Dr' Paul S' Monks

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Photoionisation Mass
Spectrometry Possible applications to problems
in atmospheric chemistry
Dr. Paul S. Monks Department of Chemistry
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Photoionisation mass spectrometry

• A h? ? A e-
• Soft-ionisation
• Relatively low energy photons (3-30eV)
• Monochromatic
• Detect using mass spectrometry
• Specificity
• Sensitivity

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• Photoionization Mass Spectrometry (PIMS)
• The Adiabatic ionization energy (IE) from the
  photoionization threshold
• The heat of formation of the ion from the IE
  and the heat of formation of neutral
•       M ? M e 
•    ?fHo0(M) ?fHo0(M) IE
• Reaction pathways, branching fractions and
  unambiguous product (isomer) identification
•      N C2H5 ? H2CN CH3 (0.4)
•    ? NH C2H4 (0.6)
• The vibrational state and Rydberg state energy
  levels

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Experimental
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Based on the discharge-flow technique
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U11 (NSLS)

• Wide-angle acceptance wadsworth monochromator
• Iridium (water-cooled) mirror (gives 35eV
  cut-off)
• Grating 4m spherical concave
• 1mm spot
• 0.7-2x1013 photons s-1 (1000 mA, 100 mA)

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The Nitrate Radical
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Night-time tropospheric chemistry
NO3     NO2 O3 ? NO3 O2 k(T298K)
3.2x10-17 cm3 molecule-1 s-1 Daytime less
important as photolysed (t ? 5s)
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Salisbury et al, JGR, 106, 12,669-12,688, 2001
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• Ground state symmetry (D3h vs. C2v)
•  
• Conflicting evidence both experimental and
  theoretical as to ground electronic state and
  geometry
• D3h planar geometry (3-fold axis of symmetry)
• C2v one long and two short
• The PES of the two configurations are low lying
  and separated by 0.1-0.33 eV dependent on the
  level of electron correlation used in the
  calculations
• Electron configuration for a 2A2 ground state
• (s1s2s3)6(1a1')(1a2)(3e)4(4e4)(1e)4(1a2)1
•  

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Production of NO3

• Low Pressure (1-10 Torr)
• F2 ? F F (Microwave discharge)
• F HNO3 ? NO3 HF
• k(T298K) 3.0x10-11 cm3 molecule-1 s-1

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Dissociative ionisation

• NO3 from HNO3
• Only ions m/z 63 (Parent) and m/z 46 (NO2)
• No NO or NO3 at ? ? 85 nm (?14.6 eV)
• HNO3 ? NO2 OH
• AE(NO2. HNO3) 12.125eV

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Monks et al, JPC, 98, 10017, 1994.
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? 98.675 ? 0.140 nm IE 12.565 ? 0.020
eV Sharp transition good FC overlap
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NO3 larger IE than predicted by linear
progression Rationalisation delocalisation of
electron on O3 ring around N atom makes more
stable Supported by MP4 calculations
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Cation symmetry D3h by 0.94eV over C2v
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Ground state symmetry (D3h vs. C2v)   ?
Conflicting evidence as to ground electronic
state and geometry  From PIMS the experimental
photoionization threshold exhibits a large, steep
initial step with no evidence of significant
structure  From Theory theoretical evidence is
unambiguous that the cation symmetry is D3h   ?
Ionisation transition is NO3(1A1?) ?
NO3(2A2?)   NO3 is most probably D3h
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BrO

• Important in stratospheric chemistry
• Found in the troposphere from space

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Polar Spring BrO Explosion
Courtesy of John Burrows Univ. Bremen
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Generation of BrO (X2?)

• Br2 O(3P) ? BrO Br
• PES vs PIMS (Dunlavey et al., CPL, 53, 382, 1978)
• Interferences - Br2, O2, O Br and HBr
• Misassigned threshold and ?(0,1) as under Br2 ?
  Br2 band

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Select ion in complex mixtures (Br2O and OBrO)
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Monks et al, CPL, 211, 416, 1993
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IE 10.46 ? 0.02 eV (? 118.54 nm)
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Why so much structure .
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And then

• Dyke et al, (JCP, 112, 6262, 2000) re-measured
  using PES and
• IE(BrO) 10.46 ? 0.02 eV in agreement with PIMS
  work

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Halogen Oxides
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FO from F NO3
Vibrational progression superimposed with
vibrational autoionization (high Rydberg states
that converge with an excited state of the cation)
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I
Zhang et al, JPC, 100, 63, 1996
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Br2O
Thorn et al, JPC, 100, 12199, 1996
OBrO
HOBr Monks et al, JCP, 1902, 1994
Thorn et al, JPC, 103, 8384, 1999
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HOI
Monks et al, JPC, 99, 16566, 1995
IO
Zhang et al, JPC, 100, 63, 1996
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Halogen Oxides
Ref 12 Thorn, R. P., Jr. Stief, L. J. Kuo,
S.C. Klemm, R.B., J. Phys. Chem., 1996, 100,
14178.
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Planetary Atmospheres
Nitrile Compounds
Titan (Voyager Image)
Titans Haze
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CH2CN Cyanomethyl radical

• Formed in
• C2H3 N(4S) ? Products
• Products and isomers of CH2CN and CH3CN
• Thermochemistry complicated
• C2H2N six (radical) isomers
• C2H3N five (molecule) isomers
• Thorn et al., JPCA, 1998, 102, 846

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• Production of cyanomethyl
• F CH3CN ? CH2CN HF
• Production of vinyl
• F C2H4 ? HF C2H3
• ? H C2H3F

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CH2CN IE 10.2810.012 eV
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x 10
F CH3CN
C2H3 N(4S) ? C2H2N H
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Obviously, not all linear ground-state but not
cyclic or NC equivocally - other isomers HCCNH
or H2CCN or HC-CHN unknown IEs
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• For adduct
• C2H3 N(4S) ? C2H3N
• PIMS confirms product is acetonitrile
• Deuterated reaction another story

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Measuring in the atmosphere
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Radicals - day and night
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PTR(CI) -TOF-MS
Developing HT-PTR-MS CI-TOF-MS for atmospheric
measurement applications
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PIMS and the atmosphere

• PIMS is a tool for probing complex mechanisms,
  identifying/ quantifying reaction products and
  channels therefore ideal for atmospheric
  oxidation mechanisms
• NO3 DMS
• Organic complexity VOC/OVOC oxidation
• Understanding radical chemistry
• Specificity and sensitivity a power

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Acknowledgements

• BNL Bruce Klemm, Steven Kuo, Zhengyu Zhang
• NASA (Goddard) Lou Stief, Peyton Thorn