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Daniel ZajfmanMax-Planck Institute for Nuclear Physics
Heidelberg, Germanyand
Weizmann Institute of ScienceRehovot, Israel
Physics with Colder Molecular Ions:The Heidelberg Cryogenic Storage Ring
CSR
Robert von HahnManfred GrieserCarsten WelschDmitry Orlov
Joachim UllrichJose CrespoClaus SchroeterHolger Kreckel
Andreas WolfDirk SchwalmMichael RappaportXavier Urbain (LLN)
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Characteristics of the Interstellar Medium and Many Body Quantum Dynamics
Crossing Barrier
Interstellar Conditions: Low temperature Low density
o Slow reaction rates are also important.o Sensitive to the initial quantum states of the reactants
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Production of cold molecules and molecular ions
Cooling Techniques: Supersonic expansion.Cold buffer gas collisions.Trapping.
Molecular ion productionin standard ion sources:
V(R)
R
V=0
AB
V=0V=1V=2
AB+
)()( )( RRvP ABAB
Vibrationally excited
Typical time scales:10 ms – 10’s seconds
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The Heavy Ion Storage Ring-MPI-Heidelberg
AB+ (hot, from the ion source)
E=~ MeV
Laser
Coulomb Explosion ImagingAB+ +X ?
Laser spectroscopyAB++hv?
Electron-molecular ioninteractionAB+ + e ?
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Vibrational cooling, the simplest case: HD+
(H2+ or D2
+ do not cool!)
0 0.5 1 1.5 2 2.5-0.5
0
0.5
1
1.5
2
R (A)
U(R
) [eV
]
D2+ (2g
+) v=0
v=2
v=4
v=6
Internuclear distance (Å)
How can we measure the vibrational population?
HD+ H2+, D2
+
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Coulomb Explosion Imaging:A Direct Way of Measuring Molecular Structure
Preparation
• Ion source• Acceleration (MeV)• Initial quantum state?
E0
Micro-scale
Collapse
• Ion target effects• Electron stripping• Multiple scattering
t=1 s to few secs t <10-15 sec
60 A thick
Measurement
• Field free region• Charge state analysis• 3D imaging detector• Reconstruction
Macro-scale
t= few s
Velocities measurement
vd )vP(
Rd )R(Rd )RP(2
v
Storage ring!Z. Vager et al.
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Coulomb Explosion Imaging for a
Diatomic Molecular Ion
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Kinetic energy release (KER) forthe Coulomb Explosion Imaging ofHD+ after various storage time in thestorage ring.
v
2vvkk (R)a dR P(R)dE )P(E
Tim
e
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v
2v (R)a dR P(R) v
Distribution of the internuclear distance distribution of HD+
as a function of storage time.
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Vibrational population as a function of storage time
Z. Amitay et al., Science, 281, 75 (1998).
Solid line:fit to the data,lifetimes as freeparameters
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Lifetime of HD+
vibrational states
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Physics with vibrationally cold molecular ions
•Basic quantum chemistry (theory-experiment)•Interesting platform for study of few particle quantum problem•Molecular dynamics on single and multi-dimensional surfaces•Benchmark for simple molecular systems•Relevant to Plasma Physics•Necessary for understanding the interstellar medium
Experiments on Storage Rings:
•Electron induced recombination•Electron induced dissociation•Electron induced excitation•Photon induced processes
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Electron-cold molecular ion reaction: Dissociative Recombination
HD+ (2g+)
HD+ (2pu)
H(1s)+D(2l)D(1s)+H(2l)
e-
Direct processIndirect process
Interference
KineticEnergyRelease
HD+ + e- H(n) + D(n’) + KER
Rydberg state
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Typical setup: Merging the molecular ion beam with the e--beam
1.5 m
AB+ + e- A + B
Ion beam
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1/22
ee
i2
ii
e
i
ecm ΔEv
v1ΔEmm
vv1ΔE
Merged Beam Kinematics
Electrons Ee,meIons Ei, mi
2
eii
e2cmecm EEm
mvm21E
Center of mass resolution:
~ meV resolution for zero relative kinetic energy!
Electron-cold molecular ion reaction: Dissociative Recombination
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Dissociative recombination cross section for HD+ (hot)
No storage
Vibrationally excited HD+
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Dissociative recombination cross section for HD+ (cold)
2 sec of storage,
Vibrationallyrelaxed
P. Forck et al., 1992
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Cryogenic Photocathode Driven Electron Beam. T~500 μeV
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HD+ + e- H+D
Advance in electron beam resolution
June 2004
kTperp =500 μeV, kTpar=20 μeV
Trot=300 oK
D. Orlov, F. Sprenger, M. Lestinski, H. Buhr, L. Lammich, A. Wolf et al.
June 1992
P. Forck et al
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H2+ DR cross section for (v,J)=(0,0)
H2+ DR cross section for (v,J)=(0,1)
H. Takagi, J. Phys. B, 26, 4815 (1993)
Only one rotational quanta ofexcitation changes the wholespectra!!
Recombination cross section fora single quantum rotational stateof H2
+ (The simplest molecular ion!)
In fact, these resonances havenever been individually observed!
•Position•Depth•Shape teach everything about the dynamics taking place during the dissociation.
Rotationally cold molecular ions!
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Rotational temperature of fast stored beam: Probing rotational population through photodissociation.
HChJvCH ,
Astrophysics relevance• Steady state models cannot reproduce CH+ abundance.• The reverse reaction is the main production process.
Photodissociation throughnon-adiabatic coupling.
Laser spectroscopy
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Photodissociation Spectrum of CH+
U. Hechtfischer et al, PRL, 80, 2809 (1998)
T=500 oKC. WilliamsJCP, 85, 2699 (1986)
3Π metastable state
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Time evolution of the rotational population and comparison to a radiative model.
Radiative transition (oscillator strength) can be extracted. “Easier” spectroscopy. New spectroscopic constants for CH+.
U. Hechtfischer et al., PRL, 80, 2809 (1998).
Asymptotic rotational temperature: T~300 (+50 -0) K.
However, some new evidences showsthat there are collisions (residual gas) induced processes which can internallyheat the beam.
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H3+ Dissociative recombination rate coefficient: 1947-2005
Experimental data
H(1s)H(1s)H(1s)
H(1s)(v)HeH 23
H3+ cannot be thermalized
in a storage ring.
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Calculations
What happen to the rotational population when you store a hot H3+ in a ring?
Simulation of radiative rotationaltransitions for H3
+ starting from Trot= 0.23 eV, and calculating 245,000 transitions (J. Tennyson web-site).
L. Neale, et al., Astrophys. J., 464, 516, (1996)B. M. Dinelli, et al., J. Mol. Spectr. 181, 142 (1997)
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Calculations
Long live states:States for which the axis of rotation is nearly parallel to the C3v symmetryaxis (K=J, K=(J-1))
Is the additional energy stored as rotational energy?
J: Angular momentumK: Projection of J onto themolecular symmetry axis
Simulation of radiative rotationaltransitions for H3
+ starting from Trot= 0.23 eV, and calculating 245,000 transitions (J. Tennyson web-site).
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Production of rotationally cold H3+ at the TSR
H. Kreckel et al. (2004)
Pre-trapping for Pre-cooling
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TSR data (kTtrans=0.5 meV)
Cryring data (kTtrans=2 meV)
Theory (C. Green, kTtrans=10 meV )
Dissociative Recombination of H3+
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Physics with rotationally cold molecular ions: “real” interstellar conditions
TSR “limits” the physics to vibrational states
To achieve rotational cooling, the ring needs to becooled to much lower temperature (~10 K)
The Cryogenic Storage Ring
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Ultra cold electron beam
Merged neutralatomic beam
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CSR and Prototype: Under design and construction at the MPIK
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Physics with colder (~ 2o K) molecular ions
Interstellar conditions Single quantum state physics Comparison with theoretical calculations
Molecular dynamics under controlled initial conditions
•Dissociative recombination (single Rydberg resonance)•Laser spectroscopy and transition strength •Cold collisions and atom exchange•State control and laser manipulation•Infrared emission spectroscopy•Biomolecules•Cluster physics• …
Highly charged ions (J. Ullrich)Antiproton physics (GSI)
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Molecular Ion-Neutral Exchange Reactions
Merged beams
The rate is usually assumed to be based on Langevin model (polarization) mechanism: σ~1/√E, where E is the collision energy
Tosi et al, Phys. Rev. Lett., 67, 1254 (1991).Tosi et al, JCP, 99, 985 (1993).
~10 meV HWHM
Almost no experiments (cross sections)
with cold molecular ions!Model reaction:
HOHHOHOH
22
3
AB+ + C AC+ + B
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State control (state manipulation) with tunable infrared laser
Extremely difficult if the initial population is made of several rotational states
300 oK situation (TSR)
Boltzmanndistribution
~5%
~2.5%
v=0
v=1
10 oK situation (CSR)
~100%
~50%
v=0
v=1
Make all previously described experimentspossible with different initial quantum state!
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Infrared emission spectroscopy
Cerny-Turner monochromator
Single Photon Cryogenic Infrared Detector (Saykally, JPC A102, 1465 (1998))
The ultimate goal:Measuring the emission linesof mass selected stored (and cooled) PAH ions and ionic clusters.