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The comparative method, called the anellation method, has shown, that the absorption spectra of aromatic hydrocarbons consist of three types of bands [ 11:
The difference between the gaseous state and the solution in alcohol or hexane amounts to -900 cm-l.
a-Bands, less intense, shifting to the red on linear and angular anellation in the series of the scenes and phenes, and hidden or partly hidden in the more jntense pra-bands in the higher scenes beginning with anthracene [2]. The red shift in going from the gaseous state to the solution in alcohol or hexane is -250 cm-'.
/?-Bands, the most intense, showing the same anellation effect and in a fixed ratio to the a-bands, lying more in the ultra-violet.
The difference between the gaseous state and the solution in alcohol or hexane is -900 cm-l.
The following absorption spectra show clearly that a further characterization for these classes is provided by the typical shift of bands, observed at low temperature (-170" to -180Β°C). Thus the p-bands and p-bands shift about 30Ocm-1 to the red, whilst the a-bands shift about 30 to 50cm-' to the violet.
Since the latter amount is almost within the limit of accuracy, the bands nearly always appear to be stationary.
Aa already observed by MAYNEORD and ROE [3] for benzene and in some qualitative experiments with naphthalene, anthracene and phenanthrene at -120" C, the bands became much narrower, the maxima and minima more intense and multiplicity of bands occurred. The distinctness of the spectra is thus much increased and enables one to make sign&ant observations.
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## Abstract The difference between the tripletβtriplet and singletβsinglet absorption spectrum of naphthalene has been measured at 90Β°K using a newly developed computer controlled spectrograph. Good spectral resolution has been obtained and a strong band was found in the ultraviolet region. Recent