294 research outputs found
Complete Photoionization Experiments via Ultrafast Coherent Control with Polarization Multiplexing II: Numerics & Analysis Methodologies
The feasibility of complete photoionization experiments, in which the full
set of photoionization matrix elements are determined, using multiphoton
ionization schemes with polarization-shaped pulses has recently been
demonstrated [Hockett et. al., Phys. Rev. Lett. 112, 223001 (2014)]. Here we
extend on our previous work to discuss further details of the numerics and
analysis methodology utilised, and compare the results directly to new
tomographic photoelectron measurements, which provide a more sensitive test of
the validity of the results. In so doing we discuss in detail the physics of
the photoionziation process, and suggest various avenues and prospects for this
coherent multiplexing methodology
Maximum information photoelectron metrology
Photoelectron interferograms, manifested in photoelectron angular
distributions (PADs), are a high-information, coherent observable. In order to
obtain the maximum information from angle-resolved photoionization experiments
it is desirable to record the full, 3D, photoelectron momentum distribution.
Here we apply tomographic reconstruction techniques to obtain such 3D
distributions from multiphoton ionization of potassium atoms, and fully analyse
the energy and angular content of the 3D data. The PADs obtained as a function
of energy indicate good agreement with previous 2D data and detailed analysis
[Hockett et. al., Phys. Rev. Lett. 112, 223001 (2014)] over the main spectral
features, but also indicate unexpected symmetry-breaking in certain regions of
momentum space, thus revealing additional continuum interferences which cannot
otherwise be observed. These observations reflect the presence of additional
ionization pathways and, most generally, illustrate the power of maximum
information measurements of this coherent observable
Coherent strong-field control of multiple states by a single chirped femtosecond laser pulse
We present a joint experimental and theoretical study on strong-field
photo-ionization of sodium atoms using chirped femtosecond laser pulses. By
tuning the chirp parameter, selectivity among the population in the highly
excited states 5p, 6p, 7p and 5f, 6f is achieved. Different excitation pathways
enabling control are identified by simultaneous ionization and measurement of
photoelectron angular distributions employing the velocity map imaging
technique. Free electron wave packets at an energy of around 1 eV are observed.
These photoelectrons originate from two channels. The predominant 2+1+1
Resonance Enhanced Multi-Photon Ionization (REMPI) proceeds via the strongly
driven two-photon transition , and subsequent
ionization from the states 5p, 6p and 7p whereas the second pathway involves
3+1 REMPI via the states 5f and 6f. In addition, electron wave packets from
two-photon ionization of the non-resonant transiently populated state 3p are
observed close to the ionization threshold. A mainly qualitative five-state
model for the predominant excitation channel is studied theoretically to
provide insights into the physical mechanisms at play. Our analysis shows that
by tuning the chirp parameter the dynamics is effectively controlled by dynamic
Stark-shifts and level crossings. In particular, we show that under the
experimental conditions the passage through an uncommon three-state "bow-tie"
level crossing allows the preparation of coherent superposition states
Bichromatic phase-control of interfering Autler-Townes spectra
We propose a new scheme to control the shape of the Autler-Townes (AT)
doublet in the photoelectron spectrum from atomic resonance-enhanced
multiphoton ionization (REMPI). The scheme is based on the interference of two
AT doublets created by ionization of the strongly driven atom from the ground
and the resonantly excited state using tailored bichromatic femtosecond (fs)
laser pulses. In this scheme, the quantum phase of the photoelectrons is
crucial for the manipulation of the AT doublet. The laser polarization state
and the relative optical phase between the two colors are used to manipulate
the interference pattern. We develop an analytical model to describe the
bichromatic REMPI process and provide a physical picture of the control
mechanism. To validate the model, the results are compared to an ab initio
calculation based on the solution of the 2D time-dependent Schr\"odinger
equation for the non-perturbative interaction of an atom with intense
polarization-shaped bichromatic fs-laser pulses. Our results indicate that the
control mechanism is robust with respect to the laser intensity facilitating
its experimental observation.Comment: 13 pages, 5 figure
Interference in the resonance fluorescence of two incoherently coupled transitions
The fluorescence light emitted by a 4-level system in to
configuration driven by a monochromatic laser field and in an external magnetic
field is studied. We show that the spectrum of resonance fluorescence emitted
on the transitions shows a signature of spontaneously generated
interference effects. The degree of interference in the fluorescence spectrum
can be controlled by means of the external magnetic field, provided that the
Land\'e g-factors of the excited and the ground state doublet are different.
For a suitably chosen magnetic field strength, the relative weight of the
Rayleigh line can be completely suppressed, even for low intensities of the
coherent driving field. The incoherent fluorescence spectrum emitted on the
transitions exhibits a very narrow peak whose width and weight depends on
the magnetic field strength. We demonstrate that the spectrum of resonance
fluorescence emitted on the transitions show an indirect signature of
interference. A measurement of the relative peak heights in the spectrum from
the transitions allows to determine the branching ratio of the
spontaneous decay of each excited state into the channel
Orbital angular momentum superposition states in transmission electron microscopy and bichromatic multiphoton ionization
The coherent control of electron beams and ultrafast electron wave packets
dynamics have attracted significant attention in electron microscopy as well as
in atomic physics. In order to unify the conceptual pictures developed in both
fields, we demonstrate the generation and manipulation of tailored electron
orbital angular momentum (OAM) superposition states either by employing
customized holographic diffraction masks in a transmission electron microscope
or by atomic multiphoton ionization utilizing pulse-shaper generated
carrier-envelope phase stable bichromatic ultrashort laser pulses. Both
techniques follow similar physical mechanisms based on Fourier synthesis of
quantum mechanical superposition states allowing the preparation of a broad set
of electron states with uncommon symmetries. We describe both approaches in a
unified picture based on an advanced spatial and spectral double slit and point
out important analogies. In addition, we analyze the topological charge and
discuss the control mechanisms of the free-electron OAM superposition states.
Their generation and manipulation by phase tailoring in transmission electron
microscopy and atomic multiphoton ionization is illustrated on a 7-fold
rotationally symmetric electron density distribution.Comment: K. Eickhoff and C. Rathje contributed equally to this wor
A composite double-/single-stranded RNA-binding region in protein Prp3 supports tri-snRNP stability and splicing
Prp3 is an essential U4/U6 di-snRNP-associated protein whose functions and
molecular mechanisms in pre-mRNA splicing are presently poorly understood. We
show by structural and biochemical analyses that Prp3 contains a bipartite
U4/U6 di-snRNA-binding region comprising an expanded ferredoxin-like fold,
which recognizes a 3′-overhang of U6 snRNA, and a preceding peptide, which
binds U4/U6 stem II. Phylogenetic analyses revealed that the single-stranded
RNA-binding domain is exclusively found in Prp3 orthologs, thus qualifying as
a spliceosome-specific RNA interaction module. The composite double-stranded
/single-stranded RNA-binding region assembles cooperatively with Snu13 and
Prp31 on U4/U6 di-snRNAs and inhibits Brr2-mediated U4/U6 di-snRNA unwinding
in vitro. RNP-disrupting mutations in Prp3 lead to U4/U6•U5 tri-snRNP assembly
and splicing defects in vivo. Our results reveal how Prp3 acts as an important
bridge between U4/U6 and U5 in the tri-snRNP and comparison with a Prp24-U6
snRNA recycling complex suggests how Prp3 may be involved in U4/U6 reassembly
after splicing
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