33 research outputs found
Alternating-Gradient Focusing of the Benzonitrile-Argon Van der Waals Complex
We report on the focusing and guiding of the van der Waals complex formed
between benzonitrile molecules (CHCN) and argon atoms in a cold
molecular beam using an ac electric quadrupole guide. The distribution of
quantum states in the guided beam is non-thermal, because the transmission
efficiency depends on the state-dependent effective dipole moment in the
applied electric fields. At a specific ac frequency, however, the excitation
spectrum can be described by a thermal distribution at a rotational temperature
of 0.8 K. From the observed transmission characteristics and a combination of
trajectory and Stark-energy calculations we conclude that the permanent
electric dipole moment of benzonitrile remains unchanged upon the attachment of
the argon atom to within \pm5%. By exploiting the different
dipole-moment-to-mass (\mu/m) ratios of the complex and the benzonitrile
monomer, transmission can be selectively suppressed for or, in the limit of 0 K
rotational temperature, restricted to the complex.Comment: to be published in JC
Laser-induced 3D alignment and orientation of quantum-state-selected molecules
A strong inhomogeneous static electric field is used to spatially disperse a
rotationally cold supersonic beam of 2,6-difluoroiodobenzene molecules
according to their rotational quantum state. The molecules in the lowest lying
rotational states are selected and used as targets for 3-dimensional alignment
and orientation. The alignment is induced in the adiabatic regime with an
elliptically polarized, intense laser pulse and the orientation is induced by
the combined action of the laser pulse and a weak static electric field. We
show that the degree of 3-dimensional alignment and orientation is strongly
enhanced when rotationally state-selected molecules, rather than molecules in
the original molecular beam, are used as targets.Comment: 8 pages, 7 figures; v2: minor update
Stark deceleration of OH radicals in low-field-seeking and high-field-seeking quantum states
The Stark deceleration of OH radicals in both low-field-seeking and
high-field-seeking levels of the rovibronic ground
state is demonstrated using a single experimental setup. Applying
alternating-gradient focusing, OH radicals in their low-field-seeking
state have been decelerated from 345 m/s to 239
m/s, removing 50 % of the kinetic energy using only 27 deceleration stages. The
alternating-gradient decelerator allows to independently control longitudinal
and transverse manipulation of the molecules. Optimized high-voltage switching
sequences for the alternating-gradient deceleration are applied, in order to
adjust the dynamic focusing strength in every deceleration stage to the
changing velocity over the deceleration process. In addition we have also
decelerated OH radicals in their high-field-seeking
state from 355 m/s to 316 m/s. For the states involved, a real crossing of
hyperfine levels occurs at 640 V/cm, which is examined by varying a bias
voltage applied to the electrodes.Comment: 8 pages, 9 figure
Quantum-state selection, alignment, and orientation of large molecules using static electric and laser fields
Supersonic beams of polar molecules are deflected using inhomogeneous
electric fields. The quantum-state selectivity of the deflection is used to
spatially separate molecules according to their quantum state. A detailed
analysis of the deflection and the obtained quantum-state selection is
presented. The rotational temperatures of the molecular beams are determined
from the spatial beam profiles and are all approximately 1 K. Unprecedented
degrees of laser-induced alignment and
orientation of iodobenzene molecules are demonstrated when the state-selected
samples are used. Such state-selected and oriented molecules provide unique
possibilities for many novel experiments in chemistry and physics.Comment: minor changes, references update
A selector for structural isomers of neutral molecules
We have selected and spatially separated the two conformers of 3-aminophenol
(CHNO) present in a molecular beam. Analogous to the separation of ions
based on their mass-to-charge ratios in a quadrupole mass filter, the neutral
conformers are separated based on their different mass-to-dipole-moment ratios
in an ac electric quadrupole selector. For a given ac frequency, the individual
conformers experience different focusing forces, resulting in different
transmissions through the selector. These experiments demonstrate that
conformer-selected samples of large molecules can be prepared, offering new
possibilities for the study of gas-phase biomolecules.Comment: 4 pages, 4 figures (Revtex
Manipulating the motion of large neutral molecules
Large molecules have complex potential-energy surfaces with many local
minima. They exhibit multiple stereo-isomers, even at very low temperatures. In
this paper we discuss the different approaches for the manipulation of the
motion of large and complex molecules, like amino acids or peptides, and the
prospects of state- and conformer-selected, focused, and slow beams of such
molecules for studying their molecular properties and for fundamental physics
studies. Accepted for publication in Faraday Disc. 142 (2009), DOI:
10.1039/b820045aComment: 12 page
Precise dipole moments and quadrupole coupling constants of the cis and trans conformers of 3-aminophenol: Determination of the absolute conformation
The rotational constants and the nitrogen nuclear quadrupole coupling
constants of cis-3-aminophenol and trans-3-aminophenol are determined using
Fourier-transform microwave spectroscopy. We examine several
and hyperfine-resolved rotational transitions for both
conformers. The transitions are fit to a rigid rotor Hamiltonian including
nuclear quadrupole coupling to account for the nitrogen nucleus. For
cis-3-aminophenol we obtain rotational constants of A=3734.930 MHz, B=1823.2095
MHz, and C=1226.493 MHz, for trans-3-aminophenol of A=3730.1676 MHz,
B=1828.25774 MHz, and C=1228.1948 MHz. The dipole moments are precisely
determined using Stark effect measurements for several hyperfine transitions to
D, D for cis-3-aminophenol and D,
D for trans-3-aminophenol. Whereas the rotational constants and
quadrupole coupling constants do not allow to determinate the absolute
configuration of the two conformers, this assignment is straight-forward based
on the dipole moments. High-level \emph{ab initio} calculations (B3LYP/6-31G^*
to MP2/aug-cc-pVTZ) are performed providing error estimates of rotational
constants and dipole moments obtained for large molecules by these theoretical
methods.Comment: 9 pages, 4 tables, 3 figures (RevTeX
Ionization of 1D and 3D oriented asymmetric top molecules by intense circularly polarized femtosecond laser pulses
We present a combined experimental and theoretical study on strong-field
ionization of a three-dimensionally oriented asymmetric top molecule,
benzonitrile (CHN), by circularly polarized, nonresonant femtosecond
laser pulses. Prior to the interaction with the strong field, the molecules are
quantum-state selected using a deflector, and 3-dimensionally (3D) aligned and
oriented adiabatically using an elliptically polarized laser pulse in
combination with a static electric field. A characteristic splitting in the
molecular frame photoelectron momentum distribution reveals the position of the
nodal planes of the molecular orbitals from which ionization occurs. The
experimental results are supported by a theoretical tunneling model that
includes and quantifies the splitting in the momentum distribution. The focus
of the present article is to understand strong-field ionization from
3D-oriented asymmetric top molecules, in particular the suppression of electron
emission in nodal planes of molecular orbitals. In the preceding article
[Dimitrovski et al., Phys. Rev. A 83, 023405 (2011)] the focus is to understand
the strong-field ionization of one-dimensionally-oriented polar molecules, in
particular asymmetries in the emission direction of the photoelectrons.Comment: 12 pages, 9 figure
Exploring the conformational preferences of 20-residue peptides in isolation: Ac-Ala 19
Alternating-gradient focusing and deceleration of large molecules
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