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Columbus, June 2008
<Dateiname>.ppt
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Zentrum für Festkörperchemie
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The Salt Molecule LiKF2
Richard MawhorterDepartment of Physics & Astronomy, Pomona CollegeJens-Uwe Grabow Institut für Physikalische Chemie. GWLU Hannover
LiKF2
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Columbus, June 2008 <Dateiname>.ppt
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alkali halides in the gas phase
Introduction, M2X2 Salt – Alkali Halide Gas Phase Molecules
Laser Ablation & Mixed Salt Kinetics
Experimental, LiKF2
Electron DiffractionMicrowave Spectroscopy
ResultsSpectroscopic Parameters, IsotopologuesStructure
Outlook, M’M’’X’X’’
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alkali halide vapor
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1/r2 force toform monomers,
1/r3 force to form dimers
=>
much stronger than ~1/r6 forces for metal cluster formation
Mixed Dimer Formation
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electron diffraction
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COBRA chamber
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production of Non-volatile Species:Coaxial Ablation Twin Source (CATS)
(precursor) / rare gas
Nd:YAG LASER I
Nd:YAG LASER II
supersonicexpansion
rotating/translating rods: elements, alloys, compoundsbulk material, compressed powder
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6LiF/KF Rods: before & after ablation
Laser Ablation Source:
Top view
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Spherical Reflector: Transmit & Receive Antennae
supersonic-jet source
high-frequency antennae
low-frequency antennae
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LiKF2 moments of inertia
a
bc
xF (fixed by IA)
yF (fixed by c.m. & IB)
yK (fixed by c.m. & IB/IC)
fixed by c.m. & IC
Simplest case:
IA = 2(mFxF2)
asymmetric top: A > B > C
planarity: IC = IA + IB
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Microwave spectroscopy of mixed alkali halide dimers: LiNaF2, S. Biermann, J. Hoeft,T. Törring, R. Mawhorter, F.J. Lovas, R.D. Suenram, Y. Kawashima, E. Hirota, J. Chem. Phys. 105 9754 (1996).analyzed lines (cm & mm wave): 49 6LiNaF2 & 46 7LiNaF2
LiNaF2
LiNaF2 spectroscopic constants
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Mixed alkali halyde dimers:LiNaF2-rods with varying NaF/LiF mole fractions
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Mixing Salt
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LiNaF2:NaF Number Density vs. LiF Mole Fraction
0.00
0.20
0.40
0.60
0.80
1.00
1.20
1.40
1.60
1.80
2.00
0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4
Mole Fraction LiF in NaF
LiN
aF2:
NaF
Rat
io
Series1
LiNaF2:NaF Ratio vs. LiF Mole Fraction
Peak ~15%
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simple kinetic model for LiF/NaF formation
Assumptions:
•LiF more likely to dimerize
•Dimerization quickly quenched
Peak ~15%
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Columbus, June 2008 <Dateiname>.ppt
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6Li39KF2 r0 Structure
A=6475.984(16) MHz
B=5590.410(19) MHz
C=2994.110(12) MHz
r0(Li-F)= 1.697Å/1.701Å
r0(K-F)= 2.394Å/2.397Å
A=6477.34(45) MHz
B=5409.892(73) MHz
C=2941.932(51) MHz
3 Isotopologues
6Li39KF2 (8 lines)7Li39KF2 (5 lines)6Li41KF2 (2 lines)
Θ(F-Li-F) = 115.25° / Θ(F-Li-F)QZ = 115.28°
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LiKF2
LiNaF2
???
LiCsF2
ED homo-dimer geometries
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Do mixed dimers relate to regular dimers?
One model: Mixed dimers consist of 2 regular half-dimers:Asymmetric M-F-N angle ≈ average of M-F-M and N-F-N angles
for Θ(Li-F-Na)=82.9°, average(Li-F-Li/Na-F-Na) = 82.7°√reasonably good agreement however:for LiKF2, Θ(Li-F-K)=85.8°, average(Li-F-Li/K-F-K) = 87.8°
Agreement not as good
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Further results
Centrifugal Distortion Constants
DJ= -.026(3) MHzDJK= .113(13) MHzDK= -.089(10) MHz
inonic clusters are less rigid than covalent species
Nuclear Hyperfine Structure due to interaction of the 39K nuclear quadrupole moment
3/2 χaa = -5.2 MHz¼ (χbb – χcc) = -1.8 MHz
Additional splittings observed from 7Li nuclear quadrupole moment
MP2/QZ predictions give excellent agreement with observed splittings and predicted dipole moment of μ = 4.25 D is consitent with observation
Nuclear quadrupole coupling tensor principle axes
Kernquadrupolkopplungstensorhauptachsensystem
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Future Work
Mixed Alkali Halide Dimers with only 1 Q≠0 Nucleus
• 6LiNaF2 – first molecule observed
•6Li39KF2 , 6Li41KF2 - this study
• 6LiRbF2, 6LiCsF2 – next in line; CsF very hygroscopic, but Cs challenging
• 6Li2FCl , 6Li2FBr, 6Li2FI – involve Cl, Br, I; too light to see LiX to tune => mix in few % NaCl, also helps in making sample rods
Asymmetric MixedDimer: LiNaFCl
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acknowledgement
Colin Pulham, EdinburgPhilip Camp, EdinburgPhil McCaffrey, Edinburg
electrical shop mechanical shop
Deutsche Forschungs-Gemeinschaft (DFG)
Land Niedersachsen
Deutscher Akademischer Austauschdienst (DAAD)
Mellon Foundation
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+ -
Figures from:Rohlf Modern Physics from α to Z0
Ionic Bond Dipole Dipoles Attract
Dimer Molecule
(SmallestCluster/Nanoparticle)
Bulk, Dimer, Monomer
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Coaxial oriented Beam-Resonator Arrangement (COBRA)
t
E
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6LiKF2: 39K–hyperfine structure