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.

Laser-induced photodissociation of NaCs molecule has been observed when a mixture of Na and Cs metal vapour in a glass cell was irradiated by most of the lines of an argon ion laser. The photodissociation results in the 3P state of Na atoms which is correlated with theF¹Σ⁺ and G¹π molecular states of NaCs. Distribution of photofragments over fine structure components 3²P_3/2 and 3²P_1/2 of Na has been studied. The ratio of intensity ofD₂ line (5890 Å) toD₁ line (5896 Å) of Na varies from around 2 at 5145 Å to about 3.5 at 4579 Å. The relative photodissociation cross-section increases monotonically as the wave-length of laser light decreases from 5145 Å to 4579 Å. It is seen that the 4579 Å photon is about 200 times more effective than the 5145 Å photon in causing the photoreaction NaCs + (Ar⁺ photon) → Na*(3P) + Cs(6S).

A¹Σ_u⁺-X¹Σ_g⁺ emission in Na₂ is observed following excitation ofB¹π_u by various lines of an argon ion laser. The excitation energy ofB¹π_u is collisionally transferred to the (2)¹Σ_g⁺ which then radiatively populates theA¹Σ_u⁺ state. The Na vapour is contained in a stainless steel crossed heat pipe with Ar buffer gas and temperature around 600°C. For all laser lines except 4579 Å, the coarse features ofA-X emission are independent of the laser wavelength. However, at high resolution the finer differences between different laser line excitation are explained. Variousv′-v″ transitions in this emission are identified. Computer simulation is presented to help explain some features of this emission.

Single colour three photon resonant ionization (2 + 1) is observed in atomic potassium vapour in a heat pipe oven using an excimer laser pumped dye laser. Using wavelengths between 570 nm and 603 nm various²S and²D Rydberg states are populated by two photon excitation. Third photon of the same wavelength ionizes the atoms. Rydberg states up ton ⋍ 50 are observed. Electric field as low as 1 V/cm causes extensive Stark mixing of the states. This results in progressively higher three photon ionization signals via the perturbed²P and²F Rydberg states. The three photon ionization process is studied using both linearly and circularly polarized incident light. The experiment shows qualitatively that the²P Rydberg states are perturbed primarily by the²D states in the prescence of an external electric field and to a much smaller extent by²S states. This is also explained theoretically by calculating the Stark mixing coefficients under the Bates and Daamgard (1949) approximation. Implication for a similar effect in other alkali elements is discussed.