Velocity map imaging studies of the photodissociation of H₂O⁺ cations

Abstract

The near-threshold photofragmentation dynamics of state-selected H₂O⁺ cations have been investigated using velocity map ion imaging methods. The cations were prepared in the v = 0 level of the ground ( ²B₁) electronic state, by 2 + 1 resonance enhanced multiphoton ionization via selected rotational levels of the H₂O, ¹B₁, v = 0 state. Subsequent two photon excitation of the resulting H₂O⁺ cations to ²B₂ state levels lying above the lowest dissociation limit (i.e. at total energies in the range 46000–50600 cm⁻¹) results in O–H bond fission and OH⁺ fragment ion formation. These fragments display isotropic recoil velocity distributions, which peak at low kinetic energy but extend to the highest speeds allowed by energy conservation. Ab initio calculations of key sections through the potential energy surfaces (PESs) for the ground and first few excited states of H₂O⁺ suggest two possible mechanisms for the observed rotational and (when energetically allowed) vibrational excitation of the OH⁺ fragments. Both require initial non-adiabatic (vibronic coupling) from the photo-prepared state level to high levels of the ²A₁ state, but involve different subsequent HO⁺−H bond fission mechanisms. One involves Renner–Teller coupling to the ground state PES, while the alternative requires spin-orbit induced coupling to the repulsive ⁴B₁(⁴A″) state PES.

Keywords:

• molecular photodissociation
• gas phase
• H₂O⁺ cations
• ion imaging

Acknowledgements

The authors are grateful to the EPSRC and the EU for financial support through, respectively, a programme grant (EP/G00224X) and a Marie Curie Initial Training Network (ICONIC, contract agreement no. 238671), and to A.D. Webb, B.B. Seebohm, J.N. Harvey, K.N. Rosser and J.A. Smith for their many and varied contributions to the work described in this paper.