Chapter 5 Multiple Choice Questions

. Which of these techniques may be used to characterise the unoccupied electronic states of adsorbed molecules?

. When carried out under some particular conditions, a LEED experiment on a certain sample results in only a small number of diffraction spots being visible on the instrument's phosphor screen. What might the experimenter do in order to ensure that more spots are viewable on the screen at any one time?

. An STM tip is scanned across a nominally flat surface at constant height, with a constant and very low bias voltage applied between it and the sample. As the tip crosses over a step edge, the measured current increases by a factor of 100. The work function of the tip is known to be 4.0 eV, and that of the surface 5.0 eV. What would be your best estimate for the height of the step?

. During an ARUPS experiment, conducted with photons of energy 40.81 eV, electrons photoemitted from a certain state at binding energy 1.00 eV are detected at a polar angle of 60⁰ relative to the surface normal. The sample work function is known to be 4.50 eV. At approximately what polar angle would photoelectrons be emitted from a state having the same surface-parallel wavenumber but a binding energy of 3.00 eV?

. In an XPS study of a PdO surface, making use of photons of energy 1253.6 eV, electrons photoemitted from the O 1s core state (binding energy 530.3 eV) are detected with a kinetic energy of 717.1 eV. What must be the work function of the surface?

. A supersonic beam experiment is conducted using pure methane and a nozzle temperature of 300 K. Aiming to study the effect of increased vibrational energy within the methane molecules, it is intended to repeat the experiment with a xenon-seeded beam and a nozzle temperature of 325 K. Take the constant-pressure and constant-volume heat capacities of methane to be roughly 4.4R and 3.4R respectively, with R being the molar gas constant, and those of xenon to be roughly 2.4R and 1.4R. Assume that the corresponding values for a methane/xenon mixture linearly interpolate between those of the individual gases. What proportion of the beam (expressed as a mole fraction) would xenon have to constitute in order to reproduce the same translational energy for the methane molecules as they had in the first experiment?

. Prior to a King & Wells experiment, a given sample is prepared in a vacuum chamber at a base pressure of 5.6 × 10-11 mbar. A supersonic beam of a certain species is then allowed to enter the chamber, but is prevented from directly striking the sample by means of an inert flag placed in its path. The pressure rises at this point, but eventually settles at 1.24 × 10-10 mbar. Upon removing the flag from the path, the chamber pressure abruptly drops to 7.3 × 10-11 mbar, before gradually rising again, reaching around 9.0 × 10-11 mbar by the time the flag is reinserted. The pressure then rises quickly back to 1.24 × 10-10 mbar, at which point the experiment is concluded. What is the initial sticking probability of the beam species on this sample?

. A certain metal surface is exposed to CO and monitored continuously by RAIRS. At first, a peak at around 2100 cm-1, associated with the C-O stretch mode of an upright molecule, is seen to increase in overall intensity in proportion with the exposure time, but eventually its intensity stops increasing and then drops significantly before settling at around three quarters of its maximum intensity. No other absorption peaks are detected. Upon subsequently heating the surface (after ceasing exposure) these intensity changes are exactly reversed as CO desorbs. What might best explain the reduction in RAIRS intensity at high exposure?

. An apparatus for conducting SFG experiments makes use of a monochromatic visible laser of wavelength 532 nm, and an infrared laser whose wavelength may be modulated within the frequency range 1000–4000 cm-1. What will be the approximate wavelength range of the resulting sum-frequency beam?

. A plane-wave DFT calculation is to be performed within a supercell for which the corresponding reciprocal-space volume of the 1BZ is roughly 125 nm-3. If a 350 eV kinetic energy cutoff is applied, roughly how many plane-wave coefficients will have to be calculated to describe those states at the centre of the 1BZ?

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