Fluorescence of humic acids (HA) and pyrene-HA complexes at ultralow temperature

The interaction of pyrene with humic acids of different origins was investi gated using fluorescence line narrowing (FIN) spectroscopy at 10 K. FLN spe ctroscopy can be used to obtain vibrationally resolved fluorescence spectra from samples that under ordinary conditions suffer from extensive spectral broadening. Due to the presence of humic acid the site distribution and th e relative intensities of the ground-state vibrations in the fluorescence s pectrum (Ham-effect) of pyrene were changed. Both effects were related to t he altered microenvironment of pyrene in the presence of humic acid. On the other hand, for humic acids no fluorescence line narrowing effect was obse rved indicating the occurrence of intra- and intermolecular processes.