825 research outputs found
Microscopic force for aerosol transport
A key ingredient for single particle diffractive imaging experiments is the
successful and efficient delivery of sample. Current sample-delivery methods
are based on aerosol injectors in which the samples are driven by fluid-dynamic
forces. These are typically simulated using Stokes' drag forces and for
micrometer-size or smaller particles, the Cunningham correction factor is
applied. This is not only unsatisfactory, but even using a temperature
dependent formulation it fails at cryogenic temperatures. Here we propose the
use of a direct computation of the force, based on Epstein's formulation, that
allows for high relative velocities of the particles to the gas and also for
internal particle temperatures that differ from the gas temperature. The new
force reproduces Stokes' drag force for conditions known to be well described
by Stokes' drag. Furthermore, it shows excellent agreement to experiments at 4
K, confirming the improved descriptive power of simulations over a wide
temperature range
Microscopic force for aerosol transport
A key ingredient for single particle diffractive imaging experiments is the
successful and efficient delivery of sample. Current sample-delivery methods
are based on aerosol injectors in which the samples are driven by fluid-dynamic
forces. These are typically simulated using Stokes' drag forces and for
micrometer-size or smaller particles, the Cunningham correction factor is
applied. This is not only unsatisfactory, but even using a temperature
dependent formulation it fails at cryogenic temperatures. Here we propose the
use of a direct computation of the force, based on Epstein's formulation, that
allows for high relative velocities of the particles to the gas and also for
internal particle temperatures that differ from the gas temperature. The new
force reproduces Stokes' drag force for conditions known to be well described
by Stokes' drag. Furthermore, it shows excellent agreement to experiments at 4
K, confirming the improved descriptive power of simulations over a wide
temperature range
Laser-induced alignment of nanoparticles and macromolecules for single-particle-imaging applications
Laser-induced alignment of particles and molecules was long envisioned to
support three-dimensional structure determination using single-particle imaging
with x-ray free-electron lasers [PRL 92, 198102 (2004)]. However, geometric
alignment of isolated macromolecules has not yet been demonstrated. Using
molecular modeling, we analyzed and demonstrated how the alignment of large
nanorods and proteins is possible with standard laser technology, and performed
a comprehensive analysis on the dependence of the degree of alignment on
molecular properties and experimental details. Calculations of the
polarizability anisotropy of about 150,000 proteins yielded a skew-normal
distribution with a location of 1.2, which reveals that most of these proteins
can be aligned using appropriate, realistic experimental parameters. Moreover,
we explored the dependence of the degree of alignment on experimental
parameters such as particle temperature and laser-pulse energy
Improved spatial separation of neutral molecules
We have developed and experimentally demonstrated an improved electrostatic
deflector for the spatial separation of molecules according to their
dipole-moment-to-mass ratio. The device features a very open structure that
allows for significantly stronger electric fields as well as for stronger
deflection without molecules crashing into the device itself. We have
demonstrated its performance using the prototypical OCS molecule and we discuss
opportunities regarding improved quantum-state-selectivity for complex
molecules and the deflection of unpolar molecules.Comment: 6 figure
The velocity dispersion and mass function of the outer halo globular cluster Palomar 4
We obtained precise line-of-sight radial velocities of 23 member stars of the
remote halo globular cluster Palomar 4 (Pal 4) using the High Resolution
Echelle Spectrograph (HIRES) at the Keck I telescope. We also measured the mass
function of the cluster down to a limiting magnitude of V~28 mag using archival
HST/WFPC2 imaging. We derived the cluster's surface brightness profile based on
the WFPC2 data and on broad-band imaging with the Low-Resolution Imaging
Spectrometer (LRIS) at the Keck II telescope. We find a mean cluster velocity
of 72.55+/-0.22 km/s and a velocity dispersion of 0.87+/-0.18 km/s. The global
mass function of the cluster, in the mass range 0.55<=M<=0.85 M_solar, is
shallower than a Kroupa mass function and the cluster is significantly depleted
in low-mass stars in its center compared to its outskirts. Since the relaxation
time of Pal 4 is of the order of a Hubble time, this points to primordial mass
segregation in this cluster. Extrapolating the measured mass function towards
lower-mass stars and including the contribution of compact remnants, we derive
a total cluster mass of 29800 M_solar. For this mass, the measured velocity
dispersion is consistent with the expectations of Newtonian dynamics and below
the prediction of Modified Newtonian Dynamics (MOND). Pal 4 adds to the growing
body of evidence that the dynamics of star clusters in the outer Galactic halo
can hardly be explained by MOND.Comment: 17 pages, accepted for publication in MNRAS; Fig. 8 surface
brightness/density data at github.com/matthiasjfrank/pal4_surface_brightnes
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