25 research outputs found
Observation of Photoion Backward Emission in Photoionization of He and N2
We experimentally investigate the effects of the linear photon momentum on
the momentum distributions of photoions and photoelectrons generated in
one-photon ionization in an energy range of 300 eV 40 keV.
Our results show that for each ionization event the photon momentum is imparted
onto the photoion, which is essentially the system's center of mass.
Nevertheless, the mean value of the ion momentum distribution along the light
propagation direction is backward-directed by times the photon momentum.
These results experimentally confirm a 90 year old prediction.Comment: 5 pages, 3 figure
Observation of enhanced chiral asymmetries in the inner-shell photoionization of uniaxially oriented methyloxirane enantiomers
Most large molecules are chiral in their structure: they exist as two
enantiomers, which are mirror images of each other. Whereas the rovibronic
sublevels of two enantiomers are almost identical, it turns out that the
photoelectric effect is sensitive to the absolute configuration of the ionized
enantiomer - an effect termed Photoelectron Circular Dichroism (PECD). Our
comprehensive study demonstrates that the origin of PECD can be found in the
molecular frame electron emission pattern connecting PECD to other fundamental
photophysical effects as the circular dichroism in angular distributions
(CDAD). Accordingly, orienting a chiral molecule in space enhances the PECD by
a factor of about 10
Opportunities for Gas-Phase Science at Short-Wavelength Free-Electron Lasers with Undulator-Based Polarization Control
Free-electron lasers (FELs) are the world's most brilliant light sources with
rapidly evolving technological capabilities in terms of ultrabright and
ultrashort pulses over a large range of accessible photon energies. Their
revolutionary and innovative developments have opened new fields of science
regarding nonlinear light-matter interaction, the investigation of ultrafast
processes from specific observer sites, and approaches to imaging matter with
atomic resolution. A core aspect of FEL science is the study of isolated and
prototypical systems in the gas phase with the possibility of addressing
well-defined electronic transitions or particular atomic sites in molecules.
Notably for polarization-controlled short-wavelength FELs, the gas phase offers
new avenues for investigations of nonlinear and ultrafast phenomena in spin
orientated systems, for decoding the function of the chiral building blocks of
life as well as steering reactions and particle emission dynamics in otherwise
inaccessible ways. This roadmap comprises descriptions of technological
capabilities of facilities worldwide, innovative diagnostics and
instrumentation, as well as recent scientific highlights, novel methodology and
mathematical modeling. The experimental and theoretical landscape of using
polarization controllable FELs for dichroic light-matter interaction in the gas
phase will be discussed and comprehensively outlined to stimulate and
strengthen global collaborative efforts of all disciplines
Closed-loop recycling of rare liquid samples for gas-phase experiments
Many samples of current interest in molecular physics and physical chemistry exist in the liquid phase and are vaporized for use in gas cells,diffuse gas targets, or molecular gas jets. For some of these techniques, the large sample consumption is a limiting factor. When rare, expensivemolecules such as custom-made chiral molecules or species with isotopic labels are used, wasting them in the exhaust line of the pumps isquite an expensive and inefficient approach. Therefore, we developed a closed-loop recycling system for molecules with vapor pressures belowatmospheric pressure. Once filled, only a few valves have to be adjusted, and a cold trap must be moved after each phase of recycling. Therecycling efficiency per turn exceeds 95%
Quasifree Photoionization under the Reaction Microscope
We experimentally investigated the quasifree mechanism (QFM) in one-photon double ionization of He and H2 at 800 eV photon energy and circular polarization with a COLTRIMS reaction microscope. Our work provides new insight into this elusive photoionization mechanism that was predicted by Miron Amusia more than four decades ago. We found the distinct four-fold symmetry in the angular emission pattern of QFM electrons from H2 double ionization that has previously only been observed for He. Furthermore, we provide experimental evidence that the photon momentum is not imparted onto the center of mass in quasifree photoionization, which is in contrast to the situation in single ionization and in double ionization mediated by the shake-off and knock-out mechanisms. This finding is substantiated by numerical results obtained by solving the systemâs full-dimensional time-dependent Schrödinger equation beyond the dipole approximation
Zeptosecond birth time delay in molecular photoionization
Photoionization is one of the fundamental light-matter interaction processes in which the absorption of a photon launches the escape of an electron. The time scale of this process poses many open questions. Experiments have found time delays in the attosecond (10 seconds) domain between electron ejection from different orbitals, from different electronic bands, or in different directions. Here, we demonstrate that, across a molecular orbital, the electron is not launched at the same time. Rather, the birth time depends on the travel time of the photon across the molecule, which is 247 zeptoseconds (1 zeptosecond = 10 seconds) for the average bond length of molecular hydrogen. Using an electron interferometric technique, we resolve this birth time delay between electron emission from the two centers of the hydrogen molecule
Angular emission distribution of O 1s photoelectrons of uniaxially oriented methanol
The angular distribution of O 1s photoelectrons emitted from uniaxially oriented methanol is studied experimentally and theoretically. We employed circularly polarized photons of an energy of hÎœ = 550 eV for our investigations. We measured the three-dimensional photoelectron angular distributions of methanol, with the CH3âOH axis oriented in the polarization plane, by means of cold target recoil ion momentum spectroscopy. The experimental results are interpreted by single active electron calculations performed with the single center method. A comparative theoretical study of the respective molecular-frame angular distributions of O 1s photoelectrons of CO, performed for the same photoelectron kinetic energy and for a set of different internuclear distances, allows for disentangling the role of internuclear distance and the hydrogen atoms of methanol as compared to carbon monoxide
Chiral photoelectron angular distributions from ionization of achiral atomic and molecular species
We show that the combination of two achiral componentsâan atomic or molecular target plus a circularly polarized photonâcan yield chirally structured photoelectron angular distributions. For photoionization of CO, the angular distribution of carbon -shell photoelectrons is chiral when the molecular axis is neither perpendicular nor (anti)parallel to the light propagation axis. In photo-double-ionization of He, the distribution of one electron is chiral if the other electron is oriented like the molecular axis in the former case and if the electrons are distinguishable by their energy. In both scenarios, the circularly polarized photon defines a plane with a sense of rotation and an additional axis is defined by the CO molecule or one electron. This is sufficient to establish an unambiguous coordinate frame of well-defined handedness. To produce a chirally structured electron angular distribution, such a coordinate frame is necessary but not sufficient. We show that additional electron-electron interaction or scattering processes are needed to create the chiral angular distribution
Observation of Nondipole-Induced Asymmetry in the Angular Emission Distribution of Photoelectrons from Fixed-in-Space CO Molecules
We investigate experimentally and theoretically the C and O 1s photoionization of fixed-in-space CO molecules at a photon energy of 905Â eV. We find a significant dependence of the photoelectron angular distributions on the direction of propagation of the ionizing radiation. It results from an interplay of nondipole effects, on one hand, and molecular effects, on the other. The nondipole effects lead to an increase of the emission probability in the forward direction along the light propagation, and the photoelectron wave being scattered by the molecular potential gives rise to a strong peak in the direction of the atom neighboring the emitter site. These effects can either conspire or extenuate each other, depending on the photoelectron emission direction and molecular orientation in space