H. Bluhme

Experimental studies of the photophysics of gas-phase fluorescent protein chromophores

To better understand the photophysics of photoactive proteins, the absorption bands of several gas-phase biomolecules have been studied recently at the electrostatic heavy-ion storage ring ELISA by a photo-fragmentation technique. In the present paper we discuss the involved photophysics and photochemistry for protonated and deprotonated model chromophores of the Green Fluorescent Protein (GFP) and the Red Fluorescent Protein (RFP). We show specifically that the delayed dissociation after photoabsorption can be understood in terms of a thermally activated process of the Arrhenius type. The rate of dissociation as a function of time after laser excitation is modeled in a calculation which is based on calculated heat capacities of the chromophores. Absorption of only one photon is enough to dissociate the deprotonated GFP chromophore on a time scale of milliseconds whereas absorption of two to three photons occurs for other chromophore ions. The difference is attributed to different activation energies, pre-exponential factors and locations of the absorption bands. We obtain activation energies for the dissociation that are of the order of 1–3 eV. Collision-induced dissociation experiments were performed to help identifying the fragmentation channels. Loss of methyl is found to be the dominant fragmentation channel for the deprotonated GFP chromophore and is also likely to be important for the protonated GFP chromophore at high temperatures. Other channels are open for the RFP chromophores. For the deprotonated RFP chromophore there is evidence that dissociation occurs through a non-trivial dissociation with substantial rearrangement.

Ionization of rare gases by particle - antiparticle impact

New data for single and double ionization of the rare gases Ne, Ar, Kr and Xe, by positron and antiproton impact are presented and compared with existing data for electron and proton impact. It is found that the current understanding of these processes obtained from studies involving low-Z targets only extends to high-Z atoms provided that certain target-dependent effects are taken into consideration. In particular, the static interaction between the projectile and the undistorted target is found to play a crucial role in the case of the lighter projectiles.

Non-dissociative and dissociative ionization of CO, CO2 and CH4 by positron impact

Cross sections for direct and total non-dissociative ionization of CO, CO2 and CH4 by positron impact have been measured for projectile energies ranging from threshold to 2000 eV. For CO and CO2 direct and total dissociative cross sections for several different fragments have been measured. The non-dissociative cross sections are observed to behave similarly to the single ionization cross sections of the light noble gases. The dissociative cross sections resemble that seen recently for N2, and can be related to models for double ionization of the light noble gases.

Ionization of argon and krypton by positron impact

Total single and double ionization cross sections for argon and krypton by positron impact have been measured for projectile energies ranging from threshold to ~1 keV. The single ionization cross sections are seen to behave as expected, showing large contributions from positronium formation at low energies. In the double ionization cross sections there seems to be a lack of positronium formation at threshold, while a clear contribution from the process is seen at intermediate energies.

Non-dissociative and dissociative ionization of nitrogen molecules by positron impact

The cross sections for direct and total dissociative and non-dissociative ionization of the nitrogen molecule by positron impact have been measured for projectile energies from threshold to 2000 eV. The results are compared with corresponding data for impact of electrons, protons and antiprotons. The comprehensive model which during recent years has been developed to explain the behaviour of the single- and multiple-ionization cross sections of atoms by charged particle impact was found to also apply for this case of a molecular target. In particular, the interference and factorization models which have been found to explain important features of the cross sections for double ionization of atoms apply also to the dissociative ionization of nitrogen molecules.

Ionization of helium, neon and xenon by positron impact

Single ionization of xenon and double ionization of helium, neon and xenon by positron impact have been studied at energies from threshold to ~1 keV. The total single ionization cross section of xenon is seen to behave as expected. In particular, single ionization with positronium formation is a strong channel at low energies. For double ionization we found that positronium formation is strongly suppressed and makes no contribution to the total cross section in the light noble gases, while some evidence for this channel is observed in xenon.

Single, double and triple ionization of Ne, Ar, Kr and Xe by 30 - 1000 keV impact

Experimental data for single, double and triple ionization of Ne, Ar, Kr and Xe by 30 - 1000 keV antiproton impact are presented and compared with existing proton data. It is found that the current phenomenological understanding of ionization developed from studies of light targets generally apply to heavy atomic targets as well. Inner-shell ionization followed by Auger decay is found to be an important channel for multiple ionization of the heaviest targets, and the projectile charge dependence of their multiple ionization cross sections must be understood as stemming from the underlying inner-shell ionization cross sections.

Channeling of antiprotons

Abstract We present experimental results for channeling of 1.4 MeV antiprotons in a 0.5 μm thick 〈1 0 0〉 silicon crystal. The ‘half-widths’ for the observed channeling effect, Δ ψ p , is significantly smaller than the Lindhard angle, ψ 1 , as is the case for electrons where typical angles are about Δ ψ e − ≃0.6 ψ 1 . In the case of protons and positrons, critical angles are of the order Δ ψ e + ≃Δ ψ p ≃ ψ 1 . Thus, we may attribute the difference between Δ ψ p , e − and Δ ψ p,e + to the sign of charge of the penetrating particle.

Measurement of the Barkas effect around the stopping-power maximum for light and heavy targets

Abstract The first direct measurements of antiproton stopping powers around the stopping power maximum are presented. The LEAR antiproton-beam of 5.9 MeV is degraded to 50–700 keV, and the energy-loss is found by measuring the antiproton velocity before and after the target. The antiproton stopping powers of Si and Au are found to be reduced by 30 and 40% near the electronic stopping power maximum as compared to the equivalent proton stopping power. The Barkas effect, that is the stopping power difference between protons and antiprotons, is extracted and compared to theoretical estimates.

Experimental demonstration of hydrogen formation following the interaction of protons with positronium

In support of the positronium based reaction scheme for antihydrogen formation an experiment has been performed which has demonstrated hydrogen formation following proton impact on positronium. The cross section has been experimentally determined at proton energies of 11.3 keV, 13.3 keV and 15.8 keV, values of σH = 26(±9), 7.8(±2.3) and 7.6(± 4.4) × 10−16 cm2 were obtained. The determination is in reasonable agreement with several recent calculations of the cross section and provides a well characterised process for antihydrogen formation.