Laser Spectroscopy

1.2 Laser spectroscopy of negative ions

Negative ions are unique quantum systems. The lack of a long range Coulomb force causes the inter-electronic interaction to become relatively more important. As a consequence, the independent particle model, that adequately describes atomic structure under normal conditions, breaks down. Experimental studies of negative ions can therefore serve as a useful probe of electron correlation and hence be used to test theoretical models that go beyond the independent particle approximation.

The overall goal of our research is to increase our fundamental understanding of atomic and molecular negative ions and their interaction with light. The research is conducted using the in house accelerator GUNILLA and at the double storage ring DESIREE located at Stockholm University. Radioactive negative ions are studied at the isotope facility ISOLDE at CERN in Switzerland. Finally, we are involved in an application where we aim to improve Accelerator Mass Spectrometry (AMS), which is the most sensitive trace element method. This project is performed in collaboration with researcher at the VERA facility at University of Vienna.

Fundamental properties of negative ions

The ion beam facility GUNILLA (Gothenburg University Negative Ion Laser Laboratory) consist of a negative ion source, a sector magnet and three different experimental beamlines. The sputter source can produced negative ions of essentially all elements that form stable atomic negative ions.

The facility is equipped with detectors for neutral atoms, positive ions and electrons that have been produced in the single or double photodetachment processes. We use the facility to measure electron affinities, photodetachment cross sections and angular distributions of electrons emitted in the photodetachment process. The aim of the research is to increase our understanding of atomic and molecular systems.

Studies of negative ions of radioactive isotopes

ISOLDE at CERN in Switzerland is an online facility for production of radioactive isotopes. A 1 GeV proton beam impinges in a target of uranium carbide where a very wide range of radioactive isotopes are produced. The different isotopes diffuse into an ion source where they are ionized. Ions produced in the ion source are extracted, accelerated and mass analyzed, and finally guided to different experimental end-stations.

We used the ISOLDE facility to measured electron affinities of radioactive isotopes. A major achievement is that we have measured the electron affinity of astatine, which is the least abundant element on earth. The interest in this element stems from the fact that the At- isotope is used the cancer treatment method Target Alfa Therapy (TAT). We will also study isotopes shifts in electron affinities. We have for these studies built a dedicated end station GANDALPH (Gothenburg ANion Detector for Affinity measurements by Laser Photodetachment) where the radioactive negative ion beam is overlapped with a laser beam. Neutral atoms produced in the photodetachment process are detected with a neutral particle detector.

Storage of negative ions at extremely low pressures and temperatures

DESIREE (Double ElectroStatic Ion Ring ExpEriment) is a world unique facility for studies of atomic and molecular collisions. Positive and negative ions can here be stored in two storage rings with a common collinear section. The temperature of the facility is 13 K and the pressure is 10-14 mbar. These experimental conditions resembles the properties in the interstellar medium. There are only three cryogenic storage rings globally, and DESIREE is the only double storage ring. From 1st of January 2018 DESIREE is a national infrastructure facility support by the Swedish Research Council, hosted by Stockholm University and operated jointly by Stockholm University, University of Gothenburg and Malmö University. We are using DESIREE to study life times of bound excited states in negative ions and to make precision measurements of electron affinities.

Mass spectroscopy

Accelerator Mass Spectrometry (AMS) is the most sensitive trace element method with demonstrated detection limits down to the 10-18 level. The most well-known application of this method is 14C - dating. We are collaborating with researcher at the VERA (Vienna Environmental Research Accelerator) to improve the sensitivity of this method by incorporating laser spectroscopic techniques in the mass spectrometer. Spectroscopic investigations of atomic and molecular negative ions of interest in mass spectroscopic applications will be conducted at the GUNILLA facility in Gothenburg. The results from these studies will then be implemented at the VERA facility.

1.3 Optical levitation

Optical manipulation was first demonstrated by the 2018 Physics prize Nobel laureate Arthur Ashkin when he 1971 at Bell Labs showed that glass beads can be levitated using a powerful continuous wave laser (App. Phys. Lett. 19 (1971) 283). His group later developed the single beam gradient trap also called the laser tweezers (Ashkin et al. Opt. Lett. 11 (1986) 288) which very soon became the standard tool in optical manipulation, mainly due its applicability in microbiology. In recent years a renewed interest in the technique of optical levitation has occurred, and we are currently involved in three different project using this method.

Studies of coalescences of free falling droplets

We have designed an experimental system where we can levitation 10-30 micrometer sized liquid droplets. The droplets can be positioned with sub-micrometer resolution. The droplets are released from the traps by turning off the lasers with fast shutters. We then used two orthogonally oriented high speed cameras to monitor the falling droplets. This allows us to record, in three dimensions, how the droplets fall and collide under gravity. The spatial resolution is below 1 m and the temporal resolution is in the s range. Our goal is to study under which conditions two water droplets which collide will coalesce. We have for these studies developed a system where we can control all experimental parameters such as droplet size and charge, particles velocity and impact parameters. The video to the right shows how two 30 m droplets which fall under gravity collide and eventually coalesce. The aim with the research project is to obtain data on binary collisions that can be used to model droplet coalescence in turbulent clouds.

Dynamics of trapped spherical particles

We are in this project studying the collective behavior of two liquid droplets trapped in an optical levitator. The image below shows how two trapped glycerol droplets orbit in the potential well created by a focused laser beam. The underlying physical laws that govern the motion of each individual particle is simple but the emergent dynamics of the collection of particles is very complex. The dielectric microsphere are influenced by a constant gravitational field and an upward-pointing intense laser field. The presence of these two opposing force fields causes the system to exhibit a range of interesting behaviors, from quasi-stationary state to chaotic motion. The figure shows two particles in the oscillatory regime, when gravity is balanced by light, where the particles move in orbits.

Fluorescence of trapped spherical particles

We investigate singlet oxygen in trapped droplets using laser induced fluorescence spectroscopy. Singlet oxygen is a strong radical that violently reacts with other molecules making it extremely dangerous for any form of molecules - whether biological or atmospheric. It can be produce via excitation energy transfer by solar light absorbing chromophores such as Polycyclic Aromatic Hydrocarbons (PAHs). It is well known that anthropological pollutants contain considerable amount of PAHs that are distributed throughout in the atmosphere.
The aim of the experiments is to gain the experience and knowledge needed to design a system for high-altitude balloon experiments, where the roles played by singlet oxygen and PAHs can be investigated systematically. We will address important questions in atmospheric sciences such as the role of anthropogenic PAHs in clouds by photochemical modifications involving oxygen molecules. This project is conducted in collaboration with Dr. Murthy Gudipati at Jet Propulsion Laboratory in the USA.

1.4 Optics and spinntronics

The group of Professor Johan Åkerman in our department is a world leading research group in the field of spinntronics. We have recently initiated a collaboration with his group where we combine their knowledge in spinntronics with our competence in the field of lasers. Our aim is to study how spinntronic devices react when exposed to femtosecond laser pulses. An intense laser pulse is focused down to micrometer sized spot. This induces spin waves in the magnetized sample which is detected using a Brillouin scattering microscope. Of particular interest is to study the system when repetition rate of the laser have the same frequency as the magnetic resonance frequency. We have, for this purpose, acquired a femtosecond laser with a repetition rate in the GHz range.

1.5 Teaching experimental physics

In this project we try to bridge the gap between experimental research conducted at a university with the physics taught in high schools. Our main goal is to develop modern experiments and physics experiments that are used either at visits to our laboratory or as remotely controlled laboratory exercises.
As an example, we have developed an optical levitation experiment. A dispenser is used to release charged microscopic liquid droplets that fall into a vertically focused laser beam where they become trapped. The picture shows a trapped 30 m glycerol droplet and the movie shows how a droplets repeatedly falls down and are trapped in the optical levitator.

A vertically aligned electrical field (AC and/or DC) can be applied over the trapped droplet, and it can be exposed to UV-light or a radioactive source. The students can use this set-up to observe various phenomena with their own eyes as the only detector. They can measure the size of the droplet by observing diffracted light. The charge of the droplet can be measured by detecting its motion in an applied electric field, and the photoelectric effect can be studied by observing how electrons are released when the droplet is exposed to UV radiation. The final goal with this project is to construct a single drop Millikan like experiment. This set-up is used to demonstrate very fundamental processes using an experiment which have great similarities with the traps used for atom cooling, which are existing only in the most advanced atomic physics laboratories.
We do also develop remote labs that will allow schools to online connect and run an advanced physics experiment via webcams and detectors. As a first step, we have modified the optical trap described above such that it can be operate remotely. We will use this system to investigate how blended learning can be used to reach higher achievements in education. This project is conducted in collaboration with researchers at UNED (Universidad Nacional de Educación a Distancia) in Madrid.

Current Members

Dag Hanstorp, Professor
Jonas Enger, University Lecturer
Kelken Chang, Postdoc
Di Lu, Postdoc
Oscar Isaksson, Ph. D. student
Olle Windelius, Ph. D. student
Jakob Welander, Ph. D. student
Ademir Aleman Hernandez, Ph. D. student
Annie Ringvall-Moberg, Ph. D. student (station at CERN)
Andreas Johansson, Ph. D. student
Julia Sundberg, Ph. D. student
Moa Kristiansson, Ph. D. student (Ph. D. position at Stockholm University)
David Leimbach Ph. D. student (Ph. D. position at CERN)

Master's and Bachelor students

Parvathy Bhaskar, Master student (from India)
Menna Raveesh, Master student(from India)
Javier Tello Marmolejo , Master student (from Mexico)
José Eduardo Navarro Navarrete, Master student (from Mexico)
Jessica Warbinek, Master student (from Germany)
Jan Hallenius, Bachelor internship
Benjamin Björnsson, Bachelor internship
Lucas Gervreau, Bachelor internship (from France)

Ph. D. Ulric Ljungblad
Research & Development Manager, Freemelt AB
www.freemelt.com
Freemelt is a startup in additive manufacturing that is developing an open source powder-bed 3D-printer optimized for material development.

Ph. D. Andreas Klinkmüller
Assekuradeur GmbH
www.deutsche-assekuradeur.de

Ph. D. Gunnar Haeffler
Team Manager Drug Product Manufacture, Astra Zeneca
www.astrazeneca.com/
Team Manager within an R&D function that manufactures drug products under development for use in clinical trials.

Ph. D. Jonas Enger
University Lecturer, University of Gothenburg
www.physics.gu.se
Teacher and researcher with specific responsibility for teacher training program and educational development.

Fil. Lic. Petter Hagberg
Technology Strategy & Planning Manager Volvo Group Trucks Technology
www.volvogroup.com
Working with strategic planning mainly focused on long term global technology strategies and the Volvo Group Technology Plan, and coordination of the research investment portfolio for the Trucks business.

Ph. D. Karin Fritioff
Senior Specialist, Vattenfall
www.vattenfall.se
Radiological protection specialist with focus on radiological consequence assessments, radiation shielding calculations and nuclear emergency preparedness and response.

Ph. D. Joakim Sandström
Senior Lecturer, Department of Engineering, University of Borås
www.hb.se
Lecturer in physics and mathematics.

Ph. D. Mattias Goksör
Profesor, Pro-Vice Chancellor, University of Gothenburg
www.gu.se
Supports the Vice-Chancellor in providing academic leadership to the University, and work in partnership with senior administrators to help drive strategy and policy development.

Ph. D. Kerstin Ramser
Professor, Luleå University of Technology
www.ltu.se
Professor in experimental mechanics with focus on real time optical spectroscopy, imaging, optical manipulation and microfluidics.

Ph. D. Pontus Andersson
Emissions Assessment and Instrument Development, FluxSense AB
www.fluxsense.se
Measurements of diffuse emission of VOC from primary petrochemical industries using a variety of optical absorption spectroscopy techniques. Instrument hardware development.

Ph. D. Anton Lindahl
Senior Software Designer, Qamcom Research & Technology
www.qamcom.se
Software development with focus on data analysis and algoritms but always close to the hardware and the application. Projects contain for example radar development, Klaman filtering, embedded systems, medtech and image processing.

Ph. D. Hannes Hultgren
System Architect, RaySafe AB
www.raysafe.com
System Architect developing X-ray sensors. Focusing on the physical detection methods and the mathematical algorithms needed to calculate the desired parameters.

Impact Unified AB, Co-founder
www.impactunified.com
We are a gaming studio doing impact based games, VR-experiences and documentaries. We are trying to make the world a better place through education and awareness.

Ph. D. Johan Rohlen
Software Configuration Specialist, Jeppesen Systems AB
ww1.jeppesen.com
Working with processes and automation of delivering software. Especially interested in bridging the gap between software developers and customers.

Ph. D. Mikael Eklund
R&D Engineer, Siemens Building Technologies, Zug, Switzerland www.buildingtechnologies.siemens.com
R&D Engineer sensor technologies for multi-criteria fire detectors and comfort sensors.

VERA (Vienna Environmetal Research Accelerator)
http://isotopenforschung.univie.ac.at/

The LARISSA group at University of Mainz

The ISOLDE facility at CERN
http://isolde.web.cern.ch/

The DESIREE facility Stockholm University
http://www.fysik.su.se/

The Spintronic research group at University of Gothenburg
https://www.gu.se/en/research/spintronics

Gothenburg Photon Factory

The Gothenburg photon factory is equipped with a wide range of fixed as well as tunable wavelength lasers. There are continuous wave lasers and pulsed lasers ranging from nanoseconds to femtoseconds. It is one out of three laboratories that constitutes the network Laserlab Göteborg (LLG), which in turn is part of the Swedish network Laser Lab Sweden (LLS). The list below shows the lasers available at Gothenburg photon factory:

• High rep-rate Ti:Saphire Laser (Laser Quantum, <30fs, 1 GHz, 800 nm)
• YAG Pumped OPO-system (Spectra Physics, 6ns, 10 Hz, 220-2000 nm)
• YAG Pumped OPO-system (LaserVision 6ns, 10 Hz, 1350-5000 nm)
• Femtosecond laser system (Clark, 30 fs, 1 kHz, 470-1600 nm)
• Narrow bandwidth Ti:Saphire laser (Home built, 100 ns, 7 kHz, fundamental + frequency doubled light)
• YAG-Pumped dye laser (Sirah, 6 ns, 10 Hz, 300-900 nm)
• 3st high power diode lasers (Laser quantum,cw 2 W, 2 st 532nm, 1 st 660 nm)
• YAG and dyelaserpumped DFG/OPA-system (Sirah, 2600-3600 nm och 5500-7100 nm)

The GUNILLA ion beam facility

Our in-house studies of negative ions are conducted at the experimental facility GUNILLA (Göteborg University Negative Ion Laser LAboratory), which is a negative ion accelerator with three beamlines. Negative ions are produced in a sputter negative ions source, mass selected in a 90° degree sector magnet and finally directed into one of three beam lines. The first is a collinear interaction equipped with a neutral particle detector. An electric potential can be applied to its interaction region in order to Doppler tune the frequency experienced by the ions. The second beamline is equipped with the angular resolved electron spectrometer PEARLS (Photoelectron Angular Resolved Linear Spectrometer). The third beamline, finally, is designed for studies of highly excited doubly excited states and double detachment studies. It is equipped with a field ionizer, an electrostatic analyzer and a position sensitive detector.

Below follows some of the technical specifications of GUNILLA:

• Ion source: Peabody sputter ion source
• Beam energy: 3-6 keV
• Mass resolution: 400 for atoms and 700 for molecules
• Vacuum: 10-9 mbar in detection regions
• Interaction regions: Crossed and collinear interaction regions
• Detectors: Neutral particle detector, Beam scanners, Faraday cups, RoentDek Time and Position sensitive MCP Delay Line Detector.

We can offer a large variety of Bachelor and Master projects within the large framework of the research projects described above. The projects can be conducted in our laboratories in Gothenburg or at the external facilities where we perform our research, i.e. the ISOLDE facility at CERN, the DESIREE storage ring at Stockholm University or at the VERA accelerator in Vienna. If you are interested in a project: please contact any of us in the group and we can tell you what we can offer.