Single-atom spectroscopy

STM has also been adapted for perfonning single-atom vibrational spectroscopy [73],... 

Chapter 3 is devoted to pressure transformation of the unresolved isotropic Raman scattering spectrum which consists of a single Q-branch much narrower than other branches (shaded in Fig. 0.2(a)). Therefore rotational collapse of the Q-branch is accomplished much earlier than that of the IR spectrum as a whole (e.g. in the gas phase). Attention is concentrated on the isotropic Q-branch of N2, which is significantly narrowed before the broadening produced by weak vibrational dephasing becomes dominant. It is remarkable that isotropic Q-branch collapse is indifferent to orientational relaxation. It is affected solely by rotational energy relaxation. This is an exceptional case of pure frequency modulation similar to the Dicke effect in atomic spectroscopy [13]. The only difference is that the frequency in the Q-branch is quadratic in J whereas in the Doppler contour it is linear in translational velocity v. Consequently the rotational frequency modulation is not Gaussian but is still Markovian and therefore subject to the impact theory. The Keilson-... 

Wahl P, Diekhoner L, Schneider MA, Kern K (2008) Background removal in scanning tunneling spectroscopy of single atoms and molecules on metal surfaces. Rev Sci Instrum 79 043104-4... 

The other method. Auger electron spectroscopy, is considered appropriate for studying the chemical makeup (composition) of surfaces, with a sensitivity down to 1% of a single atomic layer (monolayer). It is also easier to perform than many other methods of surface studies of the present group. It is based on the principle that if an... 

Binh, V. T., Purcell, S. T., Garcia, N., and Doglini, J. (1992). Field emission spectroscopy of single-atom tips. Phys. Rev. Lett. 69, 2527-2530. 

Conventional analytical techniques generally operate at the part per million or higher levels. Some techniques such as laser photo acoustic spectroscopy are capable of measuring phenomena at the 10-8-10-6 mol/L level. The most sensitive conventional analytical techniques, time-resolved laser-induced fluorescence, and ICP-MS are capable of measuring concentrations at the part per trillion level, that is, 1 part in 1012, but rarely does one see detection sensitivities at the single atom level as routinely found in some radioanalytical techniques. While techniques such as ICP-MS are replacing the use of neutron activation analysis in the routine measurement of part per billion concentrations, there can be no doubt about the unique sensitivity associated with radioanalytical methods. 

Different metals and different processes can be used to prepare the tips. Mechanically cleaved platinum/iridium tips (4 1) provide very sharp atomically resolved images, and furthermore they are cheap, easy to prepare, and stable. However, the exact shape of the tip differs from one experiment to another the high resolution is achieved by randomly created minitips of potentially atomic size rather than by a perfect cone decreasing to a single atom end. In addition, the general shape of the tip is not conical, which can be necessary in some optical setups for coupling with spectroscopy. Therefore, a lot of effort has been done to produce reproducible electrochemically etched tips. The basic setup is depicted in Fig. 11. 

Laser-Induced Fluorescence. Laser-induced fluorescence (Lif) provides, much as does ir spectroscopy, fingerprints of different organic molecules, which can be quantified by measuring fluorescence intensities. Selectivity is excellent, as both pump and fluorescence frequencies can be individually chosen for optimum performance, and it can be improved with measurements of fluorescence lifetimes and polarization behavior. The enhanced null-background sensitivity can achieve single-atom or single-molecule detection (256—258). Lif has important applications in gas analysis (259) and combustion and plasma diagnostics (260). 

The problem of N bound electrons interacting under the Coulomb attraction of a single nucleus is the basis of the extensive field of atomic spectroscopy. For many years experimental information about the bound eigenstates of an atom or ion was obtained mainly from the photons emitted after random excitations by collisions in a gas. Energy-level differences are measured very accurately. We also have experimental data for the transition rates (oscillator strengths) of the photons from many transitions. Photon spectroscopy has the advantage that the photon interacts relatively weakly with the atom so that the emission mechanism is described very accurately by first-order perturbation theory. One disadvantage is that the accessibility of states to observation is restricted by the dipole selection rule. 

In the study of the surface phases of the Pt-Sn system, as well as of other binary systems, a variety of experimental methods are available. Surface spectroscopies based on ion or electron interaction with the surface provide composition information with a depth resolution that can go from a few atomic layers (X-ray photoelectron spectroscopy, XPS and Auger electron spectroscopy, AES) to single atomic layer resolution. The latter can be obtained by low energy ion scattering (LEIS) a method which has been extensively used for the study ot the Pt-Sn system. Since surface spectroscopic methods are rather well known we will not review them in detail here. 

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