Photodissociation of nitrogen dioxide adsorbed on lithium fluoride (001)

Abstract

The photochemistry of NO2 physisorbed on single-crystal LiF(001) at 100 K has been studied at lambda1 = 248 nm. The adsorbate was examined by polarized FTIR in both the presence and absence of lambda1 radiation. In the absence of UV irradiation the adlayer is composed of dimeric (NO2)2. In the presence of UV, FTIR shows that some N2O3 is formed. Photodissociations (PDIS) giving both NO(g) and molecular NO2(g) were the predominant mechanisms as determined by time-of-flight mass spectrometry (TOF-MS) and resonantly enhanced multiphoton ionization (REMPI). The main objective of this work was the characterization of the photoproduct, NO, internal state distribution by 1 + 1 REMPI. Vibrational levels from v'' = 0 to 9 were probed with rotational resolution using a tunable laser, lambda2. The rotational distributions for each vibrational level could be described by one Boltzmann temperature. The spin-orbit states of NO(g) were equally populated in all vibrational levels. The lambda doublet states, PI(A') and PI(A''), were equally populated. The principal observation was that the vibrational distribution in NO(g) was inverted and bimodal with a peak in v'' = 0 and a second substantial peak in v'' = 3-4, qualitatively resembling but quantitatively different from that for photolysis of NO2(g). Time delays between the two lasers were used to probe the translational energy of the NO(g) photofragment in specified states of internal excitation. The translational energy distributions were invariant over all vibrational levels, except v'' = 0 for which much slower fragments were observed. This complete determination of the energy distribution in the degrees of freedom of the NO(g) from photodissociation of adsorbate has implications for the identity of the photolyzing species and the dynamics of photodissociation. Two mechanisms for photoformation of NO2(g) were found: one at low coverages and one at higher coverages, the former giving peak translational energies approximately 1.2 kcal/mol and the latter 0.4 kcal/mol.