Knowledge Bank

Perpendicular bands of several symmetric and near symmetric top molecules like $CH3I,CH2DI,NO2$, etc., have been studied using a tunable diode laser spectrometer in the $12-14 \mu m$ region. The limited tunability of the diode laser is compensated by its high resolution and sensitivity so that from the analysis of the Q branches alone precise band centers and rotational constants could be determined. It is known from microwave studies that $CH3I$ has a large nuclear quadrupole coupling constant. The nuclear spin (I) of $5/23$ of iodine combines with J and splits the rotational levels into 2I + 1 = 6 components for J$ > 2$.SelectionrulesshowthatforJ$ \simeq K, a\ ^{P}Q(J,K)$ line of the $\nu 6$ band is split into several components which should be resolved under Doppler-limited resolution. Such splittings were seen in the spectrum of $CH3I.1$ The effect of diode laser line width on the observed spectrum will also be discussed. In the case of $CH2DI$ the presence of a single D atom removes the degeneracy of $\nu 6$ in $CH3I$ and splits it into $\nu 6\prime$ and $\nu 6\prime$, Although the asymmetry is extremely small (x = -0.9983), spectacular asymmetry effects were observed in the $\nu \prime \prime 6$ band for $K\prime =0,1,2$ and 3. A sixth order Watson reduced Hamiltonian was used to explain these effects. In $NO2$, the electron spin couples with the rotational angular momentum J of the molecule and splits it into two components Such, splittings have been observed in the $\nu 2$ band spectrum of $NO2$ from which accurate spin-rotation interaction constants have been $determined.2$