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Dissociative double photoionization of CO2 molecules in the 36–49 eV
energy range: angular and energy distribution of ion products
Published on 01 April 2010. Downloaded by New York University on 25/10/2014 07:33:35.
M. Alagia,ab P. Candori,c S. Falcinelli,c M. Lavollée,d F. Pirani,e R. Richter,f
S. Strangesbg and F. Vecchiocattivi*c
Received 22nd December 2009, Accepted 2nd March 2010
First published as an Advance Article on the web 1st April 2010
DOI: 10.1039/b926960f
Dissociative double photoionization of CO2, producing CO+ and O+ ions, has been studied
in the 36–49 eV energy range using synchrotron radiation and ion–ion coincidence imaging
detection. At low energy, the reaction appears to occur by an indirect mechanism through the
formation of CO+ and an autoionizing state of the oxygen atom. In this energy range the
reaction leads to an isotropic distribution of products with respect to the polarization vector
of the light. When the photon energy increases, the distribution of products becomes anisotropic,
with the two ions preferentially emitted along the direction of the light polarization vector. This
implies that the molecule photoionizes when oriented parallel to that direction and also that the
CO22+ dication just formed dissociates in a time shorter than its typical rotational period. At low
photon energy, the CO+ and O+ product ions separate predominantly with a total kinetic energy
between 3 and 4 eV. This mechanism becomes gradually less important when the photon energy
increases and, at 49 eV, a process where the two products separate with a kinetic energy between
5 and 6 eV is dominant.
I.
Introduction
The first observation of the CO22+ dication, formed by
electron impact ionization of carbon dioxide was reported in
1961.1 Since that experiment, the double ionization of CO2
has been studied in several laboratories.2–5 These kinds of
processes are of great interest because of the involvement
of CO2 in several atmospheric phenomena of the Earth and
of other planets and in plasma environments. In Mars’ atmosphere, where CO2 is the main component, the importance of
the CO22+ dication and its dissociation has been recently
demonstrated.6
Like most of molecular dications, CO22+ is unstable and,
when it is formed just above the double ionization threshold
energy, can dissociate to give CO+ and O+. Such a dissociative
reaction has been investigated with a variety of techniques like
electron impact ionization mass spectrometry,7,8 double charge
exchange,9 and photoionization mass spectrometry.10–17 In some
cases, coincidence detection techniques have been applied.5,12–19
We have recently reported some results on the double
photoionization of carbon dioxide, in the 34–50 eV photon
energy range, by the use of the synchrotron radiation.20
Those results, together with other experimental data that
recently appeared in the literature,4 allow us to assert that the
molecular CO22+ dication is unstable and dissociates to give
rise to CO+ and O+ ions,
CO2 + hn - CO+ + O+ + 2e,
(1)
with a wide range of lifetimes.4,20 Although the vertical threshold
for the formation of the molecular CO22+ dication is 37.3 eV,
it has been found that the reaction (1) starts at around
35.6 eV,4 indicating the presence, in this lower energy range,
of an indirect mechanism leading to the production of the
CO+ ion together with an autoionizing state of the oxygen
atom.4 However, many features of the possible mechanisms
promoting the reaction (1) are still not well characterized.
It is well known that the measurement of the angular and
energy distribution of products, with respect to the polarization vector of the ionizing light, is a powerful tool for understanding details of the photodissociative microscopic process.21,22
Such measurements provide the angular distribution of formed
ions, usually given as I(y)sin y,
stot
b
IðyÞ sin y ¼
1 þ ð3 cos2 y 1Þ sin y;
ð2Þ
4p
2
a
ISMN-CNR Sez. Roma1, P.le A. Moro 5, Rome,
00185, Italy
b
IOM CNR Laboratorio TASC, I-34012 Trieste, Italy
c
Dipartimento di Ingegneria Civile ed Ambientale, 06125 Perugia,
Italy. E-mail: [email protected]
d
CNRS, Universite´ Paris-Sud, LIXAM UMR8624, Bâtiment 350,
Orsay Cedex, F-91405, France
e
Dipartimento di Chimica Università di Perugia, 06123 Perugia, Italy
f
Sincrotrone Trieste, Area Science Park, 34149 Basovizza, Trieste,
Italy
g
Dipartimento di Chimica, Università di Roma ‘‘La Sapienza’’, 00185
Roma, Italy
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where I(y) and stot are the differential and the total cross
section of the process, respectively, while y is the angle
between the velocity vector of the fragment ion and the light
polarization direction. The anisotropy parameter b runs between 1, for the orientation of the dissociating molecule
perpendicular to the electric field vector (e.g. S to P transition), up to a value of 2 for the parallel transition (e.g. S state
to S state). An isotropic distribution of fragment ions is
characterized by b = 0.
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Measurements of b for the dissociative double ionization of
CO2 have been performed by Masuoka and coworkers16,17 in
the photon energy range from 41 to 100 eV, exploiting
synchrotron radiation coupled with time of flight analysis of
the dissociation products, and a photoion–photoion coincidence detection. Some other studies have been also carried out
at higher photon energies, where core orbitals of the target
molecules are involved.23,24 However, detailed information on
the possible mechanisms promoting reaction (1) in proximity
to the threshold, and on their evolution with rising photon
energy is still lacking. In particular detailed experimental
characterization of the b parameter for double photoionization at the threshold provides unique information on the
mechanisms in terms of molecular orbitals. In recent years, the
high potential for coupling photoion–photoion coincidence
with imaging techniques has been shown.24,25 For photon
energies around the double ionization threshold, this technique
has provided valuable information about the N2O system,
where the angular distribution has been found rather anisotropic, with the b parameter ranging from b = 0 at low energies
up to b = 1.5 and b = 1.6 for the dissociation leading to
N+ + NO+ and to N2+ + O+, respectively.25 Recently Pesic
et al. studied the photodissociation of CO2, at a photon energy
of 308 eV, exciting C(1s) core orbital electrons.24
In this paper we report the angular and energy distribution of ion products for reaction (1) in the 36–49 eV photon
energy range. The experiments have been carried out using
synchrotron radiation and a time of flight mass spectrometer
equipped with a position sensitive ion detector. The data has
been analyzed in order to obtain, as a function of the photon
energy, the anisotropy parameter b and the kinetic energy
released (KER) in the ion products.
Preliminarily, it is important to point out a substantial
difference between the dissociative process as studied in our
previous paper20 and the one investigated and discussed here.
In Fig. 1 we show a typical coincidence map as obtained in our
previous work20 for a photon with energy 44 eV. The relative
cross sections for CO22+ dication formation reported in that
paper come from the analysis of the photon energy dependence
of the relevant peak in the mass spectrum. The relative cross
sections for the ‘‘instantaneous’’ CO+ + O+ dissociation
result from the analysis of the coincidence CO+/O+ spot in
the map, while those for the metastable (CO22+)* formation
were obtained from the analysis of the trace of coincidence
points connecting the CO+/O+ spot with the CO22+ dication
position. The word ‘‘instantaneous’’ used above is related to
the typical ion flight times in the measured spectra. Thus we
can define dissociations occurring within B50 ps of the
photoionization event as ‘‘instantaneous’’ .20 In the present
work we have studied in detail such ‘‘instantaneous’’ dissociations
with the ion imaging technique.
II.
Experimental
The experiment has been carried out at the synchrotron
light laboratory ELETTRA (Trieste, Italy) by the use of
the ARPES end station of the Gasphase Beamline. Details
about the beamline and the end station have been already
reported elsewhere,27 while the apparatus used in the present
5390 | Phys. Chem. Chem. Phys., 2010, 12, 5389–5395
Fig. 1 A typical coincidence map as obtained at 44 eV photon energy
(see ref. 20). The delay times t1 and t2 define the ion–ion coincidence
points. In addition to the mass spectrum peaks, the CO+/O+ spot for
the instantaneous dissociation and the metastable trace leading from
that spot to the CO22+ dication location are evident.
experiment has been specified previously.20,25,26 Only some
features relevant for the present investigation are described
here.
Fig. 2 shows schematically the experimental set up. The
energy selected synchrotron light beam crosses an effusive
molecular beam of CO2 and the product ions are then detected
in coincidence with photoelectrons. The monochromator uses
a 400 lines mm1 spherical grating in first diffraction order.
The entrance and exit slits of the monochromator have been
adjusted in order to give a photon energy resolution between
1.5 and 2 meV in the investigated 36–49 eV range. In order to
avoid spurious effects, due to ionization by photons of higher
energy, a magnesium film filter was placed in the synchrotron
radiation beam. The molecular and the light beams cross at
right angles.
The ion extraction and detection system has been assembled
following the design described by Lavollèe.28 Such a time-offlight spectrometer, with an ion position sensitive detector, is
particularly designed in order to measure the spatial momentum
components of the ionic dissociation products. The electron
detector, located just below the interaction volume (see Fig. 2),
consists of a stack of three micro-channel plates followed by a
copper anode. The ion detector also consists of a stack of
three-micro channel plates located at the end of the drift tube.
The signals are read by an array of anodes arranged in 32 rows
and 32 columns.28 Such an arrangement allows the detection
of the position of the ion arriving on the detector plate. In the
experiment the photoelectron signals are used as start pulses,
and then ions are counted as a function of their arrival time
and position on the detector. The delay times are measured by
an array of time-to-digital converters connected with the multi
anode. Therefore, the obtained images are resolved in time,
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Fig. 3 Typical 3D representation of the angular distribution of
products at photon energy of 49 eV.
Fig. 2 Experimental set-up. The effusive CO2 beam direction and the
synchrotron light beam are perpendicular, with the light polarization
direction on the same plane. The time of flight tube is oriented
perpendicularly to both beams.
allowing an easy transformation to angle and energy distributions. All experiment components are controlled by a computer that also records experimental data.
The incident photon flux and gas pressure have been
monitored and stored in separate acquisition channels. Ion
yields have then been corrected for pressure and photon flux
changes while varying the photon energy.
Carbon dioxide is supplied to the needle beam source from a
commercial cylinder at about 10 atm at room temperature.
The gas has a 99.99% nominal purity and has been used
without any further treatment. An adjustable leak valve along
the input gas pipe line is used in order to control the gas flow,
which is monitored by checking the pressure in the main
vacuum chamber.
In order to test the efficiency and the performance of the
experimental set up, we have measured the angular distribution of CO+ and O+ product ions in the double photoionization of CO2 at 308 eV. The results compare well with those
available in the literature23,24 recorded with a similar experimental set up. In particular, we have obtained an anisotropy
parameter b = 0.5, in good agreement with that obtained by
Pesic et al.24 The data in the 36–49 eV photon energy range
were recorded using the same experimental conditions.
Fig. 3 shows a typical 3D representation of the angular
distribution of CO+ and O+ product ions at 49 eV, obtained
in two hours of coincidence counting. In this plot, the two
distributions – one for CO+, the other one for O+ ions – have
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been normalized and summed. During data analysis the
measured events are checked for total momentum conservation, to ensure that all parameters of the experiment, e.g.
the shape of the interaction region, the extraction potential in
the ion optics, and many others are correctly set and constant.
The time needed for an accumulation of statistically
meaningful data typically ranges from 2 to 4 h. Therefore data
were collected only at a number of selected photon energies: 36,
39, 41, 44, and 49 eV.
III.
Results and data analysis
The angular distributions of product ions of reaction (1) are
reported in Fig. 4 as I(y)sin y (see eqn (2)). Like in the 3D
distribution of Fig. 3, the data have been again collected for
CO+ and O+ in coincidence, normalized and summed. They
have been used to determine the value of the anisotropy
parameter b, giving b = 0 at 36 eV, with an increase to
b = 1.1 at 39 eV and finally a decrease to b = 0.7 at 41 eV and
b = 0.6 for the last two energies, 44 and 49 eV.
The kinetic energy distribution of the two product ions, as
obtained from the analysis of the ion images, is reported in
Fig. 5 as a function of the photon energy. The energy
distributions at 36 eV are not very clear because of the
low intensity of the signal, as can be also argued from the
large scatter of the data in the angular distribution at 36 eV
(see Fig. 4), due to the low cross section at this energy.20
For the other energies the distributions are much better
defined and the total kinetic energy released can be more
meaningfully plotted as shown in Fig. 6. Looking at such
distributions it is evident that, in the photon energy range here
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Fig. 4 The angular distributions of product ions for the dissociative
photoionization of CO2, reported as I(y)sin y at different photon
energies. In the Figure the best fit b values are also reported.
considered, they mostly arise from the contribution of two
components, indicating the presence of two mechanisms for
the reaction (1). We have analyzed these distributions by
fitting them with simple Gaussian functions, as shown by the
dotted lines in the Figure. This analysis allowed us to obtain
the mean total kinetic energy released as a function of the
photon energy, for each one of the two mechanisms, the one
dominant at the lowest energy (labelled A), and the other
dominant at the higher energies (labelled B). The kinetic
energy released for the two mechanisms is plotted in Fig. 7.
In a more rigorous and quantitative treatment, in place of
simple Gaussian functions for such an analysis, one should use
Gaussian functions convoluted with exponential decays to
account for the lifetime of the dissociating oxygen atom
autoionizing state and the interatomic distance distribution
of the two ions during the Coulomb explosion. However, we
do not have any detailed information about such lifetimes and
therefore we cannot carry out a more rigorous analysis.
Nevertheless, we made some rough attempts by using reasonable
lifetime values and the results still confirm the occurrence of a
double mechanism, one dominating at low photon energies
and the second one emerging at higher energies.
IV.
Discussion
The anisotropy parameter b not only describes the asymmetry
of the angular distribution of product ions for reaction (1), but
also allows definition of the anisotropy in the dissociative
5392 | Phys. Chem. Chem. Phys., 2010, 12, 5389–5395
Fig. 5 The kinetic energy distributions of product CO+ and O+ ions
as obtained from the analysis of ion images as a function of the photon
energy.
double photoionization cross section. Specifically, with respect
to the light polarization direction, the parallel cross section, sJ,
is related to the perpendicular one, s>, by the equation21,22
s? 2 b
:
¼
sk 1 þ b
ð3Þ
In the photon energy range investigated in this experiment, we
have found that the ratio ranges from a minimum of 0.4 up to
0.9. This means that the parallel cross section is always larger
than the perpendicular one, except at 36 eV photon energy,
where the ratio is 2. In that case the ionization is isotropic and
the product ions are distributed in all directions with a
spherically symmetric probability. It has to be noted that such
a low energy is below the vertical threshold for double photoionization of 37.34 eV,4 but above the thermodynamic level of
ground state products CO+ + O+ that lies at 33.15 eV.
Despite of the low cross section the isotropy of the distribution
appears clear from the Fig. 4. One can explain the presence of
reaction products at this low energy by assuming an indirect
mechanism, as suggested by Slattery et al.4 and also supported
by our recent results.20 Although the isotropy appears
well evident, the low intensity does not allow to extract
(see previous section) a meaningful energy distribution of
product ions. At higher energies, the ionization cross section
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Fig. 6 Total kinetic energy of ion products in the 39–49 eV photon
energy range. The dotted lines represent the Gaussian functions used
for the analysis (see text).
is zero, that is the reaction occurs isotropically with respect
to the light polarization direction, but when the photon
energy increases the b parameter increases up to 1.5. Therefore
for N2O the angular distribution of ion products for both
N+ + NO+ and N2+ + O+ channels is more anisotropic
with respect to the CO2 case and the parallel cross section is
larger than the perpendicular one. An anisotropic distribution
of products indicates the fulfilment of two requirements: the
double photoionization occurs if the molecule is appropriately
oriented, with respect to the polarization vector, when it absorbs
the photon and ejects two electrons. Moreover, the dissociation must take a time shorter than the typical rotational period
of the formed dication.25
The kinetic energy distributions of CO+ and O+ products
provide further interesting information about the reaction (1).
As already discussed in the previous section, the simple
analysis of the total kinetic energy distributions in terms of
Gaussian functions indicates, in the photon energy range
between 39 and 49 eV, the presence of two mechanisms, one
producing ions with a total kinetic energy between 3 and 4 eV
and a second one with energy between 5 and 6 eV (see Fig. 6).
These energy features are of interest for trying to understand
the role of states involved in the dissociation process. Some
low lying electronic energy levels of the CO22+ dication have
been reported by Slattery et al.4 on the basis of the threshold of
the photoelectron coincidence spectrum. In Fig. 8 these states
are shown with an energy scale that starts from the ground
state of the neutral CO2 molecule and are compared with the
electronic energy levels for the dissociation products. Hochlaf
et al.29 calculated the potential energy curves along the
collinear CO+–O+ separation coordinate, keeping the C–O
distance fixed at the equilibrium one in the ground state
CO22+ dication, that is 1.22 Å. The calculation has shown
that all electronic states of the dissociation products are
connected with those of the molecular dication through
potential barriers. In particular, the ground state CO22+(X3S
g)
dication is connected, along the collinear CO+–O+ separation
coordinate, with the ground state ion products, CO+(X2S)
and O+(4S0), through a barrier that is about 1.4 eV above
the bottom of the potential well. For the electronic states
immediately above, the potential barriers, correlating with the
ion products, are even higher.29
Looking at the energy levels in Fig. 8, taking into account
some information from the calculation by Hochlaf et al.29 and
the results of the present analysis, some considerations can be
drawn about the reaction (1). As we have already discussed,
below the vertical threshold of 37.34 eV, the dissociation
occurs through the formation of a CO+ ion and an autoionizing
state oxygen atom that ionizes later on:
Fig. 7 Total kinetic energy released in the dissociative process as
obtained from a Gaussian function analysis of data reported in Fig. 6.
CO2 + hn - CO+ + O* + e - CO+ + O+ + 2e. (4)
increases and therefore the signal becomes more intense,
making obtaining anisotropy parameters b easier. Our values
of b appear to be in agreement with the early values obtained
by Masuoka.17
In a recent study, we have found that the N2O molecule
behaves in a similar way when doubly ionized at about the
same photoenergy range:25 at very low energy, the b parameter
In the case when the double photoionization happens at the
energies around the vertical threshold and the dissociation
leads to CO+ and O+ in their ground electronic and
vibro-rotational states (33.15 eV4), then one expects an
energy release of the product ions of 4.2 eV. Values of the
kinetic energy released lower than 4.2 eV should indicate
the involvement of some excited internal modes of the final
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released kinetic energy distribution from the analysis of the
metastable trace in the coincidence maps. For an excitation
energy of 50 eV they found an average value of 4.4 0.4 eV.
Such a value must be compared not with the present results,
referring to an ‘‘instantaneous’’ channel, but with the value of
4 1 eV that we have reported in our previous paper,20
obtained from a trajectory calculation analysis of the metastable
trace in the coincidence maps.
V. Conclusions
Fig. 8 Energy levels of some low lying CO22+ dication states as
reported by ref. 4 and referred to the ground state neutral CO2
molecule together with some low lying asymptotic levels of the CO+
and O+ products.
diatom. This seems to be the case of the mechanism A
mentioned in the previous section.
The second mechanism, labeled as B, which dominates in
the higher energy range and leads to a kinetic energy release
larger of about 2 eV, with respect to the other mechanism A
(see Fig. 6 and 7), must be due to the population of an
electronically or vibro-rotationally (or both) excited state
of the CO22+ dication, and possibly leading to an excited
electronic state of ion products. However, on the basis of the
sequence of dication states determined by Slattery et al.4 and
considering the calculations by Hochlaf et al.29 it is difficult to
assign the involved electronic states unequivocally in this
second mechanism, B.
Moreover, under the assumption of a simple Coulombic
repulsion responsible for the kinetic energy release of product
ions, the two characteristic energies for the two mechanisms
correspond to two average distances where the two ions start
separating. These distances are at around 4.5 Å for mechanism
A and around 2.5 Å for mechanism B. At such distances one
expects a transition from the internal rearrangement of the
dication to the opening of the Coulomb explosion channel.30
It is also interesting to compare the kinetic energy release
obtained in the present paper with the one reported by
Masuoka et al.16 in the 40–100 eV photon energy range.
Looking at that paper it appears that the authors obtained
the kinetic energy release in the CO+ + O+ dissociation by
analysing the shape of the time-of-flight peaks of CO+ and
O+ detected in coincidence. Therefore, that data should be
comparable with our ‘‘instantaneous’’ dissociation channel. By
using a curve fitting procedure Masuoka et al. have obtained an
average kinetic energy release ranging from about 5 to 6 eV in
the 40–50 eV photon energy, in reasonable agreement with our
results. However, the higher resolution achievable in our ionimaging measurements allowed us to also observe an additional
mechanism at lower energy (mechanism A), not evident in the
earlier work.
Finally, the recent electron impact experiment reported by
King and Price8 also provided further values of the kinetic
energy release due to the competition of several dissociation
mechanisms. However, in that paper the authors obtained the
5394 | Phys. Chem. Chem. Phys., 2010, 12, 5389–5395
The results here reported demonstrate some important
aspects of the dissociative double photoionization of CO2 in
the 36–49 eV energy range. At low energy, the reaction occurs
by an indirect mechanism, that is through the formation of
CO+ and an autoionizing oxygen atom state. In this energy
range the reaction occurs with an isotropic distribution
of products with respect to the polarization vector of the
light. When the photon energy increases, the distribution of
products becomes anisotropic, with the two ions preferentially
emitted along the direction of the light polarization vector.
This implies that the molecule photoionizes when orientated
parallel to that direction, and also that the CO22+ dication just
formed dissociates in a time shorter than its typical rotational
period.
At low photon energy, the CO+ and O+ product ions are
separating predominantly with a total kinetic energy between
3 and 4 eV. However, such a mechanism becomes gradually less
important when the photon energy increases and at 49 eV a
different mechanism, where the two product ions are separating
with a kinetic energy between 5 and 6 eV, is dominating.
The present paper demonstrates the high potential of the
experimental technique used: the coupling of ion–ion coincidence with ion imaging. The angular distribution of the ionic
products with respect to the light polarization vector and their
kinetic energy distribution reveals important new information
about the dynamics of the dissociative reactions of type (1).
It has to be stressed that, recently, a large variety of
experimental data became available for the double photoionization of CO2 in the vacuum ultraviolet region. This data
provided many and complementary details about the dynamics
of the ionization process and the following dissociative channels.
It would be desirable to obtain a unified description of the
potential energy surfaces of this systems at a level of accuracy
achievable with current computational techniques, together with
accurate dynamics calculations.
Acknowledgements
Financial contributions from the MUR (Ministero dell’Università e della Ricerca) are gratefully acknowledged. F. V. and P.
C. gratefully acknowledge a travel support by the ‘‘Sincrotrone
Trieste S.C.p.A.’’
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