Electronic relaxation as a cause of diffuseness in electronic spectra

Abstract

Diffuseness, as seen in the vapour absorption spectra of polyatomic molecules, is related with rapid electronic relaxation (radiationless transitions). The causes common to both are perturbations between electronic states. Line broadening is related to excited state lifetimes through the uncertainty principle. To this end, twelve causes of diffuseness are first listed, and their incidence examined. A distinction is drawn between intrinsic diffuseness, associated with excitation to or perturbation by a (quasi-)continuum and including the electronic relaxation mechanism here proposed, and trivial diffuseness due to overcrowding of individually sharp lines. Trivial diffuseness may be temperature dependent (structureless rotational envelopes; overcrowded sequence bands) or, in very large molecules, temperature independent. I t has been shown that rotational envelopes should contain sharp features in a number of transitions which prove, experimentally, to be diffuse. Because of sequence bands, however, there is for any polyatomic molecule a fairly well-defined critical temperature, calculable from ground state vibrational data, above which the entire electronic spectrum must become structureless. This critical temperature decreases with increasing molecular size and with increasing molecular flexibility, and in conjunction with vapour pressure considerations sets definable limits to the kinds of molecule for which structured spectra can be obtained in the gas phase. But this mechanism is insufficient to explain the frequent occurrence of structureless spectra in small rigid molecules.

The literature on medium to high resolution spectra of polyatomic molecules is then reviewed, to establish the generalization that the only transitions with structure (if any) will normally be the first triplet (if observable) and the first singlet, and possibly (for special reasons) Rydberg transitions. This survey is complemented by measurements on the following molecules: pyridine N-oxide, indene, indazole, purine, quinoline, isoquinoline, 1,6-naphthyridine, quinazoline, quinoxaline, 1,4,5- triazanaphthalene, biphenylene, fluorene, acridine, tetramethylcyclobutane-1,3- dione, and all monoalkylbenzenes from ethyl to t-butyl. The proposed generalization is found to have wide validity, though there are one or two undeniable exceptions and a few marginal cases. It is associated with Kasha's rule concerning states capable of emitting radiation, though the correlation between the two is not necessarily one-to- one. Taken together, these rules and the evidence on which they are based leave the electronic relaxation mechanism as the only satisfactory explanation so far offered for the widespread occurrence of diffuse electronic transitions in polyatomics.

This matter can also be studied in pure and mixed crystals, if it is assumed, as now seems very probable, that the crystalline matrix induces no line-narrowing.

Recent systematic work on line widths in crystal spectra a t 4°K by Hochstrasser and Marzzaccol is in full accord with the viewpoint advanced here.