Multiphoton dissociation dynamics of BrCl and the BrCl+cation

Abstract

Ion imaging methods have enabled identification of three mechanisms by which ⁷⁹Br⁺ and ³⁵Cl⁺ fragment ions are formed following one-color multiphoton excitation of BrCl molecules in the wavelength range 324.6 > λ > 311.7 nm. Two-photon excitation within this range populates selected vibrational levels (v′ = 0–5) of the [X ²Π_1/2]5sσ Rydberg state. Absorption of a third photon results in branching between (i) photoionization (i.e. removal of the Rydberg electron—a traditional 2 + 1 REMPI process) and (ii) π* ← π excitation within the core, resulting in formation of one or more super-excited states with Ω = 1 and configuration [A ²Π_1/2]5sσ. The fate of the latter states involves a further branching. They can autoionize (yielding BrCl⁺(X ²Π) ions in a wider range of v⁺ states than formed by direct 2 + 1 REMPI). Further, one-photon absorption by the parent ions resulting from direct ionization or autoionization leads to formation of Br⁺ and (energy permitting) Cl⁺ fragment ions. Alternatively, the super-excited molecules can fragment to neutral atoms, one of which is in a Rydberg state. Complementary ab initio calculations lead to the conclusion that the observed [Cl**[³P_J]4s + Br/Br*] products result from direct dissociation of the photo-prepared super-excited states, whereas [Br**[³P_J]5p + Cl/Cl*] product formation involves interaction between the [A ²Π_1/2]5sσ and [X ²Π_1/2]5pσ Rydberg potentials at extended Br–Cl bond lengths. Absorption of one further photon by the resulting Br** and Cl** Rydberg atoms leads to their ionization, and thus their appearance in the Br⁺ and Cl⁺ fragment ion images.