Surface aligned photochemistry: Photodissociation of Cl2 and Cl2⋯Cl adsorbed on LiF(001)

Abstract

Photodissociation of chlorine adsorbed on a LiF(001) surface at 25–70 K has been investigated by means of angularly resolved resonantly enhanced multiphoton ionization spectroscopy (REMPI). The translational-energy distributions and angular distributions for forming Cl(g) photofragments were determined. Photolysis was performed employing 351 nm radiation, with laser pulse energies of 0.3–1.2 mJ/cm2. A peak in the translational energy of Cl(g) at about 0.4 eV was identified as being due to the direct photodissociation of the Cl2(ad) molecule by 3.5 eV photons. Particular interest attached to the observation of a further channel (termed “A”) for photodissociation leading to Cl(g) with translational energy peaking at ∼1 eV and extending to 1.5 eV. The available photon energy renders it highly unlikely that this “high-energy” Cl(g) originates in Cl2(ad). Channel A had the same linear dependence of Cl-atom flux on laser pulse-energy as did the lower energy (0.4 eV) channel, termed “B,” but differed from it in exhibiting a slow approach to steady state. It appears that channel A requires the prior build-up of Cl(ad) concentration due to the photodissociation of Cl2. It is proposed that this leads to the formation of a steady-state concentration of Cl2⋯Cl which when photolyzed yields high-energy Cl(g) via channel A. Channel A exhibits a distinctive angular distribution at low coverage and a characteristic Cl*/Cl ratio, as compared with channel B. The suggested mechanism for channel A is Cl2⋯Cl+hν→Cl3*→Cl2•Cl→Cl2+Cl (where* is an electronically excited state and • represents repulsion in the lower electronic state to which Cl3* reverts). This mechanism is interpreted in terms of an extensive diatomics-in-molecules (DIM) model for the trichlorine radical, shown to be in agreement with high level ab initio multireference internally contracted configuration interaction (MRCI) calculations, and consistent with the observations.