Knowledge Bank

The vibration-rotation spectrum of the $X2\Pi \Omega$ electronic state of $H79Br+$ and $H81Br+$ has been studied in the frequency range from $1975cm-1$ to $2360cm-1$ with an a.c. glow discharge using velocity modulation. Transitions involving $\nu$ ranging from 0 to 5 and J up to 18.5 were observed in both the $\Omega =1/2$ and $\Omega =3/2$ spin substates. For the two isotopomers, a total of over 600 vibration-rotation lines were measured. Since the vibrational spacings are of the same order of magnitude as the spin-orbit splitting in this inverted $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi$ system, significant non-resonant mixing occurs between rotational levels of ($\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 1/2,\nu$) and ($\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 3/2,\nu +1$) states. This gives rise to anomalous A-doubling in the rotational energy levels of all vibrational states except the $(\nu =0)$ state of the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 3/2$ spin substate, which is isolated and unique. A striking example of this effect on the observed spectrum is the unusual J-dependence of the splitting of the two A-doublets belonging to the fundamental band of the $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi 3/2$ substate. The structure of the usual Hamiltonian $\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ for a $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}\Pi \Omega$ state has been modified and the third order vibrational Van Vleck transformation included to account for this interaction, so that reliable molecular parameters could be determined. In addition, a complete analysis of the hyperfine splittings arising from magnetic dipole and electric quadrupole interactions has been carried out for low J transitions of both the $\Omega$ substates. The hyperfine coupling constants have been determined and an improved description of the distribution of the unpaired electron in the molecule has been obtained. Details of the experiment, the modified Hamiltonian expressions and the fitted molecular parameters will be presented.