Phase Measurement of Resonant Two-Photon Ionization in Helium

We study resonant two-color two-photon ionization of Helium via the 1s3p 1P1 state. The first color is the 15th harmonic of a tunable titanium sapphire laser, while the second color is the fundamental laser radiation. Our method uses phase-locked high-order harmonics to determine the {\it phase} of the two-photon process by interferometry. The measurement of the two-photon ionization phase variation as a function of detuning from the resonance and intensity of the dressing field allows us to determine the intensity dependence of the transition energy.Comment: 4 pages, 5 figures, under consideratio

Application des harmoniques générées dans un jet de gaz : mesure de sections efficaces d´ionisation des états excités de l´hélium

Les sections efficaces d'ionisation des états excités de l'hélium 1s2p 1P et 1s3p 1P ont été mesurées au voisinage du seuil d'ionisation en observant la saturation de l'ionisation. Les états excités de l'hélium sont préparés par l'absorption résonnante d'une harmonique d'ordre élevée produite par un laser picoseconde accordable. Ces états sont ensuite ionisés par un faisceau sonde en absorbant un photon. La modification de la fréquence du faisceau sonde, du proche infrarouge à l'ultraviolet, nous a permis de déterminer la dépendance de la section efficace d'ionisation en fonction de l'énergie de l'électron arraché. Les résultats expérimentaux confirment quantitativement les travaux théoriques effectués auparavant

Photoionization in the time and frequency domain

Ultrafast processes in matter, such as the electron emission following light absorption, can now be studied using ultrashort light pulses of attosecond duration ($10^{-18}$s) in the extreme ultraviolet spectral range. The lack of spectral resolution due to the use of short light pulses may raise serious issues in the interpretation of the experimental results and the comparison with detailed theoretical calculations. Here, we determine photoionization time delays in neon atoms over a 40 eV energy range with an interferometric technique combining high temporal and spectral resolution. We spectrally disentangle direct ionization from ionization with shake up, where a second electron is left in an excited state, thus obtaining excellent agreement with theoretical calculations and thereby solving a puzzle raised by seven-year-old measurements. Our experimental approach does not have conceptual limits, allowing us to foresee, with the help of upcoming laser technology, ultra-high resolution time-frequency studies from the visible to the x-ray range.Comment: 5 pages, 4 figure

Role of the Renner-Teller effect after core hole excitation in the dissociation dynamics of carbon dioxide dication.

The fragmentation of the doubly-charged carbon dioxide molecule is studied after photoexcitation to the C 1s(1)2π(u) and O 1s(1)2π(u) states using a multicoincidence ion-imaging technique. The bent component of the Renner-Teller split states populated in the 1s→ π∗ resonant excitation at both the carbon and oxygen 1s ionization edges opens pathways to potential surfaces in highly bent geometries in the dication. Evidence for a complete deformation of the molecule is found in the coincident detection of C(+) and O(2) (+) ions. The distinct alignment of this fragmentation channel indicates rapid deformation and subsequent fragmentation. Investigation of the complete atomization dynamics in the dication leading to asymmetric charge separation shows that the primary dissociation mechanisms, sequential, concerted, and asynchronous concerted, are correlated to specific fragment kinetic energies. The study shows that the bond angle in fragmentation can extend below 20°

Diffuse large B-cell lymphoma of Waldeyer's ring has distinct clinicopathologic features: a GELA study

Background Diffuse large B-cell lymphomas (DLBCLs) arising in specific extranodal sites have peculiar clinicopathologic features. Patients and methods We analyzed a cohort of 187 primary Waldeyer's ring (WR) DLBCLs retrieved from GELA protocols using anthracyclin-based polychemotherapy. Results Most patients (92%) had stage I-II disease. A germinal center B-cell-like (GCB) immunophenotype was observed in 61%, and BCL2 expression in 55%, of WR DLBCLs. BCL2, BCL6, IRF4 and MYC breakpoints were observed in, respectively, 3 of 42 (7%), 9 of 36 (25%), 2 of 26 (8%) and 4 of 40 (10%) contributive cases. A variable follicular pattern was evidenced in 30 of 68 (44%) large biopsy specimens. The 5-year progression-free survival (PFS) and the overall survival (OS) of 153 WR DLBCL patients with survival information were 69.5% and 77.8%, respectively. The GCB immunophenotype correlated with a better OS (P=0.0015), while BCL2 expression predicted a worse OS (P=0.037), an effect overcome by the GCB/non-GCB classification. Compared with matched nodal DLBCLs, WR DLBCLs with no age-adjusted international prognostic index factor disclosed a better 5-year PFS rate (77.5% versus 70.7%; P=0.03). Conclusions WR DLBCLs display distinct clinicopathologic features compared with conventional DLBCLs, with usual localized-stage disease, common follicular features and a high frequency of GCB immunophenotype contrasting with a low rate of BCL2 rearrangements. In addition, they seem to be associated with a better outcome than their nodal counterpar

Attosecond timing of electron emission from a molecular shape resonance

Shape resonances in physics and chemistry arise from the spatial confinement of a particle by a potential barrier. In molecular photoionization, these barriers prevent the electron from escaping instantaneously, so that nuclei may move and modify the potential, thereby affecting the ionization process. By using an attosecond two-color interferometric approach in combination with high spectral resolution, we have captured the changes induced by the nuclear motion on the centrifugal barrier that sustains the well-known shape resonance in valence-ionized N$_2$. We show that despite the nuclear motion altering the bond length by only $2\%$, which leads to tiny changes in the potential barrier, the corresponding change in the ionization time can be as large as $200$ attoseconds. This result poses limits to the concept of instantaneous electronic transitions in molecules, which is at the basis of the Franck-Condon principle of molecular spectroscopy.Comment: 24 pages, 5 figure