Spectroscopy and photodissociation of molecular chlorine in argon matrices

Abstract

A structureless band observed at 180 nm in the absorption and excitation spectra of Cl2/Ar matrices is tentatively assigned as the spin forbidden 3Σ+u ← X 1Σ+g transition of molecular chlorine. Having an absorption cross section of approximately 10−18 cm2 in the solid, a twofold order of magnitude increase in the transition probability is observed relative to the gas phase. Wavelength specific measurements of the photodissociation of molecular chlorine in crystalline argon samples showed that a dominant threshold exists in the 130 nm band at 9.2 eV corresponding to absorption into the bound 1 1Σ+u state. The maximum quantum yield for permanent dissociation in the 130 nm band was found to be 0.3. Luminescence evidence indicates that this dissociation does not involve a charge-transfer mechanism but a crossing from the bound 1 1Σ+u state to a repulsive potential on which an impulsive cage escape occurs. Photoexcitation in the 180 nm band also results in the permanent dissociation of chlorine as well as the molecular A→X emission. The dissociation efficiency of this band was found however to be significantly less than in the 130 nm band and very sample preparation dependent, viz., sample crystallinity. In crystalline samples dissociation efficiencies were typically two orders of magnitude less than in noncrystalline samples. No dissociation occurs following excitation into the lower energy 1Π(1u) band at 300 nm irrespective of sample preparation conditions.