a GLYPH<RParen1> 200 298 326 343 0 0.5 1 1.5 2 0 20 40 60 T GLYPH<LParen1> °C GLYPH<RParen1> f B f$_{D}$ b GLYPH<RParen1> 200 298 326 343 0. 0.2 0.4 0.6 0.8 0 20 40 60 T GLYPH<LParen1> °C GLYPH<RParen1> power leakage f$_{D}$ Fig. 6. (Color online) a) Fractional broadening and b) power leakage versus vapor temperature (as well as the corresponding fractional delay) for a 250- fs-long pulse propagating through a 7 cm cell. The long dashed line, short dashed line, and solid line correspond to theoretical results for a pulse with a Gaussian spectrum centered at λ$_{c}$ =788.4 nm, a rectangular spectrum centered at λ$_{c}$ , and a rectangular spectrum centered at 787 nm, respectively. The points are experimental data for a pulse with a rectangular spectrum centered at 787 nm. Ga ussian pulse is almost 45% less than that of the sinc pulse due to the fact that the spectral ba ndwidth of the Gaussian pulse is less than that of the sinc-shaped pulse for identical F WHM temporal pulse widths. Gaussian pulses therefore appear to be advantageous for limiting cross-talk between closely-spaced pulses. Furthermore, the effects of propagation on Ga ussian pulses can be determined analytically, which allows for additional insight into the me chanisms responsible for distortion. 5. Conclusions We measure the group index spectrum of a warm Rubidium vapor in a multipass geometry using a monochromatic laser source in the vicinity of the D1 and D2 lines as well as the medium’s pulse response using ultrashort pulses on the order of several hundred femtosec- onds. We demonstrate that calculations involving a simple, Lorentzian frequency profile for 12