3. Analysis and discussion
The energy expression for a linear molecule in its ground vibrational state is given by
Bending modes in linear molecules are doubly degenerate since the molecule is free to bend in two orthogonal planes. If the molecule is bending and rotating simultaneously, the degeneracy is lifted giving rise to a phenomenon denoted -type doubling : The bending state can be regarded as having components of an additional angular momentum about the internuclear axis with . Hence, for a first excited bending mode causes the splitting of every rotational level into two sublevels. Within these substates (either or , according to the notation of Brown et al. 1975) transitions with can be observed. Thus, each ground state transition is accompanied by two further transitions for each first excited bending state. These considerations lead to an energy expression for the ground state transitions and the so-called vibrational satellites of the state that is given by
with q being the -type doubling constant and and being its centrifugal distortion corrections. For the ground vibrational state is zero and for the first excited bending mode is one.
The spectra of two lines near two terahertz are shown as examples in Fig. 1. The list of experimental transition frequencies obtained in the present investigation is given in Table 1. The analysis has been performed fitting both states simultaneously using Pickett's program SPFIT (1991) under consideration of previously reported rovibrational data of the band by Burkholder et al. (1987), the very recent data by Maki and Mellau (submitted), and the pure rotational transitions given in Okabayashi and Tanimoto (1993). Sextic centrifugal distortion constants could be determined for both states. The complete set of molecular parameters obtained is shown in Table 2.
The present data allows for prediction of accurate transition frequencies far beyond 2 THz. All data related to the results presented here (complete line list, fit file etc.) can be found online in the Cologne Database for Molecular Spectroscopy (CDMS) through http://www.ph1.uni-koeln.de/vorhersagen/daten/ . Predictions of the pure rotational spectrum for both states presented here are available online from the CDMS via http://www.ph1.uni-koeln.de/vorhersagen/catalog/ . Generally, the CDMS is accessible free of charge through http://www.ph1.uni-koeln.de/vorhersagen/ or via its shortcut http://www.cdms.de/ .
© European Southern Observatory (ESO) 2000
Online publication: December 5, 2000