76 research outputs found
Emerging beam resonances in atom diffraction from a reflection grating
We report on the observation of emerging beam resonances, well known as
Rayleigh-Wood anomalies and threshold resonances in photon and electron
diffraction, respectively, in an atom-optical diffraction experiment.
Diffraction of He atom beams reflected from a blazed ruled grating at grazing
incidence has been investigated. The total reflectivity of the grating as well
as the intensities of the diffracted beams reveal anomalies at the Rayleigh
angles of incidence, i.e., when another diffracted beam merges parallel to the
grating surface. The observed anomalies are discussed in terms of the classical
wave-optical model of Rayleigh and Fano.Comment: 4 pages, 3 figure
Second-harmonic phonon spectroscopy of -quartz
We demonstrate midinfrared second-harmonic generation as a highly sensitive
phonon spectroscopy technique that we exemplify using -quartz (SiO)
as a model system. A midinfrared free-electron laser provides direct access to
optical phonon resonances ranging from to $1400\
\mathrm{cm}^{-1}T_c=846\ \mathrm{K}\alpha\beta$-quartz,
demonstrating the technique's potential for studies of phase transitions
Second Harmonic Generation from Critically Coupled Surface Phonon Polaritons
Mid-infrared nanophotonics can be realized using sub-diffractional light
localization and field enhancement with surface phonon polaritons in polar
dielectric materials. We experimentally demonstrate second harmonic generation
due to the optical field enhancement from critically coupled surface phonon
polaritons at the 6H-SiC-air interface, employing an infrared free-electron
laser for intense, tunable, and narrowband mid-infrared excitation. Critical
coupling to the surface polaritons is achieved using a prism in the Otto
geometry with adjustable width of the air gap, providing full control over the
excitation conditions along the polariton dispersion. The calculated
reflectivity and second harmonic spectra reproduce the full experimental data
set with high accuracy, allowing for a quantification of the optical field
enhancement. We also reveal the mechanism for low out-coupling efficiency of
the second harmonic light in the Otto geometry. Perspectives on surface phonon
polariton-based nonlinear sensing and nonlinear waveguide coupling are
discussed
AC electric trapping of neutral atoms
We study the dynamic behavior of ultracold neutral atoms in a macroscopic ac
electric trap. Confinement in such a trap is achieved by switching between two
saddle-point configurations of the electric field. The gradual formation of a
stably trapped cloud is observed and the trap performance is studied versus the
switching frequency and the symmetry of the switching cycle. Additionally, the
electric field in the trap is mapped out by imaging the atom cloud while the
fields are still on. Finally, the phase-space acceptance of the trap is probed
by introducing a modified switching cycle. The experimental results are
reproduced using full three-dimensional trajectory calculations.Comment: 10 pages, 13 figures, updated version, added journal referenc
Direct Identification of Acetaldehyde Formation and Characterization of the Active Site in the [VPO4].+/C2H4 Couple by GasâPhase Vibrational Spectroscopy
The gasâphase reaction of the heteronuclear oxide cluster [VPO4].+ with C2H4 is studied under multiple collision conditions at 150â
K using cryogenic ionâtrap vibrational spectroscopy combined with electronic structure calculations. The exclusive formation of acetaldehyde is directly identified spectroscopically and discussed in the context of the underlying reaction mechanism. In line with computational predictions it is the terminal P=O and not the V=O unit that provides the oxygen atom in the barrierâfree thermal C2H4âCH3CHO conversion. Interestingly, in the course of the reaction, the emerging CH3CHO product undergoes a rather complex intramolecular migration, coordinating eventually to the vanadium center prior to its liberation. Moreover, the spectroscopic structural characterization of neutral C2H4O deserves special mentioning as in most, if not all, ion/molecule reactions, the neutral product is usually only indirectly identified.DFG, 390540038, EXC 2008: UniSysCatDFG, 234149247, SFB 1109: Molekulare Einblicke in Metalloxid-Wasser-Systeme: Strukturelle Evolution, GrenzflĂ€chen und AuflösungTU Berlin, Open-Access-Mittel - 201
Trapping of Rb atoms by ac electric fields
We demonstrate trapping of an ultracold gas of neutral atoms in a macroscopic
ac electric trap. Three-dimensional confinement is obtained by switching
between two saddle-point configurations of the electric field. Stable trapping
is observed in a narrow range of switching frequencies around 60 Hz. The
dynamic confinement of the atoms is directly visualized at different phases of
the ac switching cycle. We observe about 10^5 Rb atoms in the 1 mm^3 large and
several microkelvins deep trap with a lifetime of approximately 5 s.Comment: 4 pages, 4 figures; updated version, added journal referenc
Resonant enhancement of second harmonic generation in the mid-infrared using localized surface phonon polaritons in sub-diffractional nanostructures
We report on strong enhancement of mid-infrared second harmonic generation
(SHG) from SiC nanopillars due to the resonant excitation of localized surface
phonon-polaritons within the Reststrahlen band. The magnitude of the SHG peak
at the monopole mode experiences a strong dependence on the resonant frequency
beyond that described by the field localization degree and the dispersion of
linear and nonlinear-optical SiC properties. Comparing the results for the
identical nanostructures made of 4H and 6H SiC polytypes, we demonstrate the
interplay of localized surface phonon polaritons with zone-folded weak phonon
modes of the anisotropic crystal. Tuning the monopole mode in and out of the
region where the zone-folded phonon is excited in 6H-SiC, we observe a
prominent increase of the already monopole-enhanced SHG output when the two
modes are coupled. Envisioning this interplay as one of the showcase features
of mid-infrared nonlinear nanophononics, we discuss its prospects for the
effective engineering of nonlinear-optical materials with desired properties in
the infrared spectral range.Comment: 16 pages, 3 figure
Quantum reflection of helium atom beams from a microstructured grating
We observe high-resolution diffraction patterns of a thermal-energy
helium-atom beam reflected from a microstructured surface grating at grazing
incidence. The grating consists of 10-m-wide Cr strips patterned on a
quartz substrate and has a periodicity of 20 m. Fully-resolved diffraction
peaks up to the order are observed at grazing angles up to 20
mrad. With changes in de Broglie wavelength or grazing angle the relative
diffraction intensities show significant variations which shed light on the
nature of the atom-surface interaction potential. The observations are
explained in terms of quantum reflection at the long-range attractive
Casimir-van der Waals potential.Comment: 4 pages, 4 figure
Observation of the Efimov state of the helium trimer
Quantum theory dictates that upon weakening the two-body interaction in a
three-body system, an infinite number of three-body bound states of a huge
spatial extent emerge just before these three-body states become unbound. Three
helium atoms have been predicted to form a molecular system that manifests this
peculiarity under natural conditions without artificial tuning of the
attraction between particles by an external field. Here we report experimental
observation of this long predicted but experimentally elusive Efimov state of
He by means of Coulomb explosion imaging. We show spatial images of
an Efimov state, confirming the predicted size and a typical structure where
two atoms are close to each other while the third is far away
Gas phase structures and charge localization in small aluminum oxide anions: Infrared photodissociation spectroscopy and electronic structure calculations
We use cryogenic ion trap vibrational spectroscopy in combination with quantum chemical calculations to study the structure of mono- and dialuminum oxide anions. The infrared photodissociation spectra of D2-tagged AlO1-4 â and Al2O3-6 â are measured in the region from 400 to 1200 cmâ1. Structures are assigned based on a comparison to simulated harmonic and anharmonic IR spectra derived from electronic structure calculations. The monoaluminum anions contain an even number of electrons and exhibit an electronic closed-shell ground state. The Al2O3-6 â anions are oxygen-centered radicals. As a result of a delicate balance between localization and delocalization of the unpaired electron, only the BHLYP functional is able to qualitatively describe the observed IR spectra of all species with the exception of AlO3 â. Terminal AlâO stretching modes are found between 1140 and 960 cmâ1. Superoxo and peroxo stretching modes are found at higher (1120-1010 cmâ1) and lower energies (850-570 cmâ1), respectively. Four modes in-between 910 and 530 cmâ1 represent the IR fingerprint of the common structural motif of dialuminum oxide anions, an asymmetric four-member Alâ(O)2âAl ring
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