The infrared spectra of HBr and HCl on LiF(001) single crystal surfaces were measured as a function of coverage at temperatures ≤83 K using Fourier-transform infrared (FTIR) spectroscopy. For each hydrogen halide three different spectral features could be distinguished. At low coverages broad absorptions centered at 2265±20 cm−1 (HBr) and at 2515±20 cm−1 (HCl) were observed. These absorptions were attributed to molecules hydrogen-bonded to F− anions of the surface, the angle between the molecular axis and the surface being 21±5° for HBr and 19±5° for HCl as determined from experiments employing polarized infrared radiation. Hydrogen bonding was evidenced by: (i) redshifts with respect to the gas phase (∼300 cm−1), (ii) broad infrared absorption (FWHM: 265±25 cm−1 for HBr, 295±15 cm−1 for HCl), and (iii) enhancement of the infrared absorption intensity compared to the gas phase by more than one order of magnitude for both HBr and HCl. With increasing coverage a second absorption was observed before the first one saturated (HBr:ν=2461±5 cm−1, FWHM=75±10 cm−1; HCl: ν=2763±5 cm−1, FWHM=80±10 cm−1). This absorption was attributed to molecules adsorbed in a second layer. The smaller redshift and spectral width for the second layer were consistent with weaker hydrogen bonding, probably to the halogen of molecules adsorbed in the first layer. Further increase in coverage resulted in the appearance of the well-known doublet absorptions due to formation of solid. Coadsorption of HBr and HCl, as well as experiments under adsorption–desorption equilibrium conditions, confirmed that the first and second layers could coexist. The isotherms could best be understood on the assumption of a repulsive interaction within the first layer.

The photochemistry of NO, physisorbed on single-crystal LiF(001) at 100 K has been studied at XI = 248 nm. The adsorbate was examined by polarized FTIR in both the presence and absence of XI radiation. In the absence of UV irradiation the adlayer is composed of dimeric (NO,),. In the presence of UV, FTIR shows that some NzO3 is formed. Photodissociations (PDIS) giving both NO(g) and molecular N02(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 vf'= 0 to 9 were probed with rotational resolution using a tunable laser, A,. 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, II(Af) and II(A'I), were equally populated. The principal observation was that the vibrational distribution in NO(g) was inverted and bimodal with a peak in u" = 0 and a second substantial peak in v N = 3-4, qualitatively resembling but quantitatively different from that for photolysis of NOz(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 u" = 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 NOz(g) were found one at low coverages and one at higher coverages, the former giving peak translational energies -1.2 kcalfmol and the latter 0.4 kcalfmol.

The photodissociation of adsorbed NO dimers on LiF͑001͒ was studied in the monolayer regime at 248 nm using resonantly enhanced multiphoton ionization ͑REMPI͒ and Fourier transform infrared ͑FTIR͒ absorption spectroscopy. Vibrationally excited NO photofragments were observed in vЈϭ0 -9. The vibrational energy distribution was found to have a maximum at vЈϭ0 and a second region of substantial population between vЈϭ2 and 9. The rotational and translational energy distributions of the photofragments showed no major change with vibrational excitation. By contrast, the translational energy displayed a systematic increase with increasing rotational excitation. Photodissociation at 1 ML ͑NO͒ 2 coverage yielded average vibrational, rotational, and translational energies of 0.48, 0.05, and 0.13 eV, respectively, in the NO fragments. The vibrational and rotational energy distributions of the fragments were unchanged for 0.06 ML, while the translational energy increased by approximately 30% in going to this lower coverage. The angular distribution was peaked in the normal direction at both coverages. The results are interpreted in terms of an excitation/deexcitation mechanism, for which the observed energy distributions can be rationalized by assuming differing equilibrium geometries between the ground and excited states of the adsorbed dimer.

No abstract

No abstract