Transcript fwOSU2006last.ppt

APPLICATION OF A COAXIALLY CONFIGURED SUBMILLIMETER PULSE JET SPECTROMETER :
The Structure and Dynamics of HI Dimer
H
I
I
H
Fabrice F. WILLAERT, Luis Rivera, Blake A. McELMURRY, Robert R. LUCCHESE, Sergey P. BELOV
and John W. BEVAN
Chemistry Department, Texas A&M University, College Station, TX 77843-3255
OSU Presentation 6/21/2006
Outline
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Submillimeter spectrometer
Morphed Potential
(HI)2 molecule
Conclusions
Outline
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Submillimeter spectrometer
Morphed Potential
(HI)2 molecule
Conclusions
Phase and Frequency Stabilized FASSST with Coaxial Supersonic Jet
Configuration - Version 1: Designed and Constructed at TAMU
A round trip frequency scan of
the F=11/2←9/2 R(3) transition
in Ar:H81Br illustrating Doppler
displaced components
Quadrupole substructure
partially resolved of the
F=5/2←3/2 component of the
hyperfine structure in the 2H79Br
J=1←0 transition
Performances
Frequency Range : 0.2 to 1 THz
Instrumental linewidth <1 kHz, 3.3 x 10-8 cm-1
Resolution <40 kHz
Outline
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Submillimeter spectrometer
Morphed Potential
(HI)2 molecule
Conclusions
Lucchese and Bevan initiated generations of Morphed Potential Energy
Surfaces.
Example with Rg-HX (X= F, Cl, Br, I) (Rg= Ar, Ne, Kr, Xe)
Initial Ab-initio Potential
The generated morphed
potential demonstrates that
the ground state structure of
the complex is hydrogen
bonded, Ar:HBr
but different from the global
minimum Van Der Waals
Ar:BrH structure.
Morphed Potential from Spectroscopic Data
MORPHED POTENTIAL OF Ar:HBr
Ground state (GS) and equilibrium (EQ)
structures of Rg:HX
[Our contributions are in red]
Rg:HX
Ne
Ar
Kr
HF
HCl
HBr
HI
GS
Ne-HF
Ne(HCl)
Ne(HBr) Ne(HI)
EQ
Ne-HF
Ne-HCl
Ne-BrH
Ne-IH
GS
Ar-HF
Ar-HCl
Ar-HBr
Ar-IH
EQ
Ar-HF
Ar-HCl
Ar-BrH
Ar-IH
GS
Kr-HF
Kr-HCl
Kr-HBr
Kr-IH
EQ
Kr-HF
Kr-HCl
Kr-BrH
Kr-IH
Outline
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Submillimeter spectrometer
Morphed Potential
(HI)2 molecule
Conclusions
Structure of (HX)2 from Four Dimensional Morphed Potential
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Accurate tunneling frequencies of
(HF)2 and (HCl)2 determined
We obtained morphed potential
with accurate datas on (HBr)2.
Are the structure and
dynamics of the (HI)2 dimer
potential similar to (HX)2
(X=F, Cl,Br) with a barrier to
interconversion?
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A 4-D morphed potential is
generated to answer these and
other questions associated with
(HI)2
The tunneling splitting frequency in HI-HI is estimated to be 17 cm-1 from IR
Using TAMU Fast Scan Submillimeter Spectrometer, accurate constants for the
band can be determined which will improve the morphed potential as well as
give structural information about the dimer from the quadrupole constants.
(Origin: 511931.42184MHz (17.07619415cm-1)
Confirmation of Quadrupole Structures
- R(0) and P(1) transitions were used to initially
identify the quadrupole structure. All components
were observed.
- After the HFS was assigned a prediction was
calculated using a standard linear hamiltonian
- Measured ~ 300 transitions from P(6) to R(6)
with an accuracy of ~1kHz for well resolved lines.
Fitting
H = H V + HR + HQ
Only first order quadrupole constants were included
The Hamiltonian used to generate initial fit gave a RMS of ~73 kHz
which is bigger than estimated accuracy of the measurement.
Parameters (B,DJ, Χaa) in upper and lower states and band origin
(0) were fitted independently.
Modified Hamiltonian
Due to large RMS we included additional terms to the Hamiltonian
HSR - Iodine spin-rotation term
Hss - Iodine Spin-spin interaction term
DΧaa - Second order iodine quadrupole term
RMS was only slightly improved from Simple Hamiltonian (RMS ~
60kHz)
Why is fit not comparable to accuracy of frequency
measurement?
Accuracy of measurements for well resolved lines is ~1 kHz as
determined from combination frequency differences.
Which terms in Hamiltonian should be included or discarded?
HBr Dimer Fits
Accuracy of measurement was ~ 5-10 kHz
Three isotopomers of HBr dimer (79:79,79:81,81:81) were fitted earlier
with an RMS ~ 8-10 kHz.
1200 lines in HBr dimer were measured and fitted to a Hamiltonian
H = H V + HR + HQ
Why does the same approach not work for HI dimer?
We compared the observed-calculated deviations for all three dimers
Comparison of the deviations of fit for HBr Dimer
IH-HI Constants and Comparison with Infrared Constants
We used the most basic Hamiltonian in the final fit with a minimum
number of parameters
B and DJ in upper and lower state
Xaa was fixed to be equivalent for both nuclei in both states
Total number of parameters : 7
 0 (MHz)
B (MHz)
D J (kHz)
c aa1 (MHz)
H127I: H127Ia 511931.451231(133) 370.8060546(296) 0.14048(113) -377.35245(44)
H127I: H127Ib
378.2997997(233) 0.38971( 53) -389.99784(44)
127I 127 a IR
H :H I
370.747(54)
0.284(28)
H127I: H127Ib IR
378.438(50)
0.402(20)
a
b
upper state
lower state
IR - Wang et. al. CHEM. PHYS. LETT. 328 (1-2): 153-159 (2000).
σ (kHz)
79
From these datas and this analysis, a 4-D morphed potential has been generated,
showing no tunneling barrier
H
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H
Unlike all of the other (HX)2
systems that have been studied
previously
the (HI)2 complex has a
minimum at the symmetric
structure.
(Note however that at larger values
of R the potential regains the twowell form found in the lighter (HX)2
dimers)
Two dimensional cuts of the morphed interaction potential of (HI) 2
(left column) with the corresponding statistical uncertainties from
the non-linear least squares fit (right column).
Further investigations
Adjust the submillimeter datas with an Hamiltonian for an
asymetric rotor with the A value fixed at it ab initio value (
K=0←0 transitions)
Development of a program in collaboration with Laurent
COUDERT and Jon T. HOUGEN using a theoretical model
accounting for :
-The interaction between the dimer interconversion large
amplitude motion and the quadrupole coupling arising from
the two iodine atoms
-The large magnitude of this quadrupole coupling
Question : Will these investigations on the spectral
analysis confirm the morphed potential?
Outlines
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Submillimeter spectrometer
Morphed Potential
(HI)2 molecule
Conclusions
Conclusions
Our rovibrational Hamiltonian for HI dimer is currently inadequate
for describing its submillimeter spectrum and is the subject of
further investigations
For the lowest J transitions in the submillimeter spectrum, the
quadrupole substructure is completely resolved
An initial 4-D morphed potential has been generated with a global
minimum at R = 4.35 Å, 1 = 43°, 2 = 137°, and = 180°, with
V = -358.6 cm–1
The structure and molecular dynamics of HI dimer are different
from other members of the homologous series (HX)2 X= F, Cl, Br
It has a single global minimum with an effective bifurcated
hydrogen-bonded structure and no tunnelling barrier
Acknowledgements
• John W. Bevan, S.P. Belov, B.A. McElmurry, R.R. Lucchese,
Luis Rivera-Rivera and the rest of the Bevan Research
Group
• L. H. Coudert, LISA, CNRS/Universités Paris 12 et 7,
61 Avenue du Général de Gaulle, 94010 Créteil, France.
• Jon T. Hougen, Optical Technology Division, NIST,
Gaithersburg, MD 20899-8441, USA
• The National Science Foundation, EPA, NIH-STTR and the
Center for Atmospheric Chemistry and the Environment are
thanked for supporting this research.