A method using successive approximation is developed for determining the rotational temperatures, when the rotational lines are overlapped. The method is applied to CH (B²Σ⁻ −X² II) band as a test.

Emission spectra of BH(A¹Π-X¹Σ⁺) system were recorded and studied using a low pressure (3.0 torr) arc in flowing hydrogen and argon + hydrogen mixture. The rotational distributions in theA¹Π state determined from the intensities of rotational lines for the 0–0 band of theA-X system conforms to a Maxwellian distribution with effective rotational temperature of 1000 ± 50°K. Intensities of Balmer lines of hydrogen were measured and used to determine electronic excitation temperature which was found to be around 2000°K.

Phosphorescence decay of 3-(2′-N-methylbenzimidazolyl)-7-N, N-diethylaminocoumarin; 2,3,5,6-1H, 4H-tetrahydro-8-methylquinolizino-〈9, 9a, 1-gh〉 coumarin; 2,3,5,6-1H, 4H-tetrahydro-8-trifluormethylquinolizino-〈9,9a,1-gh〉 coumarin; 9(10H)-acridone; 9-aminocridine hydrochloride, 1-hydrate and 2-(4-biphenylyl)-5-(4-t-butylphenyl)-1,3, 4-oxadiazole dyes in EPA glass at 77 K have been recorded using N₂ laser. The results were used to determine lifetimes. Emission intensities from the first two dyes have been examined in relation to dye concentration and excitation intensity. The results are discussed.

Triplet-singlet energy transfer in laser dyes have been studied in EPA at 77K using N₂ laser as an excitation source. Phosphorescence of the donor (D) and the delayed fluorescence of the acceptor (A) and their lifetimes have been measured for coumarin 102 (D)-rhodamine B(A) and 9(10H)-acridone (D)-rhodamine 6G(A) dye systems as a function of acceptor concentration. These data yield energy transfer rate constants of ∼10³ dm³ mol⁻¹ s⁻¹ for the donor acceptor combinations, consistent with the Forster mechanism. The phosphorescence quantum efficiency and other spectral parameters are also reported.

Rotation-free transmission measurements governed by ground-state recovery of coumarin 102, coumarin 138 and coumarin 339 dyes in ethanol solutions were performed for the first time by pump-probe technique using 35 ps pulses of Nd:YAG laser at 355 nm. Recovery of absorption is characterized by simple exponential behaviour with relaxation times of 4.0, 3.7 and 3.0 ns respectively for C 102, C 138 and C 339. These values are comparable to the lifetimes of the first excited singlet states.

The time-resolved laser emission of coumarin 102 was investigated in various aprotic and protic solvents at picosecond resolution by frequency upconversion technique. The spectral shift of the transient emission spectrum is attributed to solvent reorientation and the results are discussed.