the same parameters as in panel (a). ](Fig_3){width="\columnwidth"}

![image](Fig_4a){width="\columnwidth"} ![image](Fig_4b){width="\columnwidth"}

Negative pump-probe delay time {#negativeTimeA}
------------------------------

Let us now turn to the situation where the probe pulse precedes the pump pulse with $\delta t < 0$. We focus our analysis on the nonadiabatic case, $\tau_{\textrm{p}} \ll \tau_{\Delta}$, since in the opposite regime, $\tau_{\textrm{p}} \gg \tau_{\Delta}$, the pump-probe signal does not show any interesting characteristics as a function of delay time $\delta t$.

### Nonadiabatic regime, $\tau_{\textrm{p}} \ll \tau_{\Delta}$

In Fig. \[FigM3\](a) we present the real part of the pump-probe response, $\textrm{Re} [ \sigma(\delta t , \omega ) ]$, versus negative delay time $\delta t$ and frequency $\omega$ for an ultrashort pump pulse with $\tau_{\textrm{p}} =0.5$ ps $\ll \tau_{\Delta}$. Both the energy gap before and after the pump pulse are clearly visible in the frequency dependence of $ \sigma(\delta t , \omega ) $. That is, the pump-probe signal displays two sharp edges as a function of frequency, one at twice the gap energy in the initial state $\hbar \omega_{\Delta ( t_i )} =  2 \Delta (t_i)=2.7$ meV and one at twice the asymptotic gap value $\hbar \omega_{\Delta_{\infty} } =  2 \Delta_{\infty}=1.677$ meV \[green and blue traces in Fig. \[FigM3\](a)\]. Between 