative intensity and position of the two transitions. The energy of the singlet transition obtained
from this analysis is 530.32 eV (0.43 eV lower than
the triplet band 1 ). Better agreement between the fit
and the experimental spectrum has been obtained by
allowing also the equilibrium distance and vibrational frequency of the singlet state to vary. The
lifetime-width of the state has been set to 150 meV,
the value determined independently for the triplet
transition [6]. The result of the fit is shown in Fig. 4
together with the experimental spectrum and summarized in Table 3. The position of the singlet state was
found to be relatively insensitive to the vibrational
constants and equilibrium internuclear distance used.
As the higher vibrational components of the singlet
transition overlap the strong triplet band, the de1
As the most intense band in the triplet spectrum corresponds to
v53 the energy of the same vibration is given for the singlet
spectrum. Due to the difference in the lower state constants the
v52 singlet band is more intense.
91
Fig. 4. Total-ion-yield spectrum of the 1s excitation of the O 2
X 3 S g2 and a 1 D g states recorded with the microwave discharge ON.
The simulations of the separate singlet and triplet transitions are
also shown. The solid line represents the best fit to the data – the
sum of the singlet and triplet curve. See text for further details.
termination of spectroscopic parameters is not very
accurate.
4. Conclusions
The K-edge photoelectron and excitation spectra
of oxygen transient species have been studied at high
resolution. For the oxygen atom, higher members of
the ( 2 P)np Rydberg series have been studied showing a large change of the observed linewidths with
increasing n. For the molecule the 2 D state of the
core hole ion has been identified following ionization
from the a 1 D metastable state. Below the ionization
Table 3
Spectroscopic parameters derived for the 1s excited valence states
of O 2 a
1
T o / eV
ve / meV
ve x e / meV
˚
Re / A
a
Pu
530.9 (0.1)
145 (10)
2.3 (1)
1.35 (0.01)
P (a)
u
a 1 D g( b)
b)
X 3 S 2(
g
530.4
138.4
1.6
1.3427
0.977
183.9
1.6
1.2156
0.0
195.92
1.485
1.2074
3
The parameters of the p* 3 P state from reference [6] are
included for comparison (a). A common lorentzian (150 meV) and
gaussian (50 meV) contribution to the line profile was used in the
analysis. (b) from reference [30]. See text for details.
92
M. Alagia et al. / Journal of Electron Spectroscopy and Related Phenomena 114 – 116 (2001) 85 – 92
threshold an estimate of the spectroscopic parameters
for the p* 1 P core excited state has been obtained.
Acknowledgements
The authors thank the staff of Elettra, in particular
the research team of the Gas Phase beamline, for
their support during the experiments. We would also
like to thank Kevin Prince and Lorenzo Avaldi for
many helpful discussions and a careful reading of the
manuscript.
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