Transcript bmosu2005.ppt

OBSERVATION AND ANALYSIS OF THE
K= 0 ← 0 ROVIBRATIONAL TRANSITIONS IN
(HI)2
BLAKE A. McELMURRY, ROBERT R. LUCCHESE, and JOHN W. BEVAN
Chemistry Department, Texas A&M University
College Station, TX 77843-3255
SERGEY P. BELOV, and IGOR I. LEONOV
Institute of Applied Physics of Russia Academy of Sciences
Uljanova Str. 46, Nizhny Novgorod 603950, Russia
Nature of Investigations
• Initial prediction of n5 (HI)2 ~ 16 cm-1 made
from near infrared spectroscopy a
• TAMU Fast Scan Submillimeter Spectrometer
used to record the ultra-high resolution
spectrum of n5 (HI)2
• Analysis yields accurate ro-vibrational
constants for the determination of an initial
morphed potential when combined with other
data.
a
Wang et. al. CHEM. PHYS. LETT. 328 (1-2): 153-159 (2000).
Band Structure as predicted from initial spectroscopic data.
17cm-1
Preliminary Analysis
• Spectral features are similar to previously
studied HBr dimer.
• A Standard Hamiltonian for a linear molecule
with 2 equivalent Iodine nuclei was used to
identify quadrupole sub-structure.
• H = Hv + HR + HQ
Confirmation of Quadrupole Sub-structure
• R(0) and P(1) transitions were used to initially identify the
quadrupole sub-structure. All components were observed (24
components for each transition).
• After the HFS was assigned a prediction was calculated and
transitions of other rotational energy levels were measured.
• Quadrupole Structure in other rotational levels fit reasonably
well to the chosen Hamiltonian.
• Measured ~ 300 transitions from P(6) to R(6) with an accuracy
of ~1kHz for well resolved lines.
Spectral Analysis
• H = Hv + HR + HQ
• Only first order quadrupole constants were included
in the initial fit.
• The Hamiltonian used to generate an initial fit gave a
RMS of ~73 kHz which is significantly larger than
the estimated accuracy of the measurement.
• Parameters (B,DJ, caa) in upper and lower states and
band origin (n0) were fitted independently.
Modified Hamiltonian
• Due to large RMS we included additional terms to the
Hamiltonian
– caa – Independent Quadrupole coupling constant for each
iodine atom
– HSR - Iodine spin-rotation term
– Hss - Iodine Spin-spin interaction term
– Dcaa - Second order iodine quadrupole term
• RMS was only slightly improved from the simple
Hamiltonian used initially (RMS ~ 60kHz)
Why does the fit not reflect the
accuracy of the measurements?
• 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?
Energy Level for J=0 in the Ground State (V=0) Determined from the Combination
Frequency Differences of Measured Transitions
5
MHz
4
3.9033
3.9049
3
3.0408
3.0417
2
4.5406
4.5380
J = 0 SG
F= 5/2 (3)
1
0
F
5.4163
5.4173
Separation of frequencies of ortho and
para HI dimer
• We attempted to fit the frequencies of the ortho
and para HI dimer separately for low J
transitions.
• Ortho was assigned to be triplet and para is
singlet.
• The RMS for the separate fits did not
significantly improve.
• To give insight we chose to examine the
previous fits of HBr dimer more closely.
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 = Hv + 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
0.015
0.01
Difference (MHz)
0.005
0
79:81
79:79
81:81
-0.005
-0.01
-0.015
-0.02
R(0)
P(1)
Comparison of the deviation of fit for HBr Dimer
0.04
0.03
0.02
Difference (MHz)
0.01
0
79:81
79:79
-0.01
81:81
-0.02
-0.03
R(1)
-0.04
-0.05
P(2)
• Comparison of HBr data for the different
isotopomers leads to a conclusion that
there are factors that are not treated and
contribute to the observed RMS deviation
of the fit.
• This effect will need to be examined in
more detail in the future.
Energy Level Splitting for J=0 in the Ground State (V=0) Determined from the Combination
Frequency Differences of Measured Transitions and the Same Values Calculated from Fit (Red)
Ratio between measured and
calculated values is 1.0435 for all
differences
5 triplet
MHz
4 singlet
3.9033
3.9049
3.7392
3
3.0408
3.0417
2.9139
2 singlet
4.5406
4.5380
4.3511
J = 0 SG
F= 5/2 (3)
1 triplet
0 singlet
F
5.4163
5.4173
5.1903
Result of Current Analysis of (HI)2
• We used the most basic Hamiltonian in the
final fit with a minimum number of parameters
– B and DJ in upper and lower state
– caa was fixed to be equivalent for both nuclei in
both states
– Total number of parameters was 7
(HI)2 Constants and Comparison with Infrared Constants
n 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)
127I
127 b IR
H :H I
378.438(50)
0.402(20)
a
σ (kHz)
79
upper state
b
lower state
IR - Wang et. al. CHEM. PHYS. LETT. 328 (1-2): 153-159 (2000).
These constants were used to generate a morphed potential for HI
dimer. Predictions of other bands for HI dimer as well as DI dimer
can be made. This will be discussed in more detail in RJ10.
Acknowledgements
• We would like to thank Phil Bunker, Ed
Cohen and Jon Hougen for discussion on
the symmetry and fitting procedure.
• NSF for funding the current project.