Lifetime of the 5s5p1P1 Level of Strontium

The lifetime of the first excited singlet level, 5s5p1P1, of strontium has been measured using the Hanle resonance of the 5s21S0 – 5s5p1P1, 4607 Å, resonance line. The lifetime of the 5s5p1P1 level is 5.29 ± 0.10 ns and the oscillator strength of the 4607 Å line is 1.80 ± 0.03. These results are compared with previous measurements.

Emission and Deactivation of the Excited States of 2,3-Pentanedione

Vibrationally excited singlet and triplet states of 2,3-pentanedione are formed by photolysis at 365 nm. The processes removing these excited states in the gas phase are studied by measuring the fluorescence and phosphorescence yields. Fluorescence can occur from the vibrationally excited, as well as the vibrationally equilibrated, singlet state. The fluorescence and phosphorescence data are considered in terms of mechanisms which involve either weak or strong collisions. Although the data cannot distinguish between the alternatives, there are two significant conclusions. The fluorescence data require that emission occur from at least two levels in the singlet manifold. To explain the phosphorescence data, the highest emitting singlet level must not lead to a vibrationally equilibrated triplet state.

The effect of environment on singlet-triplet transitions of organic molecules

It is well known that many molecules, especially those containing one or more multiple bonds, have excited triplet levels in which there are two unpaired electrons (Kasha & McGlynn 1956; Reid 1958). The first triplet level normally lies lower in energy than the first excited singlet level, and emission of radiation from this triplet level gives rise to the familiar long-lived phosphorescence spectra of organic molecules in rigid media. Singlet-triplet transitions are formally forbidden by the selection rule prohibiting transitions between states of different multiplicity. That they occur at all is due to a process known as spin-orbit coupling. The effect of this coupling is to give the triplet level a small amount of singlet character by mixing it with one or more of the singlet levels of the molecule. It is also possible for the coupling to give the ground singlet level a small amount of triplet character, but it appears that the former mechanism is normally more important. For many molecules, and in particular those containing an aromatic system, the spin-orbit interaction is very small. In such cases the singlet-triplet absorption spectra will be exceedingly weak, and consequently difficult to observe. In fact, the literature contains many examples of bands which were originally assigned as singlet-triplet transitions, and were subsequently shown to be singlet-singlet in nature, or even vibrational overtones. Genuine examples of singlet-triplet absorption spectra (and correspondingly short phosphorescence lifetimes) have been observed by McClure, Blake & Hanst (1954) in aromatic molecules containing heavy atoms such as iodine and bromine. The spin-orbit interaction now becomes appreciable owing to the increase in the non-uniform magnetic field associated with an electron moving in the vicinity of the heavy nucleus. Similarly, in molecules containing a paramagnetic ion, a marked reduction in phosphorescence lifetime has been observed by Yuster & Weissman (1949) and by Becker & Kasha (1955), which is attributed to the inhomogeneous magnetic field of the ion.

Fine structure in the mercury singlet terms

The fine structure occurring in a number of line spectra has been reasonably accounted for by the theory of nuclear spin, but up to the present the complexity of the fine structure in mercury lines has remained unexplained, more lines occurring than theory demands. Because of the weakness of the singlets in the sources usually employed most of the investigations on mercury fine structure have been carried out on triplet and intercombination lines. The fine structures in the singlest series should yield more readily to analysis than in the triplet series, being simpler, and for this reason are investigated here. A comprehensive survey of the experimental work done up to 1926 in mercury was given by Ruark who succeeded in accounting for the majority of the components in several triplet lines by giving a number of levels as threefold multiplicity. Only one singlet level 6 1 P 1 was discussed and the multiplicity attributed to this was seven. In a previous communication an account was given of a method for increasing the relative and intrinsic intensity of the singlet series and of inter-combination lines involving upper singlet levels. The series 6 1 P 1 — m 1 S 0 , 7 1 S 0 — m 1 P 1 and 7 3 S 1 — m 1 P 1 are particularly strengthened and these have been examined for fine structure with a Fabry-Perot interferometer. It was stated in a preliminary note that λ 4916 (6 1 P 1 — 8 1 S 0 ) had four components and that the members of the series 7 1 S 0 — m 1 P 1 , 7 3 S 1 — m 1 P 1 were sextet. These observations are now somewhat extended, fainter components having been found. Whilst this paper was in the course of preparation, a communication was received from Venkatesachar and Sibaiya giving the structures of some of these lines. Venkatesachar also stated, that in a short note to 'Nature,' which was overlooked by the writer, Hansen found that λ 4916 was a quintet. These results will be discussed later.