1 Spectral signatures of non-adiabatic dynamics Richard N. Dixon School of Chemistry, University of Bristol, Bristol BS8 1TS, UK International Symposium on Molecular Spectroscopy 61st Meeting - - June 19-23, 2006

2 Outline of non-adiabatic spectroscopic signatures Perturbations of energy levels Intensity stealing in “forbidden transitions” Wide amplitude dynamics on multiple surfaces Quantum interferences The opening of dissociation pathways The control of energy disposal in the products of dissociation brought about through electronically off-diagonal matrix elements of  e /  q v,  e / , or  e /  J etc.

3 Spin-orbit doubling constants for NH 2 levels with K a = 1 G Duxbury & RN Dixon. Molec. Phys. 43, 255 (1981)

4 K a -dependent linewidths in HCO Ã-X̃ PHOFEX bands JC Loisin, SH Kable, PL Houston & I Burak. J. C. P. 94, 1706 (1991)

5 22 Dissociation of HCO via Internal Conversion Time-independent coupling of the bound state with the dissociation continuum - the spectroscopic approach Time-dependent decay via surface hopping close to linearity – the dynamic approach HCO Alternative descriptions

6 Renner-Teller induced pre-dissociation widths (time-dependent theory) R.N. Dixon. Chem. Soc Rev.. 23, 375, (1994)

7 SH.Kable, JC Loison, DW Neyer, PL Houston, I Burak & RN Dixon,. J. Phys. Chem. 95, 8013 (1991) (time-independent theory) Observed and calculated recoil anisotropy parameter  for HCO Ã 2 A" → H + CO X 1  +

8 The HCO Ã 2 A“ - X̃ 2 A' transition is perpendicular, but the greatest recoil anisotropy is +ve (parallel)

9 Molecular beam of R-H Molecular Beam “Rydberg Tagging” 366 nm “Rydberg Tagging” 366 nm  Photolysis beam High resolution H-atom (Rydberg state) photofragment translational spectroscopy Detector n=60-90 n=2 n=1 Lyman-α (121.6 nm) Tagging (366nm) Ionisation Limit Lyman α Record the time-of-flight spectrum Transform to Total Kinetic Energy Release

1010 The internal energy distribution within NH 2 or ND 2 following excitation of jet-cooled beams of NH 3 and ND 3 through Ã, 1 A 2 (v 2 ') – X̃, 1 A 1 (v=0) Note the alternation of profile for low v 2 ' NH 2 ND 2 E NH 2 = E NH 3 + h - D TKER

1 NH 2 internal energy spectra from photolysis of NH 3 at cm -1 (2 1 0 band) in perpendicular polarisation Note the predominance of levels with high K a  N

1212 The planar excited state evolves over a low barrier from having 3s Rydberg character at short R to having an NH anti-bonding character at long R 2-D cuts of the potentials for the X̃ and à states of NH 3

1313 Wavefunctions for dissociation of NH 3 Ã, v 2 ′ = 0:3 v 2 ’ = 0 v 2 ’ = 2 v 2 ’ = 1 v 2 ’ = 3 RN Dixon, Molec. Phys., 88, 949, (1996)

1414 Recoil anisotropy of H + NH 2,v = 0 from NH 3 Ã,v 2 ′ = 0 DH Mordaunt, MNR Ashfold & RN Dixon, J. C. P., 104, 6460, (1996)

1515  The photochemistry of H 2 O plays an important role in atmospheric chemistry and in interstellar masers, particularly for excitation at the Lyman-  wavelength.  Three electronic states of the water molecule are implicated in this process.  The major branching is to yield H + OH in a wide range of states. The photodissociation of H 2 O at Lyman-  (121.6 nm)

1616 The energy release from photolysis of H 2 O at nm Note the alternations in intensity in both spectra

1717 The population and recoil anisotropy for OH(X,v=0)  The oscillations at low N arise mainly from dissociation via the A 1 B 1 state, and are most prominent in perpendicular polarisation.  The oscillations at high N arise mainly from dissociation via the B 1 A 1 state, and are most prominent in parallel polarisation.  What gives rise to these oscillations ?

1818 The resonant H 2 O wavepacket for = nm uu

1919 The link to quantum interference An angular function of  can be constructed from the experimental populations for OH(X) v=0 according to: SA Harich, DWH Hwang, XF Yang, JJ Lin, XM Yang & RN Dixon, J. Phys. Chem., 113, 10073, (2000). Populations reconstructed from  (  ), with and without attenuation of its amplitude in the range 90° to 180°, demonstrate the link of the alternation of the population to quantum interference in the outgoing wave.

2020 Dissociation dynamics of some heteroaromatic molecules  Domcke and co-workers have investigated the excited state potential energy surfaces of chromophores of biological interest using multi- reference ab initio methods.  They have found characteristic features of these heteroaromatic molecules: Strongly absorbing diabatically bound singlet  * excited states. Optically weak but photochemically reactive  * excited states. Conical intersections which provide a mechanism for ultrafast deactivation of excited states.  Experimental and/or accurate ab initio vibration frequencies often available both for parent molecules and their dissociation products. A. Sobolewski, & W. Domcke, Chem. Phys., 259, 181, (2000)

2121 General Features of 1  * States Energy / eV R (X-H) / Å  * states S0S0 1  * 1  * state above the 1  * states in phenol and indole. Low oscillator strengths. Rydberg at short range; dissociative in the R (X-H) coordinate at long range. Form conical intersections with lower energy singlet states. Energy / eV R (X-H) / Å  *  * states S0S0 1  * state below the 1  * states in pyrrole and imidazole.

2 Photodissociation of Pyrrole TKER / cm -1 Intensity / Arb. Units 234 nm 244 nm 252 nm The 1  * state gives rise to a very weak and diffuse absorption, and dissociation by loss of an H atom.. The active pyrrolyl vibrational modes in the TKER spectrum vary with the energy of the excitation. 240 nm

2323 TKER / cm -1 Intensity v=0 14 (b 2 ) Combination bands =244 nm Pyrrole – assignment of modes at 244 nm 21 (b 1 ) 20 (b 1 ) Dissociation via the diffuse 1 A 2 (   ) state leads to population of pyrrolyl modes of all three non-totally symmetric classes, reflecting their presumed retention following vibronically induced excitation. 16 (b 2 ) 9 (a 2 ) These modes have differing polarisation behaviours.

2424 The disposal of the available energy upon dissociation Excitation of any of the three disappearing modes results in V → T transfer, leading to population of Pyrrolyl in v = 0.. The rapid dissociation favours adiabatic retention of modes excited in Pyrrole. Consequently the internal energy of Pyrrolyl rises with increase in the excitation energy such that the mean TKER remains close to  E, the fall in the potential energy from the small exit channel barrier to the dissociation asymptote. Mode conserved from pyrrole to pyrrolyl. Disappearing mode. B Cronin, MGD Nix, RH Qadiri & MNR Ashfold, PCCP, 6, 5031, 2004

2525 The 1 A 2 (  *) S 1 -state of 2,5-Dimethyl Pyrrole The S 1 ←S 0 spectrum shows some resolved vibrational structure, indicating a higher barrier to dissociation than in Pyrrole. The methyl torsional modes are vibronically active in S 1 ←S 0. The band origin and dissociation energies are lower than in Pyrrole. The fragmentation process is more structured than in Pyrrole. B Cronin, MGD Nix, AL Devine, RN Dixon & MNR Ashfold, Phys Chem Chem Phys, 8, 599, 2006

2626 Features of the TKER spectra for 2,5-DMP 1. Modes which track with the excitation frequency 2. A persistent peak at ~ 5100 cm -1 : the value of  E for 2,5-DMP.

2727 Features, and adiabatic potentials for 2,5-DMP 3. Strong features are accompanied by three lower KE satellite peaks. Adiabatic potentials Part of E int for a promoting mode must be channelled into a disappearing mode to reduce the barrier and permit dissociation. The S 1 and S 0 states can be coupled at the circled conical intersection by a 2 vibrational modes.

2828 Part of this wavepacket will remain on the 1 A 1 surface and decay via the S 0 continuum. Coupling back to the 1 A 2 surface will lead to population of two quanta of some a 2 mode. ( 19 or 20 ), or a combination.. A second portion will evolve onto the 1 A 1 surface. The 1 A 1 and 1 A 2 states are coupled by a 2 vibrational modes. 1 A 1 (S 0 ) 1A21A2 (a 2 ) 2 (a 2 ) Dynamical coupling at the Conical Intersection One portion of the wavepacket will be unaffected by this interaction (v(a 2 ) = 0).

2929 Photolysis of Phenol (low energy: 275 – 246 nm) Deduced position for the origin n 16a 18b + n 16a TKER observed via the 0-0 band at λ phot = nm 16a (a 2 ) 18b (b 2 ) Phenol has a highly structured S 1 ( 1  *) state, with a 2ns lifetime. Unexpected (high KE) resolved structure in TKER spectrum? Excitation is in several vibrational modes. The origin level is not observed. All the observed levels have a″ vibrational symmetry n  18b + n 16a 1

3030 Interpretation for low energy excitation of Phenol 1.The lower energy levels of the  *S 1 state are predissociated by internal conversion (IC) to high levels of the S 0 ground state having predominant vibration in OH stretching (13-16 quanta). 2.Mode 16a (a cm -1 ) in odd quanta mediates coupling to the  * S 2 ( 1 A”) state at the outer conical intersection, thus facilitating dissociation to ground state products. Phenoxyl is only populated in a” vibrational levels, but: the disappearing OH a“ torsional mode is not active.

3131 Phenol low energy excitation: 275 – 270 nm Some modes excited in Phenol S 1 become inter-converted in carrying through to Phenoxyl. This may result from a Duschinsky rotation of these normal coordinates, either in the IC step to S 0, or at the S 0 to S 2 conical intersection. Excitation

3232 Phenol Results (high energy continuum: 244 – 208 nm) nm photolysis Coupling mode 16b (b 1 ) The phenoxyl radical is formed with population in a single vibrational progression in mode 18b (447 cm -1 ), the C=O in plane wag (a’ symmetry). This progression is built on one quantum of 16b (476 cm -1 ), the lowest frequency b 1 mode, which couples the 1 A’ and 1 A’’ electronic states at the 1  *  1  * conical intersection. 16b + n 18b 8 n 18b (b 2 ) origin

3 Interpretation for high energy excitation of Phenol Mode 16b (b 1, 476 cm -1 ) mediates coupling to the  * S 2 ( 1 A”) state at the inner conical intersection, thus facilitating rapid dissociation to ground state products. The long progression in mode 18b(b 2 ), the CO wag, can be interpreted as a Franck-Condon projection from an in-plane distortion of 13° from C 2v. This is longer than can be attributed entirely to O-H recoil. However a CASSCF calculation of the  * orbital indicates possible non- local forces:

3434 Reconciliation of the TKER for low and high energy The recognition that different promoting modes are involved in photodissociation of phenol following low and high energy excitation to S 1 was critical to reconciling the TKER values for the full data set. The plot of the predicted position of the TKER for v=0 against the excitation energy then gives a single straight line of unit slope. This leads to: D 0 (O-H) = ± 40 cm -1. MNR Ashfold, B Cronin, AL Devine, RN Dixon & MGD Nix, Science, 312, 163, 2006.

3535 Summary of non-adiabatic effects

3636 Bristol University Molecular Science Group Professor Michael ASHFOLD Professor Gabriel BALINT-KURTI Professor Richard DIXON Professor Andrew ORR-EWING Dr Paul MAY Mr Keith ROSSER Dr Colin WESTERN Plus many gifted collaborators, including current postgraduate students Brid CRONIN Adam DEVINE Mike NIX

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