An overview of projects in gasphase chemistry at Birmingham University

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An overview of projects in gas-phase chemistry
at Birmingham University (Bham-Leicester-Warw
ick meeting, 3.3.05) Drs Chris Mayhew and
Richard Tuckett   Dr Viktor Mikhailov Michael
Parkes Sahangir Ali Margaret OHara Shane
OHehir Matt Simpson, Jason Butler EPSRC,
CCLRC, NERC, RSC (Analytical Division), Wellcome
Trust.
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Research Interests of Richard Tuckett (Chemistry)
and Chris Mayhew (Physics) Interaction of
ionising species with gas-phase molecules.
Tunable vacuum-UV radiation, cations, low-energy
electrons, and protons. Applied aspects
Atmospheric chemistry (e.g. CF3SF5) and plasma
chemistry (e.g. CCl4). Pure aspects When do
molecules (or ions) fragment according to
statistical rules (large species), and when do
non-statistical rules operate (small
species). Tunable vacuuum-UV studies
(Daresbury SRS, Berlin Bessy, France SuperAco
) Uses tunable photons in the range 50-200 nm,
resolution ca. 0.1 nm (vibrational only).
Vacuum-UV absorption spectroscopy MXn hn ?
(MX)n Vacuum-UV fluorescence excitation
spectroscopy dispersed studies with ccd
detectors Threshold photoelectron
spectroscopy MXn hn ? (MXn)() e- ?
MXn-1 X e- Threshold photoelectron
photoion coincidence spectroscopy. KE releases
?mechanism. Selected Ion Flow Tube studies
Reactions of cations and anions with target
molecules. Kinetics and product states.
Mechanisms of Charge Transfer. Low-energy
electron attachment in thermal and non-thermal
SWARMs (Birmingham), and under molecular beam
conditions (Innsbruck). MXn e- ? (MXn)- ?
MXn-1- X ______________________________ Ion-Pai
r formation in molecular photodissociation
MXn ? MXn-1 X- Proton transfer studies
MXn H3O ? MXnH H2O Medical
applications breath analysis
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1. Absorption spectroscopy (Spectroscopy)
Scan hn1. Detect absorption of vacuum-UV
photons. Gives information on the primary
VUV photoexcitation. 2. Fluorescence
excitation spectroscopy (Spectroscopy) Scan
hn1. Detect undispersed fluorescence. Gives
information on the primary VUV photoexcitation,
using fluorescence as a 'sensor' of VUV
absorption. 3. Action spectroscopy
(Dynamics) Scan hn1, but define hn2. Gives
the excitation spectrum of MX4 leading to
fluorescence in a particular fragment. 4.
Dispersed emission spectroscopy
(Spectroscopy) Fix hn1, scan hn2. Gives
low-resolution information on the
electronic spectroscopy of MX3, MX2, MX, M etc.
or MX4. 5. Radiative lifetime measurements
(Dynamics) Fix both hn1 and l2. Pulse the
VUV photoexcitation source. Record decay of
fluorescence in 'real time'. Information on k1
and k2.
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Fluorescence excitation spectrum of Rydberg
states of PF3 ? He(I) photoelectrum spectrum
of PF3 J. Chem. Phys. (1998) 108 857
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Multi-purpose coincidence apparatus (Daresbury)
Paul Hatherly (Reading University) Meas Sci Tech
(1992) 3 891
MXn hn ? (MXn)() e- (MXn)() ? (MXn-1)
X Measure a zero-energy (threshold)
photoelectron (START) in coincidence with an ion
of known time-of-flight (STOP), all as a function
of hn. High-time-resolution analysis of the TOF
mass spectrum gives the translational KE released
into the two fragments. Can inform on the
dynamics of the state-selected dissociation.
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SF5CF3 atmospheric background

• ? Sturges et al. Science (2000) 289, 611
  report observation of SF5CF3 in the Antarctic.
  Believed to be anthropogenic.
• ? IR absorption measurements (Wallington) show
  it has the highest radiative forcing per molecule
  of any greenhouse gas, 0.57 W m-1 ppb-1.
• ? Current concentrations are low (0.12 ppt), but
  growing at 6 per annum.
• Stratospheric profiles suggest it is long-lived.
  The value of the SF5-CF3 bond strength is
  important to determine if this greenhouse gas can
  be photolysed in the stratosphere.
• Or, is the sink route determined by ionic
  processes in the mesosphere ?
• ? Does SF5CF3 behave as a perturbed SF6, a
  perturbed CF4, or neither ?
• ? Measure DrHo0 (SF5CF3 ? CF3 SF5 e-) using
  TPEPICO spectroscopy, as a route to determine the
  S-C bond strength.
• DIE (SF5CF3) Do0(SF5?CF3) Adiabatic IE
  (CF3)

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Dissociative Ionisation Energy (DIE) of AB-CD
(SF5-CF3) definition
ABCD
Eavail
AB CD e-
DIE
hn Ethermal
AIE (AB)
AB CD
Do0 (AB-CD)
ABCD
0
R (AB-CD)
 
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Results for CF3 / SF5CF3 Do0(SF5?CF3) 3.86 ?
0.45 eV DfHo0(SF5CF3) -1750 ? 50 kJ mol-1

• No parent ion is observed SF5CF3 behaves as
  CF4 and SF6.
• ? Single extropolation over the complete
  Franck-Condon region of the X state of SF5CF3
  yields
• DIE (SF5CF3 ? CF3 SF5 e-)
• 12.9 ? 0.4 eV.
• SF5CF3 is not broken down in the stratosphere by
  UV photolysis. It is probably removed by
  reactions with cations, by electron attachment
  (to SF5-), or by vacuum-UV photodissociation with
  Lyman-a (10.2 eV) radiation in the mesosphere.
• J. Phys. Chem. A., (2001) 105, 8403

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Sink routes for SF5CF3 in the mesosphere (h gt 60
km) No excited states of SF5CF3 below 8 eV (l gt
150 nm). Therefore, SF5CF3 is not broken down in
stratosphere by UV or VUV photolysis. It must be
removed by reactions with small cations or
electrons, or by Lyman-a photodissociation, in
the mesosphere such reactions must also produce
a fragment which is not re-cycled. Rate
SF5CF3.(Skionion kee- s121.6J121.6)
molecules cm-3 s-1 ( . ) is the sum of
pseudo-first-order rate constants. Need to know
rate constants, concentrations of ions and
electrons, and the solar flux in the mesosphere.
Also, ideally need to know the products of these
reactions to see whether they remove SF5CF3 from
the atmosphere. Measurement of the products of
ion reactions is much more facile than of neutral
reactions.
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Selected Ion Flow Tube (SIFT)
Determines rate constants and product ions for
the reactions A or A- B ? C or C- D 2k
have to be faster than ca. 10-12 cm3 molecule-1
s-1
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Typical results from SIFT experiments on
SF5CF3 Phys Chem Chem Phys (2002) 4 2206

1 ?fHo0(SF4CF3) is unknown.
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Vacuum-UV absorption apparatus (Bessy I, Berlin)
Measure cross-sections in the range 10-19 to
10-16 cm2 Wavelength range 40 200 nm,
resolution lt 0.1 nm
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Vacuum-UV absorption spectrum of SF5CF3
(Chem. Phys. Letts., (2003) 367, 697)
photon resolution 0.08 nm s (121.6 nm) 1.3 ?
0.2 x 10-17 cm2
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Glass Cylinder
10 M? Resistors
63Ni Source
Collector
Gate
1 Bar Buffer Sample Flow
To MS
Forward Flow
Jarvis et al. Int. J. Mass Spectrom. Ion
Processes 205 (2001) 255-272
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Kennedy et al., Int. J. Mass Spectrom., (2001)
206, vii-x
kexp(298 K) (7.7 ? 0.6) ? 10-8 cm3 s-1
s-wave capture gives kth(298 K) 3.2 ? 10-7
cm3 s-1
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• Lifetime of SF5-CF3 in the earths atmosphere
• ? No reaction with OH or O (1D). The S-C bond
  too strong to be photolysed by UV radiation in
  stratosphere (200-300 nm) or troposphere (l gt 300
  nm).
• ? SF5CF3 most likely to be removed in the
  mesosphere (h gt 60 km) by reactions with photons
  (especially Lyman-a), electrons or ions.
• ? Rate constants and products from reactions of
  SF5CF3 with small cations (e.g. N, N2, O, O2,
  NO) have been measured in a Selected Ion Flow
  Tube at 298 K. But concentrations of cations
  are too low in the mesosphere.
• SF6 SF5CF3
• k (e- attachment) / cm3 s-1 2.3 x 10-7 7.7 x
  10-8 (i.e. 3 times slower)
• s (121.6 nm) / cm2 1.76 x 10-18 1.3 x 10-17
  (i.e. 7.4 times bigger)
• __________________________________________________
  ___________________
• ? Fractional loss of SF6 by electrons vs.
  photons 1998 / 1 Morris J Geophys Res D
  (1995) 100 1287. Therefore loss of SF5CF3 by
  electrons vs. photons 91 / 1.
• But lifetime still dominated by global
  climate that transports SF5CF3 to the mesosphere.
  Typically, t ca. 800-1000 years.
• Chem. Phys. Letts., (2003) 367, 697 Adv.
  Fluorine Science (2005) in press