Imaging of oriented and quantum-state selected photodynamics

We use velocity map ion-imaging to study the quantum state-to-state photodynamics for several gasses which play a role in atmospheric chemistry. New experimental results on the photochemistry of OCS, which is considered to be a source of sulphur in the stratosphere, have been acquired recently. In collaboration with visiting guest T.P. Rakitzis (FORTH, Crete) we obtained a microscopic view of bond breaking of OCS in the molecular frame. OCS molecules in a molecular beam were spatially oriented in the laboratory frame using an electrostatic quantum vice. A polarized laser dissociated the OCS molecules in CO and S fragments. The three-dimensional recoil of CO fragments, measured with ion imaging, was strongly asymmetric. For the first time, an unambiguous picture of the molecular bond breaking was obtained revealing the curved trajectory of the recoiling CO fragment. This new experimental approach can similarly be applied to study and control the three-dimensional dynamics of photoinitiated reactions of fixed molecules or molecules oriented by emerging techniques like shaped ultrafast laser pulses. The experimental apparatus is equipped with a fast high-voltage gate on the two-dimensional detector which makes it possible to slice through the three-dimensional recoil distribution. This new experimental design opens up new experiments on oriented molecular photodissociation where there are no implicit symmetry requirements.

Contact: Maurice Janssen, e-mail:
Physical Chemistry

A. Experimental set-up to spatially orient the molecular axis of freely rotating molecules.
B. An experimental image showing the striking up-down asymmetry in the recoil and the
various molecular frame vectorial properties and correlations that can be extracted from
these experiments (Science 303 (2004), 1852).