First soft matter conference

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First soft - matter conference

• Hydrogen molecules, atoms, and negative ions and
  what they may experience in intense laser pulses
  and near surfaces

Uwe Thumm Department of Physics Kansas State
University
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Probing and controlling the nuclear motion in
H2 with intense ultrashort laser pulses

• Introduction
• Numerical model
• One pulse results
• -dissociation
• ionization
• Pump-probe results
• pump-probe imaging
• wavefct reconstruction
• revivals dephasing
• Control
• control pulses
• vibration. quenching
• quality control

Uwe Thumm
Kansas State University

laser pulse
molecule
vibration/rotational excitation dissociation ioniz
ation ( fragmentation), .
?
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Basic processes
H0 H
H2
H2
H H

• Introduction
• Numerical model
• One pulse results
• -dissociation
• ionization
• Pump-probe results
• pump-probe imaging
• wavefct reconstruction
• revivals dephasing
• Control
• control pulses
• vibration. quenching
• quality control

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Dissociation and Ionization paths
p p
Coulomb explosion
Charge resonance enhanced ionization
E a.u.
H2
?u
1w
?g
2(3)w
R a.u.
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Single pulse (0.2 PW/cm2, 25 fs)
Initial state H2(v 4)
PCE
Dissociation
PD
Laser
Norm
time / fs
total fragment energy eV
R / a.u.
log scale
Contours jz(R,t)
time / fs
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Revival of the wave packet theory vs.
experiment
D2 7-8 fs pulse
Experiment T.Ergler et al. PRL 97, 193001 (2006)
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Manipulating nuclear motion with control pulses

• Introduction
• Numerical model
• One pulse results
• -dissociation
• ionization
• Pump-probe results
• pump-probe imaging
• wavefct reconstruction
• revivals dephasing
• Control
• control pulses
• vibration. quenching
• quality control

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Stopping a wavepacket with TWO control
pulses (0.1
PW / cm2, 6 fs FWHM)
ak2
Control pulse delay / fs
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Quality control for coherent control scheme

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Neutralization of H- near metal surfaces Image
and surface state dynamics
Uwe Thumm, Himadri S. Chakraborty, Thomas
Niederhausen, Boyan Obreshkov
J. R. Macdonald Laboratory Department of Physics
Kansas State University
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surface

• Applications
• Basic surface chemistry and analysis
• Development of ion sources
• Control of ion-wall interactions in fusion plasma

Computation Direct numerical solution of the
Schrödinger equation
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Charge-transfer scenario
Ion-surface distance
H-
Conduction band
Affinity level
Image states
Band gap
Surface state
Affinity level explores band gap and structure
therein .
Valence band
Effects on charge transfer ?
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Surface electronic structures Ag(111)
Pd(111) Pd(100)
Image states decay to bulk Surface state
long lived
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Evolution of electronic prob. density
Ag(111)
H- at 50eV and 60 Incidence
Ag(100)
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Surface state effects on Cu
Wave-packet probability density as a function of
time 1 time-step 5 a.u.
H- Cu(111) D5a.u.
H- Cu(100) D5a.u.

Band-gap induced reflections at surface
Increase in surface state population
Strong surface accumulation for (111)
Strong normal decay for (100)
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Image state effects
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Image state effects
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H--survival probability
2D
...strong dependence on IS dynamics...
...on SS dynamics..
Chakraborty, Niederhausen, Thumm, Nucl. Instr.
Meth. B, in print.
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Ion neutralization at vicinal surfaces
Can designer nano-structured surfaces improve
catalytic converters ?
faceted of Cu surface
from I.G. Hill and A.B. McLean, PRL 82 (1999)
2155 T. Torsti et al.,
arXibcond-mat/0209087 (2002)
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H survival probability
EK 1 keV

• Mono-atomic step

• Double-atomic step

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Summary

• Neutralization dynamics of H- near flat Ag, Cu,
  and Pd (100/111) surfaces
• larger ion survival near (111) surfaces due to
  re-capture from surface state.
• image states that are degenerate with the
  conduction band favor the recapture of electrons
  by outgoing ions.
• localized image states that are degenerate with
  the band gap hinder the re-capture process and
  enhance the ionneutralization probability.

Supported by the Division of Chemical Sciences,
Office of Basic Energy Sciences, Office of Energy
Research, US DoE and NSF.
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Thank you !
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Pump-probe D2, 0.3 PW/cm2, 2 x 25 fs
delay 70 fs
Norm(t)
Dissociation
Coulomb explosion
PCE(t)
- - - - - (Coulomb only)
Laser
PD (t)
b
c
a
log scale
Contours jz(R,t)
c
b
a
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Pump-probe experiment H2 25 fs pulse
E / eV
delay / fs
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What are image states ?

-









D
D
What are surface states ?
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DFT Thomas-Fermi-von Weizsacker model
Kinetic energy Exchange-correlation
energy Coulomb energy

• Thomas-Fermi-von Weizsacker approximation for Ts

• Local density approximation (LDA) for Exc

• Electrostatic Coulomb energy in jellium
  approximation

• Euler-Lagrange equation,
  

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Model potentials

• H- effective potential for active electron and
  polarizable core (H).
• Surface single - electron model potential (z),
  free motion (x,y)
• based on self-consistent LDA calculation
• adjusted to measured band edges, surface, and
  image states

V(z)
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Surface and image states
Band gap 5.0 ... 0.6 eV (-4.44 ... 2.16
eV for Pd(111) ) Surface state 4.56 eV
(-4.14 eV for Pd(111)
) 1st Image state 0.77 eV
(-0.55eV for Pd(111) )
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Fixed-ion scenario
Chakraborty, Niederhausen, Thumm, Phys. Rev. A70,
052903 (2004)
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Projected density of states H- Ag
D 6 a.u.
D 1 a.u.
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Electronic probability density ...
... 50 a.u. after the projectile has reached the
position of closest approach (log. scale) 1keV
H- 50o incidence (with respect to the surface)
Pd(111) image states evolve to
vacuum Ag/Pd (111)surface state remains localized
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Surface state effects on Ag
bulk
vacuum
H- on Ag(111) D5 a.u.
Time between frames 50 a.u.
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Projected density of states
D 6 a.u.
H- Ag
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Ion survival theory and experiment
3D
Larger ion survival for Ag(111) due to (re-)
capture from surface state.
Chakraborty, Niederhausen, Thumm, Phys. Rev. A
69, 052901 (2004)
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Electronic structure of vicinal surfaces
Kronig-Penney model
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