H. Mäder

High resolution rotational spectroscopy on D2O up to 2.7 THz in its ground and first excited vibrational bending states

We present highly accurate laboratory measurements on the pure rotational spectrum of doubly deuterated water, D2O, in selected frequency regions from 10 GHz up to 2.7 THz. Around 140 rotational transitions in both the vibrational ground and first excited bending states (upsilon2=0,1) were measured in total, involving energy levels with unexcelled high J and Ka rotational quantum numbers. The data give valuable information for the spectroscopic analysis of this molecule. In the case of the light and non-rigid water molecule, standard methods for its analysis are limited due to large centrifugal distortion interactions. Here, we present a global analysis of rotational and rovibrational data of the upsilon2=0 and 1 states of D2O by means of an Euler expansion of the Hamiltonian. In addition to the newly measured pure rotational transitions, around 4000 rotational and rovibrational lines have been included from previous work. It was possible to reproduce the extensive dataset to nearly its experimental uncertainty. The improved predictive capability of the model compared to previous work will be demonstrated.

The spectrum of ethylene from microwave to submillimetre-wave

87 transitions between the vibrational states v 7 = 1 and v 8 = 1 of ethylene have been measured in the range going from microwave to submillimetre-waves. The v 7 = 1 state is already known with an accuracy better than 10 − 6 cm−1 thanks to sub-Doppler CO2 laser sideband spectra. Combining these new measurements with the existing measurements on the ν3, ν4, ν7, ν8 and ν10 bands allows us to obtain accurate parameters for the v 8 = 1 state, including all sextic centrifugal distortion constants and the Coriolis interaction constants up to third order.

Spectroscopy of the formaldehyde isotopomer H213CO in the microwave to terahertz region

The pure rotational spectrum of H213CO in its ground vibrational state has been investigated extensively between 8 and 1900 GHz using samples of natural isotopic abundance as well as samples enriched in 13C. The accurate new line frequencies were subjected to a combined fit with previously published data, resulting in distinctly improved spectroscopic constants, which include several octic and one decic centrifugal distortion constants. Fits employing Watsons S and A reductions will be compared. The following rotational constants have been obtained: AS=281993.0397 (31), BS=37809.106966 (213), CS=33215.941417 (207) and AA=281993.0148 (32), BA=37811.090040 (219), CA=33213.976745 (207) MHz, respectively.

Rotational spectrum of the isotopically substituted van der Waals complex Ar–CO2 investigated with a molecular beam Fourier transform microwave spectrometer

Pure rotational spectra of the isotopomers Ar–16O 13C16O, Ar–17O12C16O and Ar–18O12C16O of Ar–CO2 have been investigated using a molecular beam Fourier-transform microwave spectrometer. The analyses of the experimental data yielded rotational and centrifugal distortion constants. These data were included in a force field analysis which has resulted in values for van der Waals stretching and bending force constants and harmonic vibration frequencies. In the case of Ar–17O12C16O, hyperfine structure of rotational transitions due to the quadrupolar 17O nucleus was observed and the corresponding nuclear quadrupole coupling constant could be determined. The obtained data are discussed in terms of structural and dynamical parameters.

The rotational spectrum of dichlorocarbene, C35Cl2, observed by molecular beam-Fourier transform microwave spectroscopy

The rotational spectrum of the most abundant isotopomer of dichlorocarbene, C35Cl2, in its 1A1 ground electronic state has been recorded using a molecular beam—Fourier transform microwave spectrometer. The CCl2 was generated by flash pyrolysis in a pulsed free jet expansion using CBr2Cl2 or CHCl2COCl as precursors. 43 hyperfine structure lines of 6 rotational transitions were recorded in the frequency range 8–40 GHz. The analysis of the observed spectrum yielded the rotational constants, the complete 35Cl nuclear quadrupole coupling tensor, and the nuclear spin-rotation interaction constants. In addition, a refined r0 structure of CCl2 was derived.

Rotational spectra of CF3CN in the centimetre, millimetre and submillimetre wave ranges Observation of direct l -type resonance transitions

Rotational spectra of the ground, ν = 1, ν7= 1 and ν8 = 1 vibrational states of trifluoroacetonitrile, CF3CN, have been measured in the millimetre and submillimetre wave range and analysed. The ground state spectrum has been extended up to 650 GHz (J = 109) and sextic centrifugal distortion constants have been determined accurately. The ν4 = 1 and ν7 = 1 spectra have been measured up to 350 GHz (J = 51), and improved molecular constants have been obtained. By means of microwave Fourier transform spectroscopy, the nuclear quadrupole hyperfine structures of the J = 1-0 transitions for the vibrational states ν8 = 1,2,3, have been measured and analysed. Using the same experimental technique 431 direct l-type resonance transitions (section rule ΔJ=0, Δk=Δl=±2) in the vibrational state ν8=1 have been observed up to J= 85 which were partly split by nuclear quadrupole coupling. The analysis of these splittings was based on the theory of Aliev, M. R., and Hougen, J. T., 1984, J. molec. Spectrosc., 106, 110. An acc...

The influence of an off-resonant pump radiation in Fourier transform microwave spectroscopy

The influence of an off-resonant pump radiation during the free induction decay in Fourier transform microwave spectroscopy is analyzed. Applying the density-matrix formalism to a three-level system, it is shown that under special conditions a single line in the frequency domain splits into four lines and that the well-known symmetric coherence splitting is included for a resonant pump in this theory. Experimental results obtained with a microwave-microwave double-resonance spectrometer employing crossed Fabry-Perot cavities are also reported. All measurements on 1,2-difluorobenzene are in good agreement with the applied theory. R´

The microwave spectrum of 5-methylthiazole: methyl internal rotation, 14N nuclear quadrupole coupling and electric dipole moment

Abstract The microwave spectrum of 5-methylthiazole has been investigated in the frequency range from 8 to 36 GHz, employing both Fourier transform and Stark spectroscopy. Double resonance MWFT techniques with broadband pump excitation were applied in order to facilitate the assignment of the spectrum. The results of methyl internal rotation, 14 N quadrupole hyperfine structure and fourth order centrifugal distortion analyses are given. The dipole moment components, with respect to the principal inertia axes system, could be determined from the Stark splittings of selected rotational lines.

Analysis of internal rotations in ethyl nitrite using molecular beam Fourier transform microwave spectroscopy

The microwave spectrum of ethyl nitrite (C2H5ONO) shows the presence of three rotational isomers, corresponding to cis–trans, cis–gauche, and trans–gauche structures. We have performed an investigation of the internal rotation of the methyl group in the cis–trans and cis–gauche rotamers and the motion of the ethyl group in the trans–gauche structure by using molecular beam-Fourier transform microwave (MB-FTMW) spectroscopy. Rotational spectra of these three isomers were recorded in the frequency regions 1–4 GHz and 7–26 GHz with two different spectrometers. For cis–trans ethyl nitrite we observed µa- and µb-type and for the cis–gauche form we detected µa-, µb-, and µc-type spectra. Due to the internal rotation of the methyl group, some lines were split and the torsional barriers could be determined, V3(CH3)=1082(2) cm-1 for the cis–trans and V3(CH3)=918(10) cm-1 for the cis–gauche form. For the trans–gauche form we found µa-, µb-, and µc-type spectra. The µb- and µc-type lines showed a doubling due to the interconversion between the two equivalent gauche conformers through rotation about the C–O axis. We determined the difference between the two lowest energy levels of the torsion around the C–O bond axis to be 68(3) kHz. Based on the observed splittings and the rotational constants, the Fourier coefficients of the potential hindering this internal rotation were determined to be V1=-1322 cm-1, V2=-1048 cm-1, V3=-412 cm-1, and V4=-109 cm-1 with a low barrier of 183(3) cm-1 in the trans position and a higher barrier of 1916(100) cm-1 in the cis position. Rotational, centrifugal distortion, and quadrupole coupling constants and some structural parameters have also been obtained from the analysis of spectra.