Intensity ratios ofμ-mesonic x-rays in some heavy elements are discussed. Intensity ratios of the$$K_{\alpha _1 } /K_{\alpha _2 } $$ and$$L_{\alpha _1 } /L_{\beta _2 } $$ lines have been calculated for²⁰⁶Pb,²⁰⁹Bi and²³⁸U with relativistic wavefunctions and retardation effect. Though for a refined calculation, it is necessary to take into consideration several features that are peculiar to mesonic atoms, the present calculations have shown that the observed intensity ratios of mesonic x-rays are not anomalous.

The ratios of the widths ofK levels of heavy elements with atomic number 70 to 92 have been calculated with relativistic wave functions, retardation, screening and field theoretical corrections to relativistic energy. The calculated values have been compared with the available observed data and good agreement has been obtained.

TheA²Π_ustate of O₂⁺ was earlier established as an inverted state contrary to previous assumptions. The rotational analysis of a few more bands of theA-X system of O₂⁺ has now been completed. These studies show that the spin-orbit coupling constantA in theA²Π_ustate gradually varies with the vibrational quantum numberν and is found to be positive forν⩾6. It has also been observed that the spia-rotation interaction is not negligible in theA²Π_ustate. The spin splitting constantγ is reported for various vibrational levels of this electronic state.

High-resolution spectra of the 0–9, 0–10 and 0–11 bands of theA²II_u—X²II_g system of (¹⁶O¹⁸O)⁺ ion have been studied for their rotational structure. This study enables a direct determination of the Λ-doubling parameters of theA²II_u andX²II_g states. The model of ‘pure precession’ explains, though not entirely, the Λ-doubling of theX²II_a state as arising out of its interaction with theB²Σ_g⁻ state. The Λ-doubling in theA²II_u state was found insignificant.

The 2-0, 1-0 and 0-0 bands of the ultraviolet system of PS have been analysed for their rotational structure. It is shown that they involve the transitionC²Σ -X² Π_r(a). TheC²Σ state shows a significant spin doubling.