Field imperfection induced axial secular frequency shifts in nonlinear ion traps

This article develops a simple analytical expression that relates ion axial secular frequency to field aberration in ion trap mass spectrometers. Hexapole and octopole aberrations have been considered in the present computations. The equation of motion of the ions in a pseudopotential well with these superpositions has the form of a Duffing-like equation and a perturbation method has been used to obtain the expression for ion secular frequency as a function of field imperfections. The expression indicates that the frequency shift is sensitive to the sign of the octopole superposition and insensitive to the sign of the hexapole superposition. Further, for weak multipole superposition of the same magnitude, octopole superposition causes a larger frequency shift in comparison to hexapole superposition.

Composite field approximations for ion traps with apertures on electrodes

This paper presents two approximate analytical expressions for nonlinear electric fields in the principal direction in axially symmetric (3D) and two dimensional (2D) ion trap mass analysers with apertures (holes in case of 3D traps and slits in case of 2D traps) on the electrodes. Considered together (3D and 2D), we present composite approximations for the principal unidirectional nonlinear electric fields in these ion traps. The composite electric field E has the form E = E-noaperture + E-aperture. where E-noaperture is the field within an imagined trap which is identical to the practical trap except that the apertures are missing and E-aperture is the field contribution due to apertures on the two trap electrodes. The field along the principal axis, of the trap can in this way be well approximated for any aperture that is not too large. To derive E-aperture. classical results of electrostatics have been extended to electrodes with finite thickness and different aperture shapes.E-noaperture is a modified truncated multipole expansion for the imagined trap with no aperture. The first several terms in the multipole expansion are in principle exact(though numerically determined using the BEM), while the last term is chosen to match the field at the electrode. This expansion, once Computed, works with any aperture in the practical trap. The composite field approximation for axially symmetric (3D) traps is checked for three geometries: the Paul trap, the cylindrical ion trap (CIT) and an arbitrary other trap. The approximation for 2D traps is verified using two geometries: the linear ion trap (LIT) and the rectilinear ion trap (RIT). In each case, for two aperture sizes (10% and 50% of the trap dimension), highly satisfactory fits are obtained. These composite approximations may be used in more detailed nonlinear ion dynamics Studies than have been hitherto attempted. (C) 2009 Elsevier B.V. All rights reserved.

Off-axis field approximations for ion traps with apertures

In recent work (Int. J. Mass Spec., vol. 282, pp. 112–122) we have considered the effect of apertures on the fields inside rf traps at points on the trap axis. We now complement and complete that work by considering off-axis fields in axially symmetric (referred to as “3D”) and in two dimensional (“2D”) ion traps whose electrodes have apertures, i.e., holes in 3D and slits in 2D. Our approximation has two parts. The first, EnoAperture, is the field obtained numerically for the trap under study with apertures artificially closed. We have used the boundary element method (BEM) for obtaining this field. The second part, EdueToAperture, is an analytical expression for the field contribution of the aperture. In EdueToAperture, aperture size is a free parameter. A key element in our approximation is the electrostatic field near an infinite thin plate with an aperture, and with different constant-valued far field intensities on either side. Compact expressions for this field can be found using separation of variables, wherein the choice of coordinate system is crucial. This field is, in turn, used four times within our trap-specific approximation. The off-axis field expressions for the 3D geometries were tested on the quadrupole ion trap (QIT) and the cylindrical ion trap (CIT), and the corresponding expressions for the 2D geometries were tested on the linear ion trap (LIT) and the rectilinear ion trap (RIT). For each geometry, we have considered apertures which are 10%, 30%, and 50% of the trap dimension. We have found that our analytical correction term EdueToAperture, though based on a classical small-aperture approximation, gives good results even for relatively large apertures.

Approximate multipole coefficients of RF ion traps as functions of aperture size

In this study we present approximate analytical expressions for estimating the variation in multipole expansion coefficients as a function of the size of the apertures in the electrodes in axially symmetric (3D) and two-dimensional (2D) ion trap ion traps. Following the approach adopted in our earlier studies which focused on the role of apertures to fields within the traps, here too, the analytical expression we develop is a sum of two terms, A(n,noAperiure), the multipole expansion coefficient for a trap with no apertures and A(n,dueToAperture), the multipole expansion coefficient contributed by the aperture. A(n,noAperture) has been obtained numerically and A(n,dueToAperture) is obtained from the n th derivative of the potential within the trap. The expressions derived have been tested on two 3D geometries and two 2D geometries. These include the quadrupole ion trap (QIT) and the cylindrical ion trap (CIT) for 3D geometries and the linear ion trap (LIT) and the rectilinear ion trap (RIT) for the 2D geometries. Multipole expansion coefficients A(2) to A(12), estimated by our analytical expressions, were compared with the values obtained numerically (using the boundary element method) for aperture sizes varying up to 50% of the trap dimension. In all the plots presented, it is observed that our analytical expression for the variation of multipole expansion coefficients versus aperture size closely follows the trend of the numerical evaluations for the range of aperture sizes considered. The maximum relative percentage errors, which provide an estimate of the deviation of our values from those obtained numerically for each multipole expansion coefficient, are seen to be largely in the range of 10-15%. The leading multipole expansion coefficient, A(2), however, is seen to be estimated very well by our expressions, with most values being within 1% of the numerically determined values, with larger deviations seen for the QIT and the LIT for large aperture sizes. (C) 2010 Elsevier B.V. All rights reserved.

Variable field ion traps

This paper presents a technique to vary the electric field within a cylindrical ion trap (CIT) mass spectrometer while it is in operation. In this technique, the electrodes of the CIT are split into number of mini-electrodes and different voltages are applied to these split-electrodes to achieve the desired field. In our study we have investigated two geometries of the split-electrode CIT. In the first, we retain the flat endcap electrodes of the CIT but split the ring electrode into five mini-rings. In the second configuration, we split the ring electrode of the CIT into three mini-rings and also divide the endcaps into two mini-discs. By applying different potentials to the mini-rings and mini-discs of these geometries we have shown that the field within the trap can be optimized to desired values. In our study, two different types of fields were targeted. In the first, potentials were adjusted to obtain a linear electric field and, in the second, a controlled higher order even multipole field was obtained by adjusting the potential. We have shown that the different potentials required can be derived from a single RF generator by connecting appropriate capacitor terminations to split electrodes. The field within the trap can be modified by changing the values of the external capacitors. (C) 2013 Elsevier B.V. All rights reserved.

TWO-DIMENSIONAL ARRAY OF ION TRAPS ON A PLANAR CHIP

We investigate a planar ion chip design with a two-dimensional array of linear ion traps for the scalable quantum information processor. The segmented electrodes reside in a single plane on a substrate and a grounded metal plate, a combination of appropriate rf and DC potentials are applied to them for stable ion confinement, and the trap axes are located above the surface at a distance controlled by the electrodes' lateral extent and the substrate's height as discussed. The potential distributions are calculated using static electric field qualitatively. This architecture is conceptually simple and many current microfabrication techniques are feasible for the basic structure. It may provide a promising route for scalable quantum computers.

Novel designs for Penning ion traps

A number of alternative designs are presented for Penning ion traps suitable for quantum information processing (QIP) applications with atomic ions. The first trap design is a simple array of long straight wires, which allows easy optical access. A prototype of this trap has been built to trap Ca+ and a simple electronic detection scheme has been employed to demonstrate the operation of the trap. Another trap design consists of a conducting plate with a hole in it situated above a continuous conducting plane. The final trap design is based on an array of pad electrodes. Although this trap design lacks the open geometry of the other traps described above, the pad design may prove useful in a hybrid scheme in which information processing and qubit storage take place in different types of trap. The behaviour of the pad traps is simulated numerically and techniques for moving ions rapidly between traps are discussed. Future experiments with these various designs are discussed. All of the designs lend themselves to the construction of multiple trap arrays, as required for scalable ion trap QIP.

An introduction to the trapping of clusters with ion traps and electrostatic storage devices

This paper presents an introduction to the application of ion traps and storage devices for cluster physics. Some experiments involving cluster ions in trapping devices such as Penning traps, Paul traps, quadrupole or multipole linear traps are briefly discussed. Electrostatic ion storage rings and traps which allow for the storage of fast ion beams without mass limitation are presented as well. We also report on the recently developed mini-ring, a compact electrostatic ion storage ring for cluster, molecular and biomolecular ion studies.

Physics of electron beam ion traps and sources

This paper presents the basic physics underlying the operation of electron beam ion traps and sources, with the machine physics underlying their operation being described in some detail. Predictions arising from this description are compared with some diagnostic measurements.

Photodissociation dynamics of tert-butylnitrite following excitation to the S(1) and S(2) states. A study by velocity-map ion-imaging and 3D-REMPI spectroscopy

Excitation of tert-butylnitrite into the first and second UV absorption bands leads to efficient dissociation into the fragment radicals NO and tert-butoxy in their electronic ground states (2)Π and (2)E, respectively. Velocity distributions and angular anisotropies for the NO fragment in several hundred rotational and vibrational quantum states were obtained by velocity-map imaging and the recently developed 3D-REMPI method. Excitation into the well resolved vibronic progression bands (k = 0, 1, 2) of the NO stretch mode in the S(1) ← S(0) transition produces NO fragments mostly in the vibrational state with v = k, with smaller fractions in v = k - 1 and v = k - 2. It is concluded that dissociation occurs on the purely repulsive PES of S(1) without barrier. All velocity distributions from photolysis via the S(1)(nπ*) state are monomodal and show high negative anisotropy (β ≈ -1). The rotational distributions peak near j = 30.5 irrespective of the vibronic state S(1)(k) excited and the vibrational state v of the NO fragment. On average 46% of the excess energy is converted to kinetic energy, 23% and 31% remain as internal energy in the NO fragment and the t-BuO radical, respectively. Photolysis via excitation into the S(2) ← S(0) transition at 227 nm yields NO fragments with about equal populations in v = 0 and v = 1. The rotational distributions have a single maximum near j = 59.5. The velocity distributions are monomodal with positive anisotropy β ≈ 0.8. The average fractions of the excess energy distributed into translation, internal energy of NO, and internal energy of t-BuO are 39%, 23%, and 38%, respectively. In all cases ∼8500 cm(-1) of energy remain in the internal degrees of freedom of the t-BuO fragment. This is mostly assigned to rotational energy. An ab initio calculation of the dynamic reaction path shows that not only the NO fragment but also the t-BuO fragment gain large angular momentum during dissociation on the purely repulsive potential energy surface of S(2).

Preparation of cold Mg + ion clouds for sympathetic cooling of highly charged ions at SPECTRAP

Die Elektronen in wasserstoff- und lithium-ähnlichen schweren Ionen sind den extrem starken elektrischen und magnetischen Feldern in der Umgebung des Kerns ausgesetzt. Die Laserspektroskopie der Hyperfeinaufspaltung im Grundzustand des Ions erlaubt daher einen sensitiven Test der Quantenelektrodynamik in starken Feldern insbesondere im magnetischen Sektor. Frühere Messungen an wasserstoffähnlichen Systemen die an einer Elektronenstrahl-Ionenfalle (EBIT) und am Experimentierspeicherring (ESR) der GSI Darmstadt durchgeführt wurden, waren in ihrer Genauigkeit durch zu geringe Statistik, einer starken Dopplerverbreiterung und der großen Unsicherheit in der Ionenenergie limitiert. Das ganze Potential des QED-Tests kann nur dann ausgeschöpft werden, wenn es gelingt sowohl wasserstoff- als auch lithium-ähnliche schwere Ionen mit einer um 2-3 Größenordnung gesteigerten Genauigkeit zu spektroskopieren. Um dies zu erreichen, wird gegenwärtig das neue Penningfallensystem SPECTRAP an der GSI aufgebaut und in Betrieb genommen. Es ist speziell für die Laserspektroskopie an gespeicherten hochgeladenen Ionen optimiert und wird in Zukunft von HITRAP mit nierderenergetischen hochgeladenen Ionen versorgt werden.rnrnSPECTRAP ist eine zylindrische Penningfalle mit axialem Zugang für die Injektion von Ionen und die Einkopplung eines Laserstrahls sowie einem radialen optischen Zugang für die Detektion der Fluoreszenz. Um letzteres zu realisieren ist der supraleitende Magnet als Helmholtz-Spulenpaar ausgelegt. Um die gewünschte Genauigkeit bei der Laserspektroskopie zu erreichen, muss ein effizienter und schneller Kühlprozess für die injizierten hochegeladenen Ionen realisiert werden. Dies kann mittels sympathetischer Kühlung in einer lasergekühlten Wolke leichter Ionen realisiert werden. Im Rahmen dieser Arbeit wurde ein Lasersystem und eine Ionenquelle für die Produktion einer solchen 24Mg+ Ionenwolke aufgebaut und erfolgreich an SPECTRAP in Betrieb genommen. Dazu wurde ein Festkörperlasersystem für die Erzeugung von Licht bei 279.6 nm entworfen und aufgebaut. Es besteht aus einem Faserlaser bei 1118 nm der in zwei aufeinanderfolgenden Frequenzverdopplungsstufen frequenzvervierfacht wird. Die Verdopplerstufen sind als aktiv stabilisierte Resonantoren mit nichtlinearen Kristallen ausgelegt. Das Lasersystem liefert unter optimalen Bedingeungen bis zu 15 mW bei der ultravioletten Wellenlänge und erwies sich während der Teststrahlzeiten an SPECTRAP als ausgesprochen zuverlässig. Desweiteren wurde eine Ionequelle für die gepulste Injektion von Mg+ Ionen in die SPECTRAP Falle entwickelt. Diese basiert auf der Elektronenstoßionisation eines thermischen Mg-Atomstrahls und liefert in der gepulsten Extraktion Ionenbündel mit einer kleinen Impuls- und Energieverteilung. Unter Nutzung des Lasersystems konnten damit an SPECTRAP erstmals Ionenwolken mit bis zu 2600 lasergekühlten Mg Ionen erzeugt werden. Der Nachweis erfolgte sowohl mittels Fluoreszenz als auch mit der FFT-ICR Technik. Aus der Analyse des Fluoreszenz-Linienprofils lässt sich sowohl die Sensitivität auf einzelne gespeicherte Ionen als auch eine erreichte Endtemperatur in der Größenordnung von ≈ 100 mK nach wenigen Sekunden Kühlzeit belegen.