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The role of (n/  )  * states in molecular photodissociation processes Mike Ashfold University of Bristol Kasteel Oud Poelgeest, Leiden 3-5 February 2015.

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Presentation on theme: "The role of (n/  )  * states in molecular photodissociation processes Mike Ashfold University of Bristol Kasteel Oud Poelgeest, Leiden 3-5 February 2015."— Presentation transcript:

1 The role of (n/  )  * states in molecular photodissociation processes Mike Ashfold University of Bristol Kasteel Oud Poelgeest, Leiden 3-5 February Leiden Observatory Workshop: Photodissociation in Astrochemistry

2 Plan for lecture Short introduction to (n/  )  * states O–H/S–H bond fission in H 2 O, H 2 S, alcohols/thiols, etc N–H bond fission in ammonia, amines C–H bond fission in methane, ethyne and HCN Larger molecules? (n/  )  * state mediated ring opening? Conclusions and future prospects

3 Excited state photochemistry: (n/ π)  * excited states HF Archetypal  *  n/  excitation. Repulsive excited state potential. Direct bond fission  H + F atoms

4 H 2 O ( singlet state potentials shown only) OH not spherically symmetric, presents p  and p  orbitals. 1  and 1  potentials cross at linear geometry. 1 A components avoid each other when bent  conical intersection (CI) at  HOH = 180  and extended R O–H. Change in  HOH with O–H bond extension  OH product rotation? conical intersection

5 How to test such predictions? In case of H 2 O: simple triatomic, light atoms, high I.P. Experiment: photofragment translational spectroscopy (PTS) / imaging. Theory: ab initio full-dimensional PESs, propagate wavepackets. What are key wavelengths to study? Experimentalists prefer > 200 nm or = nm, but almost any wavelength is possible if the problem merits it. Absorption cross-sections  ( ; T) generally not available  an issue for light molecules with structured Rydberg regions.

6 Tagging (366nm) Molecular Beam Photolysis “High n” tagging (366nm) Lyman-  (121.6nm) Detector H Rydberg atom PTS {Karl Welge (Bielefeld)} Hydrogen Atom Rydberg State (H*) n=2 n=1 Lyman-  (121.6nm) Cation H*

7 H 2 O + h ( =121.6 nm)  H + OH(X/A, v, N) Yuan et al., PNAS (Mordaunt et al., JCP )  Product recoil anisotropy, electronic branching in products, immune to effects of OH predissociation, confirm massive OH product rotation. (see also Dr Kaijun Yuan presentation, Wed 4 pm).

8 Hydrides (and halides) Similar ideas go a long way to explaining/predicting photoinduced excited state bond fission in all gas phase hydride molecules: H 2 O  CH 3 OH, C 6 H 5 OH, … H 2 S, CH 3 SH, C 6 H 5 SH, …… NH 3  CH 3 NH 2, cyclic amines (pyrroline, morpholine, etc), heterocycles (azoles, indoles, adenine, etc), C 6 H 5 NH 2, ….. HCN, HCCH, etc alkylated analogues (e.g. ethers, thioethers, secondary amines, etc) (PCCP ) families of halides (e.g. hydrogen halides  alkyl halides, aryl halides, halophenols,..) (PCCP ; JCP )

9 H2SH2S Wilson et al., Mol Phys Similarities (but also differences) with H 2 O. I.P.(H 2 S) < I.P.(H 2 O), D 0 (H–SH) < D 0 (H–OH)  observe given photodissociation behaviour at longer in H 2 S. Near UV photolysis  *  3p x (HOMO) continuum spanning nm. H + SH(X) products formed predominantly in v = 0, low N states Anisotropic recoil Similar behaviour to that shown by H 2 O in wavelength range 150 < < 190 nm. = nm

10 H 2 S + h ( =121.6 nm)  H + ? Cook et al., J Chem Phys Excite just below 1 st I.P. – high density of states. Populate (or couple to) second n  * state. Dissociate to H + SH(A) with: v  5 (and low N), and v = 0 with high N. No H + SH(X) products. Dissociating molecules fail to sample relevant CI in R H–SH at linear geometries. 3-body fragmentation  H + H + S. H 2 + S yields? (Mingli Niu presentation, Thurs 11 am)

11 VUV photolysis of alkyl alcohols and thiols? * Yuan et al, Chin. J. Chem. Phys HRA-PTS studies of MeOH, EtOH photolysis at = nm.* Fast H atoms from H–OMe, H–OEt bond fission on n  * PES; slower H atoms attributed to primary C–H bond fission and to secondary decay of vibrationally ‘hot’ OMe and OEt products. MeSH studied at = nm (and longer wavelengths) (Butler, Wittig, ourselves, Yang, Parker, ….). H–SMe and HS–Me bond fissions studied in some detail nm photolysis of such larger polyatomic systems rarely studied in a quantitative manner. In many cases, photoexcitation would project molecule above first I.P., myriad fragmentation pathways (in principle), not that appealing to photodissociation dynamicists.

12 NH 3 + h  H + ? Mordaunt et al., J Chem Phys s  n excitation gives structured A – X absorption band centred at ~200 nm, dominated by progression in excited state umbrella-bend vibration. Conical intersection between ground and first excited PESs in R H2N–H dissociation coordinate, at planar geometries. Upon dissociation, parent out-of- plane vibrational motion maps into a-axis rotation of NH 2 fragments.

13 NH 3 + h ( = 216 nm)  H + NH 2 (X) Mordaunt et al., J Chem Phys Excess energy channelled into product translation and rotation Broadly similar behaviour seen at all wavelengths  193 nm. NH 2 (A) products also identified once above relevant energy threshold. Similar studies of NH 2 D, NHD 2 and ND 3 photolysis at these near UV wavelengths. No similar quantitative study at = nm (above I.P.) MeNH 2 : Me–NH 2 and MeNH–H bond fission following near UV excitation, but nothing quantitative at shorter wavelengths.

14 HCN + h ( = nm)  ? Cook et al., J Chem Phys H + CN(A) products dominate, bimodal rotational state population distribution. No H + CN(X) products identified. Fully consistent with dissociation via  * PES. Predict same for HC 2n CN, given same X 2  vs A 2  ordering in C 2n CN radicals. (  HCN = 180  )

15 C 2 H 2 + h  ? Mordaunt et al., J Chem Phys ; Loeffler et al., J Chem Phys ~210 nm: Excite (bent) valence states, ‘Slow’ dissociation (ISC) via triplet states  H + C 2 H(X) products. Beautifully quantum state resolved problem nm: Region of high state density, Efficient coupling to  * PES Dissociate to H + C 2 H(A) products, with obvious activity in C=C stretch mode.

16 CH 4 + h ( = nm)  ? A long standing challenge. CH 4 only absorbs at  137 nm. H + CH 3 identified as major primary products when exciting at nm as long ago as Also see slow H atoms from three-body dissociation. Mechanism? H atoms show speed dependent recoil anisotropy. (Wang et al., J. Chem. Phys ). Mordaunt et al

17 CH 4 + h ( = nm)  ? Recent clarifications. Experiments at ~130 nm ( Zhang et al., J. Phys. Chem. Letts Structure in TKER spectrum confirms H + CH 3 products; latter carry high N (and v) excitation. Theory (van Harrevelt, J. Chem. Phys ) Identifies conical intersections between S 1 and S 0 PESs at planar geometries that offer potential routes to the observed fragmentation products.

18 Summary Focus of talk – photodissociation dynamics of hydride molecules, using H (Rydberg) tagging methods. Ion imaging methods applicable to many other small fragments. In almost all cases, level of study (and understanding) much better for near UV wavelengths than at = nm. (n/  )  * PESs enable excited state photofragmentation. Radiationless transfer to S 0 PES, and unimolecular decay of vibrationally ‘hot’ S 0 molecules becomes ever more important for larger polyatomic molecules. Outstanding issues – for experiment and theory: identification of all productsproduct branching ratios (T) dependence of  total (and  partial for forming possible products)

19 Acknowledgements Bielefeld: Karl Welge, Ludger Schnieder, Eckart Wrede (Bielefeld) Bristol: PhD students: Greg Morley, David Mordaunt, Steve Wilson, Claire Reed, Phil Cook, Brid Cronin, Mike Nix, Adam Devine, Graeme King, Tom Oliver, Tolga Karsili, Barbara Marchetti, Rebecca Ingle. PDRAs: Ian Lambert, Steve Langford, Emma Feltham. Academic colleagues: Richard Dixon, Colin Western, A ndrew Orr-Ewing.







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