GLOBAL FIT ANALYSIS OF THE FOUR LOWEST VIBRATIONAL STATES OF
ETHANE: THE 12 9 BAND L. Borvayeh and N. Moazzen-Ahmadi Department
of Physics and Astronomy University of Calgary V.-H. HORNEMAN
Department of Physical Sciences, University of Oulu, Finland Slide
2 Ethane on Titan F.M. Flasar et al., Science 308 (2005). Slide 3
Ethane on Saturn in emission F.M. Flasar et al., Science 307
(2005). Slide 4 To retrieve reliable abundances from low resolution
spectra: Obtain a high-resolution laboratory spectrum of the band.
Obtain a good fit of the line positions (frequency model). Measure
intensities for selected lines. Model the intensity. Apply the
frequency and intensity models to the whole band. Slide 5 High
Resolution spectrum of the 9 band at room temperature 9 9 4 4 9 2 4
2 4 Slide 6 High Resolution spectrum of the 9 band at T = 133 K 9 9
4 4 Slide 7 Resolution 0.0025 cm -1 Path Length 172 m Pressure 10
Torr Temperature 296 K The torsional bands N. Moazzen-Ahmadi et
al., JMS (2001). Slide 8 V 9 = 1 gs Two-band model Torsion-mediated
Coriolis interaction Level crossing at K = 18 in the P branch N.
Moazzen-Ahmadi et al., JCP (1999). Slide 9 Three-band model D.
Bermejo, et al., JCP (1992); N. Moazzen-Ahmadi, JMS (2002). Strong
torsion-mediated Fermi interaction Slide 10 The hot band 9 + 4 4 +
Torsion Slide 11 Difference between calculated (three-band model)
and observed frequencies in the P branch J.R. Cooper and N.
Moazzen-Ahmadi, JMS (2006). Slide 12 The hot band 9 + 4 4
Interaction with v 12 =1 Slide 13 The hot band 9 + 4 4 12 9 band 12
9 band Slide 14 PQ5PQ5 Wavenumber / cm -1 H2OH2O Resolution = 0.003
cm -1, 15 Torr, 172 m Slide 15 12 9 and 6 9 bands 12 9 and 6 9
bands Wavenumber / cm -1 RQ2RQ2 H2OH2O H2OH2O Slide 16 The hot band
9 + 4 4 Four-band model Slide 17 Comparison of simulated and
experimental spectra using List3 line parameters Slide 18 Slide 19
Slide 20 Ethane on Titan GEISA LIST3 List3 with abundance adjusted
Slide 21 Ethane on Titan GEISA LIST3 List3 with abundance adjusted
Slide 22 Ethane on Titan at high resolution Slide 23 Effect of
ethane on acetylene band Slide 24 Thank you! Slide 25 + 2 Torsional
splitting in 9 + 4 4 due to internal rotation Slide 26 Effect of
skeletal bond vibration on the bath states Strong torsion-mediated
Fermi interaction Slide 27 Effect of skeletal bond vibration on the
bath states Strong torsion-mediated Fermi interaction Slide 28
Comparison of simulated and experimental spectra using List3 line
parameters Slide 29 The hot band 9 + 4 4 Slide 30 Summary Using
results from a global analysis of data involving the four lowest
vibrational states of ethane, we have generated a new set of line
parameters which we have shown to provide much more accurate
description of the experimental spectrum of ethane in the 12 m
region. An isolated band analysis which often works in the case
semi-rigid molecules is not appropriate in the case of ethane and
ethane-like molecules. (1) The torsional mode is very low frequency
and strongly anharmonic. (2) Torsion results in a high density of
torsional bath states which mix strongly with the small amplitude
vibrational fundamentals and dramatically alters the fine structure
of the vibrational bands. Slide 31 Torsional energy CH 3 Slide 32
Slide 33 Two-band model Change in the barrier height and shape,
vibrational frequency,..... Slide 34 Three-band model Change in the
barrier height, vibrational frequency,..... Slide 35 Torsional
motion Near the bottom of the barrier Above the top of the barrier
Slide 36 C-C stretching fundamental A. Al-Kahtani et al., JCP
(1993); D. Bermejo, et al., JCP (1992); N. Moazzen et al., JMS
(2002). Slide 37 The simplest molecule with internal rotation
Ethane is a benchmark molecule exhibiting internal rotation about a
single bond. The Hamiltonian describing the vibration-torsion-
rotation for ethane is much simpler than that for a less
symmetrical molecule. As a result, much fewer interactions are
allowed between the vibrational states. Slide 38 To retrieve
reliable abundances from low resolution spectra: Obtain a
high-resolution laboratory spectrum of the band. Obtain a good fit
of the line positions (frequency model). Measure intensities for
selected lines. Model the intensity. Apply the frequency and
intensity models to the whole band. Slide 39 Torsional motion is
strongly anharmonic. It is also of much lower frequency than the
remaining modes. Torsion results in a high density of torsional
bath states which mix strongly with the small amplitude vibrational
fundamentals and dramatically alters the fine structure of the
vibrational bands. Complications arising from large amplitude
internal rotation Ground vibrational state Excited vibrational
state Torsional sublevels Slide 40 Complications arising from large
amplitude internal rotation An isolated band analysis which often
works in the case semi-rigid molecules is doomed to failure in the
case of ethane and ethane-like molecules.
