Dynamical and stereodynamical studies of alkaline-earth atom–molecule reactions

Abstract

Alkaline-earth atom molecule reactions have been extensively investigated since the early days of reaction dynamics. Though these reactions have many similarities to those of corresponding alkali atoms, as one would expect from their low ionization potential, there are important differences due to the presence of the two valence electrons in the alkaline-earth atom. This feature confers a distinct dynamical or stereodynamical behaviour whose basic description and analysis is the main objective of this article. The adopted approach is phenomenological based on the huge body of data revealed by modern crossed-beam and laser pump and probe techniques with no intention to be comprehensive but rather to put into context both authors’ work. The paper starts with a resumé of the experimental approaches followed by a visit to full-collision studies under gas cell, beam–gas and crossed-beam arrangements. Detailed analysis and discussion is given of the M + H₂ (M = Ca, Mg) → MH + H reaction family whose dynamics is discussed using their Potential Energy Surfaces. In these studies care was taken to discuss the most relevant features of the molecular reaction dynamics with emphasis on the energy selectivity and on the stereodynamical (vectorial) nature of the reaction. To this end the available data are discussed in light of topological features of the underlying potential energy surface favouring either insertion or abstraction mechanisms. Particular attention is paid to the discussion of alkaline-earth atom reactions with halogen containing molecules. In these reactions the presence of two valence electrons leads to reaction channels, as chemiluminescent or chemi-ionization reactions, which are specifically associated with the transfer of the more energetic (inner harpooning) electron, whose dynamics or stereodynamics pattern differs from those of reaction channels associated to the transfer of the first, less energetic electron as, for example, in ion-pair reactions. A full section of the paper is dedicated to photo-induced intracluster reactions and, in particular, to the transition state dynamics of the prototype Ba···FCH₃ + hν → BaF + CH₃ reaction investigated in great detail by both nanosecond and femtosecond photo–ion and photo–electron pump and probe techniques. For this system the combination of these ultrahigh resolution techniques with isotopic labelling and ab initio calculations allowed the deciphering of its ultrafast reaction mechanism. Finally, the paper ends with some concluding remarks based on the observed propensity rules or general trends found in these alkaline-earth atom reactions.

Acknowledgements

KCL is grateful for the financial support of the National Science Council, Republic of China, under contract no. NSC94-2113-M-002-013. AGU acknowledges a visiting Professorship from the National Academy of Sciences of Taiwan during which part of this review was prepared. He is also grateful for the current financial support of his research group by the Ministerio de Educación y Ciencia of Spain (grant CTQ2004-3468) and from the Government of the Comunidad de Madrid, the typing of part of the manuscript by Arantxa Vera, and the assistance of Dr Skowronek with figures 3 and 33.