A. Cheesman, D.M.E. Davies, J.A. Smith, J.N. Harvey, Yu.A. Mankelevich1 and M.N.R. Ashfold, Element Six Industrial CASE student School of Chemistry, University of Bristol, Bristol, BS8 1TS 1Nuclear Physics Institute, Moscow State University, 119992 Moscow, RussiaImaging the photolysis of molecular ions
Mike Ashfold
School of Chemistry, University of Bristol, Bristol, U.K. BS8 1TS
http://www.chm.bris.ac.uk/pt/laser/laserhom
Dalian, China, 21-23 July 2004
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H Rydberg atom photofragment translational spectroscopy
Measure time-of-flight (TOF) spectrum of H/D atom products from instant of creation in the interaction region to detector located at a known distance, d.
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Jet-cooled supersonic molecular beam of target hydride, seeded in Ar.
Dissociation initiated by photolysis laser.
H atom photofragments ‘tagged’ ~10 ns later, by Lyman- and 366 nm laser pulses.
Record TOF spectrum of ‘tagged’ H atoms reaching the detector.
Investigate recoil anisotropy (by rotating phot),
and dependence on phot.
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H(D) atom fragments are “tagged”, at source, by two-photon double resonant excitation to a Rydberg state with high principal quantum number, n.
Resulting Rydberg atoms are
neutral, and long-lived.
H atoms that recoil along detection axis are field-ionised immediately prior to detection.
This strategy obviates the blurring
(from space-charge effects) that limits the ultimate resolution of ion tagging methods, and the imprecision in d that limits the KE resolution achieved with universal detection methods.
Rydberg tagging
Measurements and data analysis
d known, so TOF spectrum distribution of H atom velocities, vH,
and thus kinetic energies, Ek(H).
Given vH and mass of RA co-fragment, momentum conservation
enables determination of Ek(RA) and thus the total kinetic energy
release (TKER).
Energy conservation
information on the
- population distribution within these product states, and
- strength of the dissociating bond, D0(RAH).
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Near UV photolysis of acetylene
Photolysis of C2H2, and the subsequent chemistry of C2H radicals, are both important in establishing the hydrocarbon balance in atmospheres of the outer planets and their moons.
Both C2H2 and C2H are important intermediates in combustion processes, and implicated in soot formation and some chemical vapour deposition (CVD) environments.
This example illustrates the exquisite energy resolution that can be obtained using the H (Rydberg) tagging method.
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Acetylene photolysis at (a) 211.75 nm and (b) 211.51 nm
Direct observation of H + C2H(X) products; latter show 2 (bending) vibrational excitation.
Precise determination of bond strength:
D0(HCCH) = 46074 ± 8 cm-1.
Identification of energy barrier in HCCH exit channel, magnitude ~600 cm-1, measured relative to the asymptotic H + C2H(X) products.
Products exhibit quantum state dependent recoil anisotropy.
Rationalise, qualitatively, in terms of S1 Tn intersystem crossing and subsequent dissociation.
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Important molecule in synthesis of biologically
active compounds (e.g. porphyrins, chlorophylls),
pesticides, organic polymers and organometallic magnets.
Major source of fuel nitrogen in coals and heavy oils production of atmospheric NOx contributing to acid rain and smog.
Much current theoretical interest
in relative photochemical behaviour
such molecules.
Previous studies:
Velocity map ion imaging study of H
atoms formed following excitation at
243.1nm and at 217nm. Observed
fast, anisotropic distribution
distribution of recoiling H atoms.
Pyrrole
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Blank, …. Y.T. Lee,
λ = 193 nm
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TKER / cm-1
various phot gives:
pyrrolyl fragment predicted from ab initio calculation.
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54.7o Magic Angle
Normalised 0o (Parallel)
Normalised 90o (Perp)
parameter (Right axis)

1B2X1A1
(3b1(π*)1a2(π))
1B1X1A1
(3px(Ryd)1a2(π))
1A2X1A1
(3s(σ*) 1a2(π))
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Sobolewski, Domcke, Chem. Phys., 259 181 (2000)
1A2 state has dominant Rydberg character at the ground state equilibrium geometry but stretching the
N-H bond leads to Rydberg antibonding valence orbital evolution (like in NH3).
1A2 potential exhibits conical intersection with the ground state potential at large RNH.
1A2 X1A1 transition is vibronically induced.
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Observe vibronically induced excitation to 1A2 state and rapid dissociation through the conical intersection to H + pyrrolyl(X2A2).
Bulk of Eint in pyrrole(1A2), in excess of that required to surmount barrier in N-H dissociation coordinate, is retained as vibrational excitation of pyrrolyl fragment.
Parent promoting mode vibration maps adiabatically through to pyrrolyl fragment. H atom recoil anisotropy depends on pyrrolyl(v) state.
Observed <TKER> (~7000 cm-1) reflects drop in potential energy between the barrier and the dissociation asymptote.
At shorter wavelengths (< 230nm)
Excitation to 1B2 state, followed by two dissociation pathways.
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Traditional imaging
Requirements:
Neutral precursor should show clearly resolved Rydberg vibronic structure in its n+1 REMPI excitation spectrum. 1
Propensity for core-conserving v = 0 transitions from the intermediate Rydberg level should then lead to ion formation in, predominantly, a single, well-defined electronic, spin-orbit and vibrational state. 2
Minimal fragment ion formation 1
such as would occur, for example:
- if parent ions autoionise,
if resonance enhancing Rydberg
as a result of unintended photolysis
of parent ions.
For Br2:
1. R.J. Donovan, et al. Chem. Phys. 1998, 226, 217.
2. B.G. Koenders, C.A. de Lange, et al.,
Chem. Phys. Lett. 1988, 147, 310.
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E.R. Wouters et al. JCP 2002, 117, 2087.
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Excitation spectra TOF spectra
2+1 REMPI via Br2 [1/2] 4d; 0g(v = 1)  X; 0g(v = 0) transition
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Images of 79Br+ fragment ions from 79Br2+(21/2,v=1)
(a) Image obtained by one colour 2+1 REMPI at 263.012 nm, and
(b) by adding a second laser pulse with phot = 26600 cm-1 (tdelay ~ 5 ns).
(c) Difference image obtained by subtracting (a) from (b).
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Analysis of many such images gives:
D0 (BrBr+, 21/2, v+=1, N ~ 5) = 23160.4 0.6 cm-1
D00(BrBr+, 23/2, v+=0, N = 0) = 26345 2 cm-1,
and A = 2817 3 cm-1 for Br2+ (2g).
[M. Beckert et al,
PCCP (2003), 5, 308]
Br2+(23/2,v=0) parent ions.
Monitor Br+(3P2) + Br(2P3/2) products
Angular anisotropy seen to vary with phot
27260 cm-1
27194 cm-1
Resonance structure evident in energy region between first and second dissociation limits.
Similar observations when exciting from 23/2,v=1, and from 21/2 spin-orbit state.
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Interpretation, I
1. MR-CI calculations of adiabatic (spin-orbit averaged) potential energy curves for ground and all possible ungerade excited states of Br2+ associated with gug*u* valence space using a vqz basis.
2. Incorporate spin-orbit effects semi-empirically, diabatize, and propagate wavepackets on coupled states with a common ’. (Richard Dixon)
3. One ’ = ½ state correlates to the spin-orbit excited limit.
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Interpretation, II
Adiabatic Diabatic
Br+ + Br
Br+ + Br*
’ = ½ potentials
Excitation from X23/2 involves parallel absorption to ’ = 3/2 continuum and perpendicular excitation to inner limbs of ’ = 1/2 states I, II and III.
Vibrational levels supported by diabatic potential III above first dissociation threshold can predissociate to ground state limit resonance structure.
Resonance lineshapes – Fano interferences.
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Rydberg absorptions to a5 (v’, v’’): [(2P3/2)5ss]
and b5 (v’, v’’): [(2P1/2)5ss] states.
[A. Hopkirk et al. ; J. Phys. Chem. ; (1989), 93, 7338]
(2+1) REMPI
degree of vibrational and spin-orbit state selectivity (with Parker).
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hn1 = 30807.6 cm-1
Intensity / arb. units
Dissociation channels observed
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Spectroscopy and thermochemistry of BrCl+
Extrapolate each product channel to the respective threshold for zero kinetic energy particles D0[Br-Cl+(X23/2)] = 25019 ± 4 cm-1;
D0[Br-Cl+(X21/2)] = 22949 ± 2 cm-1 and A = 2070.4 ± 4 cm-1
IP(BrCl) =
One colour experiment
at 31818.4 cm-1
Image Cl+ ions
Strikingly different
angular distribution
Resonance enhanced 3 photon dissociation to Cl** and subsequent one photon ionisation.
b2 = 1.79 ± 0.06
b2 = 1.79 ± 0.06
b4 = -0.93 ± 0.06
b6 = -1.07 ± 0.06
H (Rydberg) PTS:
Ion imaging:
Olivier Vieuxmaire, Josephine Jones
(Nijmegen, BrCl PE imaging)
Near UV photolysis of allene and propyne
Two isomers of C3H4 (cyclopropene is another).
Both are important in combustion processes, and are present in interstellar clouds and in the atmospheres of the outer planets.
Allene contains four identical CH bonds,
H2CCCH2 + h H2CCCH + H (1) D0 ~ 30000 cm-1.
Propyne contains two types of CH bond, with different strengths:
H3CCCH + h H2CCCH + H (2) D0 ~ 30000 cm-1.
H3CCCH + h H3CCC + H (3) D0 ~ 45000 cm-1.
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Both molecules can also dissociate by eliminating H2.
Isomerisation on the ground (S0) state PES is known to occur.
Previous photolysis studies give conflicting conclusions:
- Ramsay and Thistlethwaite, Can. J. Phys. 44, 1381 (1966): UV flash photolysis of allene and propyne. Same transient product absorption detected in each case, since shown to be due to propargyl radical, H2CCCH.
- Satyapal and Bersohn, JCP 95, 8004 (1991): CH3CCD + 193 nm Detect D atoms only, by LIF.
- Seki and Okabe, JCP 96, 3345 (1992): CD3CCH/Cl2 + 193 nm HCl but not DCl.
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Near UV photolysis of allene and propyne
Ni, .., Jackson, JCP 110, 3320 (1999): Molecular products from 193 nm photolysis of allene and propyne detected by 118 nm photoionization + TOF-MS. Apparent differences in C3H3/C3H2 product ratios taken as evidence for direct acetylenic CH bond fission in excited state of propyne.
Sun, …, Neumark, JCP 110, 4363 (1999): As Ni et al, but used tunable VUV photoionization. Apparent differences in C3H3 fragment photoionization efficiency curves rationalised by assuming that propyne dissociates by acetylenic CH bond fission.
Chen, ..., Rosenwaks, JCP 113, 5134 (2000): 243.1 nm photolysis of CD3CCH(vC-H=3) molecules. H and D atoms observed, with very similar (low) kinetic energy releases.
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H(R)PTS studies of allene and propyne photolysis
Investigated allene and propyne photolysis at various wavelengths in the range 193.3 - 213.3 nm, and at 121.6 nm, using H2CCCH2, H3CCCH and D3CCCH precursors.
Most of the products are formed with low TKER (i.e. the partner C3H3 products are formed with high levels of internal excitation).
The products show no recoil anisotropy.
Most of the products appear with TKERs that are only compatible with propargyl radical formation, i.e. channel (1) or (2), not (3).
Earlier studies were likely affected by secondary photolysis of the primary C3H3 and C3H2 fragments.
TKERmax(3)
More on propyne photolysis
TKER spectra of H(D) atom products from propyne photolysis at 193.3 nm monitoring:
D atoms from D3CCCH
H atoms from D3CCCH
H atoms from H3CCCH
are all very similar.
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H from H2CCCH2
D from D3CCCH
to aid comparison)
H from H3CCCH
H from D3CCCH
i.e. do find some selective, direct fission of CH3CCH bond at 121.6 nm.
Parallels with results of Yang and co-workers, at 157 nm.
[PCCP 2, 1187 (2000), JCP 112, 6656 (2000)]
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Present TOF spectra resulting from pyrrole photolysis
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Overview of TKER results in the range 216 < λ < 254nm
R
A
H
-10 2000 4000 6000 8000 10000 12000 14000 16000
14.0
16.0
18.0
20.0
24.0
22.0
from excited state ~30%