Atomic Physics Group Stockholm University Experimental Projects Instrumentation seminar November 28, 2002 Presented by Sven Mannervik

Experimental work is primarily performed at the Manne Siegbahn Laboratory National Facility at Stockholm University

G A,E: Laser spectroscopy B,G: Atomic collision C: Ion- surface collision D: Mass measurements F: Ion-electron recombination

In the ring we perform experiments with cooled stored ions on electron-ion recombination and laser assisted excitation and recombination on fast atomic collisions (single and multiple electron capture, ionization, recoil momentum spectroscopy) with internal target. on laser spectroscopy, and lifetime measurements of metastable states at stored heavy ions. We use beams of highly charged ions from the electron beam ion source (EBIS), the ECR ion source and stored ions in a synchrotron ion-cooler storage ring (CRYRING). With slow highly charged ions we study: surface and cluster interactions multiple electron transfer reactions mass spectrometry of highly charged ions in a Penning trap

Lifetime measurements Why are radiative lifetimes needed? A ik Intensity I=N i A ik The radiative lifetime (  is determined by the sum of the transition probabilities (A ik ) for all decay channels  A ik Excited state Metastable state Allowed transition A=10 8 s -1 Forbidden transition A=1 s -1

Why stored ion beam? fast ion beam 5 mm A=1 s -1 A=10 8 s km ! Laser High spectral resolution (laser) Time-resolution and long observation time Pure light source (isotope separation) Ultra high vacuum

laser obs M E G Probing of the meta- stable population by laser excitation + higher efficiency + high selectivity + high flexibility PM Laser PM Observation of spontaneous decay obs M G passive method Instead of the passive method we use the active laser probing method We gain a factor of 5000 and can reduce detector background. forbidden line

Lifetime: Ca + 3d 2 D 3/2 moving laser probe pulse Laser probing technique (LPT) developed at CRYRING for lifetime measurements Shutter synchronized with the ring creates laser pulses at variable time delays

Laser probing technique – in summary Photon counts Time after injection Lifetime curve cycle 2 cycle 1 cycle 3 Laser pulses Fluorescence yield Moving probe pulse Time Metastable level Higher level Lower level Laser light Fluorescence CRYRING Laser Photomulti plier Number of injected ions has to be constant!

a6DJa6DJ 1/2 3/2 5/2 7/2 9/2 a 6 S 5/2 z 6 D 7/ Å Laser probing of a 6 S 5/2 Eta Carinae blob The FERRUM project 62 metastable levels

Results Fe II Rostohar et al Phys Rev Lett 86(2001)1466

Experimental results Ti II c 2 D 3/2 : 0.35  s Long lifetime – very sensistive to corrections Preliminary 27  s 34 metastable levels Ground level a 4 F 3/2 c 2 D 3/2 b 4 P 5/2

Ion beam Laser beam Collinear geometry gives subDoppler line width  200 MHz  1 MHz Laser and Radio-Frequency double resonance spectroscopy

Ion-electron interaction Electron Cooler Dipole Magnet SBD cooling recombination

Dielectronic Recombination is a resonant process in which a continuum electron is captured as it excites a target electron, forming a short lived intermediate state which decays by photon emission Radiative Recombination is an direct spontaneous process in which a continuum electron is captured with the subsequent release of a photon Recombination Overview

ExperimentExperiment CRYRING Electron Cooler Dipole Magnet SBD

Energy (eV) Si 14+ He ++ Used for radiative recombination studies of ions with free electrons at CRYRING B Influence of external (electromagnetic) fields on recombination rate E Laser induced recombination into specific quantum states  enhancement factor  200 p+e -  H(n=3) Si 14+ E

Laser Ring with an Implemented Amplifier HVP PD1 Excimer- dye laser Nd:YAG Laser 2 nd harmonic AC WP PC FS2 FS1 PD2 The optical laser ring with the implemented amplifier gives a total gain of about 23 in comparison with a single passage of the pulse through the interaction zone. T. Mohamed, G. Andler, R. Schuch, subm. to J. Opt. Com. Electron Cooler Dipole Magnet SBD

Spin-orbit interaction e-e- 1s 2 2s  f 1 F o 1s 2 2p4d 3 D o 1s 2 2p 2 3 P e-e- Coulomb interaction 1s 2 2p4f 3 G e 1s 2 2s  g 3 G e 1s 2 2p3d 3 F o Ionization limit Dielectronic recombination (DR) process ‘allowed’ process ‘forbidden’ process Radiative stabilization C 3+ + e C 2+ First ionization limit

Experiment

Importance of knowing the ring length S6 Beam profile monitor Two cooler scrapers Relative position Laser induced recombination Measurement of the difference between DR resonances Absolute length

Li-like Kr 2s2s 2 p 1/2 15 l Energy Splitting QED effects are small for high-n, so these states can be calculated accurately Madzunkov et al., Phys. Rev. A65, (2002) Theory Uncertainty Exp. Uncertainty (19) (8) We are now doing… “Quantum electrodynamics in the dark” Physics World, Aug. 2001

Ni eNi 16+

p-He  H 0 +He 2+ +e - at MeV Transfer Ionization in MeV p-He Collisions Studied by Pulsed Recoil-Ion-Momentum Spectroscopy in a Storage Ring/Gas Target Experiment Fast ion-atom collisions

Fast Ion-Atom collisions in CRYRING CRYRING: CRYRING: High Current (100  A H + ) Cold and narrow beam (  1 mm) The Gas-Jet Target: Density: up to cm -3 Jet diameter: 1.0 mmLuminosity: 6  cm -2 s MeV:  1 min MeV:  10 7 s -1. GAS TARGET Gas jet Ring beam pR║ pR║ PROJECTILE DETECTOR

Transfer Ionization in fast H + -He collisions: Thomas p-e-e scattering p v He 2 v 0.55 mrad v v 45 o The He nucleus is not directly involved in the collision pR0pR0 RIMS! The He nucleus is emitted in the backward direction as a result of the kinematical capture. p R  -Q/v p -m e v p /2 Kinematical Transfer Ionization (KTI) Kinematical capture through momentum overlap. Shake-off He v p v H He 2+ e-e- p R = MeV (E R ~50 meV)

The pulsed spectrometer:

Recoil detector images.

KTI/(SC+TI) [%]

Highly charged ions produced in CRYSIS an EBIS

q+ e-e- e-e- e-e- Slow Highly Charged Ions Colliding with C 60 – stability and fragmentation

A q+ C 60 Experimental set-up V ex Multi-hit TDC START TRIG STOP 0 V -100 V Collimated C 60 Jet Time-of-flight Cylindrical analyzer PSD T=500  C A q+ (q-s) C C C C C Time-of-flight Xe C 60 Xe ….

26 keV Ar 8+ + C 60  Ar (8-s) s = 1 Cold s = 4 s = 2 s = 3 Hot

Evaporation of small neutral fragments Asymmetric fission Activation energy E a for evaporation of a C 2 unit ~ 10 eV Decay channels of excited C 60 : (C 60 r+ )*  C 60-2m r+ + C 2m (m=1,2,3,4…) (C 60 r+ )*  C 60-2m (r-1)+ + C 2m + (m=1,2,3,4…) dominate for r  4 Multifragmentation: (C 60 r+ )*  many small fragments in low charge states dominate for r  3 k  1/exp( B fis / k B T) Depends on internal energy or temperature of C 60 Decay rate: decrease T  increase lifetime U(R) C 60 r+ E kin B fis R kinetic energy releases  fission barriers  stability C 58 (r-1)+ + C 2 +

175 mm 8 mm - electrostatic - simple - small - easy to cool ConeTrap: An Electrostatic Ion Trap for Atomic and Molecular Physics H.T. Schmidt, H. Cederquist, J. Jensen, and A. Fardi, NIMB 173, 523 (2001). Storage and lifetime measurements of C 60 ions using

Mass determinations with highly charged ions relevant for fundamental physics SMILE Trap Ions from CRYSIS

Principle : Measurement of the cyclotron frequency of an ion trapped in a homogeneous magnetic field : How to measure atomic mass with very high precision? -  810 Hz +  36 MHz z  240 kHz  using HCI the precision increases linearly

Frequency Detection c is scanned and the ion TOF is measured A resonance is detected : Relative uncertainty = 0.57 ppb To avoid the B dependence the unknown mass is deduced from the ratio: The atomic mass m is obtained by correcting for the missing q electrons and binding energies the reference ion is 12 C q+ or H 2 +

28 Si for Atomically Defined Kilogram Mass Standard 76 Ge an 76 Se gives the Q value for the neutrino-less double beta decay 133 Cs, for Accurate Determinations of the Fine Structure Constant  24 Mg and 26 Mg for bound-electron g factor determination in hydrogen-like ions Hg to solve the “mercury problem” in Audi/Wapstras mass table binding energies from A q+, A q-1, A q-2... … a relative mass accuracy of  m/m = is required Where does the mass of an atom or ion matter

Highly Charged Ions on Surfaces

ECR – a new ion source medium high charge state on high voltage platform

Filling and cascading mechanism ? How fast charge-state equilibrium reached? Time until hollow atom is relaxed ? 8.5 q keV Pb 55+ : t  6 fs Auger transitions X-ray transitions Side-feeding Pb 55+ on Ta: R c  72 a.u. n c  53

-Below surface relaxation ? Auger and X-ray spectroscopy, transmission exp -Above surface relaxation ? Grazing Angle Scattering Large angle Scattering Ar q+  Au(111) X-ray Measurement Pb q+  Ta Neutralization Charge State Distribution Energy loss Absorption Method

d Back d Front Right Si(Li) detector Ta foil Left Si(Li) detector Ion beam Moveable Faraday cup Focusing system 8.5 q keV Pb 53+ Mean Emission Depth  44 nm (about 100 monolayers) Foil thickness determined by Rutherford backscattering technique Below surface relaxation