## 3 Results and Discussion
The double peak spectra then agrees with our dressed state picture that allows gain at both the Raman transition and at the two-photon Stark shifted transition, which would have only a small dependence on time delay. In both the adiabatic and transient experiments enhancement in the number of generated orders only occurs for red-detuning because the single-color Stark shift allows only red-detuned pulses to be resonant with the transition. We have also observed in the transient regime, that there are no blue shifted shoulders with blue detuning indicating that there was no transfer to the coherent Stark shifted states. Pump radiation that is blue-detuned from the low intensity Raman frequency would be further detuned from the Stark shifted transition. To further confirm that the red-shifted peaks are a result of an intensity dependent Stark shift, we performed the same experiment at lower energies. In Figure 3, we show the time delay spectra for a total energy of 1.5 mJ and in Figure 4, we show the spectra of the 7 th to 11 th anti-Stokes Raman orders at a time delay of 0.667 ps, for three different pump energies. The total energy of the two pumps was 1.0, 1.5 and 2.0 mJ. The spectra have been normalized so that the pump spectrum would have a peak intensity of 1 on the vertical scale. In Figure 3, we can see that the maximum number of orders occurs at the delay of 0.333 ps. This time difference from the 2 mJ result is limited by the resolution of the time step. However it does confirm that at lower intensity, the required red-shift to remain resonant with the single-color Stark shifted transition is reduced. In Figure 4, the red shifts of the orders are larger with increased pump energy, which is expected if the shifts are due to two-photon Stark shifting. If the extra peaks were due to non-resonant four-wave mixing, the height of the peaks should increase, but not the frequency shift. The peaks that appear at the expected multiples of the Raman transition frequency remain narrow, but the red shifted peaks get broader at the higher orders. At 2 mJ, the average spacing of the red-shifted peaks is 22.7 THz,  
Figure 3: The MRG spectra are shown for two positively chirped pump pulses, with total energy of 1.5 mJ as a function of time delay between the two pulses. which lies between the pump frequency separation and the Raman spacing. This could be due to the fact that each order can produce a Stark shifted Stokes frequency, which would result in a small shift from the Raman order. Each of these shifted orders can then undergo Stimulated Raman scattering at either the Raman frequency or the Stark shifted frequency leading to broad peaks after multiple scattering events. This broadening could also be a result of the transient nature of the Stark shifting resulting in different shifts at different intensity points. It is surprising that with just 1 ps pulse duration, the results agree with the nanosecond pump duration results, where an adiabatic transition to the coherent state is expected. We measured the MRG spectrum for unchirped pulses that had duration of \(\sim\)400 fs with zero time delay between the pulses. We tuned the peak frequencies of the laser pulses to vary the frequency separation across the Raman resonance. We show three spectra, each with a total energy of 2 mJ in Figure 5. The top panel shows the spectrum for the two pumps slightly red detuned with a peak separation of 22.5 THz. The middle panel shows the MRG spectrum for on-resonance pumping and the bottom panel shows the spectrum for the blue-detuned pump frequency separation of 24.6 THz. Unlike the linearly chirped pump pulses, the blue shifted pumping displays blue shifted peaks. With on-resonance pumping, the first few orders show red-shifted peaks, but the higher orders appear at the expected Raman frequency. With red-detuned pumps, large red-shifted shoulders appear. The continuum that appears under the MRG spectra with these short pulses also changes with pump detuning.