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?

Aik

Intensity I=NiAik

The radiative lifetime (is determined by the sum of the transition probabilities (Aik) for all decay channels

Aik

Excited state

Metastable stateAllowed transition

A=108 s-1

Forbidden transition

A=1 s-1

Why stored ion beam?

fast ion beam

5 mm

A=1 s-1A=108 s-1

500 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-Probing of the meta-stable population bystable population bylaser excitationlaser excitation

+ higher efficiency+ high selectivity+ high flexibility

PMPM

LaserLaser

PMPM

Observation ofspontaneous decay

obsM

G

passive methodInstead of the passive method we use the active laser probing method

We gain a factor of 5000 and can reduce detector background.

forbidden line

Ca_02_rp

1000

10000

100000

0 1 2 3 4 5 6

Time [s]

Inte

nsity

Lifetime: CaLifetime: Ca+ + 3d 3d 22DD3/23/2

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

Photoncounts

Time after injection

Lifetime curve

cycle 2

cycle 1

cycle 3

Laser pulsesLaser pulses

Fluorescence yield

Moving probe pulse Time

Metastable level

Higher level

Lower level

Laser light

Fluorescence

CRYRING Laser

Photomultiplier

Number of injected ions has to be constant!

a6DJ

1/23/25/27/29/2

a6S5/2

z6D7/2

6516 ÅLaser probing of a 6S 5/2

Eta Carinae blob

The FERRUM project

62 metastable levels

Level Garstang Nussbaumer et al

Quinet et al HFR

Quinet et al SST

Experiment

a6S5/2 326 ms 235 ms 220 ms 262 ms 230(30) ms

b4D7/2 618 ms 500 ms 567 ms 530(30) ms

a4G9/2 856 ms 694 ms 755 ms 650(20) ms

b2H11/2 10.1 s 5.20 s 6.59 s 3.8(0.3) s

Results Fe II

Rostohar et al Phys Rev Lett 86(2001)1466

c2D3/2 base pressure

0

1000

2000

0 0,5 1 1,5

Time [s]

Experimental results Ti II

c 2D3/2 : 0.35 s

b4P5/2 base pressure

10

100

1000

0 10 20 30 40 50

Time [s]

Long lifetime – very sensistive to corrections

Preliminary 27 s

34 metastable levels

Ground level a 4F3/2

c 2D3/2

b 4P5/2

Ion beam

Laser beam

Collinear geometry gives subDoppler line width

F=9/2

F=7/2

F=5/2

F=3/2F=1/2

0 2 4 6 80

1000

2000

3000

4000

Flu

ores

cenc

e In

tens

ity [c

ount

s]

Relative Frequency [GHz]

151Eu+

=5818 Å

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

hAAe qq 1

hAAAe qqq *1**1

Rec

ombi

natio

n O

verv

iew

ExperimentExperimentExperimentExperimentCRYRING

Electron Cooler Dipole Magnet

SBD

||

2||

2

|| 22exp

22

21

Tk

vm

Tk

mv

Tk

m

Tk

mvf

BBBB

Relative Energy (eV)

Rate

Coeff

icie

nt

(10

-12 c

m3 /s)

10-1

100

101

102

103

104

Si14+

N7+

He2+

D+

0 10-6 10-5 10-4 10-3 10-2 10-1 100

10-6 10-4 10-2 100 Energy (eV)

Si14+

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)

Si14+

E

Laser Ring with an Implemented Amplifier

HVP

PD1

Excimer-dye laser

Nd:YAGLaser2nd

harmonic

AC

WPPC

FS2FS1

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- 1s22sf 1Fo 1s22p4d 3Do

1s22p2 3P

e- Coulomb interaction

1s22p4f 3Ge1s22sg 3Ge

1s22p3d 3Fo

Ionization limit

Dielectronic recombination (DR) process

‘allowed’ process

‘forbidden’ process

Radiative stabilization

C3+ + e C2+

First ionization limit

Channel

0 1000 2000 3000 4000 5000 6000

CM

Ene

rgy

[eV

]

0

2

4

6

Lab

Ene

rgy

[eV

]

4000

4200

4400

4600

4800

5000

Cou

nts

0

50

100

150

200

250

300

350

(a)

(b)

(c)

Experiment

Importance of knowing the ring lengthImportance of knowing the ring lengthImportance of knowing the ring lengthImportance of knowing the ring length

•S6

•Beam profile monitor

•Two cooler scrapers

Relative positionRelative positionRelative positionRelative position

•Laser induced recombination

•Measurement of the difference between DR resonances

Absolute lengthAbsolute lengthAbsolute lengthAbsolute length

Lff 01v

Li-like KrLi-like KrLi-like KrLi-like Kr

2s

2p1/2

15 l

nlps EE 2121 22

Energy Splitting

QED effects are small for high-n, so these states

can be calculated accurately

Madzunkov et al., Phys. Rev. A65, 032505 (2002)

Theory Uncertainty

Exp. Uncertainty

71.248(19)

71.243(8)

We are now doing…

“Quantum electrodynamics in the dark”Physics World, Aug. 2001

Ni17+ + e Ni16+

p-Hep-HeHH00+He+He2+2++e+e- - at 2.5-4.5 MeVat 2.5-4.5 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 CRYRINGFast Ion-Atom collisions in CRYRINGCRYRING:CRYRING:High Current (100 A H+)Cold and narrow beam ( 1 mm)

The Gas-Jet Target:The Gas-Jet Target:

Density: up to 1011 cm-3

Jet diameter: 1.0 mm

Luminosity:Luminosity:

61024 cm-2s-1.

TI-rate @ 4.5 MeV:TI-rate @ 4.5 MeV:

1 min-1.

SI-rate @ 4.5 MeV:SI-rate @ 4.5 MeV: 107 s-1.

GAS TARGET

Gas jet

Ring beam

p R║

PROJECTILE DETECTOR

Transfer Ionization in fast HTransfer Ionization in fast H++-He collisions:-He collisions:Thomas p-e-e scatteringThomas p-e-e scattering

pv

He

2 v

0.55 mrad

v

v

45 o

The He nucleus is notdirectly involved in thecollision

pR0

RIMS!RIMS!

The He nucleus is emitted in the backward direction as a result of the kinematical capture.

pR-Q/vp-mevp/2

Kinematical Transfer Ionization (KTI)Kinematical Transfer Ionization (KTI)Kinematical capture through

momentum overlap.

Shake-off

He

vp v

H

He2+

e-

pR=5.0 au @ 2.5 MeV(ER~50 meV)

The pulsed spectrometer:The pulsed spectrometer:

Recoil detector images.Recoil detector images.

0

1

2

3

4

5

0 5 10 15 20

Total TIKTI

Shake-off limit:1.63% ???

v(v0)

He

2+/(

He

++

He

2+)

KTI/(SC+TI)

0

1

2

3

4

5

0 5 10 15 20

Total TIKTI

Shake-off limit:1.63% ???

v(v0)

He

2+/(

He

++

He

2+)

0

1

2

3

4

5

0 5 10 15 20

Total TIKTI

Shake-off limit:1.63% ???

v(v0)

He

2+/(

He

++

He

2+)

[%]

Highly charged ions produced in CRYSIS an EBIS

qq++

e-

e-

e-

Slow Highly Charged Ions Colliding Slow Highly Charged Ions Colliding with Cwith C6060 – stability and fragmentation – stability and fragmentation

Aq+

C60

Experimental set-up

Vex

Multi-hit TDC

START

TRIG

STOP

0 V

-100 V

Collimated C60 Jet

Tim

e-o

f-fl

igh

t

Cylindrical analyzer

PSD

PSD

T=500 C

Aq+

(q-s)+

29+28+ 27+

C605+ C60

4+

C603+

C606+

C607+

Time-of-flight

Xe30+ + C60

Xe28 ++ ….

2000 3000 4000 50000

20

40

60

80

100

120

140

C+

4

C+

3

C7+

60

C6+

60

C5+

60

C4+

60

C3+

60

Coun

ts

time-of-flight (channel number)

2000 3000 4000 5000 60000

50

100

150

200

250

C4+

60-2mC

3+

60-2m

C2+

60C6+

60

C4+

60

C5+

60 C3+

60

Coun

ts

time-of-flight (channel number)2000 3000 4000 5000 6000 7000 8000

0

50

100

150

C+

60

C2+

60

C4+

60

C3+

60

Coun

ts

time-of-flight (channel number)

2000 3000 40000

10

20

30

C+

11C

+

9

C+

10C

+

12

C+

8

C+

7

C+

6

C+

5

C+

4

C+

3

Coun

ts

time-of-flight (channel number)

26 keV Ar26 keV Ar8+8+ + C + C6060 Ar Ar(8-s)+(8-s)+ + ... + ...

s = 1

Cold

s = 4

s = 2

s = 3

Hot

Evaporation of small neutral fragments

Asymmetric fission

Activation energy Ea for

evaporation of a C2 unit ~10 eV

Decay channels of excited C60:

(C60r+)* C60-2m

r+ + C2m (m=1,2,3,4…)

(C60r+ )* C60-2m

(r-1)+ + C2m+

(m=1,2,3,4…)

dominate for r 4

Multifragmentation:

(C60r+ )* many small fragments in low charge states

dominate for r 3

k 1/exp( Bfis / kBT)

Depends on internal energy or temperature of C60

Decay rate:

decrease T increase lifetime

U(R)

C60r+

Ekin

Bfis

R

kinetic energy releases fission barriers

stability

C58(r-1)+ + C2

+

175 mm

8 mm

-electrostatic

-simple-small

-easy to cool

ConeTrap:An Electrostatic Ion Trapfor Atomic and Molecular Physics

H.T. Schmidt, H. Cederquist, J. Jensen, and A. Fardi, NIMB 173, 523 (2001).

Storage and lifetime measurements of C60 ions using

Mass determinations with highly Mass determinations with highly charged ions relevant for charged ions relevant for

fundamental physicsfundamental physicsSMILE

Trap

Ions from CRYSIS

• Principle : Measurement of the cyclotron frequency of an ion trapped in a homogeneous magnetic field :

m

qeBc

2

1

c

c

m

m

How to measure atomic mass with very high precision?

- 810 Hz+ 36 MHzz 240 kHz

using HCI the precision increases linearly

Frequency DetectionFrequency 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

]s[T0.1

]Hz[exc

c

mq

qmR

REF

REF

cREF

c

34.5 35.0 35.5 36.0 36.5 37.0 37.5 38.0

50

55

60

65

70

75

80

85

RESONANCE SPECTRUM FOR 76

Se25+

1s excitation / 15 hours

1.0000 ± 0.0399 Hz

23 852 936.0726 ± 0.0135 Hz

Tim

e-o

f-fli

gh

t (µ

s)

23 852 9XX.XX [Hz]

the reference ion is 12Cq+ or H2+

• 28Si for Atomically Defined Kilogram Mass Standard

• 76Ge an 76Se gives the Q value for the neutrino-less double

beta decay

• 133Cs, for Accurate Determinations of the Fine Structure

Constant

• 24Mg and 26Mg for bound-electron g factor determination in

hydrogen-like ions

• 198-204Hg to solve the “mercury problem” in Audi/Wapstras

mass table

• binding energies from Aq+, Aq-1, Aq-2 ...

… a relative mass accuracy of m/m = 10-9-10-10 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 Pb55+: t6 fs

Auger transitionsX-ray transitions

Side-feeding

Pb55+ on Ta:

Rc72 a.u.

nc 53

-Below surface relaxation ? Auger and X-ray spectroscopy, transmission exp

-Above surface relaxation ? Grazing Angle Scattering

Large angle ScatteringArq+ Au(111)

X-ray MeasurementPbq+ Ta

Neutralization

Charge State Distribution

Energy loss

Absorption Method

dBack

dFront

Right Si(Li) detector

Ta foil

Left Si(Li) detectorIon beam

Moveable Faraday cup

Focusingsystem

8.5 q keV Pb53+

Mean Emission Depth 44 nm(about 100 monolayers)

Foil thickness determined by Rutherford

backscattering technique

Below surface relaxation