Fragmentation pathwaysfor selected electronic states of theacetylene dication Page: 2 of 4
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30FIG. 1: (Color online) KER vs. Auger electron energy for
(a) CH++ CH+ and CH2++ C+ and (b) deprotonation
C2H++H+ break up channels. Diagonal lines are determined
by energy conservation (KER + EAug = constant) with x-
axis intercepts indicating the energy of the various product
species at infinite separation (see text). The location of the
curves was chosen approximately (+0.5eV) through the peak
position (not the midpoint) of the appropriate spots. The ev-
idence of more density to the left of the diagonal lines than to
the right is due to the presence of the rovibrationally excited
product fragments
a beam of circularly polarized photons from the LBNL
Advanced Light Source. A COLTRIMS (cold target re-
coil ion momentum spectroscopy) spectrometer [5] was
used to extract positively charged particles towards one
side of the spectrometer and electrons towards the other
side. RoentDek position sensitive delay line detectors [8]
were placed at both ends of the spectrometer for mea-
suring the position and the time-of-flight (TOF) of the
charged particles. By using a weak magnetic field (~ 10
gauss) collinear with the spectrometer extraction field,
we were able to collect all Auger electrons emitted within
15 degrees of the spectrometer axis (transverse to the
photon beam) while all positive recoils of the reaction
were measured with 47r solid angle efficiency. For each
ionization event the vector momenta of both fragments
and the Auger electron were determined by the times
and positions of arrival of the particles on the detectors.
From these the ion species, the Auger energies and the
kinetic energy release were calculated for whichever chan-
nel (A,V or P) was populated in the event.
K-shell photoionization of C2H2 creates an energetic,
singly-charged, quasi-degenerate state(s) of the core-FIG. 2: (Color online) Potential energy curves in linear geom-
etry for C2H2++ (a) symmetric breakup and (b) deprotona-
tion. All energies relative to neutral acetylene at equilibrium
geometry. Note that the bond distances other than those dis-
played are fixed at the equilibrium geometry of neutral C2H2.
Shaded portions indicate Franck-Condon regions and vertical
lines mark the equilibrium geometry of the neutral.
ionized molecule, 1lo--(2+), with 291.1 eV energy rel-
ative to the neutral, which subsequently decays by filling
the core vacancy and ejecting a fast electron. The Auger
energy measurement determines the electronic state of
the dication C2Hz+ from which the dissociation starts.
The KER is a measure of the difference in energy be-
tween the starting point of the dissociation and the final
state of the fragments. In Fig. 1 we display a density
plot of Auger energy versus KER for the symmetric and
quasi-symmetric channels A and V (Fig.la) and the de-
protonation channels (Fig.ib). The g/u splitting of the
initial state(s) is of order - 100 meV [9], smaller than the
Auger energy resolution of our apparatus (~ 0.5 eV) and
of the same order of magnitude as the natural linewidths
of the states (~ 90 meV), which means that they cannot
be resolved in principle. The sum of the KER and the
Auger energy represents the difference in energy between
the initial core-hole state(s) and the final state of the
fragments. This is shown as the x-axis intercepts of the
solid lines in Fig. 1. Each intercept represents a different
state of the fragments at the asymptotic limit.
Turning to Fig. la, we see two intense spots at Auger
energies of 255.5 and 250 eV. Since the sum momentum
of the fragment ions plus the recoil momentum must ap-
proximately add to zero (the photoelectron energy and
photon momenta are much smaller) we were able to de-
termine that the 255.5 eV peak corresponds almost ex-2
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Osipov, Timur; Rescigno, Thomas N.; Weber, Thorsten; Miyabe,Shungo; Jahnke, T.; Alnaser, A. et al. Fragmentation pathwaysfor selected electronic states of theacetylene dication, article, December 18, 2007; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc902540/m1/2/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.