INTERRING TORSIONAL MODULATION IN MOLECULAR LASERS - ULTRAVIOLET LASING VIA AMPLIFIED SPONTANEOUS EMISSION-SPECTROSCOPY OF PHENYLIMIDAZOLES

The intramolecularly torsion-capable molecules 2-phenylimidazole (I), 2-phenylbenzimidazole (II), 1-methyl-2-phenylimidazole (III), and 1-me thyl-2-phenylbenzimidazole (IV) are shown to yield efficient UV lasing action. The amplified spontaneous emission (ASE) laser spikes wavelen gths (and gain coefficients) are for I, 321 nm (alpha = 7 cm(-1)); for II, 341 Mn (alpha = 10.5 cm(-1)); for III, 324 nm (alpha = 8 cm(-1)); and for IV, 345.5 nm (alpha = 9 cm(-1)) (ASE cell optical length of 0 .8 cm). The laser spikes represent for each molecule the normal simple case of wavelength coincidence with the fluorescence maximum. Theoret ical calculations are presented to correlate electronic structural cha nges with observed spectra and for theoretical torsional potential fun ctions. In cases I and II, the torsional mode is active only in the ex cited-state S-1, and serves merely to modulate by a large shift (simil ar to 3000 cm(-1)) the ASE laser spike position, driven by stretching mode vibronic excitation. In the cases of III and IV, the torsional mo de is interpreted to be the driving mode, with an ASE laser spike Delt a nu of 5000 cm(-1) or more (measured as the Franck-Condon shift of la mbda(max) from absorption to fluorescence, and the ASE laser spike pos ition), deduced from the theoretically calculated torsional potentials offering a four-level population inversion system.