UV-induced photoelectron

There exist a number of laser-induced techniques for ionizing these neutrals, such as resonance-enhanced multiphoton ionization (in ion source I or II, Figures lA and IB), laser-induced vacuum UV ionization (e.g. by 118 nm from a 3x355 nm/ Nd YAG laser), electron attachment (in ion source I) and electron ionization (in ion source II). Electrons may be supplied by laser-induced photoelectron emission from metal surfaces, preferably from thin wires made out of material having a low work function (e.g. hafnium). Ions formed in source I have to drift into the ion optics of the mass spectrometer (together with the neutral molecular beam) and can be extracted by a pulsed electric field while ionization in source II may be performed within a static electric field with instantaneous ion extraction. 

Building on the initial findings described in Section 4, we will acquire photoelectron images of 0CS (H20)i, cluster anions at different wavelengths in the visible and UV and investigate the dynamics of hydration and hydration-induced stabilization of... 

A review of the Journal of Physical Chemistry A, volume 110, issues 6 and 7, reveals that computational chemistry plays a major or supporting role in the majority of papers. Computational tools include use of large Gaussian basis sets and density functional theory, molecular mechanics, and molecular dynamics. There were quantum chemistry studies of complex reaction schemes to create detailed reaction potential energy surfaces/maps, molecular mechanics and molecular dynamics studies of larger chemical systems, and conformational analysis studies. Spectroscopic methods included photoelectron spectroscopy, microwave spectroscopy circular dichroism, IR, UV-vis, EPR, ENDOR, and ENDOR induced EPR. The kinetics papers focused on elucidation of complex mechanisms and potential energy reaction coordinate surfaces. 

An important issue associated with molecular machines is the detection of actuations on the nanoscale level. When a chemical stimulus induces movement in a machine, several spectroscopic techniques, such as nuclear magnetic resonance (NMR) spectroscopy, UV-Vis spectroscopy, emission spectroscopy and X-ray photoelectron spectroscopy (XPS) can be used to detect their outputs. More intri-guingly, electrochemical and photochemical inputs often provide [6, 8g] a two-fold advantage by inducing the mechanical movements and detecting them. Additionally, the dual actions of the these two types of stimuli can be exploited when the time-scale of the molecular actuations, which ranges from picoseconds to seconds, falls within the detection time-scale of the apparatus. 

Molecular orbital (CNDO/2) theoretical calculations have been carried out on [Cr(PF3)6], [Ni(PF3)J, and [Fe(PF3)5] (320), and the results compared with experimental ionization energies determined by UV photoelectron spectroscopic measurements of these complexes in the gas phase. The metal-phosphorus bonds show large ct(P— M) and ji(M—>P) charge transfers but small total charge transfers (M— P) which induce on the metal a small positive charge. 

Figure 8 Multiphoton excitation and ion spectroscopy Bottom spectrum cold, mass selected UVspectmm (S, <- So transition) of fluorobenzene, providing wavelengths for efficient and selective ionization. Middle spectra photoelectron spectra induced by UV-resonance enhanced two-photon absorption. Choosing different intermediate states [S,(0,0) or S,(6b )] results in different populations of the final fluorobenzene radical cations. Top spectrum spectroscopy of the excited ionic state of the fluorobenzene radical cation measured by muKiphoton dissociation spectroscopy. The ions have been prepared via the neutral 0°o transition. A special excitation scheme has been used to optimize cation spectroscopy (for further details see text).

Fig. 6.16. Probing the transient electronic structure of TbTes in the course of its ultrafast insulator-to-metal transition induced by femtosecond-laser excitation (a) Time- and angle-resolved photoemission spectroscopy. A TbTcs sample was excited by an IR pulse (hi Pump = I.SeV, about 50 fs duration) and probed after a time delay At with a UV pulse (hi/pump = 6 eV, about 90 fs duration). The photoelectron intensity and kinetic energy E ,i were measured as a function of the emission angles (a, 9). (b) Insulator-to-metal transition Above the critical temperature Tc (or 100fsafterlaserexcitation)the band gap of the CDW phase closes, (c) "Snapshots" of the electronic band structure E(k)in TbTej fordifferenttimedelaysAt.Afterlaserexcitation, the gap has closed and the band dispersion near the Eermi level, Ep, changed after a time delay of 100 fs. Such a delayed collapse of the band gap is characteristic of the "Peierls type" mechanism (see text).

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