Abstract
The dependence of fluorescence intensity on the wavelength of exciting radiation has been recorded for anthracene vapour at 0·09 mm and 170°C over the range 230–366 mμ. It is shown that whereas the photon cascade theory cannot account for the much higher intensity excited in the 250 mμ band than excited in the band at 363 mμ, this behaviour is precisely that expected if the fluorescent state is populated by internal conversion following excitation in the short-wave band. Since the excited molecules are collisionally unperturbed under these conditions, the fluorescence spectrum exhibits a marked dependence on the wavelength of exciting radiation; an increase in the energy of the absorbed quantum produces an appreciable red-shift and loss of vibrational structure.

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