Photoionization of three isomers of the C9H7 radical

Article abstract of DOI:10.1021/jp9068569

Three resonance-stabilized radicals, 1-indenyl (Ind), 1-phenylpropargyl (1PPR), and 3-phenylpropargyl (3PPR), all isomers of the composition C ₉H₇, were generated by jet flash pyrolysis. Their photoionization was examined by VUV synchrotron radiation. The mass spectra show a clean and efficient radical generation when the pyrolysis is turned on. To study the photoionization, photoion yield measurements and threshold photoionization spectroscopy techniques were applied. We determined adiabatic ionization energies (IE_ad) of 7.53 eV for Ind, 7.20 eV for 3PPR, and 7.4 eV for 1PPR. Ab initio calculations show no major change in geometry upon ionization, in agreement with ionization from a nonbonding molecular orbital. The IEs were also computed and are in agreement with the measured ones. The difference in the IE might allow a distinction of the three isomers in flames. In the indenyl spectrum, an excited a^{+ 3}B₂ state of the cation was identified at 8.10 eV, which shows a low-energy vibrational progression of 61 meV. Furthermore, we have examined the dissociative photoionization of the precursors. The indenyl precursor, 1-indenyl bromide, undergoes dissociative photoionization to Ind⁺. An appearance energy (AE_0K) of 10.2 eV was obtained from fitting the experimental breakdown diagram. A binding energy of 1.8 eV can thus be determined for the C-Br bond in 1-indenyl bromide. The phenylpropargyl precursors 1PPBr (1-phenylpropargyl bromide/3-phenyl-3-bromopropyne) and 3PPBr (3-phenylpropargyl bromide/1-phenyl-3-bromopropyne) also lose a bromine atom upon dissociative photoionization. Approximate appearance energies of 9.8 eV for 3PPBr and 9.3 eV for 1PPBr have been determined.

Full text of DOI:10.1021/jp9068569

4698
J. Phys. Chem. A 2010, 114, 4698–4703
Photoionization of Three Isomers of the C9H7 Radical†
Patrick Hemberger,‡ Michael Steinbauer,‡ Michael Schneider,‡ Ingo Fischer,*,‡
Melanie Johnson,§ Andras Bodi,§ and Thomas Gerber*,§
Institute of Physical Chemistry, UniVersity of Wu¨rzburg, Am Hubland, D-97074 Wu¨rzburg, Germany, and Paul
Scherrer Institut, Villigen 5232, Switzerland
ReceiVed: July 20, 2009; ReVised Manuscript ReceiVed: August 24, 2009
Three resonance-stabilized radicals, 1-indenyl (Ind), 1-phenylpropargyl (1PPR), and 3-phenylpropargyl (3PPR),
all isomers of the composition C9H7, were generated by jet flash pyrolysis. Their photoionization was examined
by VUV synchrotron radiation. The mass spectra show a clean and efficient radical generation when the
pyrolysis is turned on. To study the photoionization, photoion yield measurements and threshold photoionization
spectroscopy techniques were applied. We determined adiabatic ionization energies (IEad) of 7.53 eV for Ind,
7.20 eV for 3PPR, and 7.4 eV for 1PPR. Ab initio calculations show no major change in geometry upon
ionization, in agreement with ionization from a nonbonding molecular orbital. The IEs were also computed
and are in agreement with the measured ones. The difference in the IE might allow a distinction of the three
isomers in flames. In the indenyl spectrum, an excited a+ 3B2 state of the cation was identified at 8.10 eV,
which shows a low-energy vibrational progression of 61 meV. Furthermore, we have examined the dissociative
photoionization of the precursors. The indenyl precursor, 1-indenyl bromide, undergoes dissociative
photoionization to Ind+. An appearance energy (AE0K) of 10.2 eV was obtained from fitting the experimental
breakdown diagram. A binding energy of 1.8 eV can thus be determined for the C-Br bond in 1-indenyl
bromide. The phenylpropargyl precursors 1PPBr (1-phenylpropargyl bromide/3-phenyl-3-bromopropyne) and
3PPBr (3-phenylpropargyl bromide/1-phenyl-3-bromopropyne) also lose a bromine atom upon dissociative
photoionization. Approximate appearance energies of 9.8 eV for 3PPBr and 9.3 eV for 1PPBr have been
determined.
Introduction
tion utilizing synchrotron radiation can be employed to monitor
their concentrations in flames online, as has been shown in the
literature.6-9 However, ionization and appearance energies of
radicals and their precursors have to be known beforehand to
distinguish between structural isomers in flames.
Only few spectroscopic data are available in the literature
for the three reactive intermediates examined in this paper. The
ionization energy of Ind was first measured by Pottie and
Lossing.10 They report an adiabatic ionization energy (IEad) of
8.35 eV by electron impact techniques. Furthermore, Izumida
et al. published an emission and excitation spectrum of indenyl
in a 3-methylhexane matrix at 7 K.11
In this paper, we present photoionization yield, threshold
photoelectron, and threshold photoelectron photoion coincidence
(TPEPICO) spectra of the 1-indenyl (Ind), 3-phenylpropargyl
(3PPR), and 1-phenylpropargyl radicals (1PPR), produced by
vacuum jet flash pyrolysis1 (Figure 1), applying synchrotron
radiation in the vacuum ultraviolet (VUV) energy range. As
visible in Figure 1, the corresponding bromides were used as
precursors. The precursor for 1-PPR is 3-phenyl-3-bromopro-
pyne (center trace), and the one for 3PPR is termed 1-phenyl-
3-bromopropyne (bottom trace). To make the relationship
between radical and precursor unambiguous, we will use from
hereon the shorthand notations IndBr (indenyl bromide), 1PPBr
(1-phenylpropargyl bromide), and 3PPBr (3-phenylpropargyl
bromide) for the precursor molecules. All of the radicals have
the same composition, C9H7, and are precursors in soot
formation. In previous experiments, we have shown the potential
of synchrotron radiation studies in connection with coincidence
spectroscopy for determining ionization or appearance energies
of reactive intermediates. For example, the thresholds for the
dissociative photoionization of allyl, ethyl, and propargyl were
identified,2-4 and recently, the ionization energies of two
phenylcarbenes and the dissociative photoionization of their
precursors were measured.5
Radicals play an important role in the formation of polycyclic
aromatic hydrocarbons (PAH) and soot in flames. Photoioniza-
† Part of the special section “30th Free Radical Symposium”.
* To whom correspondence should be addressed. E-mail: ingo@phys-
chemie.uni-wuerzburg.de (I.F.); Thomas.Gerber@psi.ch (T.G.).
‡ University of Wu¨rzburg.
Figure 1. The three C9H7 isomers were produced by vacuum jet flash
pyrolysis from their bromine precursors. Also, the point group of each
radical is given on the right-hand side.
§ Paul Scherrer Institut.
10.1021/jp9068569  2010 American Chemical Society
Published on Web 10/08/2009

Photoionization of Three Isomers of the C9H7 Radical
J. Phys. Chem. A, Vol. 114, No. 14, 2010 4699
Only approximate experimental ionization energies are avail-
able for either of the phenylpropargyl radicals. Andrews and
Kelsall first observed 3PPR in a discharge experiment with
phenylacetylene in a 20 K argon matrix and identified it by its
UV spectrum.12 The laser-induced fluorescence (LIF) and single
vibronic level fluorescence spectrum (SVLF) of 1PPR were
reported by Reilly et al.13,14 They synthesized 1PPR in an
electrical discharge source with benzene as a precursor and
identified it by multiphoton ionization (MPI) and LIF spectros-
copy. Apart from C2, C3, and CH, 1PPR was the most brightly
fluorescing substance in the LIF spectrum. According to the
authors, the ring breathing, the ring deformation, and the C-C
stretching vibrations in the side chain have the highest
Franck-Condon factors, and the transition is of mixed πn/nπ*
character. Both the ground (D0) and excited states (D1) are of
A′′ symmetry. With these techniques, the authors could assign
almost half of the 42 frequencies in each state. From the MPI
experiments, they concluded that the IE has to be between 5.1
and 7.3 eV.
The most likely step in the formation of PAHs is the
dimerization of two propargyl radicals with a hydrogen abstrac-
tion forming an aromatic C6Hn ring.15-17 Further addition of
C3H3 and H-abstraction creates a phenylpropargyl radical, which,
according to Stein et al.,18 can be involved in a number of further
reactions, two examples of which are the generation of o-
terphenyl in a dimerization process and the formation of a
doubly substituted 1,5-hexadiyne.
Figure 2. Photoionization mass spectra of IndBr (lower trace) and
Ind (m/z ) 115) with active pyrolysis (upper trace) at 8.5 eV.
electrons was subtracted following the method outlined by Baer
and co-workers.24,25 Ions are extracted by a field of 40 or 80
V/cm and further accelerated to -550 V within a distance of
10 mm. At the end of a 550 mm long-field free drift tube, the
ions are detected by MCPs.
On the vacuum chamber, we mounted a flange with a
continuous molecular beam source coupled with a pyrolysis
tube.1 A heated silicon carbide (SiC) tube was mounted onto
the faceplate (orifice 100 or 150 µm) of a General Valve,
operating without a poppet and augmented by a water cooling
system. In order to avoid bimolecular reactions, the heating
electrodes were fixed together as close as possible at the end of
the tube. Due to the continuous gas flow and the small orifice,
the beam temperature was higher than that in the case of a pulsed
molecular beam, and cooling of the internal degrees of freedom
was less efficient.
It is the aim of this study to determine IEs of the three C9H7
isomers, which may permit an isomer-selective detection of these
radicals in flames.
We used 3PPBr and 1PPBr as precursors for the PPR and
IndBr for the generation of indenyl radicals. All precursors were
synthesized as published in the literature.26-28 The compounds
were seeded in either 0.25 or 0.6 bar (absolute) of argon and
expanded through the SiC tube. We adjusted the heating power
in order to get full conversion of the precursors (strong orange
glowing of the SiC tube). Most of the spectra were averaged
for 120 s per data point and normalized to the photon flux, which
was measured previously by a photodiode.20
Experimental Section
The experiments were carried out at the VUV beamline of
the Swiss Light Source at the Paul Scherrer Institut (PSI) in
Villigen, Switzerland. Only a brief overview is given here
because the setup of the beamline is described in detail
elsewhere.19,20 The X04DB bending magnet provides synchro-
tron radiation, which is collimated by a platinum-coated copper
mirror and sent to a plane grating monochromator. The 600
grooves·mm-1 grating is optimized for the 5-15 eV energy
range and has a resolving power of 104. The beam is focused
by another mirror onto the 110-1000 µm vertical exit slits of
the monochromator, which are located inside of a rare gas filter.
This filter operates at a pressure of 10-13 mbar to suppress
the higher harmonic radiation. For photon energies below 8 eV,
either a MgF2 window or a gas mixture (Ne, Ar, Xe) is used,
and between 8 and 10.5 eV, an Ar-Ne mixture is used. The
flux is approximately 1011 photons/s in the energy range between
6.5 and 10.5 eV employed in the present study.20 A photon
energy resolution of 5 meV was achieved at 15.764 eV,
measured at the 11s′ peak of argon with fully open horizontal
slits.
Experiments were performed in a differentially pumped
vacuum chamber employing a supersonic molecular beam
apparatus. The (T)PEPICO technique was used to study the
photoionization of the radicals and the dissociative photoion-
ization of the precursors.21,22 The spectrometer combines a
velocitymapimagingelectronspectrometer23 andaWiley-McLaren
TOF mass spectrometer.19 Electrons are accelerated with a
constant field of 40 or 80 V/cm and focused by a potential of
144 or 273 V onto the microchannel plates (MCPs) of a position-
sensitive detector with a delay line anode (Roentdek DLD40).
Threshold electrons were typically selected with an energy
resolution of 5 meV. The contribution of hot background
Results and Discussion
a. Indenyl Radical. In a first step, we recorded mass spectra
in order to ensure that the reactive intermediates were formed
cleanly. Figure 2 shows mass spectra of IndBr at 8.5 eV photon
energy with pyrolysis on (upper trace) and off (lower trace). In
the lower trace, the precursor (IndBr) with its bromine isotopic
pattern at m/z ) 194 and 196 is visible. Note that no dissociative
photoionization of the precursor was observed at this energy.
When the pyrolysis was turned on, the precursor signal almost
disappeared, and a strong signal at m/z ) 115 (upper trace) was
detected, which was assigned to the Ind. We found no evidence
of dimerization of the radical or other side products coming
from the pyrolysis, as has been reported in earlier experiments
using synchrotron radiation.3 Only a small precursor signal
remained; therefore, almost full conversion into the radical was
achieved. Small traces of indene from the synthesis were also
visible.
Before studying the photoionization of the reactive intermedi-
ate, the photoionization and dissociative photoionization of the
precursor have to be analyzed because it can generate fragment
ions that are mistakenly assigned to the cation of the radical.
Therefore, we measured the signal intensity of the precursor as
a function of the photon energy while the pyrolysis was turned

4700 J. Phys. Chem. A, Vol. 114, No. 14, 2010
Hemberger et al.
and the crossover point between the parent and daughter signal
lies at about 10 eV. The breakdown diagram is modeled by eqs
2-4 to extract the appearance energy at 0 K, AE0K. SParent(hν)
and SDaughter(hν) denote the signals of the precursor and the
fragment, which means that AE0K is reflected by the disappear-
ance of the parent ion in the breakdown diagram.24,29-31
Sparent(hν) ) AE-hν P(E)dE
for hν < AE
(2)
∫
0
∞
SDaughter(hν) ) AE-hν P(E)dE
for hν < AE (3)
∫
Figure 3. Photoion yield and TPES of IndBr. The signal sets in at 8.4
eV, which constitutes an aproximate IEad.
SParent(hν) ) 0
SDaughter(hν) ) 1
for hν > AE
(4)
P(E) is the normalized thermal energy distribution of the parent
molecule, which is given by P(E) ∼ F(E) exp(-E/kT). The
density of vibrational states F(E) was calculated according to
the Beyer-Swinehart algorithm.32 The frequencies of the
precursor were calculated by DFT methods described below.
From a least-squares fit of eqs 2-4, AE0K ) 10.2 eV is
obtained.33,34 The best fit was achieved for a temperature of 300
K. This procedure assumes that all parent ions that have enough
energy to dissociate will do so, that is, the dissociation is fast,
and there is no kinetic shift. Parent ions that dissociate slowly
(103 < k < 107 s-1) in the acceleration region of the TOF analyzer
contribute to asymmetric daughter ion peak shapes. At low
energies, the Ind+ is slightly asymmetric, but due to the large
step size and the small signal levels, the presumably small
kinetic shift was not included in the modeling.
Figure 4. Breakdown diagram of IndBr+/Ind+. A fit to the experimental
data (solid line) yields AE0K ) 10.2 eV
Because of the large photon energy step size, the accuracy is
probably limited to 0.1 eV, worse than both electron and photon
resolution. With the appearance energy of the fragment and the
ionization energy of the precursor known, the C-Br binding
energy of the precursor cation IndBr+ can be calculated,
assuming that there is no reverse barrier along the reaction
coordinates. Taking an IE(IndBr) of 8.4 eV and AE(IndBr, Ind+)
) 10.2 eV, one arrives at 1.8 eV as the bond energy of C-Br
in IndBr+.
Once the necessary data of the precursor photoionization and
dissociative photoionization are known, the photoionization of
indenyl can be investigated. The pyrolysis is turned on, and
the radical is produced by thermal cleavage of the C-Br bond.
In Figure 5, the TPE signal of Ind+ is plotted as a function of
photon energy. Data were recorded with a 5 meV step size.
The signal increased strongly at about 7.5 eV. At 7.7 eV, an
additional broad band appeared, and at around 8.1 eV, a second
band appeared.
We assign the first peak in the TPES to the ionization
threshold. We took the maximum of the peak to obtain the
adiabatic ionization energy (IEad). Hence, the measured IEad is
7.53 eV, with an estimated error of 20 meV. The full width at
half-maximum (fwhm) of the peak of 23 meV is rather broad
compared to the one observed for Ar, which showed a fwhm
of only 5 meV. Thus, the broadening is not due to the bandwidth
of the synchrotron radiation or the electron spectrometer. We
regard this as an effect coming from insufficient cooling of the
radical in the molecular beam. The small signal below 7.5 eV
is assigned to hot bands, but sequence bands and the rotational
envelope will contribute to the TPE signal as well. Because the
spectrum is dominated by one intense peak, the geometry does
not change strongly upon ionization, and Franck-Condon
factors favor the transition from the vibrational ground state of
Figure 5. TPES of the 1-indenyl radical. We assigned the first band
to IEad ) 7.53 eV. The first excited electronic state of the cation appears
at 8.10 eV and shows a low-energy progression of around 61 meV.
off. In Figure 3, such a spectrum is depicted. The photon energy
was scanned with a step size of 0.1 eV. The ion signal of IndBr
increased at about 8.4 eV. The ionization energy can be derived
more accurately if we look only at the ion signal in coincidence
with threshold photoelectrons (TPE). The dashed line in Figure
3 shows this mass-selected TPE signal as a function of photon
energy. One can extract a value of 8.4 eV for the adiabatic
ionization energy of indenyl bromide.
The analysis of the dissociative photoionization of IndBr is
based on the breakdown diagram depicted in Figure 4. It shows
the fractional abundance of the parent and the daughter ion
(Ind+). Note that we used the threshold electron signals of
IndBr+ and Ind+ in this diagram. The dissociation proceeds
according to eq 1.
IndBr + hν f Ind+ + Br + e-
(1)
As visible in Figure 4, energy-selected IndBr ions show
fragmentation in the energy range between 9.7 and 10.2 eV,

Photoionization of Three Isomers of the C9H7 Radical
J. Phys. Chem. A, Vol. 114, No. 14, 2010 4701
TABLE 1: Comparison of the Calculated IEs with the
Experimental and Earlier Literature Values for Indenyl,
1-Phenylpropargyl, and 3-Phenylpropargyl
IEexp/eV
IEad, calc/eV
IEvert, calc/eV
IELit./eV
Ind
3PPR
1PPR
7.53
7.20
7.4
7.50
7.13
7.14
7.54
7.21
7.19
8.3510
6.813
6.8 (comp)/
5.1-7.3
eV (exp)13
the neutral to the vibrational ground state of the cation. Another
rather broad band appears with lower intensity at around 7.7
eV. It possibly originates from excited vibrational states of the
cation. The third band around 8.10 eV is too high in energy for
a vibrationally excited state. We exclude any contribution from
indene (C9H8) to these peaks, which is present in small
concentrations as a side product from the synthesis of IndBr
and has an IE of 8.14 eV.35 As supported by calculations (see
below), the band corresponds most likely to the electronically
excited a+ 3B2 state of the indenyl cation, where the electron is
removed from the HOMO-1 orbital. Closer inspection reveals
further peaks separated by 61 meV, as indicated in Figure 5.
They are assigned to a low-energy vibrational progression in
the a+ 3B2 state. Computations (see below) yielded a ring-
deformation mode at 478 cm-1, which is thus a possible carrier
of the progression. When the ionization of the indenyl radical
was first studied by electron impact ionization, the IE was
determined to be 8.35 eV.10 Probably, the first excited state of
the cation was found in this earlier work and not the ionic
ground state. We also attempted to find the low-lying thresholds
for dissociative photoionization of indenyl and recorded mass
spectra at selected energies up to 15 eV. However, no fragments
appeared that could be unambiguously assigned to this process.
In order to support the analysis of our experiments, we
performed density functional theory (DFT) calculations. We
used the Gaussian03 program package with the B3LYP exchange-
correlation functional and the 6-311++G** basis set.36,37 For the
geometry optimization, a modified GDIIS algorithm (geometry
optimization using direct inversion in the iterative subspace)
was employed, and tight convergence criteria with an ultrafine
grid were used.38 The ground-state geometries of the radical
and cation (singlet and triplet) were calculated, and the
difference between the zero-point energies was taken as the IEad.
As a guess for the vertical ionization energy, the energy of the
cation was computed at the equilibrium geometry of the neutral
ground state. To calculate the higher excited states of the cation
at the ground-state geometry, we applied time-dependent DFT
(TD-DFT) with a B3P86 functional and 6-311++G** basis
set.39,40
Table 1 summarizes the results of our calculations and
compares them to the experimental ionization energies and
earlier literature values. A summary of bond lengths and angles
is given in the Supporting Information. C2V symmetry was found
for both the radical and the cation in the calculations. The
calculated ionization energies are in good agreement with the
experiment and also give a substantially smaller value than
the earlier experimental number.10 Also in agreement with the
experiment, the computations suggest no major geometry change
upon ionization of the 1-indenyl radical. Correspondingly, the
difference between the IEad and IEvert (7.50 and 7.54 eV) is small.
The highest occupied molecular orbital (HOMO) of a2 symmetry
is distributed over the whole molecule. The nonbonding
character of this HOMO explains the low change in geometry
during ionization. We also found three vibrations of the cation
in the energy range at around 1200 cm-1 (0.149 eV) which might
Figure 6. Photoionization and dissociative photoionization of the 1-
and 3PPBr. The ion signal as a function of the photon energy is given
in the graphs. Both precursors yield m/z ) 115 as the product according
to the reaction indicated in the graph. A fitting procedure yields the
onset of fragmentation and the ionization energies.41,42
be responsible for the band at around 7.7 eV. They correspond
to C-H bending vibrations and to a ring breathing vibration.
At least for the latter one, some activity is expected. Computa-
tions of the cationic excited singlet and triplet states yield a
low-lying 3B2 state at 7.94 eV (adiabatic ionization energy). This
supports the assignment of the band at 0.6 eV above the
ionization threshold to this excited triplet state. With respect to
the radical, the electron is removed from the HOMO-1 orbital,
which has b1 symmetry. To summarize the results of the
calculations, a comparison with the experimental data indicates
that the method and basis set employed are adequate for the
calculation of ionization energies of C9H7 isomers.
b. Phenylpropargyl Radicals. Phenylpropargyl radicals
were generated in a similar way as indenyl. We used 1PPBr
and 3PPBr as precursors for both intermediates. A clean and
almost complete conversion of the precursor could be confirmed.
The mass spectra of both species exhibit similar features to those
of the indenyl radical. The precursors also show an isotopic
pattern at m/z ) 194/196 and disappear when the pyrolysis is
turned on, while a signal at m/z ) 115 arises that can be assigned
to 1PPR or 3PPR. To exclude that the phenylpropargyl cations
are formed by dissociative photoionization of the precursors,
the photoionization and fragmentation of these molecules were
examined as well. Figure 6 depicts the photoion yield of 3PPBr
and 1PPBr and their dissociative photoionization products (3PP+
and 1PP+). The reaction equation for the dissociation is also
given in the figure. All ions were recorded, not only those
associated with threshold electrons, in contrast to the earlier
experiments on indenyl bromide (cf. Figure 4); therefore, no
breakdown diagram is available.
The upper part of Figure 6 shows the ion signal intensities
of the 3PPBr parent and the 3PP+ daughter as a function of the
photon energy. Ionization of the precursor sets in at about 8.6
eV, while the daughter mass (3PP+) appears above 9.8 eV. The
ionization energy and onset of fragmentation were determined
by a simple fitting procedure, convoluting a thermal energy
distribution with a linear function, as suggested in the literature.41,42
The best fits were found for a temperature of 150 K. In a similar
way, values for the onset of ionization and dissociative ionization
can be obtained for 1PPBr and 1PP+. Here, the precursor ionizes
at around 8.5 eV and starts dissociating above 9.2 eV. An
approximate appearance energy of 9.3 eV is obtained from the
fit. Coincidence data based on the detection of threshold
electrons would yield more accurate data, as in the case of IndBr.
However, the signal-to-noise ratio in the experiments on PPBr

4702 J. Phys. Chem. A, Vol. 114, No. 14, 2010
Hemberger et al.
Figure 8. Photoion yield spectrum of 1PPR. The ionization energy
was determined to be 7.4 ( 0.1 eV.
Figure 7. Threshold photoelectron spectrum of 3PPR showing a signal
increase at 7.20 eV, assigned to the ionization threshold. The ion yield
spectrum is shown in the inset. A fit of the photoion signal yielded the
same value of 7.2 eV.
it is also possible that Reilly et al. ionized the radical through
an intermediate Rydberg state slightly below the ionization
threshold.
was too low, and a breakdown diagram of sufficient quality
could not be recorded. Therefore, exact values for binding
energies can also not be derived. Estimated binding energies
of the C-Br bond in 3PPBr+ and 1PPBr+ are 1.2 and 0.7 eV,
respectively, significantly lower than the C-Br binding energy
in IndBr. Despite their lower accuracy, the data show that we
can exclude any influence of dissociative photoionization below
9 eV for both radicals.
The IEs of both phenylpropargyl radicals have also been
calculated by Reilly et al.13 They also used the B3LYP functional
but a smaller basis set and calculated for both phenylpropargyl
species an IEvert of about 6.8 eV. Our calculations yielded
slightly higher values that agree well with the experimental ones
(see Table 1). Again, the HOMO is distributed over the complete
radical in both species, a common observation for resonance-
stabilized radicals. The difference in the adiabatic and vertical
IEs in 3PPR is the most pronounced for any of the C9H7 species,
in agreement with the experiment, but the computed geometry
change is also small. For more information on the computations,
the reader is referred to the Supporting Information. TDDFT
calculations of both phenylpropargyl cations yield no excited
electronic states in the experimentally studied energy range.
Again, we also studied the dissociative photoionization, but no
evidence of this process in 3-phenylpropargyl was observed up
to a photon energy of 15 eV.
We also determined the ionization energies of the 3PPR and
1PPR radicals. As for the indenyl radical, we turned the
pyrolysis on and monitored the ion and threshold electron
intensity as a function of the photon energy. In Figure 7, the
TPE spectrum of 3PPR is depicted, and the ion yield is shown
as an inset in the figure.
We assume that the part of the spectrum between 6.9 and
7.2 eV corresponds to hot bands or sequence bands of the
radical. At around 7.2 eV, the signal increases strongly. We
assign this onset to the adiabatic ionization energy, IEad ) 7.20
eV. From the rising edge of the signal, we estimate an error of
about 50 meV. The TPE signal maximizes at 7.25 eV and stays
roughly constant from thereon, which indicates unresolved low-
energy vibrational modes, corresponding to a moderate geometry
change. The ion signal of 3PPR is given as an inset in Figure
7. One also finds a signal onset at 7.2 eV for the photoionization
process, using the same fitting procedure as that described above
for the precursor.41,42 Thus, the ionization energies obtained from
the total ion signal and the threshold electron signal are in
agreement.
Conclusion
We investigated the photoionization of three resonance-
stabilized radicals, 1-indenyl (Ind), 1-phenylpropargyl (1PPR),
and 3-phenylpropargyl (3PPR), using VUV synchrotron radia-
tion. Threshold photoelectron photoion coincidence, photoion
yield, and threshold photoelectron spectroscopy were applied.
All three are isomers with the same molecular formula, C9H7.
They have been generated cleanly by jet flash pyrolysis from
their bromine precursors as demonstrated by time-of-flight mass
spectrometry. Our main focus was to determine the ionization
energies of these radicals. We determined an adiabatic ionization
energy of IEad ) 7.53 eV for indenyl. The TPES is dominated
by one strong peak, indicating a negligible geometry change.
This is also confirmed by computations. Another band at around
8.1 eV is assigned to the a+ 3B2 excited electronic state of the
cation based on comparison with computations. It shows a
vibrational progression of around 61 meV. The computed
ionization energies agree well with the measured ones (IEad,calc
) 7.50 eV; IEvert,calc ) 7.54 eV). For the phenylpropargyl
radicals, the IEad could also be determined. 3PPR and 1PPR
are ionized at 7.20 and 7.4 eV, respectively. The different
ionization energies determined for the three isomers rule out
isomerization in the pyrolysis nozzle. No excited electronic
states of the cation were found in the observed energy range.
Our calculations also indicate a small change in geometry during
ionization. However, experimental data suggest some vibrational
activity. The possibility to distinguish isomers of the same
We also measured the photoionization of 1PPR, but due to
the lower vapor pressure of the precursor and a lower photon
flux, only a photoion yield spectrum could be obtained. In these
experiments, a MgF2 window was used to cut off higher
harmonics instead of a gas filter. The spectrum is given in Figure
8. It shows that the ion signal starts at about 7.3 eV and increases
over the whole energy range. A fit of the energy range from
7.25 to 8 eV gives an adiabatic ionization energy of IEad )7.4
eV.41,42 Although the error is larger than that for the other two
radicals and estimated to be 100 meV, the IE is definitely higher
than that of 3PPR. The experimental IE also deviates more from
the computed one than in the case of both Ind and 3PPR. In
their MPI experiments, Reilly et al. placed the IE of 1PPR
between 5.1 and 7.3 eV, lower than the present value. On the
one hand, the error bars of the IE extracted from our 1PPR
spectra are quite large, and the value presented here is thus
consistent with the upper limit of this range. On the other hand,

Photoionization of Three Isomers of the C9H7 Radical
J. Phys. Chem. A, Vol. 114, No. 14, 2010 4703
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molecular formula by photoionization was shown to be relevant
for online monitoring of radicals in flames.6,7 The difference in
the ionization energy between 3-phenylpropargyl and 1-indenyl
is about 0.3 eV, and hence, a differentiation of these two isomers
in flames should be possible, while a selective identification of
1PPR might be more difficult. No dissociative photoionization
was found for Ind and 3PPR up to 15 eV. We also investigated
the photoionization and dissociative photoionization of the
precursors because they dissociatively photoionize by Br-atom
loss under the influence of VUV light and can also produce
+
C9H7 species. Appearance energies AE(C9H7Br,C9H7+) were
determined for all three precursors. For Ind+ from IndBr, we
obtained AE0K ) 10.2 eV from an analysis of the breakdown
diagram and an ionization energy (IE) of 8.4 eV for the IndBr
precursor from the TPES. Approximate appearance energies
were also determined for the phenylpropargyl precursors. For
the formation of 3PP+ from 3PPBr, we obtained a value of 9.8
eV, and for 1PP+ from 1PPBr, we obtained 9.3 eV. Approximate
ionization energies of 8.5 (1PPBr) and 8.6 eV (3PPBr) were
determined for the precursors.
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Acknowledgment. This work was financially supported by
the Deutsche Forschungsgemeinschaft (Fi575/7-1). Travel sub-
sidies for P.H. were provided by the European Commission
program “Transnational access to research infrastructures”. We
also thank Bastian Noller and Wolfgang Roth for many useful
discussions and Christian Alcaraz for his contributions to
preliminary experiments on C9H7. This work was also funded
by the Swiss Federal Office of Energy (BFE Contract Number
101969/152433). This paper is dedicated to Prof. J. Weis for
his contributions to Silicon chemistry on the occasion of his
65th birthday.
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Supporting Information Available: Optimized geometries
of the C9H7 radicals and cations and full ref 37. This material
is available free of charge via the Internet at http://pubs.acs.org.
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