SMR/1831-16
Spring College on Water in Physics, Chemistry and Biology
10 - 21 April 2007
Neutral water clusters: infrared spectroscopy of pure and doped nanoparticles
Udo BUCK
MPI for Dynamics & Self Organization,
Goettingen, Germany
Max-Planck-Institut für Dynamik und
Selbstorganisation Göttingen
Neutral water clusters: infrared
spectroscopy of pure and doped
nanoparticles
Udo Buck
Funded by DFG, MPG, Humboldt
Outline
Pure water nanoparticles:
- Size determination
- Results
„ Na doped water clusters
- Size selection
- Results
„
Size determination
System with similar neutral and ionic configurations
ion
tunable dye laser scanned
through the IP close to the
threshold:
fragmentation free detection
neutral
→ size determination
correlations with source
conditions
doping by one Na atom: applications rare gases, ammonia, water
EPJD 27, 223 (2003)
•Sizes by doping with Na: log-normal distributions
Size distribution of water
clusters
• single photon
ionization at threshold
• fragmentation free
detection
• scaling law Γ(p,T,d)
p0 :
11.0
19.6 bar
T0 : 495 K conical
Scaling of size
scaling law:
<n> = D (Γ* /1000)a
Γ* =( n0 dq T0 α)red
water:
q = 0.643
α = - 2.655
D= 2.63
a = 1.872
Whitehead
Torchet
Vostrikov
Large water clusters
at angle Θ
Depletion too small
Detection of the products:
EPJD 24, 53 (2003)
photofragment
spectroscopy
preferential: hexamers
Large water clusters
Detection of the product: surprise
most intensity (H2O)4H+
result not statistical
(H2O)6
special coupling
Size dependence
„
„
energetically favored: under-coordinated molecules
connected under-coordinated molecules
n
48
90 123
%
43
41
33
293
600
9
10
Increase: more possibilities
decrease: 4 - coordinated
IRPC 23, 375 (2003)
intensity over the whole
frequency range
Size dependence
of IR spectra
DDAA cryst
S4 ASW int
S4 ASW surf
DAA
Calculation
Buck, Buch
IRPC 23, 375 (2003)
Ice particles: FTIR spectra
calculation
measurement
condensation cell
surface and
volume separated
Buch
Devlin
293
123
600
931
40
• spherical in shape
• surface always amorphous
• crystalline core
completely amorphous
Sensitivity of fragments:
2Å
bulk
surface
from outer surface
JPC A 108, 6165 (2004)
Comparison: FTIR/coll.cell - pho-frag/beam
complete
calculations: red
48
outer surface
measurements: black
Bauerecker
3400 cm-1 amorphous
123
931 Devlin
3200 cm-1 crystalline
Coordination and structures
DA,
DA free
DDA
DAA,
DAA free
3350
3715
3550 HB: 3100
free: 3715
DDAA
3200 crystal.
3400 amorph
unique correlation between frequencies (cm-1) and coordination
Calculation: (H2O)20
free: 6
DAA: 6; 6 DDA neighbors
DDA: 6
DDAA: 8; 2 different types
neighbors
DAA
DDAA
DAA
DAA
DDA
free
DAA
DDA
DAA
DDAA
DAA
DDAA
DDAA
DAA
DAA
DAA
DAA
Xantheas JCP 122,134304 (2005)
DDAA
DAA
Edge sharing pentagonal prisms
Raman spectrum of liquid water
297 K 128 bar
shoulders: 3 coord
DAA and DDA
two peaks: 4 coord DDAA
Carey JCP 108, 2669 (1998)
3400 cm-1
amorphous
3200 cm-1
collective excitation
of a symmetric mode
Summary
ordered structures of small clusters allow
us to identify the spectral fingerprints of
the coordination
„ the fragment signal of (H2O)kH+ is mainly
sensitive to the amorphous structure of
the outer surface
„ key: connected under-coordinated
molecules
„ agreement with calculations for outer
surface spectra
„
Outline
Pure water nanoparticles:
- Size determination
- Results
„ Na doped water clusters
- Size selection
- Results
„
Size selection for ionization:
Example: sodium doped
cluster
ion
tunable dye laser scanned
through the IP close to the
threshold:
fragmentation free detection
neutral
→ size determination
correlations with source
conditions
Spectroscopy of size selected clusters
coupling of
ionization and
vibration
First example:
Na(H2O)n
add energy:
enhancement
JPCA 110, 3128 (2006)
Experimental
water
Signal enhancement
optimum: 400 nm
difference
UV
UV + IR
Na doped water clusters
Doped water clusters
single sizes
3100: DAA?
3715: free
3550: DDA?
3630: ?
3400 : ?
DDAA but
n too small
Comparison with calculations for ions
Na+(H2O)n
calculation
DAA
DDAA
DDAC
DDC
experiment
Hartke PCCP 5, 5021 (2004)
The solvated electron
Dissolve a sodium metal chip in liquid
ammonia or water
„ Characteristic electronic spectrum
appears around 0.8 to 1.6 eV
„ Explanation: uncoupled electron in a
cavity of 3.3 Å radius formed by the
liquid
„ models: dielectric continuum, explicit
solvent dynamics, PIMC and MD
„
Comparison with calculations CPMD
Na(H
O)
2
n
dipole moment
Velocity autocorrelation function
projected on stretch coordinate.
delocalized electrons
3 Å compared to 0.7 Å in the liquid
Mundy JACS 122, 4837 (2000)
Experiments with (H2O)n
–
at 3400 cm-1 !
n=6
The excess electron binds two H donor bonds (AA)
Johnson JCP 123, 244311 (2005)
Calculations
Sampling of representative structures from real-time
simulations at finite temperatures:
ab initio MD with density functional theory
B3LYP/6-31+G**; plane wave for e- ; pseudopotential for Na+
Na+ (H2O)m (H2O)l (H2O)-n-m-l
solute – solvent
e – solvent
solvent – solvent
Zhifeng Liu JCP 126, 084501 (2007)
OH {e} HO
Results for (H2O)n
mode
cal
exp
factor
n=3
free
3894
3726
0.957
n=4
DA
free
3608
3884
3533
3714
0.979
0.956
DA
3416
3414
0.999
free
DA
free
3888
3365
3885
3714
3360
3727
0.955
0.998
0.959
DDA
DDA
DAA
3689
3595
3243
3557
3528
3087
0.964
0.981
0.952
n =5
n=8
Ingo Dauster
Comparison n=8
free 6
e-
3
DA
5
DAA 1
DAC 1
eeAA
DA
eDAC
eDAC
free
DAA
DAC
DAA
H
Comparison n=20
A eeA
eDA
DAA
eDA
eDAC
DDAC
DAA
DAA
Interaction with the electron
molecule 8D
eeAA
eeA
eDAC
eDAA
eDA
eDC
10A
-
3354/3560
3349/3440
-
16B
-
20A
3236/3592
3558
3539
3295/3438
3126-3419
-
-
3391/3438
-
3525
3578; 3541
3626
Single electron: 3525-3578 moderate shift
Coupling to D and to other modes for larger clusters
Double electron: weaker e- distributions
Johnson: 3260/3400
Comparison
eDAC eDA
DeAC eeA
DAA
DDAA
Na
eOH+
Johnson
(H2O)n n=20
Most structured spectrum
Clear differences
Doped water clusters
3715: free
more complicated
3100: DAA
3400 : delocalized
electron
single sizes
go to larger sizes
Summery : sodium-water clusters
„ size
selected IR spectra up to
n=80 by a new method
„ spectral fingerprints of the
delocalized electron
„ similarities and differences with
the spectra of anionic water
clusters
„ electron is located at the surface