Knowledge Bank

“Duchesne and $Rosen1$ have suggested that the vibronic bands of this region belong to several separate electronic transitions. The present paper presents evidence that all these bands belong to a single transition. Spectrograms taken in the third order of a two meter grating spectrograph show at least thirty partly resolved bands distributed throughout this region which show similar structure. These bands exhibit P and R branches degrading to the red with a second difference corresponding to $\Delta (A-B¯)\approx -0.70$ $cm-1$. Each subband is characterized by a sharp intensity peak which is probably a Q branch. For most bands a weak subhead extends $8-14cm-1$ toward the violet side of the principal head. Interpretation of these subheads in terms of R branches leads to $\Delta B¯\sim -0.01$ $cm-1$. These results indicate a fairly large increase in bond distance and a significant decrease of angle. This corresponds to Walsh’s $predictions2$ for a $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}B1\leftarrow 1A1$ transition in this region. The large change is geometry indicated by the rotational structure is consistent with the interpretation that the corresponding band system extends over a considerable wavelength range. Although the bands have sharp heads which can usually be isolated from each other, the system resists interpretation in terms of symmetrical frequencies alone. Some progress is being made in the vibrational analysis by assuming that bands involving the antisymmetrical mode appear because of a double minimum potential in the antisymmetrical coordinate.”