Christian Stråhlman

Angle-resolved time-of-flight spectroscopy applied to multi-bunch operation at MAX-lab: a design study

Angle-resolved time-of-flight (ARTOF) spectrometers have found use in a number of applications, including ARPES. However, the fundamental requirement of an external start trigger matching the read-out time of the instrument limits its usability at many storage rings. Hitherto all reported experiments have been performed at storage rings capable of running in single-bunch mode. To eliminate this restriction, we propose a method where a pulsed electronic gate is introduced to allow for ARTOF usage at normal multi-bunch operation of the MAX II storage ring. This paper will show the working principle and outline the design for this technique.

Student participation in developing student feedback

Sweden has a high level of student influence. At Lund University, students are not viewed as counterparts but partners in the university’s activities. Lund University has carried out Student Satisfaction Surveys (baromoters) since the 1990s and an overview has shown that an evaluation culture has grown during the past decade. It is, however, time to pose the critical question: do we ask the right questions? It is important that course evaluations and the institution-wide barometers complement each other without too much overlap. This article proposes a different way to carry through a barometer, where students’ experience mirrors the responses of alumni.

A Tandem Time--of--Flight Spectrometer for Negative--Ion/Positive--Ion Coincidence Measurements with Soft X-ray Excitation

We present a newly constructed spectrometer for negative-ion/positive-ion coincidence spectroscopy of gaseous samples. The instrument consists of two time-of-flight ion spectrometers and a magnetic momentum filter for deflection of electrons. The instrument can measure double and triple coincidences between mass-resolved negative and positive ions with high detection efficiency. First results include identification of several negative-ion/positive-ion coincidence channels following inner-shell photoexcitation of sulfur hexafluoride (SF6).

Negative-Ion/Positive-Ion Coincidence Yields of Core-Excited Water

We report yields of mass-resolved negative ions and positive ions measured in coincidence after core excitation of water molecules. The analysis of negative-ion/positive-ion and negative-ion/positive-ion/positive-ion coincidence events provides new information on pathways leading to negative ion production, enhancing the present understanding of the dissociation processes of the water molecule. Dissociation following (resonant) Auger decay dominates negative ion production, but radiative decay is shown to contribute above the O 1s ionization threshold. A peak in the H(-)/O(+) yield above the O 1s threshold is attributed to decay from doubly excited states.

Workshop on Timing Modes for Low-Emittance Storage Rings

A three-day workshop was held in Lund, Sweden, from March 25 to 27, 2015, with a focus on timing modes for low-emittance storage rings. The MAX IV Laboratory, currently under construction in Lund, will provide a 1.5 GeV storage ring for soft X-rays and a 3 GeV storage ring with an ultralow emittance down to 0.2 nm rad for hard X-rays [1]. Both rings are designed to operate with a uniform multibunch filling pattern with a 100 MHz RF system and employ passive harmonic cavities [2] to damp instabilities and increase the Touschek lifetime. For the 3 GeV ring, the harmonic cavities are also required to conserve the ultralow emittance at high bunch charge [3]. The facility will include a short pulse facility delivering 100 fs FWHM X-ray pulses at a repetition rate of 100 Hz.

Field ionization of high-Rydberg fragments produced after inner-shell photoexcitation and photoionization of the methane molecule.

We have studied the production of neutral high-Rydberg (HR) fragments from the CH4 molecule at the C 1s → 3p excitation and at the C 1s ionization threshold. Neutral fragments in HR states were ionized using a pulsed electric field and the resulting ions were mass-analyzed using an ion time-of-flight spectrometer. The atomic fragments C(HR) and H(HR) dominated the spectra, but molecular fragments CH(x)(HR), x = 1-3, and H2(HR) were also observed. The production of HR fragments is attributed to dissociation of CH4(+) and CH4(2+) ions in HR states. Just above the C 1s ionization threshold, such molecular ionic states are created when the C 1s photoelectron is recaptured after single or double Auger decay. Similar HR states may be reached directly following resonant Auger decay at the C 1s → 3p resonance. The energies and geometries of the parent and fragment ions have been calculated in order to gain insight into relevant dissociation pathways.

Preparing the MAX IV storage rings for timing-based experiments

Time-resolved experimental techniques are increasingly abundant at storage ring facilities. Recent developments in accelerator technology and beamline instrumentation allow for simultaneous operation of high-intensity and timing-based experiments. The MAX IV facility is a state-of-the-art synchrotron light source in Lund, Sweden, that will come into operation in 2016. As many storage ring facilities are pursuing upgrade programs employing strong-focusing multibend achromats and passive harmonic cavities (HCs) in high-current operation, it is of broad interest to study the accelerator and instrumentation developments required to enable timing-based experiments at such machines. In particular, the use of hybrid filling modes combined with pulse picking by resonant excitation or pseudo single bunch has shown promising results. These methods can be combined with novel beamline instrumentation, such as choppers and instrument gating. In this paper we discuss how these techniques can be implemented and employed at MAX IV.

Fragmentation of Methanol Molecules after Core Excitation and Core Ionization Studied by Negative-Ion/Positive-Ion Coincidence Experiments

We have studied the fragmentation of the methanol molecule after core excitation and core ionization by observing coincidences between negative and positive ions. Five different negative ions (H-, C-, CH-, O-, and OH-) were observed at both the C 1s and O 1s edges. As negative ion formation occurs after resonant and normal Auger decay of core-hole states, it is necessarily linked with the release of positively charged fragments. Our data show that such fragmentation can happen in many different ways: We found approximately 30 negative-ion/positive-ion/positive-ion coincidence (NIPIPICO) channels. All involve only singly charged positive ions. Fragmentation channels leading to atomic ions are the most probable, but positive molecular ions are also frequently found in the context of anion formation. Coincidence yields as a function of photon energy were determined for the most intense NIPIPICO channels. Adding together the data measured at different photon energies, we could also verify the occurrence of four-ion coincidences, which involved one negative ion (H- or O-) and three positive ions.

Yields and Time-of-Flight Spectra of Neutral High-Rydberg Fragments at the K Edges of the CO2 Molecule

We have studied the production of neutral fragments in high-Rydberg (HR) states at the C 1s and O 1s edges of the CO2 molecule by performing two kinds of experiments. First, the yields of neutral HR fragments were measured indirectly by ionizing such fragments in a static electric field and by collecting resulting singly charged positive ions as a function of the photon energy. Such measurements reveal not only excitations below the core ionization thresholds but also thresholds for single core-hole and shakeup photoionization. Second, we obtained the mass spectra of neutral HR fragments at selected photon energies by exploiting pulsed field ionization; they show atomic fragments C(HR) and O(HR). We discuss dissociation pathways leading to the production of neutral HR fragments in core excitation and ionization of CO2.

Negative-ion/positive-ion coincidence spectroscopy with a novel spectrometer

A novel instrument for negative-ion/positive-ion coincidence spectroscopy (NIPICO) is presented. The instrument consists of a double TOF spectrometer with a common extraction region. Electrons are deflected by means of a magnetic field. We demonstrate detections of double (NIPICO) and triple (NIPIPICO) coincidences.