Valence shell electronically excited states of norbornadiene and quadricyclane.

The absolute photoabsorption cross sections of norbornadiene (NBD) and quadricyclane (QC), two isomers with chemical formula C7H8 that are attracting much interest for solar energy storage applications, have been measured from threshold up to 10.8 eV using the Fourier transform spectrometer at the SOLEIL synchrotron radiation facility. The absorption spectrum of NBD exhibits some sharp structure associated with transitions into Rydberg states, superimposed on several broad bands attributable to valence excitations. Sharp structure, although less pronounced, also appears in the absorption spectrum of QC. Assignments have been proposed for some of the absorption bands using calculated vertical transition energies and oscillator strengths for the electronically excited states of NBD and QC. Natural transition orbitals indicate that some of the electronically excited states in NBD have a mixed Rydberg/valence character, whereas the first ten excited singlet states in QC are all predominantly Rydberg in the vertical region. In NBD, a comparison between the vibrational structure observed in the experimental 11B1-11A1 (3sa1 ← 5b1) band and that predicted by Franck-Condon and Herzberg-Teller modeling has necessitated a revision of the band origin and of the vibrational assignments proposed previously. Similar comparisons have encouraged a revision of the adiabatic first ionization energy of NBD. Simulations of the vibrational structure due to excitation from the 5b2 orbital in QC into 3p and 3d Rydberg states have allowed tentative assignments to be proposed for the complex structure observed in the absorption bands between ∼5.4 and 7.0 eV.

Secondary ionization of pyrimidine nucleobases and their microhydrated derivatives in helium nanodroplets.

Radiation damage in biological systems by ionizing radiation is predominantly caused by secondary processes such as charge and energy transfer leading to the breaking of bonds in DNA. Here, we study the fragmentation of cytosine (Cyt) and thymine (Thy) molecules, clusters and microhydrated derivatives induced by direct and indirect ionization initiated by extreme-ultraviolet (XUV) irradiation. Photofragmentation mass spectra and photoelectron spectra of free Cyt and Thy molecules are compared with mass and electron spectra of Cyt/Thy clusters and microhydrated Cyt/Thy molecules formed by aggregation in superfluid helium (He) nanodroplets. Penning ionization after resonant excitation of the He droplets is generally found to cause less fragmentation compared to direct photoionization and charge-transfer ionization after photoionization of the He droplets. When Cyt/Thy molecules and oligomers are complexed with water molecules, their fragmentation is efficiently suppressed. However, a similar suppression of fragmentation is observed when homogeneous Cyt/Thy clusters are formed in He nanodroplets, indicating a general trend. Penning ionization electron spectra (PIES) of Cyt/Thy are broad and nearly featureless but PIES of their microhydrated derivatives point at a sequential ionization process ending in unfragmented microsolvated Cyt/Thy cations.

Electron energy loss and angular asymmetry induced by elastic scattering in superfluid helium nanodroplets.

Helium nanodroplets are ideal model systems to unravel the complex interaction of condensed matter with ionizing radiation. Here we study the effect of purely elastic electron scattering on angular and energy distributions of photoelectrons emitted from He nanodroplets of variable size (10-109 atoms per droplets). For large droplets, photoelectrons develop a pronounced anisotropy along the incident light beam due to a shadowing effect within the droplets. In contrast, the detected photoelectron spectra are only weakly perturbed. This opens up possibilities for photoelectron spectroscopy of dopants embedded in droplets provided they are smaller than the penetration depth of the light and the trapping range of emitted electrons in liquid helium.

Dopant ionization and efficiency of ion and electron ejection from helium nanodroplets.

Photoionization spectroscopy and mass spectrometry of doped helium (He) nanodroplets rely on the ability to efficiently detect ions and/or electrons. Using a commercial quadrupole mass spectrometer and a photoelectron-photoion coincidence spectrometer, we systematically measure yields of ions and electrons created in pure and doped He nanodroplets in a wide size range and in two ionization regimes-direct ionization and secondary ionization after resonant photoexcitation of the droplets. For two different types of dopants (oxygen molecules, O2, and lithium atoms, Li), we infer the optimal droplet size to maximize the yield of ejected ions. When dopants are ionized by charge-transfer to photoionized He nanodroplets, the highest yield of O2 and Li ions is detected for a mean size of ∼5×104 He atoms per nanodroplet. When dopants are Penning ionized via photoexcitation of the He droplets, the highest yield of O2 and Li ions is detected for ∼103 and ∼105 He atoms per droplet, respectively. At optimum droplet sizes, the detection efficiency of dopant ions in proportion to the number of primary photoabsorption events is up to 20% for charge-transfer ionization of O2 and 2% for Li, whereas for Penning ionization it is 1% for O2 and 4% for Li. Our results are instrumental in determining optimal conditions for mass spectrometric studies and photoionization spectroscopy of molecules and complexes isolated in He nanodroplets.

X-ray Induced Fragmentation of Protonated Cystine.

We demonstrate site-specific X-ray induced fragmentation across the sulfur L-edge of protonated cystine, the dimer of the amino acid cysteine. Ion yield NEXAFS were performed in the gas phase using electrospray ionization (ESI) in combination with an ion trap. The interpretation of the sulfur L-edge NEXAFS spectrum is supported by Restricted Open-Shell Configuration Interaction (ROCIS) calculations. The fragmentation pathway of triply charged cystine ions was modeled by Molecular Dynamics (MD) simulations. We have deduced a possible pathway of fragmentation upon excitation and ionization of S 2p electrons. The disulfide bridge breaks for resonant excitation at lower photon energies but remains intact upon higher energy resonant excitation and upon ionization of S 2p. The larger fragments initially formed subsequently break into smaller fragments.

Hospital outcome of coronary artery bypass grafting and coronary endarterectomy.

To compare hospital mortality and postoperative complications in patients with severe coronary artery disease undergoing coronary artery bypass grafting with or without coronary endarterectomy, 100 consecutive patients were divided into 2 groups of 50 patients each. Group 1 had coronary endarterectomy, and group 2 had no coronary endarterectomy. There were 87 men and 13 women, with a mean age of 54.79 +/-7.8 years; 48% had stable angina. The right coronary artery was endarterectomized in 22 patients, the left anterior descending artery in 21, and the left circumflex in 7. There were no significant differences in outcomes. There was 1 hospital death in each group. Perioperative myocardial infarction occurred in 2 patients in group 1 and 1 in group 2. Endarterectomy is a suitable option to achieve complete revascularization in patients with refractory angina and severe diffuse disease.

Anticoagulation in pregnancy with mechanical heart valves: 10-year experience.

Anticoagulation in pregnancy was evaluated in 33 women with a mechanical heart valve prosthesis who had 53 pregnancies between 1994 and 2006. Their mean age at valve operation was 24.4 +/- 5.4 years, and 22 (67%) had isolated mitral valve disease. Of these patients, 22 had a single pregnancy, 5 had 2 pregnancies, 3 had 3, and 3 had 4. In 43 pregnancies, the patients took warfarin throughout; in the other 10, heparin was used in the first trimester followed by warfarin until the last 15 days. Mean international normalized ratio and warfarin levels before, during, and after pregnancy were similar. Complications occurred in 3 (6%) women who had thrombosed valves: 2 (20%) in the heparin group and 1 (2%) who had warfarin only. Live births resulted from 37 (70%) pregnancies. There were significantly more abortions in the heparin group (6; 60%) than the warfarin group (8; 19%). Hemorrhage requiring transfusion occurred in 2 (5%) patients in the warfarin group. All live births resulted in healthy babies. It was concluded that anticoagulation with warfarin is safe during pregnancy in women with mechanical heart valves.

Prosthetic valve replacement in adolescents with rheumatic heart disease.

To assess long-term survival and anticoagulant-related complications after mechanical valve replacement in adolescents with rheumatic heart disease, 88 patients aged