Postgraduate Conference Poster Template

1
Above Barrier Resonances in HCl
D.J. Brown1, A.J. Yencha2, M.R.F. Siggel-King3,
Sunil Patel3 and G.C.M. King1
1Dept of Physics and Astronomy, Schuster Labs,
University of Manchester, U.K. 2Dept of
Chemistry, State University of New York at
Albany, Albany, U.S.A. 3 CCLRC Daresbury
Laboratories, Warrington UK
1
Introduction Doubly ionised molecular states
(dications) have many current and possible future
applications including lasing media for an XUV
laser to energy storage for hydrogen fuel cells.
Theoretically these systems are hard to
calculate. This is because the properties of the
dication are critically dependent on the height
and width of the existence of a potential
barrier1 as shown in figure 1. Experimentalists
can take motivation from the importance attached
this class of molecule in ionospheric and
interstellar chemistry.
Results The top spectrum in figure 5 shows a
threshold photoelectrons coincidence (TPEsCo)
spectrum of HCl2. The photon energy interval is
20 meV and each point has a cumulative dwell time
of 20 minutes per point. There is clear evidence
of three vibrational states numbered 0,1 and 2 in
the spectrum. The lower spectrum in figure 5 is
the threshold photoelectron spectrum of HCl
obtained simultaneously.
Dication States The potential energy curve of
hydrogen chloride (HCl) is a combination of long
range Coulomb repulsion and molecular bonding
forces. These two effects combine to produce a
local minimum that allows the formation of
discrete vibrational states. The molecule can be
excited from its ground state into a doubly
ionised quasi-bound X(3S-) state within the
Franck-Condon overlap region (see figure 1).
Figure 3 Main line and satellite states in a
threshold photoelectron spectrum of neon.
The Coincidence Technique With both electron
analysers tuned to threshold and both extraction
fields operational it is possible to look for the
two photoelectrons from a double-ionisation
event. In this mode it is important that both
analysers are looking at the same part of the
interaction region. The time between the
detection of one electron (start) and the next
electron (stop) is measured using a Time to
Amplitude Converter (TAC). Events which
contribute to random coincidence counts, occur
randomly in time and lead to a flat background in
the time spectrum as shown in figure 4. However,
true coincidences due to correlated events form a
peak sitting on top of this flat background as
can clearly be seen. The true coincidence count
rate is about 1 Hz and the true to random ratio
is about 101.
Figure 2 Schematic of coincidence spectrometer
Above Barrier Resonances It is well known that
barrier penetration can occur below a potential
barrier via quantum tunnelling. It is also
possible that above the local maximum in the
barrier the outgoing wavepacket must also have a
finite probability of reflection. Thus allowing
vibrational resonances to exist above the coulomb
barrier.
Figure 5 (top) A TPEsCO of the X(3S-) state of
HCl, (bottom) TPE spectrum of HCl.
Experiment A schematic diagram of the apparatus
is presented in figure 2 which shows the
coincidence electron spectrometer. A beam of
hydrogen chloride gas emanates from a molybdenum
capillary. The effusing gas is illuminated with a
beam of synchrotron radiation from beamline MPW
6.1 at the Daresbury SRS. The photon energy of
this beam can be set or ramped over a region of
interest which for this excited state is from
35.4 to 37.2eV. The lenses and extraction
elements of the two energy analysers are tuned to
efficiently collect and transmit only zero energy
(threshold) photoelectrons from the interaction
region. This was done by observing the the
(1D)3s(2D) satellite state in neon which has a
maximum cross-section at its threshold due to the
existence of a nearby resonance state 2. A
typical example of a threshold photoelectron
spectrum (TPES) in neon showing the satellite
state and the mainline is shown in figure 3.
A scan over a narrower range with longer dwell
time of almost 43 minutes per point is presented
in figure 6. The top of the barrier has been
calculated to occur at 36.1 eV 2. The data
suggest that state 4 in the spectrum exists above
the barrier.
Figure 4 A time coincidence spectrum obtained
in HCl.
Figure 6 A TPEsCO spectrum in the region of the
top of the barrier of HCl dication.
Figure 1 The potential energy curve of the X3S-
state of HCl1.
REFERENCES 1 F.R. Bennet et al Molecular
Physics (1999) 97, No.1/2 35-42. 2 R I Hall et
al J Phys B (1991) 24, 4133-4146.
ACKNOWLEDGEMENTS This work is being carried out
with financial support from ESPRC.