Transformation of strong picosecond pulses in radiation with an extended quasirotational spectrum during self-focusing in high-pressure hydrogen

Radiation with a spectrum representing a discrete analog of the extended sp ectrum observed in the generation of a supercontinuum in gases is generated in the self-focusing of 30-ps pulses with a wavelength of 1.06 mu m in hyd rogen at pressures up to 120 atm. The spectrum contains lines with similar intensities, an average frequency spacing approximately equal to the rotati onal transition frequency in hydrogen (587 cm(-1)), and a smooth spatial pr ofile. The lines consist of several vibrational-rotational components. As t he pressure is increased, the spectral lines are transformed so that at a p ressure above 60 atm each line in the spectrum contains one or two componen ts formed as a result of the smaller number of cascade (rotational and vibr ational) processes. Self-focusing is manifested in the occurrence of a radi ating channel up to 12 cm in length. The formation of a channel of this len gth is associated mainly with the variation of the refractive index in vibr ational excitation of the hydrogen molecules by electrons heated in the pum p field. (C) 1999 American Institute of Physics.