Jie Chen

The formation of metal–oxygen species at low temperatures

The interaction of solid molecular oxygen with Li, Cs, K, La, Ag, Cu, and Ba has been studied at 35 K or below using photoemission. A feature near 535 eV in the O 1s core‐level spectra was observed when Li, Cs, K, and La were deposited on solid oxygen. This feature was identified with one electron being donated to an oxygen molecule, i.e., the superoxide species, which as far as we know has not been previously reported for La and Li. A feature at about 531.5–533 eV was identified as a peroxide species where two electrons were donated to an oxygen molecule. Finally, features at about 528–530.5 eV were identified as oxide phases where the molecular oxygen was dissociated into atomic O with formal oxidation state of −2. These identifications are crucial in the determinations of the exotic features in the x‐ray photoelectron spectroscopy (XPS) O 1s spectra of the high Tc superconductors.

Auger and x‐ray absorption studies of solid molecular oxygen

We have studied the x‐ray absorption spectrum (XAS) and the Auger electron spectrum (AES) of condensed molecular oxygen in the vicinity of the O 1s (K) core level. The XAS spectrum, which is similar to that previously reported for gas phase O2, has sharp peaks at about 530.8 and 540.3 eV, which we attribute to excitations from the 1s level into π* and σ* states, respectively. We assign a third peak at 543.3 eV to Rydberg transitions. Compared with gas phase O2, the σ* and Rydberg states are shifted to higher energies, an effect that we attribute to solid state effects in the condensed layer. The AES spectra of the ionized molecule and excited molecule are both shifted relative to the spectra of gas phase O2, but by different amounts, 10 and 2 eV, respectively, which may be due to different final state screening effects. A tentative assignment of the features in the AES spectra is also made.

Orientational behavior of thin films of poly(3-methylthiophene) on platinum: A FTIR and NEXAFS study

Abstract Near edge x-ray absorption fine structure (NEXAFS) and infrared reflection-absorption spectroscopy (IRRAS) have been used to study the orientational behavior of thin films of poly(3-methylthiophene) electrochemically polymerized on a platinum surface. Clear orientational effects, with the thiophene rings predominantly oriented parallel to the platinum surface, were observed when the thickness of the polymer films were within a few hundred A. It was found that more highly ordered films were produced at lower polymerization potential (1.4V vs SCE) than at higher potential (1.8V vs SCE).

The identification of oxygen related species in the XPS and near edge spectra of the high Tc superconductors

Abstract In this brief note we describe data on various oxygen-metal species as well as near edge data on electron doped superconductors.

X‐ray absorption studies of YBa2Cu3O6+x, La2−xSrxCuO4, and Nd2−xCexCuO4

O K‐edge absorption studies show that the density of O 2p empty states near the Fermi level increases with increasing oxygen content for YBa2Cu3O6+x, and Sr concentration in La2−xSrxCuO4. With Zn substitution for Cu in YBa2Cu3O7, Tc does not correlate with the density of O 2p empty states. In this case Tc dramatically decreases with Zn substitution while the pre‐edge feature remains unchanged. The O K‐edge spectrum of Nd2−xCexCuO4 barely changes from the undoped, insulating, to the Ce doped superconducting sample. The lack of unoccupied O 2p states near the Fermi level in this case is consistent with naive expectations for electron‐doping.

Langmuir-Blodgett films of electroactive polymers

Electroactive Langmuir-Blodgett (LB) films have been assembled from mixtures of poly(3-alkyl thiophenes) and stearic acid. Stable monolayers and highly ordered multilayers were formed with varying alkyl chain lengths from four carbons to eighteen carbons. X-ray diffraction revealed that the multilayers form well-defined two-dimensional structures. X-ray absorption experiments revealed that the polythiophene backbone is disordered in a matrix of highly ordered and vertically oriented stearic acid molecules. With long (eighteen carbon) alkyl chains, the chains were oriented along the stearic acid molecules.<<ETX>>