## 3 Results and Discussion
Although the continuum is dependent on the pump pulse duration, the continuum cannot simply be due to SPM of the pump beams as the largest continuum appears under the higher anti-Stokes Raman orders rather than under the stronger pump frequencies. The continuum appears to be strongest for the red-detuned pumps, indicating that the generation of the continuum is linked with the red-shifted shoulders. The continuum is also centered at higher frequency for the red-detuned pump beams. Figure 4: The MRG spectra for the 7 th to 11 th anti-Stokes orders for three different pump energies. The spectral intensities have arbitrary units, but the plots are normalized to the pump spectrum. In Figure 6, we have again plotted the 7 th through 11 th orders for both the red-shifted and blue-shifted short pulse pumping at three different total energies of 1, 1.5 and 2.0 mJ. The short pulse red-detuning of 0.8 THz is less than the red-shift of the chirped pulse at a delay of 0.667 ps, which corresponds to a detuning of 1.3 THz and yet the red-shifting is stronger for each of the pump energies with short pulse pumping compared with the chirped pumping. This is expected from Stark shifting as the pump intensity would be a factor of 2.5 higher for the 400 fs pulses compared to the 1 ps pulses at the same energy. The red-shifted shoulders are also much broader with the short pumps than with the long. This is mostly because the orders extend from the Raman order to the maximum shift, which for 2 mJ, unchirped pulses equals have the transition frequency at the higher orders. The broadening increases with pump energy so that the orders begin to merge and create the continuum that appears under the higher anti-Stokes orders. The fact that the broadening is stronger with the shorter pulses so that the two peaks merge to one broad peak indicates that the broadening is effected by the transient nature of the Stark shifting. This broadening could also come from SPM and cross-phase modulation (XPM) between the orders. One would expect the largest SPM to occur at the pump frequency that has an order of magnitude more intensity and yet the pump frequencies have the narrowest spectra. From the lower plot in Figure 6, it can be seen that the blue shifted shoulders behave differently from the red-shifting in the upper plot. The position of the blue-shifted peaks is independent of pump energy indicating that the shift is not from the Stark effect. The average spacing of the peaks remains at the low intensity Raman frequency of 23.3 THz. The shifted frequency is stronger than the frequency at the Raman order of the pump frequency. The average shift of the peaks away from the Raman order is 5 THz, which is larger than the 1.3 THz detuning of the pump frequencies. The orders do get broader with intensity leading to a continuum under the orders. This broadening is presumably  
Figure 5: The MRG spectra for 2 mJ, unchirped pump pulses. Pump frequencies are **(top panel)** red detuned by 0.8 THz, **(middle panel)** on-resonance and **(bottom panel)** blue-detuned by 1.3 Thz. from SPM and cross-phase modulation between the orders, but still the pump frequencies have the most intensity and narrowest spectra. It is not clear at this point what causes the 5 THz shift or why it is this shifted frequency that then undergoes multiple Raman scattering at the low power Raman transition frequency. The 5 THz shifted frequency could be produced by either higher order non-resonant frequency mixing or by SPM/XPM. We have not observed blue shifting by time delaying chirped pulses that have peak frequency separation equal to the Raman transition, but we have observed blue shifted shoulders with chirped pulses that had peak frequencies separated by more than the Raman transition [8]. In the chirped case, the shift from the Raman frequency did not increase with order, which is the same as we have now observed with the unchirped pulses.