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PHOTOFRAGMENT TRANSLATIONAL SPECTROSCOPY H Rydberg atom PTS: pyrrole

PHOTOFRAGMENT TRANSLATIONAL SPECTROSCOPY H Rydberg atom PTS: pyrrole Imaging 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

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PHOTOFRAGMENT TRANSLATIONAL SPECTROSCOPY H Rydberg atom PTS: pyrrole

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  1. PHOTOFRAGMENT TRANSLATIONAL • SPECTROSCOPY • H Rydberg atom PTS: pyrrole • Imaging 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 Institute of Chemical Physics Symposium on Molecular Dynamics, • Dalian, China, 21-23 July 2004

  2. H Rydberg atom photofragment translational spectroscopy Jet-cooled sample of hydride molecules, RAH, absorb photons of energy Ephot and subsequently fragment to yield H (or D) atoms and a radical co-fragment RA. 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.

  3. Experimental • 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.

  4. Rydberg tagging • 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.

  5. 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 • Ephot = D0 (RAH) + TKER + Eint(RA) •  information on the • - internal (electronic, vibrational, rotational) energy states of RA, • - population distribution within these product states, and • - strength of the dissociating bond, D0(RAH). • Angular distributions obtained by rotating εphot relative to TOF axis.

  6. 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. • The next slide shows TKER spectra of the H + C2H product yield following photolysis of jet-cooled C2H2 molecules at (a) 211.75 nm and (b) 211.51 nm. The obvious structure in the TKER spectrum reflects the relative probability of forming C2H radicals in different vibrational (v) and rotational (N) states.

  7. 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(HCCH) = 46074 ± 8 cm-1. • Identification of energy barrier in HCCH 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.

  8. Pyrrole Near UV photolysis of pyrrole • 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 • of * and * excited states in • such molecules. • Previous studies: • Wei, ….. Temps, PCCP 5 315 (2003); • Velocity map ion imaging study of H • atoms formed following excitation at • 243.1nm and at 217nm. Observed • fast, anisotropic distribution • (β = 0.37 ± 0.05) and a slow, isotropic • distribution of recoiling H atoms.

  9. Previous studies of pyrrole photochemistry, II H HCN Blank, …. Y.T. Lee, Chem. Phys., 187 35 (1994) λ = 193 nm NH HCN Only H atom loss observed at λ = 248 nm.

  10. Illustrative TKER spectrum at long phot (244 nm) v9 (a2) v20 (b1) Intensity Combination bands Observed TKERs of v=0 peaks at various phot gives: D0(C4H4NH) = 32850 ± 40 cm-1 f H0o(C4H4N) = 301.9 ± 0.5 kJ mol-1 v=0 TKER / cm-1 Vibrational frequencies of ground state pyrrole and the pyrrolyl fragment predicted from ab initio calculation.

  11. Angular distributions of H + pyrrolyl(v) dissociation channels at 244nm +2 -1 v9(a2) = 1, parallel +1  • 54.7o Magic Angle • Normalised 0o (Parallel) • Normalised 90o (Perp) •  parameter (Right axis) -1 v=0, perp

  12. Illustrative TKER spectrum at short phot (210.7nm) • 54.7o Magic Angle • Normalised 0o Parallel • Normalised 90o Perpendicular •  parameter (Right axis)  fast, anisotropic slow, isotropic

  13. Pyrrole absorption spectrum and assignments 1B1X1A1 (3px(Ryd)1a2(π)) 1B2X1A1 (3b1(π*)1a2(π)) 1A2X1A1 (3s(σ*) 1a2(π)) Roos, Malmqvist and Molina, JCP116 7526 (2002)

  14. Relevant potentials (schematic) • 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. • 1B2 1A2 coupling mediated by out of plane (b1) vibrations. Sobolewski, Domcke, Chem. Phys., 259 181 (2000)

  15. Interpretation • At long wavelengths (≥ 230nm) • 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. • Efficient vibronic coupling to 1A2 PES and dissociation as at longer wavelengths, yielding an anisotropic distribution (β ~ -0.5) of fast H atoms. • Coupling to ground state potential and subsequent unimolecular decay to yield a slow, isotropic distribution of H atoms (+ cyanoallyl co-fragments) and alternative dissociation products.

  16. Imaging the photolysis of molecular ions Traditional imaging of neutral fragments Imaging dissociation of a molecular ion

  17. Br2 and BrCl as suitable demonstration systems • 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 • state has valence contamination, or • 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.

  18. Experimental See E. Wrede et al., JCP 2001, 114, 2629; E.R. Wouters et al. JCP 2002, 117, 2087.

  19. Excitation spectra for forming parent ions Excitation spectraTOF spectra 2+1 REMPI via Br2 [1/2] 4d; 0g(v = 1)  X; 0g(v= 0) transition

  20. Images of 79Br+ fragment ions from 79Br2+(21/2,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).

  21. 79Br+ fragment ion images as a function of phot • Analysis of many such images gives: • D0(BrBr+, 21/2, v+=1, N ~ 5) = 23160.4  0.6 cm-1 • D00(BrBr+, 23/2, v+=0, N = 0) = 26345  2 cm-1, and A = 2817  3 cm-1for Br2+ (2g). [M. Beckert et al, PCCP (2003), 5, 308]

  22. Recoil anisotropy parameter as a function of phot • 2+1 REMPI at 274.214 nm to form • Br2+(23/2,v=0) parent ions. • Monitor Br+(3P2) + Br(2P3/2) products • Angular anisotropy seen to vary with phot D0[Br-Br+(3P2)] 27260 cm-1 27194 cm-1 • Resonance structure evident in energy region between first and second dissociation limits. • Similar observations when exciting from 23/2,v=1, and from 21/2 spin-orbit state. D0[Br-Br+(3P1)]

  23. 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 gug*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.

  24. Interpretation, II ’ = ½ potentials Br+ + Br* Br+ + Br Adiabatic Diabatic • Excitation from X23/2 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. • (Vieuxmaire et al, PCCP (2004), 6, 543).

  25. BrCl and BrCl+

  26. Preparation of BrCl+ parent ions. (2+1) REMPI mass spectrum • No documented BrCl REMPI transitions. • One photon absorption spectrum  known • Rydberg absorptions to a5 (v’, v’’): [(2P3/2)5ss] • and b5 (v’, v’’): [(2P1/2)5ss]states. Br+ Cl+ BrCl+ Intensity / arb. units m / z [A. Hopkirk et al. ; J. Phys. Chem. ; (1989), 93, 7338] • Photoelectron imaging at 324.6 nm confirms high • degree of vibrational and spin-orbit state selectivity (with Parker).

  27. One colour photolysis of BrCl+: Process (a) hn1 = 30807.6 cm-1 79Br+ image

  28. Two colour photolysis of BrCl+: Process (b) Dissociation channels observed in range 23000 - 27000 cm-1: (i) BrCl+(2P1/2)  Br+(3P2) + Cl(2P3/2) (ii) BrCl+(2P1/2)  Br+(3P2) + Cl(2P1/2) (iii) BrCl+(2P3/2) Br+(3P2) + Cl(2P3/2) (iv) BrCl+(2P3/2) Br+(3P2) + Cl(2P1/2) (v) BrCl+(2P1/2)  Br+(3P1) + Cl(2P3/2) Intensity / arb. units

  29. Spectroscopy and thermochemistry of BrCl+ Extrapolate each product channel to the respective threshold for zero kinetic energy particles  D0[Br-Cl+(X23/2)] = 25019 ± 4 cm-1; D0[Br-Cl+(X21/2)] = 22949 ± 2 cm-1 and A = 2070.4 ± 4 cm-1 IP(BrCl) = D0(BrCl) + IP(Br) – D0(BrCl+) • D0(BrCl) and IP(Br) known, • IPadiabatic for forming BrCl+: X2P3/2: 88292 ± 6 cm-1 X2P1/2: 90362 ± 4 cm-1

  30. Photofragment angular distributions. • One photon dissociation • BrCl(X1S+) + hn • Br(2P3/2) + Cl(2P3/2) • Image Br by 2+1 REMPI b2 = 1.79 ± 0.06 • 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 b4 = -0.93 ± 0.06 b6 = -1.07 ± 0.06

  31. Acknowledgements Andrew Orr-Ewing, Colin Western, Keith Rosser, Richard Dixon H (Rydberg) PTS: Emma Feltham, Rafay Qadiri, Mike Nix, Bríd Cronin Ion imaging: Eckart Wrede, Eloy Wouters, Hendrik Nahler, Marco Beckert, Olivier Vieuxmaire, Josephine Jones Dave Parker, Andre Eppink,Marcela Coroiu (Nijmegen, BrCl PE imaging) Funding: EPSRC portfolio partnership LASER EU TMR Network IMAGINE, EU IHP Network PICNIC Leverhulme Trust

  32. 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 CH bonds, • H2CCCH2 + h H2CCCH + H (1) D0 ~ 30000 cm-1. • Propyne contains two types of CH bond, with different strengths: • H3CCCH + h H2CCCH + H (2) D0 ~ 30000 cm-1. • H3CCCH + h H3CCC + H (3) D0 ~ 45000 cm-1.

  33. Near UV photolysis of allene and propyne • 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, JCP95, 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. • - Jackson, .., Lee, JCP95, 7327 (1991): H2CCCH2+ 193 nm. Angle resolved TOF-MS measurements of molecular products. Dominant primary process identified as H atom loss and propargyl radical formation following internal conversion to S0 state.

  34. Near UV photolysis of allene and propyne • Ni, .., Jackson, JCP110, 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 CH bond fission in excited state of propyne. • Sun, …, Neumark, JCP110, 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 CH bond fission. • Chen, ..., Rosenwaks, JCP113, 5134 (2000): 243.1 nm photolysis of CD3CCH(vC-H=3) molecules. H and D atoms observed, with very similar (low) kinetic energy releases. • DeSain and Taatjes, JPC A107, 4843 (2003): CH3CCH + 193 nm  Monitor propargyl radical by IR kinetic absorption spectroscopy, time dependence suggests it is a primary product, quantum yield ~ 0.5.

  35. phot = 209.0 nm phot = 193.3 nm 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) TKERmax(1 or 2)

  36. 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. Conclude that, in all cases, electronically excited C3H4 molecules undergo internal conversion (IC) to high vibrational levels of the ground (S0) state, and then isomerise at a rate that is faster than their rate of unimolecular decay.

  37. C3H4 fragmentation channels – a summary

  38. Allene and propyne photolysis at 121.6 nm H from H2CCCH2 D from D3CCCH (TOF rescaled by 2-1/2 to aid comparison) H from H3CCCH H from D3CCCH i.e. do find some selective, direct fission of CH3CCH bond at 121.6 nm. Parallels with results of Yang and co-workers, at 157 nm. [PCCP2, 1187 (2000), JCP112, 6656 (2000)] See Qadiri et al., JCP116, 906 (2002); 119, 12842 (2003).

  39. Present TOF spectra resulting from pyrrole photolysis - at 22 different wavelengths in the range 193 < λ < 254 nm.

  40. Overview of TKER results in the range 216 < λ < 254nm

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