35 research outputs found
Ultrafast dynamics of neutral superexcited Oxygen: A direct measurement of the competition between autoionization and predissociation
Using ultrafast extreme ultraviolet pulses, we performed a direct measurement
of the relaxation dynamics of neutral superexcited states corresponding to the
nl\sigma_g(c^4\Sigma_u^-) Rydberg series of O_2. An XUV attosecond pulse train
was used to create a temporally localized Rydberg wavepacket and the ensuing
electronic and nuclear dynamics were probed using a time-delayed femtosecond
near-infrared pulse. We investigated the competing predissociation and
autoionization mechanisms for superexcited molecules and found that
autoionization is dominant for the low n Rydberg states. We measured an
autoionization lifetime of 92+/-6 fs and 180+/-10 fs for (5s,4d)\sigma_g and
(6s,5d)\sigma_g Rydberg state groups respectively. We determine that the
disputed neutral dissociation lifetime for the \nu=0 vibrational level of the
Rydberg series is 1100+/-100fs.Comment: 5 pages, 4 figure
Time-Resolved Ultrafast Transient Polarization Spectroscopy to Investigate Nonlinear Processes and Dynamics in Electronically Excited Molecules on the Femtosecond Time Scale
We report a novel experimental technique to investigate ultrafast dynamics in
photoexcited molecules by probing the third-order nonlinear optical
susceptibility. A non-colinear 3-pulse scheme is developed to probe the
ultrafast dynamics of excited electronic states using the optical Kerr effect
by time-resolved polarization spectroscopy. Optical heterodyne and optical
homodyne detection are demonstrated to measure the third-order nonlinear
optical response for the S1 excited state of liquid nitrobenzene, which is
populated by 2-photon absorption of a 780 nm 35 fs excitation pulse.Comment: 12 pages, 4 figures. Changes from previous version: added panel
labels to figures 3-
Attosecond Entangled Photons from Two-Photon Decay of Metastable Atoms: A Source for Attosecond Experiments and Beyond
We propose the generation of attosecond entangled bi-photons in the
extreme-ultraviolet regime by two-photon decay of a metastable atomic state as
a source similar to spontaneous parametric down-conversion photons. The 1s2s
metastable state in helium decays to the ground state by emission of
two energy-time entangled photons with a photon bandwidth equal to the total
energy spacing of 20.62 eV. This results in a pair correlation time in the
attosecond regime making these entangled photons a highly suitable source for
attosecond pump-probe experiments. The bi-photon generation rate from a direct
four photon excitation of helium at 240 nm is calculated and used to assess
some feasible schemes to generate these bi-photons. Possible applications of
entangled bi-photons in attosecond time scale experiments, and a discussion of
their potential to reach the zeptosecond regime are presented.Comment: 6 pages, 3 figures, and supplementary materia
Electric Field Measurement of Femtosecond Time-Resolved Four-Wave Mixing Signals in Molecules
We report an experiment to measure the femtosecond electric field of the
signal emitted from an optical third-order nonlinear interaction in carbon
dioxide molecules. Using degenerate four-wave mixing with femtosecond near
infrared laser pulses in combination with the ultra-weak femtosecond pulse
measurement technique of TADPOLE, we measure the nonlinear signal electric
field in the time domain at different time delays between the interacting
pulses. The chirp extracted from the temporal phase of the emitted nonlinear
signal is found to sensitively depend on the electronic and rotational
contributions to the nonlinear response. While the rotational contribution
results in a nonlinear signal chirp close to the chirp of the input pulses, the
electronic contribution results in a significantly higher chirp which changes
with time delay. Our work demonstrates that electric field-resolved nonlinear
spectroscopy offers detailed information on nonlinear interactions at ultrafast
time scales.Comment: 8 pages, 4 figures, and supplemental documen
Photoionization dynamics in the presence of attosecond pulse trains and strong fields
We present experimental results and a theoretical framework for understanding the ionization dynamics in atoms exposed to XUV attosecond pulse trains and strong multi-cycle infrared (IR) fields. We invoke the Floquet formalism to model dressed atomic states as a manifold of Fourier components spaced by the laser frequency. In XUV-IR pump–probe measurements, we observe that the ionization yield oscillates due to quantum interference between photo-excitation paths to a Floquet state. We show that the intensity-dependent shifts of atomic structure modify the ionization channels and the associated interference phase. We extract this phase variation and compare it with simulations. These results provide a comprehensive description of the two-color ionization process and enable new schemes for control of attosecond ionization and fragmentation dynamics
Ultrafast Dynamics of Excited Electronic States in Nitrobenzene Measured by Ultrafast Transient Polarization Spectroscopy.
We investigate ultrafast dynamics of the lowest singlet excited electronic state in liquid nitrobenzene using ultrafast transient polarization spectroscopy, extending the well-known technique of optical Kerr effect spectroscopy to excited electronic states. The third-order nonlinear response of the excited molecular ensemble is measured using a pair of femtosecond pulses following a third femtosecond pulse that populates the S1 excited state. By measuring this response, which is highly sensitive to details of the excited state character and structure, as a function of time delays between the three pulses involved, we extract the dephasing time of the wave packet on the excited state. The dephasing time, measured as a function of time delay after pump excitation, shows oscillations indicating oscillatory wave packet dynamics on the excited state. From the experimental measurements and supporting theoretical calculations, we deduce that the wave packet completely leaves the S1 state potential energy surface after three traversals of the intersystem crossing between the singlet S1 and triplet T2 states
Helicobacter pylori-induced gastric cancer is orchestrated by MRCK beta-mediated Siah2 phosphorylation
Background Helicobacter pylori-mediated gastric carcinogenesis is initiated by a plethora of signaling events in the infected gastric epithelial cells (GECs). The E3 ubiquitin ligase seven in absentia homolog 2 (Siah2) is induced in GECs in response to H. pylori infection. Posttranslational modifications of Siah2 orchestrate its function as well as stability. The aim of this study was to evaluate Siah2 phosphorylation status under the influence of H. pylori infection and its impact in gastric cancer progression. Methods H. pylori-infected various GECs, gastric tissues from H. pylori-infected GC patients and H. felis-infected C57BL/6 mice were evaluated for Siah2 phosphorylation by western blotting or immunofluorescence microscopy. Coimmunoprecipitation assay followed by mass spectrometry were performed to identify the kinases interacting with Siah2. Phosphorylation sites of Siah2 were identified by using various plasmid constructs generated by site-directed mutagenesis. Proteasome inhibitor MG132 was used to investigate proteasome degradation events. The importance of Siah2 phosphorylation on tumorigenicity of infected cells were detected by using phosphorylation-null mutant and wild type Siah2 stably-transfected cells followed by clonogenicity assay, cell proliferation assay, anchorage-independent growth and transwell invasion assay. Results Siah2 was phosphorylated in H. pylori-infected GECs as well as in metastatic GC tissues at residues serine(6) (Ser(6)) and threonine(279) (Thr(279)). Phosphorylation of Siah2 was mediated by MRCK beta, a Ser/Thr protein kinase. MRCK beta was consistently expressed in uninfected GECs and noncancer gastric tissues but its level decreased in infected GECs as well as in metastatic tissues which had enhanced Siah2 expression. Infected murine gastric tissues showed similar results. MRCK beta could phosphorylate Siah2 but itself got ubiquitinated from this interaction leading to the proteasomal degradation of MRCK beta and use of proteasomal inhibitor MG132 could rescue MRCK beta from Siah2-mediated degradation. Ser(6) and Thr(279) phosphorylated-Siah2 was more stable and tumorigenic than its non-phosphorylated counterpart as revealed by the proliferation, invasion, migration abilities and anchorage-independent growth of stable-transfected cells. Conclusions Increased level of Ser(6) and Thr(279)-phosphorylated-Siah2 and downregulated MRCK beta were prominent histological characteristics of Helicobacter-infected gastric epithelium and metastatic human GC. MRCK beta-dependent Siah2 phosphorylation stabilized Siah2 which promoted anchorage-independent survival and proliferative potential of GECs. Phospho-null mutants of Siah2 (S6A and T279A) showed abated tumorigenicity.Peer reviewe
COLECCIÓN ANTONIO GONZÁLEZ. CRONISTA OFICIAL DE TELDE [Material gráfico]
Copia digital. Madrid : Ministerio de Educación, Cultura y Deporte. Subdirección General de Coordinación Bibliotecaria, 201