The ionization of a beam of H2 Rydberg molecules in collision with a metal surface (evaporated Au or Al) is studied. The Rydberg states are excited in an ultraviolet-vacuum ultraviolet double-resonant process and are state selected with a core rotational quantum number N+=0 or 2 and principal quantum numbers n=17-22 (N+=2) or n=41-45 (N+=0). It is found that the N+=0 states behave in a very similar manner to previous studies with atomic xenon Rydberg states, the distance of ionization from the surface scaling with n2. The N+=2 states, however, undergo a process of surface-induced rotational autoionization in which the core rotational energy transfers to the Rydberg electron. In this case the ionization distance scales approximately with nu0(2), the effective principal quantum number with respect to the adiabatic threshold. This process illustrates the close similarity between field ionization in the gas phase and the surface ionization process which is induced by the field due to image charges in the metal surface. The surface ionization rate is enhanced at certain specific values of the field, which is applied in the time interval between excitation and surface interaction. It is proposed here that these fields correspond to level crossings between the N+=0 and N+=2 Stark manifolds. The population of individual states of the N+=2, n=18 Stark manifold in the presence of a field shows that the surface-induced rotational autoionization is more facile for the blueshifted states, whose wave function is oriented away from the surface, than for the redshifted states. The observed processes appear to show little dependence on the chemical nature of the metallic surface, but a significant change occurs when the surface roughness becomes comparable to the Rydberg orbit dimensions.

In this paper we report the fabrication of model catalysts prepared to understand the structure of the BaO surface. This utilises the 'inverse' catalyst method, that is, the oxide layer is fabricated onto the top of a metal single crystal surface. We show that we can atomically resolve the surface structure of BaO(111) and that it presents a (2·2) reconstruction at its surface. Under other dosing conditions we can produce a layer which is metastable at 573K, which we believe to be the peroxide, BaO 2 . We have shown that the BaO layer can store NO x from a mix of NO and oxygen, even under the extremely low exposure conditions of UHV, proving that the NOx storage process is a facile one. The results indicate that it is not necessary to have NO2 in the gas phase in order to store NO x .KEY WORDS: NSR; NOx storage and reduction; SCR; STM; model catalysts; BaO model catalysts.NSR (NOx storage and reduction) catalysis is an important strategy to aid in the removal of pollutants from lean-burn type engines [1,2]. There is a considerable amount of work relating to this process in reactors and from infra-red measurements [3,4], but little which is devoted to the resolution of these processes at the ultra-nanoscale. To that end we here report on the first studies related to NSR using STM.Our STM has been described in detail elsewhere [5], but for brevity, suffice it to say that it has the capability of achieving atomic resolution, but also a near-unique capability of imaging at high temperature (up to 1000K), with relatively little thermal drift. It also has facilities for surface analysis (Auger electron spectroscopy) and surface cleaning, sputtering and gas treatment. All of the work below was carried out under ultrahigh vacuum conditions to ensure the purity of the surfaces studied and of the gases dosed.The strategy here is to attempt to make inverse model catalysts by depositing BaO onto the surface of Pt(111). The reason for using this strategy is that STM relies on having conductivity in the imaged material in order to obtain a tunnelling current. It has been shown that thin layers of wide band gap materials (even alumina [6,7]) are suitable for imaging, provided they are fabricated onto conducting support materials, usually single crystal metals [8]. Figure 1a and 2 shows two images obtained after the adsorption of Ba, followed by oxidation under different conditions. In Figure 1a we believe we have formed the BaO(111) surface [9], whereas in figure 1b we propose that BaO 2 is formed [10]. The reason for these assignments is that the structure in 1a) is thermally stable, and the atomic spacing corresponds with that expected for the BaO(111) surface with a (2 · 2) reconstruction. Note that the (111) BaO-(1 · 1) surface is polar and unstable and is therefore not expected to form, whereas theory predicts a (2 · 2) reconstruction for this surface, producing a near-neutral layer with low surface energy, figure 1b [9]. Other structures are identified under varying oxygen treatment conditions, which may be further recon...

A continuous wave quantum cascade laser (cw-QCL) operating at 10 μm has been used to record absorption spectra of low pressure samples of OCS in an astigmatic Herriott cell. As a result of the frequency chirp of the laser, the spectra show clearly the effects of rapid passage on the absorption line shape. At the low chirp rates that can be obtained with the cw-QCL, population transfer between rovibrational quantum states is predicted to be much more efficient than in typical pulsed QCL experiments. This optical pumping is investigated by solving the Maxwell Bloch equations to simulate the propagation of the laser radiation through an inhomogeneously broadened two-level system. The calculated absorption profiles show good quantitative agreement with those measured experimentally over a range of chirp rates and optical thicknesses. It is predicted that at a low chirp rate of 0.13 MHz ns(-1), the population transfer between rovibrational quantum states is 12%, considerably more than that obtained at the higher chirp rates utilised in pulsed QCL experiments.

The ionization of H(2) Rydberg states at a metal surface is investigated using a molecular beam incident at grazing incidence on a gold surface. The H(2) molecules, excited by stepwise two-color laser excitation, are selected in each of the accessible Stark eigenstates of the N(+) = 2, n = 17 Rydberg manifold in turn and the ionization at the surface is characterized by applying a field to extract the ions formed. Profiles of extracted ion signal versus applied field show resonances that can be simulated by assuming an enhancement of surface ionization at fields corresponding to energy-level crossings between the populated N(+) = 2 manifold and the near-degenerate N(+) = 0 Stark manifolds. It is concluded that the slow (microsecond time scale) rotation-electronic energy transfer to N(+) = 0 states occurring at these crossings takes place in the time interval following application of the field ramp when the molecule is still distant from, and unperturbed by, the surface. However, the energy levels are strongly perturbed by image-dipole interactions as the molecule approaches close to the surface, leading to additional energy-level crossings. Adiabatic behavior at such crossings affects the intensity of the observed resonances in the surface ionization signal but not their field positions. Resonances are also observed in the surface ionization profiles at fields above the field-ionization threshold; some of these show asymmetric "Fano-type" line shapes due to quantum interference in the nonradiative coupling to degenerate bound and continuum states.

A high power continuous wave quantum cascade laser operating around 1900 cm(-1) has been used to conduct Lamb dip spectroscopy on a low pressure sample of NO. The widths of the Lamb dips indicate that the laser linewidth is 800 ± 60 kHz and the power sufficient to induce significant population transfer of up to 35%. While the Lamb dip signals are symmetric at low laser chirp rates, they become increasingly asymmetric as the chirp rate increases, further confirming the significant degree of population transfer. In addition rapid passage structure on the Lamb dip signal is observed after the weak probe beam is swept through the line center. This structure is sensitive to both the probe chirp rate and the underlying hyperfine structure of the rovibrational transition, and is accurately modeled using the optical Bloch equations.