Knowledge Bank

$HF-CO2$ is quasilinear at the vibrational ground $statea$ and becomes a semi-rigid bent species at $\nu HF=3$ with the $CO2$ intermolecular bending frequency of $24.75(10)cm-1$. The ab initio potential surface is very flat from $\theta =0$ to $40\circ$ at $\nu HF=0$ while at $\nu HF=3$ the potential energy of linear geometry is about $50cm-1$ higher than that at the minimum near $\theta =40\circ$. The observed features at $11174.45$, $11168.10$, and $11181.82cm-1$ have been assigned to parallel transition to the $K=0,1$, and 2 levels of the second overtone HF valence band, respectively, indicating a vibrational redshift of $198.36(5)cm-1$ compared to the HF monomer. The relatively strong transition intensities of the K subbands are due to the inertial axes $switchingb$. The rotational constants of the (3000000) state are $A=2.96(2)cm-1,(B+C)/2=0.0742(10),0.0717(10)$, and $0.0696(10)cm-1$ for the $K=0,1$, and 2 levels. The centrifugal distortion $Dk=0.270(5)cm-1$ is extremely large but in good agreement with the expectation, as a result of very soft $CO2$ intermolecular bending. The spectral linewidths are 9.0(9), 7.2(6), and 4.5(6) GHz for the above levels, showing dramatic dependence of vibrational predissociation lifetime upon K. A perpendicular transition of the HF bending combination band at $11538.92cm-1$ provides a bending frequency of $362.77(15)cm-1$, while $(B+C)/2=0.0668(10)cm-1$ and $\Gamma =4.2(6)$ GHz.