Transcript Document

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
D: Mass measurements
B,G: Atomic collision
F: Ion-electron recombination
C: Ion- surface collision
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
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.
Lifetime measurements
Why are radiative
lifetimes needed?
The radiative lifetime (t) is
determined by the sum of the
transition probabilities (Aik) for all
decay channels
t = S 1/ Aik
Aik
Excited state
Metastable state
Allowed
transition
A=108 s-1
Intensity I=NiAik
Forbidden
transition
A=1 s-1
Why stored ion beam?
5 mm
fast ion
beam
Laser
500 km !
•High spectral resolution (laser)
•Time-resolution and long
observation time
A=108 s-1
A=1 s-1
•Pure light source (isotope
separation)
•Ultra high vacuum
Instead of the passive
method we use the
active laser probing
method
E
passive method
M
G
PM
Observation of
spontaneous decay
M
G
forbidden line
Probing of the metastable population by
laser excitation
+ higher efficiency
+ high selectivity
+ high flexibility
PM
Laser
We gain a factor of 5000 and can
reduce detector background.
100000
Intensity
Laser probing
technique (LPT)
developed at
CRYRING for
lifetime
measurements
Ca_02_rp
Lifetime:
Ca+ 3d
2
D3/2
moving laser probe pulse
10000
1000
0
1
2
3
4
5
6
Time [s]
Shutter synchronized with
the ring creates laser pulses
at variable time delays
Laser probing technique – in summary
Laser light
Photon
counts
Higher
level
Lifetime curve
Fluorescence
Metastable
level
Lower
level
Moving probe pulse
Time
cycle 1
cycle 2
Photomulti
plier
CRYRING
Laser
cycle 3
Laser pulses
Fluorescence yield
Time after injection
Number of
injected ions
has to be
constant!
The FERRUM project
Photon counts
10000
62 metastable levels
4
1000
b D7/2
100
a 6S5/2
10
1
0
1000
2000
Time [ms]
Laser
probing of
a 6S 5/2
z6D7/2
6516 Å
a6S5/2
a6DJ
1/2
3/2
5/2
7/2
9/2
Eta Carinae
blob
3000
Results Fe II
Level
Quinet
et al
SST
262 ms
Experiment
a6S5/2
Garstang Nussbaumer Quinet
et al
et al
HFR
326 ms
235 ms
220 ms
b4D7/2
618 ms
500 ms
567 ms
530(30) ms
a4G9/2
856 ms
694 ms
755 ms
650(20) ms
5.20 s
6.59 s
3.8(0.3) s
b2H11/2 10.1 s
230(30) ms
Rostohar et al Phys Rev Lett 86(2001)1466
Experimental results Ti II
2
c D3/2 base pressure
2000
c 2D3/2 : 0.35  0.05 s
34 metastable
levels
1000
0
0
0,5
1
1,5
Time [s]
c 2D3/2
b4 P5/2 base pressure
Long lifetime – very sensistive to
corrections
1000
b 4P5/2
100
10
0
10
20
30
40
Tim e [s]
Preliminary 27  10 s
50
Ground level a 4F3/2
Ion beam
Collinear geometry gives
subDoppler line width
Laser beam
Laser and Radio-Frequency double
Fluorescence Intensity [counts]
4000
F=9/2
F=7/2
3000
Dn=200 MHz
F=5/2
2000
151Eu+
=5818 Å
F=1/2
F=3/2
1000
0
resonance spectroscopy
0
2
Observed
Peak 1
Peak 2
Generated Curve
2100
2000
Dn<1 MHz
Counts
1900
1800
1700
1600
1500
795085
795585
796085
796585
797085
Frequency [kHz]
797585
798085
4
Relative Frequency [GHz]
6
8
Ion-electron interaction
cooling
SBD
Electron Cooler
recombination
Dipole Magnet
e   Aq   Aq 1)  hn
Recombination Overview
Radiative Recombination is an direct
spontaneous process in which a
continuum electron is captured with
the subsequent release of a photon
e   Aq   Aq 1)**  Aq 1)*  hn
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
Experiment
CRYRING
2
2
 mv2
mv||  D ) 
m  m 
f v ) =
exp 


2k BT  2k BT|| 
2
k
T
2
k
T
B 
B ||


1
SBD
Electron Cooler
Dipole Magnet
Used for radiative recombination studies
of ions with free electrons at CRYRING10
14+
Si
Si14+
4
B
Laser induced recombination into specific
quantum states  enhancement factor 200
 = 90o
o
 = 45
o
 = 0
 = -45o
o
15 V/cm - 0
o
15 V/cm - 45
o
15 V/cm - 90
200
p+e-  H(n=3)
Gain
150
50
0
0
10
Relative Energy (meV)
103
N7+
He++
102
He2+
101
D+
100
10-1
0
10-6
10-6
10-4
10-5
20
10-4
10-3
10-2
10-1
10-2 100
Energy
Relative Energy (eV)
E
100
-10
Rate Coefficient (10-12 cm3/s)
Influence of external
(electromagnetic) fields
on recombination rate
Si14+
E
100
(eV)
Laser Ring with an
Implemented Amplifier
AC
FS2
FS1
Nd:YAG
Laser
2nd
harmonic
PC
WP
HVP
PD1
PD2
Excimerdye laser
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.
SBD
Electron Cooler
Dipole Magnet
T. Mohamed, G. Andler, R. Schuch, subm. to J. Opt. Com.
Dielectronic recombination (DR) process
C3+
+e
‘allowed’ process
C2+
1s22sg 3Ge
e-
1s22p4f 3Ge
Coulomb
interaction
‘forbidden’ process
1s22p4d 3Do
1s22sf 1Fo
4d 1Po
Spin-orbit
4d 1Fo
4f 1De interaction
4p 1Se
4f 1Ge
0,25
1 e
4f F
Energy [eV]
a
.
First ionization
limit 4p 1De 4d 1Do
-0,25
3
0
0
4f 3De
4d 3Do
4d 3Po
4p 3Se
4p 1Pe
3
-0,75
4s 1Po
-1,25
4f 3Ge
Ionization 1
limit
0
0
4f 3Fe
3 e
4d 3stabilization
Fo
4pRadiative
P
4p D
1s22p2 3P
DRrate[10 -12 cm 3/s] DRrate[10 -12 cm 3/s]
e-0,75
5
0
0
e
4
0
b
. 1
5
0
0
1
0
0
6
0
3
0
0
2
1s 2p3d 3Fo
2
0
0
.
2
0
.
4
0
.
6
1
0
0
4s 3Po
0
.
1
0
.
3
0
.
5
E
n
e
r
g
y
[
e
V
]
0
.
7
Experiment
350
(a)
300
Counts
250
200
150
100
50
0
(b)
Lab Energy [eV]
5000
4800
4600
4400
4200
CM Energy [eV]
4000
(c)
6
4
2
0
0
1000
2000
3000
Channel
4000
5000
6000
Importance of knowing the ring length
Relative position
•S6
•Beam profile monitor
•Two cooler scrapers

Dv = f  f
Absolute length
•Laser induced recombination
•Measurement of the difference
between DR resonances
1
0
)L
Li-like Kr
Madzunkov et al., Phys. Rev. A65, 032505 (2002)
QED effects are
small for high-n,
so these states
can be calculated
accurately
2p1/2
2s
Rate Coefficient x 10 -9 [cm3 s-1]
15 l
12
71.243(8)
Exp. Uncertainty
Exp.
71.248(19)
10
Theory Uncertainty
QED
8
6
4
2
0
4.0
4.2
4.4
4.6
4.8
5.0
5.2
5.4
5.6
Energy [eV]
Energy Splitting
E2 s1 2 2 p1 2 =   Enl
We are now doing…
“Quantum electrodynamics in the dark”
Physics World, Aug. 2001
Ni17+ + e
Ni16+
Fast ion-atom collisions
Transfer Ionization in MeV p-He Collisions
Studied by Pulsed Recoil-Ion-Momentum
Spectroscopy in a Storage Ring/Gas Target
Experiment
p-HeH0+He2++e- at 2.5-4.5 MeV
Fast Ion-Atom collisions in CRYRING
CRYRING:
High Current
(100 A H+)
Cold and
narrow beam
(  1 mm)
The Gas-Jet Target:
Density: up to 1011 cm-3
Jet diameter: 1.0 mm
Luminosity:
61024 cm-2s-1.
TI-rate @ 4.5 MeV:
 1 min-1.
SI-rate @ 4.5 MeV:
 107 s-1.
PROJECTILE
DETECTOR
GAS TARGET
Gas jet
Transfer Ionization in fast H+-He collisions:
Thomas p-e-e scattering
He
p
v
45
2v
0.55 mrad
o
v
v
pR0
The He nucleus is not
directly involved in the
collision
RIMS!
Kinematical Transfer Ionization (KTI)
Kinematical capture through
momentum overlap.
p
e-
v
v
He
Shake-off
H
He2+
pR-Q/vp-mevp/2
The He nucleus is emitted in the
backward direction as a result of the
pR=5.0 au @ 2.5 MeV
kinematical capture.
(ER~50 meV)
The pulsed spectrometer:
Recoil detector images.
KTI/(SC+TI)
5
4
2+
He /(He +He )
[%]
Total TI
KTI
2+
+
3
2
Shake-off limit:
1.63% ???
1
0
0
5
10
v(v0)
15
20
Highly charged ions produced in
CRYSIS an EBIS
Slow Highly Charged Ions Colliding with C60
– stability and fragmentation
q+
e-
e-
e-
Aq+
Collimated C60 Jet
T=500 C
Vex
Experimental
set-up
Time-of-flight
0 V
-100 V
Cylindrical analyzer
TRIG
(q-s)+
START
PSD
STOP
q+
A
Multi-hit TDC
4+
C605+ C60
C60
C60
C607+
6+
PSD
29+
C603+
28+ 27+
Xe30+ + C60
Xe28 ++ ….
Time-of-flight
26 keV Ar8+ + C60  Ar(8-s)+ + ...
Counts
100
4+
s =
1
3+
C60
250
2+
C60
200
4+
C60
50
+
C60
Cold
0
2000
3000
4000
5000
6000
7000
Counts
150
s = 3
8000
Counts
100
0
60
40
20
0
6+
C60
+
C3
C60
3+
+
C60
C4
3000
3000
+
C5
4000
time-of-flight (channel number)
5000
C3
+
C4
4000
+
5000
6000
C6
s = 4
+
C7
+
C9
+
C8
10
0
2000
2000
+
4+
2+
C60
C60
30
20
80
3+
C60
C60-2m
time-of-flight (channel number)
C60
7+
5+
C60
6+
50
5+
C60
3+
100
Counts
120
4+
C60-2m
150
time-of-flight (channel number)
140
s = 2
C60
2000
+
+
C10
C11
Hot
+
C12
3000
time-of-flight (channel number)
4000
Decay channels of excited C60:
Evaporation of small neutral fragments
(C60r+)* C60-2mr+ + C2m (m=1,2,3,4…)
Activation energy Ea for
evaporation of a C2 unit ~10 eV
dominate for r  3
Asymmetric fission
r+
(C60
)* 
C60-2m(r-1)+
+
C2m+
U(R)
(m=1,2,3,4…)
dominate for r  4
Decay rate:
k  1/exp( Bfis / kBT)
Depends on internal energy or temperature of C60
decrease T  increase lifetime
Bfis
C60r+
Ekin
C58(r-1)+ + C2+
R
kinetic energy releases
 fission barriers
Multifragmentation:
 stability
(C60r+ )*  many small fragments in low charge states
Storage and lifetime measurements of C60 ions using
ConeTrap:
An Electrostatic Ion Trap
for Atomic and Molecular Physics
-electrostatic
-simple
-small
-easy to cool
8 mm
175 mm
H.T. Schmidt, H. Cederquist, J. Jensen, and A. Fardi, NIMB 173, 523 (2001).
Mass determinations with highly
charged ions relevant for
fundamental physics
Ions from
CRYSIS
SMILE
Trap
How to measure atomic mass with very high precision?
n-  810 Hz
n+  36 MHz
nz  240 kHz
• Principle : Measurement of the cyclotron frequency of an ion
trapped in a homogeneous magnetic field :
1 qeB
nc =
2 m
Dm Dn c
=
m
nc
 using HCI the precision increases linearly
Frequency Detection
• nc is scanned and the ion TOF is measured
• A resonance is detected :
R ESO N A N C E SPEC TR U M FO R
76
Se
25+
1s excitation / 15 hours
85
80
Relative uncertainty = 0.57 ppb
Tim e-of-flight (µs)
75
1.0000 ± 0.0399 H z
70
1.0
Dn c [Hz] 
Texc [s]
65
60
55
23 852 936.0726 ± 0.0135 H z
50
34.5
35.0
35.5
36.0
36.5
37.0
37.5
38.0
23 852 9XX.XX [Hz]
•
To avoid the B dependence the unknown mass is deduced from the ratio:
R=
•
nc
n cREF
qmREF
=
qREF m
the reference ion is 12Cq+ or H2+
The atomic mass m is obtained by correcting for the missing q electrons
and binding energies
Where does the mass of an atom or ion matter
•
28Si
•
76Ge
for Atomically Defined Kilogram Mass Standard
an 76Se gives the Q value for the neutrino-less double
beta decay
•
133Cs,
•
24Mg
•
198-204Hg
•
binding energies from Aq+, Aq-1, Aq-2 ...
for Accurate Determinations of the Fine Structure
Constant 
and 26Mg for bound-electron g factor determination in
hydrogen-like ions
to solve the “mercury problem” in Audi/Wapstras
mass table
… a relative mass accuracy of
dm/m = 10-9-10-10 is required
Highly Charged Ions on Surfaces
ECR – a new ion source
medium high charge state on
high voltage platform
Pb55+ on Ta:
Rc72 a.u.
nc 53
Filling and cascading mechanism ?
How fast charge-state equilibrium reached?
Time until hollow atom is relaxed ?
Auger transitions
X-ray transitions
8.5 q keV Pb55+: t6 fs
Side-feeding
-Below surface relaxation ? Auger and X-ray spectroscopy, transmission exp
-Above surface relaxation ? Grazing Angle Scattering
Large angle
Scattering
Arq+ Au(111)
Neutralization
Charge State Distribution
Energy loss
X-ray
Measurement
Pbq+  Ta
Absorption
Method
Below surface relaxation
Ion beam
Left Si(Li) detector
dFront
dBack
Ta foil
Moveable
Faraday cup
Pb53+
Foil thickness determined
by Rutherford
backscattering technique
Right Si(Li) detector
8.5 q keV
Mean Emission Depth  44 nm
(about 100 monolayers)
0,0018
Intensity [per incident ion]
Focusing
system
Front-spectra
Back-spectra
0,0016
0,0014
0,0012
0,0010
0,0008
0,0006
0,0004
0,0002
2,5
3,0
3,5
Energy [eV]
4,0
4,5