Transcript Slide 1

Excess Electrons in Water:
Clusters, Interfaces, and the Bulk
Laszlo Turi
Adam Madarasz
(Eotvos Loring U., Budapest)
Wen-Shyan Sheu
(Fu-Jen University, Taipei)
Daniel Borgis
(Universite d’Evry / ENS Paris)
Funding
•National Science Foundation
•R. A. Welch Foundation
•Hungarian Science Foundation
•Eötvös Fellowship
•Bolyai János Fellowship
•Széchenyi Professor Fellowship
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Water Cluster Anions: distinct “isomers”
Systematic variations

What are the characteristic properties
which distinguish the different
classes?
Common sets of structural motifs?

Backing pressure/thermodynamic
conditions. Non-equilibrium?
J. R. R. Verlet, A. E. Bragg, A. Kammrath, O. Cheshnovsky,
and D. M. Neumark, Science, 307, 93 (2005).
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Anionic clusters and hydrated electrons:
localization mode/”binding motif ” and structure
clusters
↔
“infinite” cluster
↔
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The Toolkit for Mixed Quantum-Classical MD Simulations
{ quantum mechanical e- + classical solvent molecules }
Components:

N classical water molecules (SPC model + internal flexibility)

the excess electron (wave function represented on dual [k,r] grid)

the electron-molecule interaction (pseudopotential*)

the force acting on the molecular nuclei:
= classical force (from the solvent) + quantum force (from the solute)
= FH2O + FQ

A sampling scheme: (adiabatic) time evolution of the system:
R(t )  Hˆ  (Q; R(t ))  E (Q; R(t ))  F  F
0
0
0
nucl
H 2O
 F  R(t )  ...
Q
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* Turi, L.; Gaigeot, M.-P.; Levy, N.; Borgis, D.; J. Chem. Phys., 2001, 114, 7805.
Turi, L.; Borgis, D. J. Chem. Phys., 2002, 117, 6186.
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Applicability of the Pseudopotential

600
Bulk: E0 = -3.12 eV 
Es-p,max = 1.92 eV (vs. 1.72)
500
RG = <r2>1/2 = 2.4 A 

VDE for n=12 clusters
MP2/6-31(1+3+)G*
vs.
the pseudopotential
VDEMP2/meV
400
300
200
100
0
0
100
200
300
400
VDEpseudo/meV
Turi, L.; Madarász, Á.; Rossky, P. J.; JCP 125, 014308 (2006).
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Cluster Simulations:
Surface states vs. internal states
n = 20, 30, 45, 66, 104, 200 + 500, 1000
nominal T = 100K, 200K, 300K
(s  p; n = 45. T = 200K)
L. Turi, W.-S. Sheu, P. J. Rossky, Science 309, 914 (2005), ibid. 310, 1719 (2005).
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Average surface state energetic behavior
vs. interior states and vs. expt.
old lines, new points: n = 200, 500, 1000
(surface and internal at 200K)
expt. (M. Johnson + coworkers)
- spectral
gap
(expt)
E0,1
300K bulk
gap
internal
~35D
(expt)
E0
m
internal
300K bulk
0
0.1
0.2
n
-1/3
0.3
0.20
n -1/3
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Electron radius and kinetic energy
Simulations:
From: David M. Bartels - J. Chem.
Phys. 115, 4404 (2001).
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surface
Rg,electron/Å
4
3
2
internal
1
0
50
100
150
200
n
2.5
internal
2.0
Ekinetic/eV
1.5
1.0
surface
0.5
0.0
0
50
100
150
200
n
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Hydrated electrons at water/vacuum interfaces:
the infinite cluster limit

Cases:
 Ambient water surface (300 K)
 Supercooled water surface (200 K)
 Hexagonal ice surface (200 K)
 Amorphous solid (quenched) water surface (100 K)


Starting point: charge-neutral equilibrium surfaces
Dynamic simulations of surface accommodation
and final states
Localization analysis
Á. Madarász, P. J. Rossky, L. Turi, JCP 126, 234707 (2007).
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Interior and surface hydrated electrons
at liquid water/vacuum interfaces
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(meta)stable surface states at 200 K
vs. spontaneous internal states at 300 K
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Dz(t)
(zcom,e - zGibbs) / Å
4
2
0
-2
-4
-6
-8
0
2000
4000
6000
t / fs
8000
10000
10 ps
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Surface vs. Internal states
Internal state –
bulk hydrated electron
Surface state –
supercooled water interface
300 K
Simulation temperature
200 K
2.4 Å
Radius of the electron
2.7 Å
-3.1 eV
Ground state energy
-2.6 eV
1.9 eV
Spectral maximum
1.5 eV
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Coordination number (<5 Å)
10
11
Alternative surface states
fully
reorganized -OH
partly
reorganized -OH
restricted reorganization
‘otherwise occupied’ -OH
partly reorganized
from dangling -OH
Bulk
Supercooled
water interface
Amorphous solid
water interface
ice Ih interface
Temperature
300 K
200 K
100 K
200 K
Electron
radius
2.4 Å
2.7 Å
3.0 Å
2.6 Å
Ground state
energy
-3.1 eV
-2.6 eV
-1.6 eV
-2.7 eV
Spectral
maximum
1.9 eV
1.5 eV
~1 eV
1.6 eV
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Donor-Acceptor characterization of water molecules
D
A
D
A
4
1
2
(Credit: Mark Johnson)
3
1
2
3
4
ice
AA
AD
DD
AD
AADD
Concept: N. I. Hammer, J.-W. Shin, J. M. Headrick, E. G. Diken, J. R. Roscioli,
G. H. Weddle, and M. A. Johnson, Science, 306, 675 (2004).
strong electron binding
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Hydrated electrons at solid water interfaces
3.0
Structure I, T=200 K
Ice Ih, 200K
2.5
2.0
H-bonding structure analysis:
AA (solid) and AAD (dashed)
AAD
1.5
AA
frequency count
1.0
0.5
0.0
3.0
2.5
ASW, 100K
Structure II, T=100 K
2.0
AAD
1.5
1.0
0.5
0.0
1
2
3
4
5
6
r/Å
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Equilibrium and non-equilibrium preparation of cluster anions

quenched clusters (QC)
Prepare warm (ambient) neutral equilibrium structures
→ quench them gradually to a sequence of lower T’s

Cluster surface site analysis

metastable clusters (MC)
Alternative preparation protocol: assemble the neutral clusters at very
low T → warm them up gradually to the desired higher T.
metastable clusters have never “seen” annealing temperatures

Add the electron and relax (for ~ 200 ps).
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