Subfemtosecond pulse generation by rotational molecular modulation

We extend a recent suggestion for the generation of subfemtosecond pulses by molecular modulation [Phys. Rev. Lett. 81, 2894 (1998)] to the rotational spectrum of molecular hydrogen $\left({\mathrm{H}}_2\right)$. When a rotational transition $|a〉\to |b〉$ is strongly driven $\left(\left|{\rho }_{\mathit{ab}}\right|=0.5\right)$ the generation and phase-slip lengths are of the same order and the Raman spectrum has approximately Bessel function sideband amplitudes. Numerical simulation predicts that this spectrum (generated in a 14-cm-long cell at 1-atm pressure of ${\mathrm{H}}_2$) will compress into a train of pulses with 94-fs pulse separation and a pulse length of 0.5 fs.