State selective ion formation effects observed in the core excited CS 2 molecule
K. Yoshiki Franzén, P. Erman, A. Karawajczyk, E. Rachlew, P. A. Hatherly, and M. Stankiewicz
Citation: The Journal of Chemical Physics 110, 3621 (1999); doi: 10.1063/1.478230
View online: http://dx.doi.org/10.1063/1.478230
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JOURNAL OF CHEMICAL PHYSICS
VOLUME 110, NUMBER 7
15 FEBRUARY 1999
LETTERS TO THE EDITOR
The Letters to the Editor section is divided into three categories entitled Notes, Comments, and Errata. Letters to the Editor are
limited to one and three-fourths journal pages as described in the Announcement in the 1 January 1999 issue.
NOTES
State selective ion formation effects observed in the core
excited CS2 molecule
K. Yoshiki Franzén, P. Erman, A. Karawajczyk, and E. Rachlew
Department of Physics I, The Royal Institute of Technology, S-10044 Stockholm, Sweden
P. A. Hatherly
J. J. Thomson Physical Laboratory, University of Reading, Whiteknights, Reading RG6 6AF,
United Kingdom
M. Stankiewicz
Instytut Fizyki im Mariana Smoluchowskiego, Jagiellonian University, Reymonta 4, 30-059 Kraków, Poland
~Received 8 May 1998; accepted 6 November 1998!
@S0021-9606~99!01506-8#
in the sulfur edge vicinity do not exhibit major differences,
except for the relative abundances of the CS1
2 ion, which for
all of the observed resonances is less than half of the values
acquired at nonresonant energies. Additionally, the abundances and peak shapes of the S1 and S21 ions measured at
the s *
3/2 resonance exhibit differences as compared to the
TOF mass spectra measured at all the other excitation energies at the sulfur edge ~see Fig. 2!, both resonant and nonresonant. As seen from this figure, the intensity of the S21
ion peak is higher at the excitation energy corresponding to
s*
3/2 resonance than at the excitation energy corresponding to
1
the p *
3/2 resonance, while the S ion peak shows the opposite
correlation. This is a clear demonstration of state selectivity
where the orbital of the promoted core electron determines
the subsequent ion fragment branching ratio. The narrow
peak shape of the S21 peak observed at the s *
3/2 resonance
indicates that this fragment is released with less kinetic energy than at the other measured excitation energies. Furthermore, measurements performed at the s *
1/2 resonance resulted in spectra very similar to those obtained at the s *
3/2
The selectivity in the fragmentation of core excited molecules using soft x-rays was first reported by Murakami
et al.1 and has continued to receive considerable attention.2–4
These and other works have mainly concerned the study of
site specific fragmentation effects, while we in a recent
work5 reported evidence of state selectivity of the fragment
process in the core excited OCS molecule. In this Note we
present further evidence of such selectivity from the case of
the core excited CS2 molecule.
This experiment was performed at the undulator beamline 51 of the MAX Synchrotron Radiation Facility in Lund,
Sweden. The radiation was monochromatized by a SX700
monochromator providing photons with a resolution better
than 5000 which crossed an effusive jet of CS2 molecules.
The CS2 gas was mixed with a small amount of Ar atoms
which were added for calibration purposes. The high instrumental resolution made it possible to selectively promote the
C (1s) or the S (2 p) electrons to either valence or Rydberg
orbitals or into the continuum. The subsequently emitted
atomic and molecular fragments were detected by a time-offlight ~TOF! mass spectrometer, which has been described
elsewhere,6 in the photoelectron–photoion coincidence
~PEPICO! mode. The experiments were performed with the
instrument positioned at the magic angle relative to the polarization vector of the photon beam. High acceleration voltages were applied to optimize the collection efficiency.
To clarify the discussion, the total ion yield spectra measured at the sulfur 2 p and carbon 1s edges utilizing the
experimental setup described above are reproduced in Fig. 1.
The analysis of these spectra was discussed in detail in a
separate paper.7 The measurements presented here were performed at energies corresponding to all the resonances observed in the total ion yield spectrum and at nonresonant
energies corresponding to the background, and at energies
above the respective ionization potentials. The branching ratios between different ion fragments acquired at the energies
0021-9606/99/110(7)/3621/2/$15.00
FIG. 1. Measured total ion yield spectrum acquired at the sulfur 2 p ~lower
panel! and carbon 1s edges ~upper panel!.
3621
© 1999 American Institute of Physics
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3622
J. Chem. Phys., Vol. 110, No. 7, 15 February 1999
Letters to the Editor
TABLE I. Excitation energies and the resulting ion fragment branching ratios from the CS2 molecule. Values
within parentheses are intensities obtained by Hayesa with energies shifted to our scale.
eV
C1
S21
S1
CS21
2
CS1
S1
2
CS1
2
171.93
174.7
283.00
C 1sp*
293.25
0.17
~0.18!
0.30
0.20~0.32!
0.27
0.05
0.48
0.20
0.08
0.14
0.40
0.51
0.47
0.11
~1.00!
0.03
0.04~1.00!
0.02
0.19
~0.22!
0.07
0.17~0.36!
0.10
,0.01
~0.0!
,0.01
,0.01~0.04!
,0.01
,0.01
~0.17!
,0.01
,0.01~0.32!
,0.01
a
Reference 9.
resonance. Other experiments5,8 have shown the existence of
decay channels where neutral dissociation is followed by
* state has indeed
atomic core hole decay. The CS2 S (2 p) s 3/2
been found to exhibit a repulsive character.7 This is to be
explained by the spatial distribution of the promoted electron
occupying the s* orbital, which due to its alignment parallel
to the internuclear axis and to the destructive interference
between its forming atomic orbitals, has a degree of localization at the outer ends of the molecular axis, thus pulling
the nuclei apart. This causes bond breaking more efficiently
than the p* and Rydberg states, the promoted electrons of
which are expected to be less localized along the axis. These
states were also shown to exhibit vibrational modes and
therefore attractive characters.7 The increase in bond breaking probability results in an increase in neutral dissociation
of CS2 to S* and CS fragments, where S* (2p 21 3p) is in a
core excited state autoionizing to S21 . This decay channel is
competing with channels yielding S1 ions; for example, the
reactions for which CS2 is doubly ionized to CS21
2 decaying
into 2S1 and C or S1 and CS1 , thus explaining the quench* resonance. The presented
ing of S1 observed at the s 3/2
model is in accordance with results from our previous work,7
where the experimental ionic asymmetry b-parameters of the
CS1 and S1 ions suggested that the transitions to the S
(2p 21 ) valence states are atomic-like, while Rydberg state
transitions are molecular-like.
Table I presents the ion fragment branching ratio measured at different excitation energies in the S (2 p) and C
(1s) excitation region. Hayes9 earlier found evidence for
site-specific fragmentation in the core excited CS2 molecule
by observing different relative intensities between ionic fragments emitted after photon induced core electron excitation
at the two edges ~see Table I!. Hayes utilized a quadrupole
mass spectrometer, which generally has poor efficiency for
collecting high kinetic energy ion compared to our hightransmission TOF mass spectrometer. A direct comparison
with our experiment is difficult since only results from two
excitation energies were previously presented ~not including
the S1 and the S21 ion intensities, which were included in
our measurements performed at several excitation energies!.
However, there is a qualitative agreement between the two
experiments since they both reveal different ion branching
ratios at the two edges. Our experiment reveals that there is a
dramatic difference in fragmentation branching ratios between measurements above the S (2 p) threshold at 171.93
eV and below the C (1s) p* resonance at 283.0 eV. The
higher degrees of multi-ionization and total three-body fragmentation at the higher energy are manifested in a higher
abundance of the S21 and the C1 ions, respectively. The
branching ratios measured above the S (2p) and C (1s)
thresholds at 171.93 and 293.25 eV, respectively, are very
different. However, it can be seen in the total ion yield spectrum ~see Fig. 1! that the carbon core excitation processes
only result in weak features at the higher energy, which then
are dominated by the S (2s,2p) electron excitations. The ion
yield from the C (1s) p* resonance is, on the other hand,
high, and gives branching ratios, which, except for the CS21
2
ion, are different than measurements at the C (1s) threshold
and similar to what is observed at the S (2 p) threshold vicinity. This actually shows that the decay processes following the C (1s) and the S (2p) electron excitations to neutral
states do not necessarily have to be very different, since they
lead to the same fragmentation reactions.
In conclusion, we have presented evidence for state selectivity observed in the ion fragmentation in the core excited CS2 molecule. In addition, we have presented branching ratios of ionic fragments emitted after sulfur and carbon
core electron excitations.
1
J. Murakami, M. C. Nelson, S. L. Anderson, and D. M. Hanson, J. Chem.
Phys. 85, 5755 ~1986!.
2
R. Murphy and W. Eberhardt, J. Chem. Phys. 89, 4054 ~1988!.
3
W. Habenicht, H. Baiter, K. Müller-Dethlefs, and E. W. Schlag, Phys. Scr.
41, 814 ~1990!.
4
T. LeBrun, M. Lavollée, M. Simon, and P. Morin, J. Chem. Phys. 98,
2534 ~1993!.
5
P. Erman, A. Karawajczyk, E. Rachlew, M. Stankiewicz, and K. Yoshiki
Franzén, J. Chem. Phys. 107, 10,827 ~1997!.
6
P. Erman, A. Karawajczyk, U. Köble, E. Rachlew-Källne, and K. Yoshiki
Franzén, Phys. Rev. A 53, 1407 ~1996!.
7
A. Karawajczyk, P. Erman, P. Hatherly, E. Rachlew, M. Stankiewicz, and
K. Yoshiki Franzén, Phys. Rev. A 58, 314 ~1998!.
8
M. Neeb, A. Kivimäki, B. Kempgens, H. M. Köppe, J. Feldhaus, and A.
M. Bradshaw, Phys. Rev. Lett. 76, 2250 ~1996!.
9
R. G. Hayes, J. Chem. Phys. 86, 1683 ~1987!.
FIG. 2. Photoelectron–photoion coincidence ~PEPICO! time-of-flight
* and s 3/2
*
~TOF! mass spectra of atomic fragments from the S (2p) p 3/2
excited CS2 molecule.
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