VUV Excitation and the Stark Effect of Molecular Hydrogen

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??????????????????????
?????????????????????????? KEK 27-28/06/2005

• ?? ??
• ????????????

??? Prof. T. P. Softley, Oxford Dr. S. R. Simon,
Oxford Prof. F. Merkt, ETH Prof. K. Ohno, Tohoku
\\\ Kiban (C) Matsuo Grant Wakate (A)
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Cold molecules
1. He buffer gas cooling in a MOT (CaH, 400 mK)
J. D. Weinstein et al., Nature 395, 148 (1998).
2. Stark deceleration of polar molecules (ND3, 25
mK)
H. L. Bethlem et al., Nature 406, 491 (2000).
3. Photoassociation of metal atoms in a MOT (Li2,
600 nK, BEC)
M. W. Zwierlein et al., Phys. Rev. Lett. 91,
250401 (2003).
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Objectives

• Ultra-high resolution spectroscopy
• Coherent beams of molecules
• Weak intermolecular forces

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Aims
Stark control of Rydberg molecules

• Origin
• Very large electric susceptibility
• (1000 times greater)
• Advantage
• Non-polar, non-magnetic neutral molecules
• Moderate electric field strength
• Limitation
• Short lifetime
• Field ionization
• Level crossing (Inglis-Teller limit)

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Inhomogeneous dipole field
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Aims

• H2 Well-defined parameters available
• Good response in manipulating motion
• Large rotational energy spacing
• Ion-detected Stark spectroscopy
• Observation of long-lived molecular Rydberg
  states
• First demonstration
• Deflection and decelaration of Rydberg
  molecules
• Lifetime properties
• How to avoid predissociation and
  autoionization
• Towards 100 deceleration
• How to cool down H2 Rydberg molecules to
  150 mK

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Excitation Scheme for para-H2
Y. Yamakita, S. R. Procter, A. L. Goodgame, T. P.
Softley, and F. Merkt, J. Chem. Phys. 121, 1419
(2004).
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Stark map for N0,2 Rydberg states
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Apparatus
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Stark effect in para-H2
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Stark effect in N2 Rydberg states
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Deflection and 2 ms delayed PFI
7 ms in dipole field
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Duration vs deflected trajectories
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Deceleration in 1 ms
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nd21 Rydberg Series

• Autoionization forbidden
• There is no J1 gerade continuum associated
• with N0 threshold.
• e.g. (np0)1 has ungerade symmetry.
• No field induced mixing with rapidly
  predissociating states
• but mixes with high-l Rydberg states.

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Towards 100 deceleration

• Present work
• Neat H2
• initial translational energy 570 cm-1
• deceleration by a single step 13 cm-1
• Seeding in cold gases of heavy atoms
• Seeding in Ar
• initial translational energy 40 cm-1
• Seeding in 77 K Ar
• initial translational energy 20 cm-1
• Optimized time-varying electric fields
• and a cold nozzle.

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Two-dipole method
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Optimum electric fields
T. P. Softley, S. R. Procter, Y. Yamakita, G.
Maguire, and F. Merkt, J. Elec. Spectrosc. Relat.
Phenom., 144-147, 113-117 (2005). .
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Stark cooling of Rydberg H2
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Conclusions

• Deflection and deceleration
• of non-polar, non-magnetic neutral molecule, H2,
• has been demonstrated.
• ?Translation control
• Observed long lifetime (gt100 ms) is due to
• the (nd2)1 states which is
• 1. accessible by selected polarization
• 2. autoionization forbidden
• 3. decoupled in rotational channel interaction
• ? Lifetime control

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Outlook

• Complete stopping of
• Rydberg H2 molecules and He atoms
• Polar molecules
• Observation of quantum effects in
• cold collision and surface scattering
• with slowed atomic/molecular Rydberg beams

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Control of motion
Inhomogeneous electric fields Symmetric top
molecules Stark deceleration and trapping ND3,
3CO Meijer and co-workers (1999,2000) Buffer-gas
cooling CaH Doyle and co-workers
(1998) Optical methods Optical dipole force,
coherent control, laser cooling etc. Other
methods Rotating nozzles
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Triplet He Rydberg states
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Stark effect in triplet He
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Field ionization
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n8, m0, k7
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Tripling efficiency
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Stark effect in N2 Rydberg states
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Rydberg states of hydrogen atoms
Johannes R. Rydberg (1854-1919)
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The Quantum Defects

• ds ? 1.0
• dp ? 0.5
• dd ? 0.1
• df ? 0.01

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cf. v1 vibration Tv15 fs, N1 rotation
Tr0.28 ps
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Ultra-high vacuum chamber
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Contents

• Introduction
• Molecular Rydberg states
• Control of motion
• VUV/UV Stark Spectroscopy
• Stark spectra of molecular hydrogen
• Control of Molecular Motion
• Deflection and deceleration of a H2 beam
• by inhomogeneous electric fields

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Rydberg States and Stark Effect
Parabolic coordinates
nn1 n2m1
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• Unusual physical properties

• Macroscopic sizes of electron orbitals
• Inverse Born-Oppenheimer regime
• cf. v1 vibration Tv15 fs, N1 rotation
  Tr0.28 ps
• Very sensitive to electric fields

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Frame transformation of Hunds cases (b) and (d)

• End2,J0 EdSE2,
• End2,J1 EdPE2,
• End2,J2 (2 EdS EdP 4 EdD) / 7 E2

Matrix diagonalisation in Hund case (d) basis
HH0eFz,
Rottke Welge (1992)
DMJ0, Dl1, DN0, DJ0,1
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Block-diagonalized Hamiltonian
The (nd2)1 states has no interaction with
the N0 Rydberg states or rapidly
predissociating states.
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H2 REMPI
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The Stern-Gerlach experiment (1922)
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• Field free spectra of Rydberg states of H2
• p states
• G Herzberg and Ch Jungen (1972) 80 K
• P M Dehmer. and W A Chupka (1976) 78 K, high
  resolution
• H H Fielding, T P Softley and F Merkt (1991)
  XUV ZEKE
• s and d states
• H Rottke and K H Welge (1992) all ionisation
  processes inclusive
• W L Glab, K Qin and M Bistransin (1995) H,
  H2 detected separately
• This study survived Rydberg states detected
• Stark spectra of H2
• H H Fielding and T P Softley (1991) XUV,
  autoionising states
• W. L. Glab and K. Qin (1993) via E1Sg states,
  p polarisation, v1
• This study via B1Su states, s polarisation ,
  v0

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Forced autoionisation of N2 Rydberg states
to N0 continuum
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Images of deflected H2
Obs 1.31 mm (2.7?) -0.58 mm (-1.2?)
against 28 mm vtrans? 140 m/s
Calc vtrans? 210 m/s
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Techniques

• VUV/UV spectroscopy
• Non-resonant tripling in rare gas mixtures
• Triplet states produced by discharge
• Time-of-flight analysis imaging
• Pulsed field ionisation
• Stark control of translational motion
• Theoretical calculation
• Perturbative treatment in Hund case (d) basis
• Rotational channel coupling

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Trajectory calculation
n16 2.95 km/s (1 K) 20 ns time step 100 mm beam
waist
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Hydrogenic Rydberg states
Electron distributions of the n8, m0 state at
F0 V/cm
n1- n27 n1- n25
n1- n23 n1- n21
n1- n2 -1 n1- n2 -3
n1- n2 -5 n1- n2 -7
NB r-weighted
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Objectives

• Cold molecules
• Ultra-high resolution spectroscopy
• Ultra-cold molecular collisions
• Detectable de Broglie wave length
• Internal freedom (vibration, rotation,
  orientation, shape)
• Small molecules
• Well-definable parameters
• Large energy spacing of rotational levels