472 research outputs found
The role of charge and proton transfer in fragmentation of hydrogen-bonded nanosystems: the breakup of ammonia clusters upon single photon multi-ionization
The charge and proton dynamics in hydrogen-bonded networks are investigated
using ammonia as a model system. The fragmentation dynamics of medium-sized
clusters (1-2 nm) upon single photon multi-ionization is studied, by analyzing
the momenta of small ionic fragments. The observed fragmentation pattern of the
doubly- and triply- charged clusters reveals a spatial anisotropy of emission
between fragments (back-to-back). Protonated fragments exhibit a distinct
kinematic correlation, indicating a delay between ionization and fragmentation
(fission). The different kinematics observed for channels containing protonated
and unprotonated species provides possible insights into the prime mechanisms
of charge and proton transfer, as well as proton hopping, in such a nanoscale
system.Comment: 9 pages, 6 figure
Routes to multiphoton double ionization in combined extreme ultraviolet and infrared laser pulses
Xenon multiphoton double ionization pathways are studied in a reaction microscope using a pump-probe arrangement of extreme ultraviolet high harmonic and infrared laser radiation. The momentum of photoelectrons is recorded in coincidence with singly or doubly charged ions. Among all possible routes to multiphoton double ionization, sequential processes using ionic excited states as intermediate steps are clearly identified
Probing single-photon ionization on the attosecond time scale
We study photoionization of argon atoms excited by attosecond pulses using an
interferometric measurement technique. We measure the difference in time delays
between electrons emitted from the and from the shell, at
different excitation energies ranging from 32 to 42 eV. The determination of
single photoemission time delays requires to take into account the measurement
process, involving the interaction with a probing infrared field. This
contribution can be estimated using an universal formula and is found to
account for a substantial fraction of the measured delay.Comment: 4 pages, 4 figures, under consideratio
Site-dependent nuclear dynamics in core-excited butadiene
Symmetry breaking and competition between electronic decay and nuclear dynamics are major factors determining whether the memory of the initial core-hole localisation in a molecule is retained long enough to affect fragmentation. We investigate the fate of core holes localised at different sites in the free 1,3 trans butadiene molecule by using synchrotron radiation to selectively excite core electrons from different C 1s sites to π* orbitals. Fragmentation involving bonds localised at the site of the core hole provides clear evidence for preferential bond breaking for a core hole located at the terminal carbon site, while the signature of localisation is weak for a vacancy on the central carbon site. The origin of this difference is attributed to out-of-plane vibrations, and statistical evaporation of protons for vacancies located at the central carbon sites
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. We show that despite the nuclear motion altering the
bond length by only , which leads to tiny changes in the potential
barrier, the corresponding change in the ionization time can be as large as
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
Photoemission time-delay measurements and calculations close to the 3s ionization minimum in Ar
We present experimental measurements and theoretical calculations of
photoionization time delays from the and shells in Ar in the photon
energy range of 32-42 eV. The experimental measurements are performed by
interferometry using attosecond pulse trains and the infrared laser used for
their generation. The theoretical approach includes intershell correlation
effects between the 3s and 3p shells within the framework of the random phase
approximation with exchange (RPAE). The connection between single-photon
ionization and the two-color two-photon ionization process used in the
measurement is established using the recently developed asymptotic
approximation for the complex transition amplitudes of laser-assisted
photoionization. We compare and discuss the theoretical and experimental
results especially in the region where strong intershell correlations in the 3s
to kp channel lead to an induced "Cooper" minimum in the 3s ionization
cross-section.Comment: submitted to PR
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