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Notes: Penning ionization electron spectra (PIES) of van der Waals clusters have been observed for the first time by means of a newly developed magnetic bottle electron spectrometer, which is 1000 times more efficient than a conventional PIES apparatus. In the PIES of Ar clusters with He*(2 3S), two bands of Ar 3p hole characters with different ionization energies (IE), band A (IE=ca.15 eV) and band B (IE=ca.13.5 eV), were observed. Band A was assigned to vertical transitions in Franck–Condon regions corresponding to the band in the ultraviolet photoelectron spectra (UPS) of clusters reported by Carnovale et al. [J. Chem. Phys. 90, 1452 (1989)]. Band B, which is an extra band missing in the UPS of clusters, can be ascribed to the autoionization into a stable structure of Arn+ via superexcited states of Arn. Band B can be considered as an adiabatic ionization which is forbidden as a direct ionization. The energy difference of ca. 1.5 eV between the vertical IE for band A and the adiabatic IE for band B was found to agree well with the reported binding energy of the trimer ion core. © 2000 American Institute of Physics.

Notes: Electronic structures in metallic and insulating phases of the Li, Cu, and Ba complexes of 2,5-R1,R2-DCNQI [R1=R2=Br (abbreviated as DBr) or R1=R2=CH3 (abbreviated as DMe); DCNQI=N,N'-dicyanoquinonediimine; 2,5- is usually omitted] have been studied by observing temperature dependencies of their infrared absorption bands between 295 and 23 K. At room temperature, the wave numbers (ν˜i) of infrared absorption bands of R1,R2-DCNQI and its Li and Ba complexes are linearly correlated with the degrees of charge transfer (ρ) (ρ=−0.5 and −1.0e for the Li and Ba complexes, respectively). The ν˜i–ρ relationships indicate that the ρ value for the Cu complexes is −0.67e. This result is consistent with the previously established view that the Cu cations in the Cu complexes at room temperature are in a mixed-valence state of Cu1.33+. In the infrared spectrum of Cu(DBr–DCNQI)2 at room temperature, no electron–molecular vibration (EMV) coupling bands are observed.Below the metal–insulator (M–I) transition temperature (TMI), EMV bands grow continuously and the ordinary infrared bands observed at room temperature gradually split into three bands with decreasing temperature. Similarly, the infrared bands of Li(DBr–DCNQI)2 split into two bands. These splittings are due to an inhomogeneous charge distribution in the DCNQI columns produced by the freezing of charge-density wave (CDW). The peak-to-peak amplitudes of CDWs in the DCNQI columns estimated by use of the ν˜i–ρ relationships are 0.08±0.04 and 0.40±0.04e, respectively, for the Li and Cu complexes of DBr–DCNQI. The state of the frozen CDW is inferred from the number of split bands. Based on the observed continuous change of the infrared spectra of Cu(DBr–DCNQI)2 and the discontinuous changes of other quantities such as x-ray satellite reflections, lattice parameters, and magnetic susceptibilities, the M–I transition in Cu(DBr–DCNQI)2 may be described as follows: (1) above TMI the charges on Cu cations (two Cu1+'s: one Cu2+) are dynamically averaged to +1.33e through the Cu...N≡C bridge. (2) At TMI the charges abruptly localize in the order of (Cu1+...Cu2+...Cu1+...)n. At the same time, the CDWs begin to be frozen in the DCNQI columns. (3) As temperature decreases below TMI, the order of the frozen CDW develops gradually. In contrast to these changes in Cu(DBr–DCNQI)2, neither EMV bands nor band splittings are observed in the infrared spectra of Cu(DMe–DCNQI)2 at low temperatures. Instead, almost all bands show negative absorption lobes on their low-wave number sides and become asymmetric. This asymmetrization is due to interactions between the vibrational levels and low-lying continuous electronic levels responsible for a broad band observed in the 1600–800 cm−1 region.

Notes: A highly sensitive electron energy analyzer which utilizes a "magnetic bottle" combined with a retarding electrostatic field has been developed for Penning ionization electron spectroscopy. A beam of metastable rare-gas atoms is crossed with a continuous supersonic sample beam in the source region of the analyzer. The emitted electrons are collected by an inhomogeneous magnetic field (the magnetic bottle effect) with a high efficiency of nearly 4π solid angle, which is more than 103 times higher than that of a conventional hemispherical analyzer. The kinetic energy of electrons is analyzed by scanning the retarding field in a flight tube of the analyzer in the presence of a weak magnetic field. The velocity of the metastable atoms can also be resolved by a time-of-flight method in the present instrument. Examples of Penning ionization electron energy spectra as a function of collision energy are presented for Ar and N2 with metastable He*(2 3S) atoms. This instrument has opened the possibility for extensive studies of Penning ionization electron spectroscopy for low-density species, such as clusters, ions, electronically excited species, unstable or transient species, and large molecules with low volatility. © 2000 American Institute of Physics.