[...]... energy barrier, height V The narrower the barrier, the smaller the mass of the particle and the smaller the difference between V and E, the greater the tunnelling probability If the amplitude of the wave has not reached zero at the far side of the barrier, it will stop decaying and resume the oscillation it had on entering the barrier (but with smaller amplitude) large amount of energy to pass from reactants... molecule and can contort the allowed energy levels of the molecule; also important is the frequency of the laser, which, together with the intensity, affords optical access to different molecular energy states The detailed physics of the light–matter interaction will of course also depend on the structure of the irradiated molecule, but whatever its identity, certain general features of the excitation of. .. bind the electrons and nuclei in a molecule to such an extent that the characteristic energy levels of the molecule are modified during the ultrashort duration of the laser pulse Each of the above phenomena is the subject of intensive research pro- Laser snapshots of molecular motions grammes in its own right Figure 1.1 offers a simplified portrayal of some of these events, showing the ionisation of an... (like other catalysts) reduce the energy required to pass over this barrier, thereby increasing reaction rate The structure of the reactant at the top of the barrier is energetically unstable, and is known as the ‘transition state’ The energy required to pass over the barrier is the so-called ‘activation energy’ – the barrier is surmounted by thermal excitation of the substrate This classical over -the- barrier... come together, exchange energy and transfer atoms in the very act of transforming one material into another To map out such processes as they happen necessitates the application of laser pulses with durations of tens, or at most hundreds, of femtoseconds to take ‘snapshots’ of the changes in real time This chapter discusses the application of femtosecond lasers to the study of the dynamics of molecular... dynamic view of enzyme catalysis, it is thus the width – and not the height (as with transition state theory) – of the energy barrier that controls the reaction rate The important criterion thus becomes the ability of the enzyme to distort and thereby reduce barrier width, and not stabilisation of the transition state with concomitant reduction in barrier height (activation energy) We now describe theoretical... escapes (ionises) from the atom by tunnelling through the barrier on the side of lower potential energy and then executes an oscillatory trajectory determined by its kinetic (or ponderomotive) energy in the electric field of the laser pulse If the electron follows a trajectory that brings it back close to the nucleus of the ionised atom, emission of a high-frequency photon can occur as the negatively charged... Michael J Sutcli≈e1 and Nigel S Scrutton2 1 2 Department of Chemistry, University of Leicester, Leicester LE1 7RH, UK Department of Biochemistry, University of Leicester, Leicester LE1 7RH, UK 2.1 Introduction Enzymes facilitate life via a plethora of reactions in living organisms Not only do they sustain life – they are also involved in a myriad of processes that affect our everyday lives These include... electron trajectory High harmonic photon emission Tunnel ionisation Figure 1.1 A sequence of events following the interaction of an intense, ultrafast laser pulse with an atom The potential energy structure of the electron, which would otherwise be symmetric either side of a minimum, thereby confining the electron to the vicinty of the atomic nucleus, is distorted by the incident laser radiation The electron... molecular motion, and attempts to portray how a synergic combination of theory and experiment enables the interaction of matter with extremely short bursts of light, and the ultrafast processes that subsequently occur, to be understood in terms of fundamental quantum theory This is illustrated through consideration of a hierarchy of laser-induced events in molecules in the gas phase and in clusters A . Visions of the Future: Chemistry and Life Science Leading young scientists, m any holding prestigious Royal Society Research Fellowships, describe their research and give their visions of the future. . puting and telecom m unications), and chem istry and life science (covering the topics described below). Topics in the present book on chemistry and life science include studies of atom s and m. record for this book is available from the British Library Library of Congress Cataloguing in Publication data Visions of the future : chemistry and life science / edited by J. M. T. Thom pson p.