Photoacoustic investigations on the photostability of Coumarin 540-doped PMMA

In this paper, we present a laser-induced photoacoustic study on the photostability of laser dye Coumarin 540 doped in PMMA matrix and modified by the incorporation of low-molecular weight additives. The dependence of photostability of the dye on various experimental conditions, such as nature of solvents, incident optical power and dye concentration, is investigated in detail. The activation rates for the bleaching process are calculated for different concentrations and they suggest the possibility of two distinct mechanisms responsible for photodegradation. Further, analysis of the data confirms the linear dependence of photodegradation on the intensity of incident radiation. The role of different externally influencing parameters, such as wavelength and modulation frequency of incident radiation, is also discussed.

Study of solvent effect in laser emission from Coumarin 540 dye solution

The results of a brief investigation of the amplified spontaneous emission and lasing characteristics of Coumarin 540 dye in as many as ten different solvents are reported. It has been found that C 540 dye solutions contained within a rectangular quartz cuvette give laser emission with well resolved equally spaced modes when pumped with a 476 nm beam. The modes were found to originate from the subcavities formed by the plane-parallel walls of the cuvette containing the high-gain medium. While the quantum yield remains a decisive factor, a clear correlation between the total width of the emission spectra and the refractive indices of the solvents of the respective samples has been demonstrated. The well-resolved mode structure exhibited by the emission spectra gives clear evidence of the lasing action taking place in the gain medium, and the number of modes enables us to compare the gain of the media in different samples. A detailed discussion of the solvent effect in the lasing characteristics of C540 in different solutions is given.

La trasposizione didattica nell'insegnamento delle scienze sociali : dalla storia degli storici alla storia per la scuola. 'La transposición didáctica en la enseñanza de las Ciencias Sociales : de la historia de los hitoriadores a la historia para la escuela'.

Se plantea la preocupación por la ausencia de una conciencia de la génesis transpositiva del saber histórico que incide en una deficiente enseñanza de la historia. La noción de transposición obliga a una nueva visión de la didáctica al considerar el saber histórico y el texto que lo comunica, como un problema y no como un supuesto previo.

'Vocabuli d'autori e di scienze e di libri' (Conv. II xii 5): ercorsi sapienziali di Dante

Discusses the trasmission of biblical sapiential sources in a variery medieval contexts and discourses, to ultimately focus on Dante's works

Erratum to: “Photoproduction of heavy quarks in next-to-leading order QCD with longitudinally polarized initial states” [Nucl. Phys. B 540 (1999) 345]

There are three trivial misprints in our paper.