Perturbations and vibrational energies in acrylonitrile from global analysis of its mm-wave to THz rotational spectrum

Zbigniew Kisiel,a Lech Pszczółkowski,a Brian J.Drouin,b Carolyn S.Brauer,b Shanshan Yu,b John C.Pearson,b

Ivan R.Medvedev,c Sarah Fortman,d Christopher Neesed

aInstitute of Physics, Polish Academy of Sciences, Warszawa, PolandbJet Propulsion Laboratory, California Institute of Technology,

4800 Oak Grove Drive, Pasadena, CA 91109-8099cDepartment of Physics, Wright State University, Dayton, OH 45435

dDepartment of Physics, The Ohio State University, Columbus, OH 43210

67th OSU International Symposium on Molecular Spectroscopy TH1067th OSU International Symposium on Molecular Spectroscopy TH10

Our previous work on THz-level spectrum of acrylonitrile:

J.Mol.Spectrosc. 2009:

g.s. n11 perturbations

J.Mol.Struct. 2011:

g.s. n11 perturbations in isotopic acrylonitrile

MH09, OSU Symposium 2011

g.s. n11 n15 fit

n10 n11n15 fit

The recorded spectra:

Multiplier of cmw-region pump frequency in cascaded multiplication

Analysis: AABS package

Spectrum handling: single spectrum (36Mpoints @ 0.05 MHz) + modified baseline subtraction

Loomis-Wood displays: assignment

Difference-frequency plots: location of pairwise perturbations

Distribution plots: essential book-keeping for large data sets

AABS = Assignment and Analysis of Broadband Spectra, available at: http://info.ifpan.edu.pl/~kisiel/prospe.htm

Improved baseline subtractionin AABS:

Raw spectrum

a) Standard subtraction: baseline determined by least-squares smoothing with a long window (red bar)

J.Mol.Spectrosc. 158, 318 (1993)

b) Narrower smoothing window leads to artefacts near very strong lines

c) Blanking of strongest lines prior to baseline derivation allows using shorter smoothing windows

Interactions and Hamiltonian:

Cs symmetry point group:

A’ A’ = A’c-axis Coriolis + Fermi

A’ A’’ = A’’a-axis + b-axis Coriolis

g.s. c+F 0 0 n11 a+b c+F n15

a+b 2n11

Some resonances are visible without special techniques:

Matching missing segments at J=55,56 in bQ-type bands in n11 and n15 states

More detailed insight is possible with resonance plots:

Scaled frequency-difference plots

aR-type transitions

n11 2n11 resonance

c-axis Coriolisand Fermi

865 GHz

n11 n15 resonance

a-axis and b-axisCoriolis

530 GHz

Perturbation induced interstate transitions:

Two pairs of strongly mixed states resulting in many different Q- and R-type interstate transitions

533 to 1800 GHz

Examples of the fourth type of pairwise resonance:

n15 2n11 resonance

a-axis and b-axisCoriolis

460 GHz

860 GHz

Mopping up operations allowed by the final fit:

g.s. n11 resonance

c-axis Coriolisand Fermi

985 GHz

n11 2n11 resonance

c-axis Coriolisand Fermi

(with Ka=3 levels in 2n11)

880 GHz

Mixing coefficient plot:

Plots of (1-Pmix) for energy levels allow spotting resonances by connecting similarly shaped islands of perturbations in the horizontal (that is the J) direction

Key results from the four-state fit of g.s. + n11 + n15 + 2n11:

Fitted Rejected Deviation Deviation DElines @10s /MHz rms /cm-1

g.s. 4990 0 0.144 0.713n11 3395 0 0.157 0.781 228.29986(2)

n15 2701 26 0.191 0.989 332.67811(2)

2n11 2183 16 0.241 1.256 457.17496(2)

TOTAL 12769 0.905

Parameters of fit: decadic level rotational Hamiltonian for each state+ only 20 interstate coupling terms

+ 3 energy differences

Advance in numbers of fitted lines for g.s.+ n11: 4990+3395 now3145+1989 in

JMS2009

All n11 lines now in fit while 91 of those lines were rejected by the JMS2009 fit

Data set distribution plot:

Quick overview of quantum number and transition type coverage in the data set

Colour marks obs-calc values greater than 3s, different colours for +ve and –ve deviations

The coverage of the room temperature rotational spectrum of acrylonitrile has been increased in extent to a total of 1170 GHz corresponding to 61.6% of the range up to the upper frequency limit to 1.9 THz

Continuous record of the 182-640 GHz segment

Broadband coverage turned out to be crucial to locating specific pairwise perturbations involving the four lowest vibrational states

Global fit of data for the gs + n11 + n15 + 2n11 now encompasses 12769 measured transitions at rms deviation of close to unity

The precise values of vibrational energies (determined entirely from rotational perturbations), x11,11, and Bv-B0 allow interesting comparisons and calibration of ab initio anharmonic force field calculations: see the broadband issue of J.Mol.Spectrosc.

Coupled fits of the n10 n11,n15 dyad and of the 2n15 n14 3n11 triad have also been successfull but connections with the other states are still to be identified

SUMMARY:

Special thanks to the Microwave Laboratory at the OSU Department of Physics :

for stimulating new approaches to the analysis of broadband spectra,

for making all my visits to the OSU Symposium possible.