The twophoton absorption spectra of 7-hydrouycoumarine (umbelliferone) in ethanol have been studied m the region 510-650 nm by monitoring the fluorescence excited by a pulsed tunable dye laser. Varrations of the two-photon absorption cross sectton, 6, in this wavelength range are presented with reference to the estimated absolute value at 608.4 nm. This value was found to be equal to (7.9 f 3.9) X 10e5' cm4 s/photon molecule.
Two
-photon absorption spectra obtained by monitoring the fluorescence of the substance investigated has received a great deal of attention in the past [l-43 _ We wish to report here on our studies concerning a solution of 7-hydroxycoumarine (umbelliferone) in ethanol (Carlo Erba reagent grade)_ This dye was supplied by Spectra Physics. The aim of the research was twofold: (i) record the two-photon spectrum in a large range of excitation energies; (ii) check simultaneously in the same range the square dependence of the fluorescence signal upon the laser incident intensity_ To accomplish this, we have used the experimental set up depicted in fig. 1. The laser source consists of a commercially available nitrogen pumped, tunable dye laser (Molectron Corp., models W-400 and DG200) provided with KDP crystals for frequency doubling and operated at a repetition rate of 10 Hz. Laser output is typically characterized by tens of kW of peak power and approximately 4 ns pulse width (fwhm). The divergence of the beam was checked experimentally and found to be 2.2 mrad. The laser beam is focussed on the fluorescence cell by means of a quartz lens. The resulting fluorescence signal is collected and focussed onto a quarter meter Jarrell-Ash grating monochromator whose spectral bandwidth was adjusted to be 3.2 nm. The signal from an RCA lP28 photomultiplier is fed via a de-* On leave from the Department of Chemical Physics,