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)

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

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

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 ?

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

+

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.

Coulomb Explosion Imaging for a

Diatomic Molecular Ion

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

v

2v (R)a dR P(R) v

Distribution of the internuclear distance distribution of HD+

as a function of storage time.

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

Lifetime of HD+

vibrational states

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

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

Typical setup: Merging the molecular ion beam with the e--beam

1.5 m

AB+ + e- A + B

Ion beam

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

Dissociative recombination cross section for HD+ (hot)

No storage

Vibrationally excited HD+

Dissociative recombination cross section for HD+ (cold)

2 sec of storage,

Vibrationallyrelaxed

P. Forck et al., 1992

Cryogenic Photocathode Driven Electron Beam. T~500 μeV

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

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!

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

Photodissociation Spectrum of CH+

U. Hechtfischer et al, PRL, 80, 2809 (1998)

T=500 oKC. WilliamsJCP, 85, 2699 (1986)

3Π metastable state

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.

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.

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)

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).

Production of rotationally cold H3+ at the TSR

H. Kreckel et al. (2004)

Pre-trapping for Pre-cooling

TSR data (kTtrans=0.5 meV)

Cryring data (kTtrans=2 meV)

Theory (C. Green, kTtrans=10 meV )

Dissociative Recombination of H3+

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

Ultra cold electron beam

Merged neutralatomic beam

CSR and Prototype: Under design and construction at the MPIK

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)

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

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!

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.