Observation of anomalous conical emission from laser-excited barium vapor

The authors have observed an anomalous emission feature from a dense ({~} 10¹⁶ cm^-3) barium vapor illuminated with a N₂ pumped dye laser (peak intensity {~} 1 MW cm^-2). The input laser wavelength was 552.5 nm, 1 nm shorter in wavelength than the barium 6s²¹S-6s6p¹P resonance transition. In the forward direction strong emission was observed emerging from the barium vapor in the form of an conical shell of half-angle {~}2^∘{} at {~}1% of the incident laser intensity. The emission was broad in wavelength (3 nm) and peaked 1 nm longer in wavelength than the Ba 6s²¹S-6s6p¹P transition. Self-focusing of the laser beam by the barium vapor was observed under the same conditions. Although the emission is intense, an adequate explanation of this phenomenon has yet to be found.