Quantum Rotor Dynamics Near the Dissociation Limit: High Resolution Spectroscopy and Calculations for $H_{2}-HCl$

Abstract

High resolution near infrared spectra of bands built on the $v_{HCl} = 1 \leftarrow 0$ stretch in $H_{2}-HCl$ are recorded between $2870 - 2930 cm^{-1}$ using a slit-jet infrared spectrometer. Only transitions that access the "low energy" intermolecular states in the $v_{HCl} = 1$ upper state have easily recognizable rotational energy level patterns; the $\Pi\leftarrow \Pi$ fundamental and the $\Sigma \leftarrow \Pi$ combination band for ortho-$H_{2}(j=1)-HCl$, and the $\Sigma\leftarrow \Sigma$ fundamental for para-$H_{2}(j=0)-HCl$. These 3 vibrational bands are reasonably well fit by semi-rigid Hamiltonians, yet the poor quality of the fits and anomalous P/Q/R- branch intensities indicate the limitations of this type of analysis. The rotational energy level pattern for another excited intermolecular state of $H_{2}-HCl$, thought to be near dissociation, exhibits strong deviations from the anticipated asymmetric top rotational progressions. Analysis of the lineshapes for these transitions reveals an abrupt increase in the predissociation broadening $(J\geq 4)$ which is attributed to rotational excitation above the binding energy. A detailed modeling of this complex spectrum requires quantum calculations that include all 4 intermolecular degrees of freedom along with end-over-end rotation to account for Coriolis interactions. Self consistent field (SCF) variational quantum calculations which treat all angular and radial intermolecular degrees of freedom are carried out on the full 4-D $H_{2}-HCl$ ab initio potential energy surface of Gerber et al. A comparison of this analysis with the experimental results will be presented. Analysis of the eigenfunctions for the different intermolecular states suggest that these complexes are excellent candidates for IR/UV double resonance photochemical studies.