Special issue: CAMOP–MOLEC XVI
Authors: Ascenzi Daniela , Franceschi Pietro , Tosi Paolo 
Published: August 2007

In this special issue of CAMOP/Physica Scripta we would like to present a picture of the state-of-the-art in the field of the dynamics of molecular systems. It contains a collection of papers submitted in association with the most recent MOLEC meeting (MOLEC XVI), which was held in September 2006 in Levico Terme (Italy) to celebrate the 30th anniversary of the MOLEC conference series. The series of biennial European Conferences on the Dynamics of Molecular Systems (MOLEC) started in 1976, when the first meeting was held in Trento (Italy). Successive conferences were organized in Brandbjerg Højskole (Denmark, 1978), Oxford (UK, 1980), Nijmegen (The Netherlands, 1982), Jerusalem (Israel, 1984), Aussois (France, 1986), Assisi (Italy, 1988), Bernkastel-Kues (Germany, 1990), Prague (Czech Republic, 1992), Salamanca (Spain, 1994), Nyborg Strand (Denmark, 1996), Bristol (UK, 1998), Jerusalem (Israel, 2000), Istanbul (Turkey, 2002) and Nunspeet (The Netherlands, 2004). This is the second time that Physica Scripta has hosted a special issue dedicated to MOLEC. The previous issue ( Physica Scripta (2006) 73 C1–C89) was edited by Steven Stolte and Harold Linnartz following the MOLEC 2004 conference.

Following the philosophy of CAMOP, we have asked invited speakers to summarize important problems in their research area, with the objective of setting forth the current thinking of leading researchers in atomic, molecular and optical physics. This comprises discussions of open questions, important new applications, new theoretical and experimental approaches and also predictions of where the field is heading. In addition to being authoritative contributions of acknowledged experts, we hope that the papers also appeal to non-specialists as each work contains a clear and broad introduction and references to the accessible literature.

The present special issue comprises 17 papers, which are arranged according to the following topics: theoretical and experimental studies of molecular collisions and chemically reactive systems (papers by Toennies, Cavalli et al, Varandas, Nyman, Allan et al, Liu et al, Boxford et al); cooling and alignment of molecular systems (papers by Kumarappan et al, van de Meerakker et al); photon–matter interactions, spectroscopy and photodissociation (papers by Fárník and Buck, Golan et al, Borghesani et al, Coreno et al); theory of complex systems (papers by Villarreal et al, Bodo et al, Yurtsever and Calvo). The final contribution concerns the dynamics of systems of biological relevance (paper by Denicke et al).

The paper by Toennies, one of the founding fathers of the MOLEC conference and winner of the MOLEC award in 1996, presents a comprehensive account of the experimental developments in the field of low energy collisions over the last 30 years, and provides a short outlook on how the most recent cooling and ultra cooling techniques (helium droplets and electro-magnetic laser traps) could open up new perspectives in the field of chemical reaction dynamics. Cavalli et al present a theoretical interpretation of overlapping resonances in the reactive cross section for the benchmark reaction F + H2→HF + H, explained by the formation of a short-lived state located in the transition state region and of a van der Waals metastable state in the exit valley. The subject of accurate ab initio calculations for potential energy surfaces is addressed by Varandas in a paper describing suitable modelling strategies to obtain intermolecular potentials at near spectroscopic resolution. In the following paper, Nyman gives an overview of how thermal rate constants for polyatomic chemical reactions can be calculated from first principles: the reaction H2 + CH3→CH4 + H is used as an example and theoretical results are compared with experimental ones. The `uncharted territory' of collision dynamics at the gas–liquid interface is explored in the pioneering contribution by Allan et al, in which experimental results on the reactivity of O atoms with long chain liquid hydrocarbons are interpreted with the aid of molecular dynamics calculations. The contribution of Liu et al shifts towards the reactivity of charged systems, by presenting results of crossed beam experiments on the reaction dynamics of ethylene with OD+ and D2O+ ions. Boxford et al continue with an experimental and theoretical study of the systematics of multiply-charged anion fragmentation in dianion–cation contact ion pairs.

The next two contributions present the latest findings in the field of cooling and alignment of molecular systems. Kumarappan et al provide an overview of adiabatic and non-adiabatic laser-induced alignment of linear and asymmetric top molecules. van de Meerakker and Meijer discuss the state-of-the-art of Stark deceleration of OH radicals and the prospects for future applications of such tools in scattering experiments are presented.

The third section of this issue comprises spectroscopy and photodissociation studies. Fárník and Buck discuss the photodissociation of HBr molecules in three different cluster environments: pure hydrogen bonded (HBr)
n
clusters, large van der Waals bonded HBr·Ar
n
and HBr·(H2O)
n
clusters. Differences in the photofragmentation of rovibrationally excited methylamine and propyne molecules are taken as an example by Golan et al to demonstrate the importance of vibrationally mediated photodissociation for understanding energy flows in molecular systems. A study of the IR luminescence in excited and weakly bound rare gas complexes is reported in the contribution by Borghesani et al. Coreno et al discuss the generation of core hole states by soft x-ray absorption of molecules by presenting results on the visible and UV fluorescence decay from H2O, NH3 and CH4 molecules following core electron excitation and by proposing a mechanism of molecular dissociation.

The fourth section includes three theoretical contributions. In the first one, by Villarreal et al, the simulation of IR spectra of molecules solvated in atomic clusters is reviewed by presenting a method to obtain energies and wave functions for systems composed of a molecular impurity immersed in He clusters. Bodo et al continue with an application of variational and diffusion Monte Carlo calculation procedure to the `solvation' of molecular and atomic ions in small He clusters. Yurtsever and Calvo describe the influence of a central point defect on the structure of 2D clusters consisting of point charge particles that are confined by an isotropic trap.

Finally, Denicke et al review the imaging technique of single-beam and multi-beam two-photon laser-scanning microscopy for biomedical research by showing applications to the study of cell metabolism via NADH imaging.

We are grateful to all the authors who participated with their contributions to this issue. We thank Physica Scripta for providing us with a platform for the publication of the MOLEC special issue and we hope that such a recently established tradition will continue to bring to readers new insights into the advances in the field of molecular dynamics.