SPECTROSCOPICALLY INACTIVE STATES FROM DIFFERENCE TRANSITIONS: THE $\nu_{17} - \nu_{20}$ BAND OF BENZENE

Abstract

Spectroscopic constants for the infrared- and Raman-inactive $E_{2u}$ state $\nu_{20}$ of benzene have been obtained from an analysis of a high resolution recording of the $\nu_{17}- \nu_{20}$ difference band at $779.643 cm^{-1}$. Over 3000 lines of the band were measured on a Fourier transform spectrometer and subsequently deconvolved to an effective linewidth of $\sim 0.0012 cm^{-1}$. Both $\nu_{17}$ and $\nu_{20}$ are strongly affected by rotational f-resonance and doubling. Even for such states involved in resonances, combination relations are found to be useful for breaking up the correlations between the upper and lower state constants in determining the coefficients of the J-dependent terms $(\Delta B, q , \eta_{J}, \Delta D_{J})$ if the interactions are explicitly taken into account. The spectroscopic constants obtained for the two states reproduce the observed line positions with an over-all standard deviation of $2.2 \times 10^{-4} cm^{-1}$.