179 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
Apoptosis in neurodegenerative diseases: to be or not to be? Absence of proof is not proof of absence
Apoptosis (from Greek falling off) is a term coined by Kerr, Wyllie and Currie in 1972 to describe a form of cell death associated with peculiar morphological changes. They contrasted apoptosis with necrosis, in which large numbers of cells undergo destruction and elicit a regional inflammatory response. In contrast, in apoptosis individual cells die and are being removed quickly, without inflammation, making their demise often difficult to detect. The initial concept of apoptosis was exclusively related to a morphological phenomenon.Biomedical Reviews 1995; 4: 103-108
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
Versatile silicon-waveguide supercontinuum for coherent mid-infrared spectroscopy
Infrared spectroscopy is a powerful tool for basic and applied science. The
molecular spectral fingerprints in the 3 um to 20 um region provide a means to
uniquely identify molecular structure for fundamental spectroscopy, atmospheric
chemistry, trace and hazardous gas detection, and biological microscopy. Driven
by such applications, the development of low-noise, coherent laser sources with
broad, tunable coverage is a topic of great interest. Laser frequency combs
possess a unique combination of precisely defined spectral lines and broad
bandwidth that can enable the above-mentioned applications. Here, we leverage
robust fabrication and geometrical dispersion engineering of silicon
nanophotonic waveguides for coherent frequency comb generation spanning 70 THz
in the mid-infrared (2.5 um to 6.2 um). Precise waveguide fabrication provides
significant spectral broadening and engineered spectra targeted at specific
mid-infrared bands. We use this coherent light source for dual-comb
spectroscopy at 5 um.Comment: 26 pages, 5 figure
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