1. The lowest vibrational states of urea from the rotational spectrum
69th International Symposium on Molecular Spectroscopy TA10
The lowest vibrational states of urea from the rotational spectrum
Zbigniew Kisiel
Institute of Physics, Polish Academy of Sciences, Warszawa, Poland
Jessica Thomas, Ivan R.Medvedev
cDepartment of Physics, Wright State University, Dayton, OH 45435

2. C2, Cs, or C2v ? The lowest vibrational states of urea: LAM with:
Praha2008
The lowest vibrational states of urea:
C2, Cs, or C2v ?
LAM with:
CN-torsion
NH2 wag
Interconversion
Rotational spectroscopy (J.Thomas et al. OSU Symposium, 2013):
g.s. + three low vibrational states (below 300 cm-1)
The two highest states of the three do not fit with a single state model

3. AABS = Assignment and Analysis of Broadband Spectra
Praha2008
Analysis: 1/ AABS package
2/ SPFIT/SPCAT package of H.M.Pickett.
AABS = available at:
AABS = Assignment and Analysis of Broadband Spectra
Summary of the more advanced applications in:
J.Mol.Spectrosc. 280, 134 (2012)
Spectrum handling: recorded spectra can be combined into a single global spectrum (different point spacing accounted for) + many other manipulations including enhanced baseline subtraction and line blanking
Display and data reduction: operation on many overlapping spectroscopic species
Assignment of perturbations: Loomis-Wood type display mode and difference frequency plots
Data distribution plots: essential book-keeping for large data sets

4. Baseline subtraction + smoothing
AABS spectrum
manipulations:
Praha2008
Differentiation
(one of many possible options)
Baseline subtraction + smoothing

5. The main AABS display: Sat 2+3 g.s. sat 1
Praha2008
Characteristic type-II band of bR-branch transitions with increasing Ka and decreasing J
Sat 2+3
(highlighted)
data in the fit
g.s.
sat 1

6. ? Assignment using Loomis-Wood type plots: g.s. bQ-branch for Kc=32←33
Praha2008
g.s. bQ-branch for Kc=32←33 ?

7. Assignment using Loomis-Wood type plots:
Praha2008
Lower perturbing state bQ-branch for Kc=32←33

8. Distribution plot of mixing coefficients:
Praha2008
Distribution plot of mixing coefficients:
Mixing coefficients Pmix are tabulated in the .EGY file produced by SPCAT and are converted into binned subsets for plotting using PMIX
Data regions

9. Resonance plots (frequency differences relative to the g.s.):
Praha2008
Resonance plots (frequency differences relative to the g.s.):
In high-J (usually aR-type) spectra of prolate molecules characteristic mirror
image spikes are observed.
In the present case transitions are between levels with different Ka (and Kc) and are for
low J (large asymmetry changes) so the behaviour is not yet familiar…
bR-branch,
Ka = 13  12
Excited state Excited state Excited state 2
(unperturbed ) (lower in the perturbing pair) (upper in the perturbing pair)

10. The Hamiltonian and the spectroscopic constants :
Praha2008
The Hamiltonian and the spectroscopic constants :
Urea is oblate, hence representation III (A-reduction)
Several coupling schemes were explored and a-axis Coriolis was found to be by far the most successful.
Vibrational subset statistics automatically derived with PIFORM

11. Comparison of constants for the assigned states :
Praha2008
Comparison of constants for the assigned states :

12. All values are below |(obs-calc)/df<3
Praha2008
Distribution plot of (obs-calc)/df values:
All values are below |(obs-calc)/df<3
Perturbation regions

13. 302 250 80 The WAG-WAG-2D model for urea: 1 state < 100 cm-1
Praha2008
The WAG-WAG-2D model for urea:
Experiment:
E = (3) cm-1
Ga = (13) MHz
1 state < 100 cm-1
2 states around 250 cm-1 80
302
250
M.Inostroza, M.L.Senent, Chem.Phys.Lett. 524, 25 (2012)

14. Praha2008
SUMMARY:
Rotational transitions in the second and third excited vibrational states of urea have been measured from 207 to 493 GHz
Single state fits failed but a coupled fit based on a-axis Coriolis interaction was very successfull and encompassed all measurable transitions to within experimental accuracy
Only 3 constants in the Coriolis Hamiltonian were required!
According to the Coriolis fit the two states are very close together and are separated by only (3) cm-1
The current empirical knowledge of the lowest vibrational states of urea (one state below 100 cm-1, two additional states at ca 250 cm-1) may resolve the uncertainties concerning the LAM
Several higher vibrational states have been assigned but are still to be properly fitted….