PICNIC Network

Product Imaging and Correlation: Non-adiabatic Interactions in Chemistry (PICNIC)

PICNIC is a research network funded under the 5th Framework Programme of the European Union: Improving Human Research Potential and the Socio-economic Knowledge Base. Our scientific objective is to understand how electronic energy in the excited states of molecules is transformed into kinetic energy of the nuclei as a result of non-adiabatic coupling. The flow of energy following electronic excitation is observable in both the translational energy of the nuclei and (following further laser excitation) in the kinetic energy spectrum of emitted photoelectrons. The ideal experimental technique for observing the kinetic energy distribution of both nuclei and electrons is charged particle imaging and the principal social objective of the proposal is to bring together leading European laboratories and instrument manufacturers with appropriate and complementary expertise in this area. Within such a network we intend also to develop a wider European, and possibly global, perspective on the dynamics of excited states. To this end we need to share experiences, technical “know-how”, and, above all, our collective knowledge with the next generation of European scientists. We aim to consolidate European expertise in the following areas

• Velocity map imaging
• Correlation and 3D imaging
• Time-resolved photoelectron imaging
• Femtosecond laser technology
• Technical innovation and novel experimental methods

Research Background

The photochemistry of polyatomic molecules in excited electronic states is largely determined by processes which transfer energy from the initial electronic excitation into kinetic energy of the nuclei. These processes can be extremely fast. Internal conversion can transform electronic excitation into high levels of vibrational excitation in less than 100 fs. The resulting internal energy is often greater than barriers to isomerisation or dissociation, and it is this that leads to the observed photolytic effects. The mechanism by which energy is transferred from one electronic state to another is most usually vibronic (i.e. vibration-electronic) coupling. Understanding such coupling requires that the nuclear motion and the electron configuration be considered together (rather than separately as assumed in the widely used Born-Oppenheimer approximation) because the time scales for nuclear motion and electronic motion are similar. Such a situation is rare for molecules on the electronic ground state potential energy surface (PES) but common in excited states since the numerous possible excited state electron configurations give rise to many electronic states that are close in energy and whose PESs intersect or avoid one another, depending on symmetry considerations, at particular nuclear configurations generically known as conical intersections (although the topology will usually be more complicated than implied by this description since the PESs are multidimensional).

The chemical consequences of non-adiabatic coupling are wide-ranging and often profound. For example, in atmospheric chemistry, ultraviolet photolysis of ozone produces mainly electronically excited, and thus highly reactive, O atoms. This is of pivotal significance in the subsequent chemistry of the troposphere. Such reactions have important technological applications also e.g. chemical lasers. The operation of photobiological systems, e.g. vision and photosynthesis, also depend on very rapid non-adiabatic coupling so that a molecule that absorbs energy through a strongly optically absorbing state, which would otherwise re-radiate, can store the excitation energy in a lower lying, and weakly radiating, state. Many other examples could be given.

In small molecules the excited state dynamics, if not direct, are likely to be controlled by a single non-adiabatic event. In larger molecules the photochemistry is more often determined by a cascade of non-adiabatic processes. The fate of the products is sealed by how the energy has been partitioned in the non-adiabatic surface crossings. We want to understand these relaxation processes better, not only because of the fundamental insight into reaction mechanism that this would bring, but also because of the technological importance of these processes in, for example, molecular electronics and nanodevices.