Surface aligned photochemistry: Photodissociation of Cl[sub 2] and Cl[sub 2]...Cl adsorbed on LiF(001).

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 Cl[sub 2](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 Cl[sub 2](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 Cl[sub 2]. It is proposed that this leads to the formation of a steady-state concentration of Cl[sub 2]...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[sup *]/Cl ratio, as compared with channel B. The suggested mechanism for channel A is Cl[sub 2]...Cl+hν→Cl[sub 3][sup *]→Cl[sub 2]•Cl→Cl[sub 2]+Cl (where* is an electronically excited state and • represents repulsion in the lower electronic state to which Cl[sub 3][sup *] 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 int... [ABSTRACT FROM AUTHOR]