Electronic relaxation as a cause of diffuseness in electronic spectra
Authors: Byrne JP , Ross IG 
Published: September 2010

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.