Knowledge Bank

Nonphotochemical hole burning (NPHB) has been investigated for the ionic dye resorufin in polymethylmethacrolate $(PMMA)1$. It has been possible to burn relatively deep holes in the inhomogeneously broadened electronic origin and vibronic features of the basic form of the dye. Optical density changes at the burn frequency are typically 0.35 and the zero-phonon holes are accompanied by phonon and ``pseudophonon'' sidebands. Spectral changes associated with the energetically displaced absorbers were also observed. The non-photochemical nature of the hole burning process has been demonstrated by laser induced hole $refilling2$ which has been shown to occur over energy differences in excess of $100cm-1$. The temperature dependence of the hole linewidth was found to be proportional to $T1.3$, consistent with previous observations of holes burned in polymer matrices. An unusual feature of the hole spectra is the appearance of an isosbestic point (a point of optical invariance) in both the hole burning and hole annealing spectra. Isosbestic behavior may be characteristic of NPHB at relatively high laser powers $(100mw/cm2)$. The relationship between isosbestic behavior in NPHB and spectral-diffusion line broadening will be discussed.