=Paper=
{{Paper
|id=Vol-452/paper-5
|storemode=property
|title=Swiss Light Source VUV beamline, Imaging photoelectron photo-ion coincidence spectroscopy
|pdfUrl=https://ceur-ws.org/Vol-452/paper5.pdf
|volume=Vol-452
}}
==Swiss Light Source VUV beamline, Imaging photoelectron photo-ion coincidence spectroscopy==
https://ceur-ws.org/Vol-452/paper5.pdf
Swiss Light Source
VUV beamline, Imaging photoelectron photoion coincidence spectroscopy
A. Bodi, M. Johnson, T. Gerber*
Paul Scherrer Institut, Molecular Dynamics,
5232 Villigen/PSI, CH
An imaging photoelectron photoion coincidence spectrometer at the vacuum ultraviolet (VUV) beamline of the Swiss
Light Source is presented and a few initial measurements are reported. Monochromatic synchrotron VUV radiation
ionizes the cooled or ambient temperature gas-phase sample. Photoelectrons are velocity focused, with a resolution
better than 1 meV in order to detect threshold electrons. The electron detection also starts the counter for the time-of-
flight analysis of the associated ion. The ions are accelerated in a relatively low, 40 – 80 V/cm field, which enables
3 7
the direct measurement of rate constants in the 10 – 10 Hz range. All electron and ion events are recorded in a
triggerless multiple-start/multiple-stop protocol enabling coincidence measurements at 100 kHz event frequencies.
Introduction disk to the appropriate position. The valve is very
The VUV beamline at the Swiss Light Source slim (~ 4 cm) and thereby minimizes the distance
had first light in fall, 2007. After the initial experi- between the beam skimmer and the experimental
ments and preliminary adjustment of the optical interaction region.
elements, the first experimental station has been The sample is ionized by the incident VUV radi-
built, brought to operation and has been used in ation from the X04DB bending magnet port of the
different configurations. Since fall 2008 the VUV SLS. In the imaging photoelectron photoion coinci-
beamline changed its operation status normal and dence setup (iPEPICO), the photoelectrons are
accepts users performing their projects [1]. energy- and the photoions are mass-analyzed, by
The main components (Fig.1) of the experimen- extraction into a flight tube. Velocity map imaging
tal chamber are the vacuum system, a continuous of photoelectrons is achieved by imaging the elec-
molecular beam source, a slim “clockwork” valve, trons onto a 40 mm diameter position sensitive
velocity map imaging electron optics, a time-of- MCP (micro channel plate) detector. Electrons with
flight analyzer for ion mass analysis, detector elec- the same absolute momentum perpendicular to the
tronics and data acquisition software. flight axis arrive on the detector in concentric cir-
The vacuum system consists of four high va- cles. After a convolution the kinetic electron energy
cuum pumps and two Alcatel ACP Roots blower can be retrieved to study the dissociation pattern of
oil-free forepumps in order to maintain a back- internal energy selected ions. With the kinetic
–4
ground pressure of < 5·10 mbar in the molecular energy of electrons and the energy of the monoch-
beam source chamber. romatized ionizing photons, the full energy balance
The molecular beam source consists of a of a dissociation process can be established if also
10 µm–100 µm diameter heatable nozzle or a the ionization threshold is either known or if it can
compound pyrolytical source. The beam of mole- be derived from an additional measurement.
cules passes from the nozzle into the experimental The ions are accelerated in slow fields to
chamber through a valve comprising six ports ar- measure rate constants of slow reactions directly.
ranged on a disk. Different skimmers can be Their mass-analysis is carried out in a two-stage
brought into the beam axis by turning the valve linear time-of-flight tube. The electron and ion op-
tics were designed and constructed at PSI. A new
data acquisition program, basing on a correlation
of electron and ion event counting [2], has been
installed including a graphical user interface and
scripting possibilities for automated data acquisi-
tion and analysis. The electronics and the software
enable a triggerless multi-start/multi-stop setup for
two-particle coincidence experiments.
The iPEPICO endstation is connected to an 8-
stage differentially pumped gas filter, which acts as
a “gas stream” window [3]. Despite the direct con-
–9
nection between the synchrotron storage ring (10
mbar ultrahigh vacuum), it is now possible to have
up to 40 mbar sample pressure in the experimental
Fig. 1: Experimental setup chamber without any danger of contamination of
* Corresponding author: thomas.gerber@psi.ch
Towards Clean Diesel Engines, TCDE 2009
vacuum parts. Absorption and VUV fluorescence varied. As shown in the insert of Fig. 3, the statis-
measurements are thus readily possible with little tical goodness of fitting the measured TOF distri-
or no system modification. butions did not vary with the assumed tempera-
Results
The iPEPICO setup was tested on methane
molecule that dissociates rapidly on the time scale
of the ion TOF. Both effusive and molecular beam
sample sources were used. The room temperature
breakdown diagram, which is a plot of the fraction-
al abundance of the parent and fragment ion sig-
nals as a function of the photon energy, is shown
in Fig. 1. The solid line through the points is the
expected breakdown diagram assuming all three
rotational degrees of freedom of methane are
available for the dissociation process. The derived
0 K onset, which corresponds to the photon energy
when the complete methane thermal energy distri-
Fig.3. Breakdown diagram of chlorobenzene in a
bution is above the methane ion dissociation limit, molecular beam. The insert shows that the goodness
was found to be 14.319 ± 0.003 eV. A similar expe- of the fit to the measured TOF signal is independent of
riment with a cooled CH4 yielded an onset of the assumed temperature (short dash). But, both the
14.321 ± 0.001 eV. This is slightly below the value goodness of the breakdown diagram (long dashed)
and the barrier height (solid line) gets worse with in-
creasing temperature (indicated by increasing depar-
ture from zero). Based on the goodness of the break-
down diagram fit, 125 K is an upper limit to the tem-
perature, resulting to a 1 kJ mol−1 uncertainty in the
barrier height.
ture. However, the fit to the breakdown diagram
gets worse at temperatures above 125 K. This
suggests that the sample temperature is below 125
K, a temperature at which the sample contains
only 2.1 kJ/mol (22 meV) of rovibrational energy,
compared to a room temperature average energy
Fig.2. Breakdown diagram of thermal methane. Empty of 104 meV. The insensitivity of the breakdown
circles correspond to measured ion abundances, the diagram to temperature below 125 K is because
continuous curves are modeled taking into account the the major broadening of the breakdown diagram is
thermal energy distribution at 298 K and that all me- a result of the slow dissociation rates. In view of
thane ions above the barrier lose a H-atom instanta- previously published studies on the cooling of vi-
neously. brations in continuous molecular beams [6], a tem-
perature around 100 K is not unreasonable, and
the total 0–125 K temperature range only trans-
reported by Weitzel et al. of 14.323 ± 0.001 eV [4].
The slowly descending parent ion signal and the
corresponding ascending methyl ion signal is a
result of the thermal rotational distribution of the
sample at 298 K.
Stevens et al. [5] measured and modeled the
dissociation rates of halogen atom loss from halo-
benzenes over a wide internal energy range. We
used 2% C6H5Cl seeded in 1 bar Ar with a 100 µm
nozzle in the molecular beam source to reproduce
their results and measure the temperature of the
sample in the molecular beam [1]. The optimized
breakdown diagram, assuming a sample tempera-
ture of 100 K and yielding a best fit RRKM barrier
of 3.235 eV, is shown in Fig. 3. The breakdown Fig. 4: Measured (○ ) and modeled (▬) daughter ion
diagram and the TOF distributions of daughter ions TOF distributions at h = 13.313, 13.399, 13.480,
were fit using the reported k(E) function. Only the 13.529, 13.649 and 13.698 eV (from bottom to top) for
internal temperature of chlorobenzene in the mole- chlorobenzene.
cular beam and the barrier to dissociation were
lates into 1 kJ/mol uncertainty in the barrier height. References
The measured and calculated TOF distributions
(see Fig.4) agree very well, which corroborates the 1. Bodi A., J.M., Gerber T., Gengeliczki Z., Sztaray
reported k(E) [5] curve as well as our ability to B., Baer T., Imaging photoelectron photoion coinci-
measure slow dissociation rates. dence spectroscopy with velocity focusing electron
optics. Review of Scientific Instruments, 2009.
The imaging PEPICO, originally proposed by 80(3): p. 034101.
Sztáray and Baer [7] in 2003 has been designed 2. Andras Bodi, B.S., Tomas Baer, Melanie Johnson
and built at the Paul Scherrer Institut and is now and Thomas Gerber, Data acquisition schemes for
operational at the VUV beamline of the Swiss Light continuous two-particle time-of-flight coincidence
Source. The main benefits of the experiment in experiments. Rev. of Sci. Instrum., 2007. 78(8): p.
comparison with the one described in [7] include a 084102.
1 meV electron kinetic energy resolution in a conti- 3. Johnson, M., A. Bodi, L. Schulz, and T. Gerber,
nuous experiment, simultaneous electron kinetic New vacuum ultraviolet beamline at the Swiss Light
energy analysis in the 0–800 meV range, the use Source for chemical dynamics studies. in prepara-
tion, 2009.
of tunable synchrotron radiation up to 30 eV pho- 4. Weitzel, K.-M., M. Malow, G.K. Jarvis, T. Baer, Y.
ton energy, and a large throughput pump system Song, and C.Y. Ng, High-resolution pulsed field io-
to allow for a continuous supersonic molecular nization photoelectron--photoion coincidence study
beam. Successful applications of the iPEPICO of CH4: Accurate 0 K dissociation threshold for
+
setup could be demonstrated with the determina- CH3 . The Journal of Chemical Physics, 1999.
tion of accurate appearance energies H atom loss 111(18): p. 8267-8270.
from methane, and with the measurement of slow 5. Stevens, W., B. Sztaray, N. Shuman, T. Baer, and
photodissociation rate constants in the context of J. Troe, Specific Rate Constants k(E) of the Dis-
dissociative photoionization of chlorobenzene. sociation of the Halobenzene Ions: Analysis by Sta-
tistical Unimolecular Rate Theories. The Journal of
Physical Chemistry A, 2009. 113(3): p. 573-582.
6. Mayer, P.M. and T. Baer, A photoionization study
of vibrational cooling in molecular beams. Interna-
tional Journal of Mass Spectrometry and Ion
Processes, 1996. 156(3): p. 133-139.
7. Sztaray, B. and T. Baer, Suppression of hot elec-
trons in threshold photoelectron photoion coinci-
dence spectroscopy using velocity focusing optics.
Review of Scientific Instruments, 2003. 74(8): p.
3763-3768.