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Deflection of neutral polar molecules using inhomogeneous electric fields

Elizabeth Todd Davies

Published Jan 1, 2012• 0 citations
OpenAlex

Abstract

The ability to deflect neutral polar molecules present in supersonic molecular beams using inhomogeneous electric fields is a consequence of these molecules possessing a dipole moment. As a dipole moment is inherently a quantum state-dependent feature, it can thus be exploited to allow spatial separation of neutral molecules in different quantum states. In this report, the Stark interaction was exploited to provide deflection of a gaseous sample of indole (C8H7N). Spatial profiles of the molecular beam at various electric field strengths were measured, showing an observed deviation in the beam path attributed to the indole molecules residing in different quantum states. These state-selected samples produced act as ideal samples for laser alignment techniques, in preparation for further experiments such as diffraction on gaseous molecular samples.

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The ability to deflect neutral polar molecules present in supersonic molecular beams using inhomogeneous electric fields is a consequence of these molecules possessing a dipole moment. As a dipole moment is inherently a quantum state-dependent feature, it can thus be exploited to allow spatial separation of neutral molecules in different quantum states. In this report, the Stark interaction was exploited to provide deflection of a gaseous sample of indole (C8H7N). Spatial profiles of the molecular beam at various electric field strengths were measured, showing an observed deviation in the beam path attributed to the indole molecules residing in different quantum states. These state-selected samples produced act as ideal samples for laser alignment techniques, in preparation for further experiments such as diffraction on gaseous molecular samples.
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 (<cos2 theta2D>=0.972) 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.
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 (<cos2 theta2D>=0.972) 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.
Abstract : An experiment that tests a novel method for deflecting molecular beams using inhomogeneous resonance fields is described. If an electric field rotates at the same frequency as a molecule placed in it, the field and molecule remain aligned and the dipole moment-electric field interaction does not average to zero. If the field is spatially inhomogeneous then there is a net force on the molecule. This force can be used in making a molecular beam deflector. Resonance deflection of molecules in the J = 0 and 1 rotational states of a molecular beam of CsF has been observed. The deflection is produced by passing a collimated molecular beam of CsF through the center of a TE(111) microwave cavity which is oscillating at the J = 0 1 transition frequency, 11.019 GHz. The theory of the effect as well as potential applications are also discussed. (Modified author abstract)
Hydrogen molecules are excited in a molecular beam to Rydberg states around n=17-18 and are exposed to the inhomogeneous electric field of an electric dipole. The large dipole moment produced in the selected Stark eigenstates leads to strong forces on the H2 molecules in the inhomogeneous electric field. The trajectories of the molecules are monitored using ion-imaging and time of flight measurements. With the dipole rods mounted parallel to the beam direction, the high-field-seeking and low-field-seeking Stark states are deflected towards and away from the dipole, respectively. The magnitude of the deflection is measured as a function of the parabolic quantum number k and of the duration of the applied field. It is also shown that a large deflection is observed when populating the (17d2)1 state at zero field and switching the dipole field on after a delay. With the dipole mounted perpendicular to the beam direction, the molecules are either accelerated or decelerated as they move towards the dipole. The Rydberg states are found to survive for over 100 micros after the dipole field is switched off before being ionized at the detector and the time of flight is measured. A greater percentage change in kinetic energy is achieved by initial seeding of the beam in helium or neon followed by inhomogeneous field deceleration/acceleration. Molecular dynamics trajectory simulations are presented highlighting the extent to which the trajectories can be predicted based on the known Stark map. The spectroscopy of the populated states is discussed in detail and it is established that the N+=2, J=1, MJ=0 states populated here have a special stability with respect to decay by predissociation.
An experiment that tests a novel method for deflecting molecular beams using inhomogeneous resonance fields is described. The idea is easily understood in classical terms. Imagine that the rotating electric dipole moment of a polar diatomic molecule is placed in an electric field. In a static field the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}$ interaction averages to zero due to the rotation of the molecule. If, however, the field rotates at the same frequency as the molecule, the field and molecule remain aligned and the $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}}$ interaction does not average to zero. If the field is spatially inhomogeneous, then there is a net force $\ensuremath{-}\ensuremath{\nabla}(\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\mu}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{E}})$ on the molecule. This force can be used in making a molecular beam deflector. Resonance deflection of molecules in the J=0and1J=0 \mathrm{and} 1J=0and1 rotational states of a molecular beam of CsF has been observed. The deflection is produced by passing a collimated molecular beam of CsF through the center of a TE111{\mathrm{E}}_{111}E111​ microwave cavity which is oscillating at the $J=0\ensuremath{\rightarrow}1$ transition frequency, 11.019 GHz. The deflection of the J=0and1J=0 \mathrm{and} 1J=0and1 molecules can be detected as a decrease in the undeflected on-axis beam or as an increase in the off-axis beam. The theory of the effect as well as potential applications are also discussed.
A strong inhomogeneous static electric field is used to spatially disperse a supersonic beam of polar molecules, according to their quantum state. We show that the molecules residing in the lowest-lying rotational states can be selected and used as targets for further experiments. As an illustration, we demonstrate an unprecedented degree of laser-induced one-dimensional alignment (cos;(2)theta_(2D)=0.97) and strong orientation of state-selected iodobenzene molecules. This method should enable experiments on pure samples of polar molecules in their rotational ground state, offering new opportunities in molecular science.