1. Could you explain the significance of your article to the non-specialist?

It is well known that ozone efficiently absorbs UV light. It is this ability which turns the stratospheric ozone layer into a pivotal shield protecting us against B- and CUV radiation of the Sun. In fact, the absorption extends - with varying efficiency - across the whole solar spectrum. Photoexcitation always destroys ozone, and the experimental absorption spectrum, meticulously documented from UV through the visible region into near-IR, consists of several broad bands, each exhibiting diffuse structures. Theoretical interpretation of the observed bands is complicated because the initially excited electronic states interact with each other and with several `dark' states. A rigorous quantum mechanical analysis of the photodissociation of ozone thus requires global potential energy surfaces of many interacting electronic states as well efficient methods for solving the multistate Schrodinger equation for the nuclei. In this article we review the combined electronic structure and quantum dynamical studies in all four absorption bands. These studies provide clear electronic and vibrational assignments of the diffuse spectral lines. For the first time, each absorption band is adequately reproduced without any adjustable parameter.

2. What has motivated you to conduct this work?

Ozone is an extremely important molecule and the amount of data available from distinct experimental approaches is overwhelming. It is an eminent challenge for theoretical molecular physics to explain the experimental findings and to unravel the underlying molecular dynamics. For example, the diffuse vibrational structures on top of the broad absorption bands are known for hundred years or longer. Nevertheless, all attempts to consistently explain them failed until recently. Understanding of the diffuse structures implies understanding of the intramolecular dynamics behind them, and requires dynamics calculations based on accurate and global potentials and the couplings between different electronic states. For a molecule like ozone the electronic structure calculations of desired accuracy became tractable only a decade ago.

3. Where do you see this work developing in the future?

There are still open questions to be solved for ozone photodissociation and it is hoped that they will be approached in future studies. The most important problems concern the strongest UV absorption band. Next, it is desirable to extend the approach presented for ozone to other triatomic molecules like NO2, N2O or CO2, for example, which also play key roles in atmospheric chemistry. Such studies would bring the understanding of the dynamics of these molecules to the level of state-of-the-art quantum chemistry.

4. Are there any particular challenges facing future research in this area?

The most outstanding challenge in the theoretical description of molecular photodissociation (or bimolecular collisions) is the treatment of non-adiabatic coupling between more than two electronic states. While practicable methods have been developed and successfully applied for two interacting states, the treatment of three or more coupled electronic states is still in its infancy. The example of ozone clearly demonstrates that multistate crossings become increasingly important with growing excitation energy, i.e. with growing density of electronic states.