Knowledge Bank

A Fraser--type $spectrometer1$was used to observe the lowest excited $\Sigma$ and $\Pi$ states of the van der Waals bending mode of ArHCN. Direct transitions from the ground vibrational state were observed. The molecular carrier and lower state rotational quantum numbers were confirmed by double resonance with known ground state rotational transitions of the $complex.2$ For the $\Pi$ state, the $Q(1)-Q(5)-P(2)-P(6)$, and $R(0)-R(3)$ transitions were observed, and for the $\Sigma$ state, $R(1)-R(3)$ and $P(1)-P(5)$ were observed, $\Pi$-$\Sigma$ double resonance signals were also obtained. The spectral resolution is about 30 kHz, which allowed the quadrupole hyperfine structure and Stark effect to be measured for each of the excited states. The vibrational states are strongly coupled by rotation. The molecular constants are (in MHZ) $\nu \Pi =181,984.42$; $B\Pi =2031.361$; $\beta \Sigma \Pi$(Coriolis coupling $parameter3)=1017.0;\nu \Sigma =164,890.80$ and $B\Sigma =1958.851$. The rotational constant of the ground state is 1609.832 MHz.$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ The rotational constants given for the excited states are the deperturbed ``geometric” ones. The similarity of $B\Pi$ and $B\Sigma$ is surprising, as is the closeness of the estimated average angle for the two states.