Power spectra atomic clusters

The plots of (Ti(t) values at low internal energy (temperature) along trajectories initialized from the three isomers Da, C2v and Cat, of the LiJ cluster are shown in Fig. 1 and serve as a guidance for identification of groups of equivalent atoms with (almost) degenerate <7i(t) values. For the same trajectories the calculated power spectra are shown on the right hand side of Fig. 1. The comparison of power spectra obtained at low excess energy for all three isomers is instructive because different features can be identified. The positions of peaks correspond to frequencies obtained by harmonic vibrational analysis (vertical lines), whereas the intensities are dependent on the particular run. The power spectrum related to the D4d is characterized by peaks located at 100, 200 - 350 cm and a particularly narrow peak at 400 cm . The latter one is absent in the power spectra of isomers of Cat, and C31, symmetry, which are characterized by peaks spread in the interval between 100 and 300 cm . ... 

Figure 9. Power spectra at several energies for Ar,3 cluster, (a) In limit rigid portion of caloric curve at = —4.92 x 10" erg/atom. (b) Rigid limit form near melting, = —4.36 x 10 erg/atom. Notice softening of distinct modes. Zero intensity at cu = 0 is still maintained, however, (c) Spectrum of high-temperature form in coexistence region, = —4.13 x 10" erg/atom. (d) Spectrum of low-temperature form at same energy, (e) Spectrum at an energy in limit liquid portion of caloric curve, = -3.61 x 10 erg/atom.

When the surface of a solid is bombarded by energetic primary particles, usually, electrons, ions, neutrals, or photons, and secondary particles are emitted. The secondary particles are mainly electrons, neutral species, atoms or molecules, atomic and cluster ions. The majority of secondary particles are neutral species. The secondary ion mass spectrometry (SIMS) is a kind of mass spectrometry, where only secondary ions are detected and analyzed by the mass spectrometer. The process yields a mass spectrum of a surface and thus the method enables a detailed chemical analysis of a surface or solid. SIMS is such a powerful technique of surface analysis and microstructural characterization of solids that a very low primary particle flux density of 1 nA/cm or even less can generate spectral data in a timescale, which is very short compared to the lifetime of the surface layer. SIMS is particularly well known for its outstanding sensitivity of chemical and isotopic detection. Quantitative or semiquantitative analysis can... 

Photoelectron spectroscopy is a powerful technique to study ionic and electronically excited levels of atoms and molecules. In the case of single photon excitation of cold molecules the photoelectron spectrum reflects the internal energy levels of the ionic system. Many experiments are performed via two photon ionization enhanced by a one-photon resonance (R2PE spectroscopy) in which transitions to intermediate electronic levels are accessed which strongly enhance the ion yield. Photoelectron spectroscopy of molecules inside superfluid helium droplets is of particular interest since the interaction of free electrons with liquid helium is known to be highly repulsive, so much so that the electrons form bubbles of about 34 A diameter. In this section, three recent photoelectron spectra will be discussed those of bare helium droplets, of Ags clusters and of single aniline molecules in helium droplets. 

Such power levels can be quite adequate for Doppler-free saturation spectroscopy, as demonstrated in a series of recent studies of ultraviolet transitions of neutral helium in our laboratory at Stanford. (35-37) Fig. 5 shows as an example a spectrum of the 2 S - 5 P transition of He near 294.5 nm, recorded by intermodulated fluorescence spectroscopy. The ultraviolet radiation was generated by a yellow cw ring cavity dye laser with cavity-enhanced external ADA (ammonium dihydrogen arsenate) frequency doubler. The absorbing metastable He atoms were produced by electron impact excitation of He gas at about 0.04 torr. The spectrum shows a cluster of resolved line components which could be assigned after the fine and hyperfine Hamiltonian had been diagonalized in an uncoupled representation. We were surprised to learn that the hyperfine structure of the 5 P state of this simple 3-body system had been neither measured nor calculated before. 

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