Experimental observation of competing pathways in the relaxation of ICl* in a He supersonic expansion

Abstract

Differing caging mechanisms of electronically excited ICl within He⋯ICl complexes are investigated using laser-induced fluorescence and two-laser, pump–probe spectroscopy. A continuum signal is observed in laser-induced fluorescence spectra recorded with excitation energies that access regions above the I ²P_3/2 + Cl ²P_3/2 dissociation limit. We show that this signal is associated with transitions of the He⋯ICl(X,v″ = 0) complex with a linear equilibrium geometry to the continuum of states correlating with the repulsive, inner wall of the ICl(A ³Π₁) potential. The dissociating ICl(A) molecule within the complex undergoes kinematic one-atom caging, ejecting the weakly bound He atom, and re-stabilizing the molecule in high lying vibrational levels within the A electronic state with very little rotational excitation. At higher excitation energies, weak LIF features superimposed on the continuum fluorescence signals are observed in preliminary investigations. When fixing an excitation laser on one of these features, at 18 012 cm⁻¹, I³⁵Cl(B,v′ = 2,3) vibrational levels are formed with little rotational excitation. This feature is associated with transitions of the linear He⋯I³⁵Cl(X,v″ = 0) complex to intermolecular states associated with the ICl(B) diabat that are coupled with those in the ICl(B′) adiabat formed by an avoided curve crossing. The I³⁵Cl(B,v′) products are formed via a non-adiabatic one-atom caging mechanism of ICl* molecules within linear He⋯I³⁵Cl* complexes.