44 research outputs found
Quantum rainbow scattering at tunable velocities
Elastic scattering cross sections are measured for lithium atoms colliding
with rare gas atoms and SF6 molecules at tunable relative velocities down to
~50 m/s. Our scattering apparatus combines a velocity-tunable molecular beam
with a magneto-optic trap that provides an ultracold cloud of lithium atoms as
a scattering target. Comparison with theory reveals the quantum nature of the
collision dynamics in the studied regime, including both rainbows as well as
orbiting resonances
Improved setup for producing slow beams of cold molecules using a rotating nozzle
Intense beams of cold and slow molecules are produced by supersonic expansion
out of a rapidly rotating nozzle, as first demonstrated by Gupta and
Herschbach. An improved setup is presented that allows to accelerate or
decelerate cold atomic and molecular beams by up to 500 m/s. Technical
improvements are discussed and beam parameters are characterized by detailed
analysis of time of flight density distributions. The possibility of combining
this beam source with electrostatic fields for guiding polar molecules is
demonstrated
Detailed study of dissipative quantum dynamics of K-2 attached to helium nanodroplets
We thoroughly investigate vibrational quantum dynamics of dimers attached to
He droplets motivated by recent measurements with K-2 [1]. For those
femtosecond pump-probe experiments, crucial observed features are not
reproduced by gas phase calculations but agreement is found using a description
based on dissipative quantum dynamics, as briefly shown in [2]. Here we present
a detailed study of the influence of possible effects induced by the droplet.
The helium droplet causes electronic decoherence, shifts of potential surfaces,
and relaxation of wave packets in attached dimers. Moreover, a realistic
description of (stochastic) desorption of dimers off the droplet needs to be
taken into account. Step by step we include and study the importance of these
effects in our full quantum calculation. This allows us to reproduce and
explain all major experimental findings. We find that desorption is fast and
occurs already within 2-10 ps after electronic excitation. A further finding is
that slow vibrational motion in the ground state can be considered
frictionless.Comment: 17 pages, 5 figure
Guiding slow polar molecules with a charged wire
We demonstrate experimentally the guiding of cold and slow ND3 molecules
along a thin charged wire over a distance of ~0.34 m through an entire
molecular beam apparatus. Trajectory simulations confirm that both linear and
quadratic high-field-seeking Stark states can be efficiently guided from the
beam source up to the detector. A density enhancement up to a factor 7 is
reached for decelerated beams with velocities ranging down to ~50 m/s generated
by the rotating nozzle technique
Very long storage times and evaporative cooling of cesium atoms in a quasi-electrostatic dipole trap
We have trapped cesium atoms over many minutes in the focus of a CO-laser
beam employing an extremely simple laser system. Collisional properties of the
unpolarized atoms in their electronic ground state are investigated. Inelastic
binary collisions changing the hyperfine state lead to trap loss which is
quantitatively analyzed. Elastic collisions result in evaporative cooling of
the trapped gas from 25 K to 10 K over a time scale of about 150 s.Comment: 5 pages, 3 figure
Wave packet dynamics of potassium dimers attached to helium nanodroplets
The dynamics of vibrational wave packets excited in K dimers attached to
superfluid helium nanodroplets is investigated by means of femtosecond
pump-probe spectroscopy. The employed resonant three-photon-ionization scheme
is studied in a wide wavelength range and different pathways leading to
K-formation are identified. While the wave packet dynamics of the
electronic ground state is not influenced by the helium environment,
perturbations of the electronically excited states are observed. The latter
reveal a strong time dependence on the timescale 3-8 ps which directly reflects
the dynamics of desorption of K off the helium droplets
Penning Spectroscopy and Structure of Acetylene Oligomers in He Nanodroplets
Embedded atoms or molecules in a photoexcited He nanodroplet are well-known
to be ionized through inter-atomic relaxation in a Penning process. In this
work, we investigate the Penning ionization of acetylene oligomers occurring
from the photoexcitation bands of He nanodroplets. In close analogy to
conventional Penning electron spectroscopy by thermal atomic collisions, the n
= 2 photoexcitation band plays the role of the metastable atomic
He. This facilitates electron spectroscopy of acetylene
aggregates in the sub-kelvin He environment, providing the following insight
into their structure: The molecules in the dopant cluster are loosely bound van
der Waals complexes rather than forming covalent compounds. In addition, this
work reveals a Penning process stemming from the n = 4 band where
charge-transfer from autoionized He in the droplets is known to be the dominant
relaxation channel. This allows for excited states of the remnant dopant
oligomer Penning-ions to be studied. Hence, we demonstrate Penning ionization
electron spectroscopy of doped droplets as an effective technique for
investigating dopant oligomers which are easily formed by attachment to the
host cluster.Comment: 22 pages, 1 png figure, 4 postscript figure
Sympathetic Cooling with Two Atomic Species in an Optical Trap
We simultaneously trap ultracold lithium and cesium atoms in an optical
dipole trap formed by the focus of a CO laser and study the exchange of
thermal energy between the gases. The cesium gas, which is optically cooled to
K, efficiently decreases the temperature of the lithium gas through
sympathetic cooling. The measured cross section for thermalizing
Cs-Li collisions is cm, for both species in
their lowest hyperfine ground state. Besides thermalization, we observe
evaporation of lithium purely through elastic cesium-lithium collisions
(sympathetic evaporation).Comment: 4 pages 3 fig
Real-time dynamics of the formation of hydrated electrons upon irradiation of water clusters with extreme ultraviolet light
Free electrons in a polar liquid can form a bound state via interaction with the molecular environment. This so-called hydrated electron state in water is of fundamental importance e.g.~in cellular biology or radiation chemistry. Hydrated electrons are highly reactive radicals that can either directly interact with DNA or enzymes, or form highly excited hydrogen (Hâ) after being captured by protons. Here, we investigate the formation of the hydrated electron in real-time employing XUV femtosecond pulses from a free electron laser, in this way observing the initial steps of the hydration process. Using time-resolved photoelectron spectroscopy we find formation timescales in the low picosecond range and resolve the prominent dynamics of forming excited hydrogen states
Observation of interatomic Coulombic decay induced by double excitation of helium in nanodroplets
Interatomic Coulombic decay (ICD) plays a crucial role in weakly bound
complexes exposed to intense or high-energy radiation. So far, neutral or ionic
atoms or molecules have been prepared in singly excited electron or hole states
which can transfer energy to neighboring centers and cause ionization and
radiation damage. Here we demonstrate that a doubly excited atom, despite its
extremely short lifetime, can decay by ICD; evidenced by high-resolution
photoelectron spectra of He nanodroplets excited to the 2s2p+ state. We find
that ICD proceeds by relaxation into excited HeHe atom-pair states, in
agreement with calculations. The ability of inducing ICD by resonant excitation
far above the single-ionization threshold opens opportunities for controlling
radiation damage to a high degree of element specificity and spectral
selectivity.Comment: 6 pages, 4 figures, to be submitted to PR