Knowledge Bank

The rotational spectrum of HDCO in the $41,51$ and $61$ excited vibrational states has been investigated in Lille and Kiel using a sample enriched in D. In Lille the measurements were performed in the millimeter ($160-600$ GHz) region. The spectra in Kiel were recorded using Fourier transform microwave spectrometers in the regions of about $8-18$ GHz and $18-26$ GHz, employing a rectangular waveguide of 12 m length and a circular waveguide of 36 m length, respectively. These results were combined with the $41,51$ and $61$ infrared energy $levelsa$ obtained in a previous analysis of FTS spectra of the $\nu 4$ band (CHD bend), $\nu 5$ band (CHD rocking) and $\nu 6$ band (out of plane bend) recorded in the $10\mu m$ region at Giessen in order to get the best possible Hamiltonian constants. The energy levels calculation of the $41,51$ and $61$ interacting states accounts for the usual A- and B-type Coriolis resonances in the $51-61$ and $61-41$ off diagonals blocks. On the other hand, in order to reproduce the observed microwave transitions and infrared energy levels, it was necessary to use a $\{Jx,Jz\}$ non orthorhombic term in the $51$ and $61$ v- diagonal blocks of the Hamiltonian matrix. Therefore, HDCO exhibits an example of vibrational induced rotational axis switching.