High-Power Tunable Pulsed Lasers and Electron-Ion Recombination

Laser stimulated electron ion recombination.
Polarization dependence.
Intracavity Experiments with High Power Pulsed Lasers.
Laser assisted dielectronic recombination.
Molecular Potentials.
Equippment.
Some recent publications.

LASER ASSISTED COLLISIONS

In these projects we want to investigate the influence of strong laser fields on collisions. We try to observe the laser field inducing capture of free electrons in well defined quantum states of ions. To this aim we use and develop high-power tunable lasers.

Opportunities exist for both Masters and Doctoral work. Masters work involves a limited number of relevant topics within the scope of one of our research projects. Doctoral work involves, as a rule, responsibility for an entire project. Work here provides invaluable hands-on experience in the techniques associated with High-Power Laser systems similar to those found in an industrial environment.

Most of our work in this area takes place at CRYRING the heavy-ion cooler storage ring at Manne Siegbahn Laboratory/Stockholm University. In the electron cooler of this storage ring three beams come together:

• A very intense laser beam.
• An intense and monoenergetic (cold) electron beam.
• A highly-charged ion beam.

Manipulation of these main ingredients leads to challenging lines of research


1. Laser stimulated electron ion recombination.

We are interested in using a laser field to bind an electron to a selected quantum state of an atom in what is called Laser Induced electron-ion Recombination (LIR). This is illustrated by the diagram on the right. LIR is charactherized by an observed "gain" over the spontaneously occurring recombination into that quantum state. This enhancement depends among other things on the external electromagnetic fields, on the polarization of the laser light and on the energy distribution of the electrons as seen from the frame of reference of the ions.

Experiments have shown that the applied electromagnetic fields present in the interaction region lead to the smearing-out of the sharp edge at zero relative energy (see diagram). However, important interaction processes between electrons and ions might be studied through the population of quantum states exactly in this region just below the continuum. It is, thus, very important to understand the LIR gain just below threshold.

To this end we will use linearly polarized laser light from a high-power tunable dye laser to study LIR. In this project, we will also manipulate the trajectories of the electron and ion beams. This way we will also have some control over the magnitude of the external electric field present in the interaction region.

Angles between external field vector, polarisation vector, and electron momentum p:

High intensity laser pulses find a broad range of applications, among them are those where the light pulses are practically not absorbed or distorted in an optically thin medium. One of such applications is the interaction of highly intense laser pulses with with gaseous media. Pulsed lasers have, on the other side, a bad "duty cycle" (pulse on to pulse off ratio), and they are 'expensive' to produce. The pulse duration varies from fs to ns and repetition rates from 0.01 to 1 kHz. A "storage device" for reusing the laser pulses seems therefore motivated.
We have realized a storage ring for laser pulses that can trap fs to ns laser pulses of high power for milliseconds (which matches the time between two pulses from the laser source). It appears that we solved the most critical part of injecting the pulse into the ring. The solution for this problem is directly related to the circumference of the ring, which is around 12m, and the speed of the optical switch. During the time for one turn of the light pulse (12m / 3 108 m/s=40 ns) the "switch", a Pockels cell triggered by a very fast high voltage pulser, rotates the polarization plane and the pulse is trapped (see Fig. 1).
In the tests we have, so far, demonstrated that we can trap ns laser pulses of MW power for approximately 100 turns (see Fig. 2) which is for several microseconds. That can be still improved (by optical and timing alignment, coated optical elements, focussing mirrors etc). We have inserted a dye amplifying cell, synchronously pumped by a external laser pulses. The tests were done at , but the optical system can be used without limitations in a broad band of 400 - 700 nm. Power consideration gives an increase of the efficiency of the optical storage ring with a dye amplifying cell, as compared to a single passage of the laser pulse through the experimental section, by 23 times. The geometry of the optical ring can be adopted for special laser-dilute-target interactions. The laser beams can be crossed so that the pulse passes the target several times on its circulation. The duty cycle in such a set-up will improve by at least an order of magnitude (depending on the decaying intensity by absorption). The reaction rate will be higher, and the increased duty factor makes hitherto not feasible laser stimulated reactions with small cross sections possible. There are plans for further developments and applications:

•The ring will be further developed to an optical resonator with elements for shaping and amplifying pulses.
•The time-correlated sequence (time separation of 10 ns) a high frequency (up to GHz) of high intensity laser pulses can be attractive for studies of time dependent reactions.

Dielectronic recombination is an electron-ion recombination mechanism where recombination occurs simultaneously with the excitation of the core of the ionic state. This process is resonant and leaves the ion in a doubly-excited state that must be radiatively stabilized by the emission of a succession of photons. This projects aims at studying the influence of laser light tuned to a stabilizing radiative transition of the doubly excited ion on the observed dielectronic recombination rate. This may, in turn, provide a tool to selectively produce a population of ions in specific chosen excited states.

5..Molecular Potentials

In typical experiments involving the photodissociation of molecular H2+ or HD+, a central difficulty is that the ions are prepared by either photoionization or electron-impact ionization. This leaves the ion of interest in high vibrationally excited states making any such experiments complicated and with limitations. To overcome these limtations, H2+ and HD+ molecular ions are stored and cooled in the CRYRING storage ring. Molecules such as HD have a dipole moment and can cool radatively to vibrationally-cold states. H2+ is used here only as a 'proof-of-concept' since it does not have a dipole moment and thus requires coolng time scales at the limit of that applicable to this type of experimental scheme.

A transverse, linearly polarized laser field of 10^13 W cm^-2 interacts with the passing ions strored in the ring. If one assumes a simplified 'bum-bell' construction of the molecule, multiphoton absorption is most likely to take place when the axis of the molecule is aligned with the laser polarization (This actually falls off with some angular dependence). Once the molecule is dissociated, a neutral and a charged portion remain. The charged particle is seperated by the dipole magnet following the interaction region while the neutral particle will pass unchanged and is detected on a position-sensitve microchannel plate detector. During the dissociation, theeffective bond energy of the molecule will be released giving the constituent components a 'kick'. This 'kick' can be measured by the distance the neutral fragment is displaced from the beam center. Since there is equal probability that the neutral fragment will be in the 'up' or 'down' position when it crosses the laser field, the imaged neutrals will provide a symmetric image around the beam center. In the case of HD+, two distinct patterns should be formed: an outer pattern from neutral H and an inner pattern from neutral D. The figures below illustrate the experimental setup and the result of a Monte Carlo calculation for the image expected on the position-sensitive detector from HD+:

Equipment:

Three high-power pulsed laser systems with variable wavelength are used in Atomic - Molecular Physics Experiments:

Some recent publications:

T. Mohamed, G. Andler, and R. Schuch
Development of an electro-optical device for storage of high power laser pulses
Optics Communications 214/1-6, 291 - 295 (2002)

R. Schuch, H. Danared, N. Eklow, M. Fogle, P. Glans, E. Justiniano, E. Lindroth, S. Madzunkov, M. Tarek, and W. Zong,
Collisions of cold electrons with cooled ions in CRYRING
In "Advances in Nuclear Physics" edt. D. Poenaru and S. Stoica, World
Scientific Publ. Co 2000, p. 378 - 390.

R. Schuch
Electron-collision experiments with cold ions in storage rings
chapt. 6 in Experimental Methods in Atomic Physics,
edit. by John Gillaspy, Nova Science Publishers, p. 165, (2000)

E. Justiniano, G. Andler, P. Glans, W. Zong, M. Saito,and R. Schuch,
Laser induced recombination with polarized photons
in Application of Accelerators in Research and Industry, AIP Conference Proceedings, Editors: J. L. Duggan, I. L. Morgan, American Institute of Physics, Woodbury, New York, p. 193, 1999

W. Spies, P. Glans, W. Zong, Gao Hui, G. Andler, E. Justiniano, M. Saito and R. Schuch,
Recombination experiments at CRYRING
Hyperfine Interaction 114, 237, 1998

E. Justiniano, G. Andler, S. Asp, D.R. DeWitt and R. Schuch,
Using an OPO laser system for recombination studies in a storage ring: LIR to n=3 of deuterium
Hyperfine Interaction 108, 283, (1997)

S. Asp, R. Schuch, D.R. DeWitt, C. Biedermann, Gao Hui, W. Zong, G. Andler, and E. Justiniano
Laser induced recombination D⁺
Nucl. Ins. Meth. B117, 31 (1996)

R. Schuch,
Cooler Storage Rings: New tools for atomic physics in Reviews of Fundamental Processes and Applications of Atoms and Ions, edited by C.D. Lin. World Scientific Publ. Singapore (1993)

U. Schramm, J. Berger, M. Grieser, D. Habs, G. Kilgus, T. Sch\"ussler, D. Schwalm, A. Wolf, R. Neumann and R. Schuch, Laser-induced recombination in merged electron and proton beams in Physics of Electronic and Atomic Collisions, edited by W.R. MacGillivray, I.E.~McCarthy, M.C. Standage, IOP Publishing Ltd. 647, 1992

U. Schramm, J. Berger, M. Grieser, D. Habs, E. Jaeschke, G. Kilgus, D. Schwalm, A. Wolf, R. Neumann and R. Schuch, Observation of laser induced recombination in merged electron and proton beams Phys. Rev. Lett. 67 (1991) 22

More information about the projects and literature can be obtained from:

Prof. Reinhold Schuch, tel. 08-5537 8621, email: schuch@physto.se
Guillermo Andler, tel. 08-16 11 16, email: andler@msi.se