An interference effect on angular correlation patterns due to exchange of Auger and photoelectrons of the same energy is discussed. The detection in coincidence of the Auger and photoelectrons may reveal a striking angular correlation pattern when a synchrotron light source is tuned so that the Auger and photoelectron energies are nearly equal. This efFect is present even when only one electron is detected, but is strongly reduced by the averaging over the direction of the undetected electron. PACS number(s): 32.80.Fb, 32.80.Hd

One-electron oxidation of zinc tetraphenylbacteriochlorin and its metal-free base yielded stable cation radicals. Electron spin resonance hyperfine splittings were assigned by selective deuterations. These results indicate that the protons of the saturated rings of the bacteriochlorins carry large spin densities, in accord with molecular orbital calculations. Comparison in vitro of the optical spectra of bacteriochlorins and their cation radicals with those of bacteriochlorophyll show close correspondence and suggest that the electron spin resonance data from the former may also prove a guide to the biological molecule. The surprising similarity in properties between the radicals of free base and zinc bacteriochlorins is maintained in the chlorophylls: cation radicals of bacteriopheophytin and methyl pheophorbide (the free bases of bacteriochlorophyll and methyl chlorophyllide, respectively) exhibit electron spin resonance properties similar to those of their magnesium-containing derivatives. The possibility that metal-free chlorophylls participate in photosynthesis is discussed.The primary photochemical step of intact photosynthesizing systems, extracted reaction centers, and chlorophylls involves electron ejection from chlorophyll a (Chl) in green plants and algae and from bacteriochlorophyll (BChl) in purple bacteria (1-9). The resultant oxidized chlorophylls exhibit properties characteristic of ir-cation radicals (7, 8), i.e., the unpaired electron is delocalized over the ir system of the aromatic molecule. No electron spin resonance (ESR) hyperfine structure has ever been resolved for the chlorophyll radicals (2, 7, 10-13). The singlet ESR signals observed narrow significantly with deuterochlorophyll (2, 7, 12) and broaden upon I3C substitution (11,12), which, while confirming the ir-radical nature of the oxidized molecules, yields little additional information about the spin-density distribution or the reactive sites of the chlorophylls. Electron-nuclear double resonance (14) has been applied to bacteriochlorophylls in vitro and in vivo. The results (15, 16) support the hypothesis regarding reaction centers (7), that the photoactivated BChl is a bridged dimeric cation, (BChl)2+, but they do not yield sufficient information to specify the sites of spin delocalization.In contrast to the ESR singlets from Chl+ and BChl+ (or from Chl2+ and BChl2+ ), oxidized zinc tetraphenylAbbreviations: Chl, chlorophyll; BChl, bacteriochlorophyll; Ph, pheophytin or methyl pheophorbide; BPh, bacteriopheophytin; TPBC, tetraphenylbacteriochlorin; TPP, tetraphenylporphyrin; ESR, electron spin resonance; AH, linewidth, peak to peak, of first derivative singlet ESR signal; aX. or As,., proton hyperfine splitting constant at position-X. bacteriochlorin (ZnTPBC+ ) yields hyperfine structure that can be readily assigned, by selective deuterations, to specific positions in the radical. BChl is a modified bacteriochlorin (tetrahydroporphyrin); comparison of its electronic spectra, both as BChl and BChl+, with those of ZnTPBC an...

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The eikonal distorted-wave method is applied to single ionization in proton-helium collisions at laboratory energies of 300 keV, 500 keV, 1 MeV, and 3 MeV. The angular distributions of scattered protons agree well with recent experimental results, showing substantial improvement over the plane-wave Born approximation especially at large scattering angles e. The momentum distributions of the ejected electrons at fixed values of e are calculated. In particular, those of the electrons ejected mainly in binary proton-electron collisions are studied in detail. The momentum distributions of the recoil He+ ions at fixed values of e are also discussed by comparing with the results of recent coincidence measurements.At such small 6 at which the plane-wave Born and the eikonal distorted-wave approximations give nearly the same cross sections and are expected to be accurate, these approximations lead to average momenta of recoil ions significantly lower than the measured values.

We present a general treatment for the effect of time ordering in the many-electron processes induced by charged-particle impact. We show that time ordering is essential in order to obtain odd power Z terms in the cross section which is necessary to get different values for positively and negatively charged projectiles. A second-order calculation is performed for the double ionization of helium. The electron-electron interaction is taken into account only by the shake-off term. The interference between the shake-off and second-order term is responsible only for a part of the dependence of the cross sections on the sign of the charge of the projectile.