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The photoelectron spectra of iodobenzene and 1,2-, 1.3-, and 1.4-di-iodobenzene have been recorded by using He1 excitation. The electronic structure of these compounds is discussed in terms of a simple model of interaction between known orbitals of the composite parts, i.e. the moJecular orbitals of benzene and the 5p atomic orbitals of iodine, taking their energy and symmetry but no spin-orbit coupling into account. The assignment is simplified by the characteristic shapes of several systems, which. holds especially for the lone-pair electrons on iodine. Orbital sequences are given down to ionisation energies of 17 eV. Assignments are supported by comparison with the photoelectron spectra di-iodo-benzene.THE effect of substituents on the x-electron levels in substituted benzenes has been the subject of a number of investigations. Turner and his co-workers have shown the effect of substituents lifting the degeneracy of the elg n-orbitals of benzene and discussed it in terms of x-electron perturbation by inductive and mesomeric effects. They also discussed in general terms the effect of multiple substitution (1,2-, 1,3-, 1,4-, and perfluorosubstitution) and the characteristics of systems associated with lone-pair electrons of substituents. Most of their conclusions are still valid. However, the progress in theoretical (computational) and instrumental methods (instruments with better resolution) made it of interest to study the substituted benzenes systematically in t ?n leave from the Ruder BoSkovi6 Institute, and the Faculty 1 A

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The He I and He I1 photoelectron spectra of CBrCI3, CHBrC12, CHBr2CI, CH2BrCl, CHFC12 and CHFzCl have been recorded with low and high resolutions. Correlation with assignments for other halogenated methanes, comparison of He II/He I intensities and vibrational fine-structure analysis allowed all the systems with ionization energies up to 24 eV to be assigned.

The valence-electronic structure of the first three members of the acene series, benzene, naphthalene and anthracene, seems to be confirmed now by their photoelectron (PE) spectra. This especially holds for the r-ionizations which, according to their specific behaviour, can be assigned to different ionization modes. The vibrational fine structure of the observed systems is discussed. The effects of a series of substituents on the electronic structure of the parent molecules were traced in their PE spectra and intercompared. The results for methyl, chloro, methoxy and cyano substitution are described in detail.The effect is most pronounced for the two lowest energy -ionizations (i.e. HOMO and SHOMO in the Koopmans' picture) and can be described by the shift of the mean energy ("center of gravity") of HOMO and SHOMO, and the additional splitting compared to the unsubstituted molecule. Both parameters show good linear dependence for the three series of compounds, thus allowing to formulate general substituent parameters. A linear dependence of the lowest ionization energy with Hammett constants is found to exist as well.

Abstract. Simultaneous measurements of Os, CO, and ultrafine aerosol particles (UFP), conducted on board of a Boeing 767-ER passenger aircraft flying from Sri Lanka to Germany (project CARIBIC), are used to study two-way cross-tropopause mixing near a subtropical tropopause fold. On the equatorward side of the fold, downward mixing of stratospheric air into the upper troposphere is identified by enhanced concentrations of O3 and xqco. Very high UFP number concentrations of up to 1.5 x 104 cm -3 (STP) were encountered inside the poleward half of the fold. This accumulation of small particles is explained by recent extensive aerosol nucleation, most likely triggered by the mixing of stratospheric air with tropospheric air injected into the fold. Further, nine particle formation events were observed outside the fold which are attributed to isolated cells of deep convection and to rising air parcels under cyclonic conditions that mix with surrounding air. In the upper troposphere Os and CO were found to be correlated with high AOa/ACO ratios of 0.6 to 1.5. In the fold the correlation was strongly negative with AOa/ACO = -3.5; but the high CO mixing ratios of 100 ppb at 03 mixing ratios of 250 ppb point to earlier injection of tropospheric air, in agreement with the UFP measurements.

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