Photofragmentation: understanding the influence of potential surfaces and exit-channel dynamics

Abstract

Photoexcitation is used to prepare species whose subsequent fragmentation can be exploited for the purposes of studying, controlling and manipulating different kinds of molecular processes. First, we show how a form of sub-Doppler resolution spectroscopy can be used to determine centre-of-mass kinetic energy distributions, thereby enabling internal energy distributions to be obtained for elementary processes which occur at a fixed total energy. We present data for H atoms monitored at the Lyman-α wavelength. Secondly, we show how such processes can be used to study binary interactions by preparing van der Waals complexes (e.g. CO₂HBr) and then photodissociating one constituent of the complex (HBr +hν→ H + Br), thereby propelling the H atom toward the other constituent with well defined initial conditions. Finally, we turn from such direct fragmentation processes to those in which radiationless decay of S₁ leads to reaction via T₁ and/or S₀ surfaces. For the case of Bu^tNO, we study both channels, and find a barrier of ca. 650 cm^–1 on T₁. Reaction via S₀ is ‘statistical’, while the T₁ path leads to dynamical bias. Below the T₁ barrier, reaction is slow (>3.5 µs) compared to radiationless decay (<50 ns), thus allowing metastable species to be prepared.