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 ${}^2\Pi_{3/2},v=0,J=3/2$ ground state is demonstrated using a single experimental setup. Applying alternating-gradient focusing, OH radicals in their low-field-seeking ${}^2\Pi_{3/2},v=0,J=3/2,f$ 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 ${}^2\Pi_{3/2},v=0,J=3/2,e$ 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

Precise dipole moment and quadrupole coupling constants of benzonitrile

We have performed Fourier transform microwave spectroscopy of benzonitrile, without and with applied electric fields. From the field-free hyperfine-resolved microwave transitions we simultaneously derive accurate values for the rotational constants, centrifugal distortion constants, and nitrogen nuclear quadrupole coupling constants of benzonitrile. By measuring the Stark shift of selected hyperfine transitions the electric dipole moment of benzonitrile is determined to $\mu=\mu_a=4.5152 (68)$ D.Comment: 6 pages, 2 tables (elsart

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 $J=2\leftarrow{}1$ and $1\leftarrow{}0$ 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 $\mu_a=1.7735$ D, $\mu_b=1.5195$ D for cis-3-aminophenol and $\mu_a=0.5563$ D, $\mu_b=0.5376$ 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

Fokussierung und Abbremsung großer Moleküle mittels alternierender Gradienten

1 Introduction 2 Molecular properties 3 Theory of alternating-gradient focusing and deceleration 4 Experimental setup 5 Fourier-transform microwave spectroscopy of benzonitrile 6 Alternating-gradient focusing and deceleration of benzonitrile 7 Alternating-gradient focusing and deceleration of OH radicals 8 Summary and outlookExploiting the Stark effect, neutral polar molecules can be focused and decelerated in an array of time-varying inhomogeneous electric fields in alternating-gradient configuration. Using this principle, a new experiment for the focusing and deceleration of large molecules from a molecular beam has been set up. With the new setup, the alternating-gradient focusing and deceleration of benzonitrile, a prototypical large molecule, have been demonstrated. Benzonitrile has been decelerated in its absolute ground state, which is not susceptible to inelastic collisions at sufficiently low temperatures, as well as in rotationally excited states. Because of the complexity of the Stark manifold with a large number of real and avoided crossings, it was not a priori clear whether benzonitrile in excited rotational states could be decelerated. However, this has been successfully demonstrated in this thesis. Furthermore, using the same alternating-gradient setup, OH radicals in both low-field-seeking and high-field-seeking quantum states have been focused and decelerated. For the deceleration of molecules in a low-field-seeking quantum state using an alternating-gradient decelerator, a new high voltage switching scheme has to be applied in order to achieve phase stability for the decelerated packets. In addition, the coupling of transverse and longitudinal motion in the alternating-gradient decelerator has been studied. All focusing and deceleration measurements agree well with the outcome of trajectory simulations. The experiments performed in this thesis demonstrate that alternating-gradient focusing and deceleration is a general method: it allows to decelerate polar molecules in both low-field-seeking and high-field-seeking quantum states as well as in ground and rotationally excited states. Furthermore, it shows that large polyatomic molecules, eventually biomolecules, are amenable to the powerful method of Stark deceleration using time-varying inhomogeneous electric fields.Neutrale polare Moleküle können unter Ausnutzung des Stark-Effekts in zeitlich veränderlichen inhomogenen elektrischen Feldern fokussiert und abgebremst werden. Unter Verwendung des Prinzips der alternierenden Gradienten (AG) wurde ein neues Experiment zur Fokussierung und Abbremsung von großen Molekülen aus einem Molekularstrahl aufgebaut. In der neuen Apparatur konnte mit Benzonitril ein Prototyp für ein großes Molekül mittels alternierender Gradienten fokussiert und abgebremst werden. Dies gelang für Benzonitril sowohl in seinem absoluten Grundzustand, der bei ausreichend tiefen Temperaturen stabil gegenüber inelastischen Stößen ist, als auch in angeregten Rotatitionszuständen. Aufgrund der Komplexität der Stark-Mannigfaltigkeit mit einer großen Zahl von echten und vermiedenen Kreuzungen war zu Beginn der Arbeit nicht bekannt, ob auch angeregte Rotationszustände von Benzonitril abgebremst werden können. Dieses konnte jedoch im Rahmen dieser Doktorarbeit gezeigt werden. Des Weiteren sind OH Radikale unter Verwendung desselben experimentellen Aufbaus sowohl in tieffeldsuchenden als auch in hochfeldsuchenden Zuständen fokussiert und abgebremst worden. Für die Abbremsung von Molekülen in tieffeldsuchenden Zuständen mit Hilfe von alternierenden Gradienten wurde ein neues Hochspannungs-Schaltschema verwendet, um die Phasenstabilität der abgebremsten Pakete zu gewährleisten. Darüber hinaus wurde die Kopplung der transversalen und longitudinalen Bewegung in dem verwendeten AG-Abbremser untersucht. Alle Fokussier- und Abbremsmessungen stimmen gut mit den Ergebnissen von Simulationen überein. Die durchgeführten Experimente demonstrieren, dass die Fokussierung und Abbremsung neutraler Moleküle mittels alternierender Gradienten eine allgemein anwendbare Methode ist: Sie erlaubt, polare Moleküle in sowohl tieffeldsuchenden wie auch hochfeldsuchenden Quantenzuständen als auch im Grundzustand und in angeregten Rotationszuständen abzubremsen. Weiterhin zeigt diese Doktorarbeit, dass die leistungsstarke Methode der Abbremsung mit Hilfe von zeitlich veränderlichen inhomogenen elektrischen Feldern auf große mehratomige Moleküle und in Zukunft sogar auf Biomoleküle angewendet werden kann

Compact in-place gate valve for molecular-beam experiments

Contains fulltext : 98833.pdf (preprint version ) (Open Access

Alternating-gradient focusing and deceleration of large molecules

Contains fulltext : 99129.pdf (preprint version ) (Open Access

ALTERNATE GRADIENT FOCUSING AND DECELERATION OF LARGE MOLECULES

Author Institution: Fritz-Haber-Institut der MPG, Faradayweg 4-6, 14195 Berlin,; GermanyOver the last years, fascinating progress has been made in the spectroscopy of large molecules in general and the \emph{building blocks of life}} \textbf{20}(3), 309--626 (2002);\par Special issue ``Bioactive molecules in the gasphase'' \textit{Phys.\ Chem.\ Chem.\ Phys.} \textbf{6}(10), 2543--2890 (2004)} in particular. Such studies allow a detailed understandig of the intrinsic physical and chemical properties of large, modular molecules. The preparation of cold, isolated samples of large molecules and the manipulation of their external degrees of freedom allow further investigations using high-resolution spectroscopy or scattering experiments. Our group has been developing methods to decelerate and store neutral, polar molecules using switched strong electric fields.} \textbf{22}, 73--128 (2003)} Here we show how these techniques can be applied to large molecules, i.\,e.\ molecules of biological relevance, for which all low-lying states are high-field seeking at the realized fields. Using a novel, modular experiment for the Alternate Gradient deceleration and trapping of molecules in high-field seeking states metastable CO and benzonitrile (C$_7$H$_5$N) have been decelerated. The results of these experiments are compared to simulations and further experiments on the manipulation of the external degress of freedom of benzonitrile and large, modular molecules are discussed