Origin of the redshifted hump in the spectrum of an ultrashort laser pulse under its filamentation in atomic and molecular gases

We experimentally compare spectrum conversion of an ultrashort laser pulse under its filamentation in various gases. The laser pulse propagation through both atomic and molecular gases is observed to result in a separated local maximum formation in the long-wavelength spectral wing. This redshifted hump is more noticeable and does have a higher amplitude under the laser pulse filamentation just in molecular gases as compared to atomic ones. Therefore, we claim that the laser pulse self-phase modulation plays a crucial role in the formation of such a hump, while the Raman scattering in molecular gases enhances this process efficiency.