1. Imaging studies of S + fragments from the UV photolysis of state selected H 2 S + cations A.D. Webb, R.N. Dixon and M.N.R. Ashfold School of Chemistry, University of Bristol, Bristol, U.K., BS8 1TS

2. Preamble Ion imaging methods used widely to study photodissociation dynamics of neutral molecules. ‘Typical’ experiment (1) employs photolysis (pump) + REMPI (probe) lasers. Choice of λ phot allows state-selected detection of fragment of interest. Fragmentation dynamics of molecular cations can be studied with same apparatus, by reversing the roles of the two lasers (scheme (2)). AB AB* A+B A A* A+A+ AB* AB + AB A + +B Scheme (1) Scheme (2)

3. Application to H 2 S: 2+1 REMPI via 1 A 2 (b 1 1 4pb 2 1 ), v=0 level Simulated spectrum, [a] T rot = 20 K (1) 1 10  1 01 (ortho-) (2) 2 11  0 00 (para-) (3) 4 23  2 12 (ortho-, high J’) REMPI-PES [b] shows strong propensity for  v=0 ionisation. Ionisation likely to involve loss of s- or d-wave electron. Resulting H 2 S + should have same ortho-/para- symmetry as intermediate Rydberg level. [a] Ashfold et al, JCSFT 86 2027 (1990) [b] Steadman et al JCP 89 5498 (1988)

4. Imaging the photodissociation of molecular cations H 2 S + cations formed by 2+1 REMPI of jet-cooled H 2 S via predissociated Rydberg levels – allows J”, J’ (and some J + ) state selection. Photodissociate H 2 S + with laser 2, 425  phot  300 nm Two fragmentation channels:  S + + H 2  SH + + H Image S + (or SH + ) fragments. S + images show structure: H 2 partner formed in different v,J states.

5. H 2 S + photolysis at λ phot = 325.13 nm H 2 (v=0), J 9 7 5 3 1 H 2 (v=1) 2+1 REMPI via line 3 (i.e. via 4 23 (ortho-) level). Image S + fragments. By energy conservation, these must be in 4 S ground state. Image shows rings  preferential (exclusive?) formation of ortho-H 2 co-fragments

6. H 2 S + photolysis at λ phot = 312.62 nm REMPI via: line 1 (ortho)  H 2 (odd J) line 2 (para)  H 2 (even J)

7. H 2 S + photolysis at longer λ phot H 2 S + preparation: 2+1 REMPI via 1 A 1 (4pb 1 ) – X 1 A 1 Q branch at 302.66 nm (i.e. not ortho-/para- state selected). Narrow spread of S + velocities:  H 2 co-fragments are formed in J  2. IF ortho-/para- nuclear spin symmetry conserved as at shorter λ phot, H 2 S + photolysis offers a route to preparing H 2 molecules with an unusually high degree of (v, J, o-/p-) state selection. 405 nm 398 nm 387 nm

8. Dissociation mechanism(s) Interpretation guided by potential energy surface (PES) calculations for ground and excited states of H 2 S + by Hirst; (JCP 118, 9175 (2003)). X 2 B 1 X 2 B 1 + h ν  A 2 A 1 4 A 2 S + ( 4 S) + H 2 B 2 B 2 Long λ phot ; Renner-Teller coupling to X 2 B 1 state, then spin-orbit coupling to 4 A 2 state. C 2v conserving. Latter seam of intersection  ‘late’ barrier  S + ( 4 S) + H 2 (rotationally cold). Shorter λ phot ; Vibronic coupling to B 2 B 2 state (enabled by b 2 asymmetric stretch), then spin-orbit coupling to 4 A 2 state and dissociation to S( 4 S) + H 2 products carrying more rotational excitation. See JCP 127, 224307, 224308 (2007).

9. Conclusions 1. n+1 REMPI via appropriate Rydberg levels allows imaging studies of the photofragmentation of state-selected molecular cations. 2. In case of H 2 S +, identify two distinct pathways to S + ( 4 S) + H 2 products, characterised by different levels of H 2 product rotation. 3. Ortho-/para- exchange symmetry conserved, surviving REMPI, photo- excitation, two radiationless transitions and eventual fragmentation. 4. Possible astrophysical implications? T ~20 K H 2 : OPR ~0 H 2 S + : OPR ~1.6 H 2 formed by photodissociation of larger hydrides would have ‘non-equilibrium’ OPR. 0 50 100 150 T / K 3 1 2 0 OPR H2H2 H2S+H2S+