Daniel Zajfman

1
Physics with Colder Molecular Ions The
Heidelberg Cryogenic Storage Ring CSR
Daniel Zajfman Max-Planck Institute for Nuclear
Physics Heidelberg, Germany and Weizmann
Institute of Science Rehovot, Israel
Andreas Wolf Dirk Schwalm Michael
Rappaport Xavier Urbain (LLN)
Joachim Ullrich Jose Crespo Claus
Schroeter Holger Kreckel
Robert von Hahn Manfred Grieser Carsten
Welsch Dmitry Orlov
2
Characteristics of the Interstellar Medium and
Many Body Quantum Dynamics

• Interstellar Conditions
• Low temperature
• Low density

• Slow reaction rates are also important.
• Sensitive to the initial quantum states of the
  reactants

Crossing
Barrier
3
Production of cold molecules and molecular ions
V(R)
Molecular ion production in standard ion sources
Vibrationally excited
AB

• Cooling Techniques
• Supersonic expansion.
• Cold buffer gas collisions.
• Trapping.

V0
Typical time scales 10 ms 10s seconds
R
4
The Heavy Ion Storage Ring-MPI-Heidelberg
AB (hot, from the ion source)
E MeV
Laser
5
Vibrational cooling, the simplest case HD (H2
or D2 do not cool!)
H2, D2
HD
Internuclear distance (Å)
How can we measure the vibrational population?
6
Coulomb Explosion Imaging A Direct Way of
Measuring Molecular Structure
Preparation
Collapse
Measurement
E0
60 A thick

• Ion source
• Acceleration (MeV)
• Initial quantum state?

• Field free region
• Charge state analysis
• 3D imaging detector
• Reconstruction

• Ion target effects
• Electron stripping
• Multiple scattering

Velocities measurement
Macro-scale
Micro-scale
t1 ?s to few secs
t lt10-15 sec
t few ?s
Storage ring!
Z. Vager et al.
7
Coulomb Explosion Imaging for a Diatomic
Molecular Ion
8
Kinetic energy release (KER) for the Coulomb
Explosion Imaging of HD after various storage
time in the storage ring.
Time
9
Distribution of the internuclear distance
distribution of HD as a function of storage time.
10
Vibrational population as a function of storage
time
Solid line fit to the data, lifetimes as
free parameters
Z. Amitay et al., Science, 281, 75 (1998).
11
Lifetime of HD vibrational states
12

• 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

13
Electron-cold molecular ion reaction
Dissociative Recombination
HD e- ? H(n) D(n) KER
Direct process
Indirect process
Interference
Rydberg state
e-
Kinetic Energy Release
H(1s)D(2l)
D(1s)H(2l)
14
Typical setup Merging the molecular ion beam
with the e--beam
AB e- ? A B
Ion beam
1.5 m
15
Electron-cold molecular ion reaction
Dissociative Recombination
Merged Beam Kinematics
Electrons Ee,me
Ions Ei, mi
Center of mass resolution
meV resolution for zero relative kinetic energy!
16
Dissociative recombination cross section for HD
(hot)
No storage
Vibrationally excited HD
17
Dissociative recombination cross section for HD
(cold)
2 sec of storage,
Vibrationally relaxed
P. Forck et al., 1992
18
Cryogenic Photocathode Driven Electron Beam.
T500 µeV
19
Advance in electron beam resolution
HD e- ? HD
D. Orlov, F. Sprenger, M. Lestinski, H. Buhr, L.
Lammich, A. Wolf et al.
20
H. Takagi, J. Phys. B, 26, 4815 (1993)
Recombination cross section for a single quantum
rotational state of H2 (The simplest molecular
ion!)
H2 DR cross section for (v,J)(0,0)
Only one rotational quanta of excitation changes
the whole spectra!!
H2 DR cross section for (v,J)(0,1)

• In fact, these resonances have
• never been individually observed!
• Position
• Depth
• Shape
• teach everything about the dynamics
• taking place during the dissociation.

Rotationally cold molecular ions!
21
Laser spectroscopy
Rotational temperature of fast stored beam
Probing rotational population through
photodissociation.

• Astrophysics relevance
• Steady state models cannot
• reproduce CH abundance.
• The reverse reaction is the
• main production process.

Photodissociation through non-adiabatic coupling.
22
Photodissociation Spectrum of CH
U. Hechtfischer et al, PRL, 80, 2809 (1998)
23

• Radiative transition (oscillator
• strength) can be extracted.
• Easier spectroscopy.
• New spectroscopic
• constants for CH.

Time evolution of the rotational population and
comparison to a radiative model.
U. Hechtfischer et al., PRL, 80, 2809 (1998).
Asymptotic rotational temperature T300 (50
-0) K. However, some new evidences shows that
there are collisions (residual gas) induced
processes which can internally heat the beam.
24
H3 Dissociative recombination rate coefficient
1947-2005
H3 cannot be thermalized in a storage ring.
Experimental data
25
What happen to the rotational population when you
store a hot H3 in a ring?
Simulation of radiative rotational transitions
for H3 starting from Trot 0.23 eV, and
calculating 245,000 transitions (J. Tennyson
web-site).
Calculations
L. Neale, et al., Astrophys. J., 464, 516,
(1996) B. M. Dinelli, et al., J. Mol. Spectr.
181, 142 (1997)
26
Is the additional energy stored as rotational
energy?
Simulation of radiative rotational transitions
for H3 starting from Trot 0.23 eV, and
calculating 245,000 transitions (J. Tennyson
web-site).
Calculations
Long live states States for which the axis of
rotation is nearly parallel to the C3v
symmetry axis (KJ, K(J-1))
J Angular momentum K Projection of J onto
the molecular symmetry axis
27
Production of rotationally cold H3 at the TSR
H. Kreckel et al. (2004)
28
Dissociative Recombination of H3
TSR data (kTtrans0.5 meV)
Cryring data (kTtrans2 meV)
Theory (C. Green, kTtrans10 meV )
29
TSR limits the physics to vibrational states
To achieve rotational cooling, the ring needs to
be cooled to much lower temperature (10 K)
Physics with rotationally cold molecular ions
real interstellar conditions
The Cryogenic Storage Ring
30
Merged neutral atomic beam
Ultra cold electron beam
31
CSR and Prototype Under design and construction
at the MPIK
32
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)
33
Molecular Ion-Neutral Exchange Reactions
AB C ? AC B
The rate is usually assumed to be based on
Langevin model (polarization) mechanism s1/vE,
where E is the collision energy
Almost no experiments (cross sections) with
cold molecular ions!
Merged beams
Tosi et al, Phys. Rev. Lett., 67, 1254
(1991). Tosi et al, JCP, 99, 985 (1993).
34
State control (state manipulation) with tunable
infrared laser
Extremely difficult if the initial population is
made of several rotational states
Make all previously described experiments possible
with different initial quantum state!
300 oK situation (TSR)
2.5
v1
Boltzmann distribution
5
v0
35
Infrared emission spectroscopy
The ultimate goal Measuring the emission
lines of mass selected stored (and cooled) PAH
ions and ionic clusters.
Single Photon Cryogenic Infrared Detector
(Saykally, JPC A102, 1465 (1998))
Cerny-Turner monochromator