} \rangle$, remain unoccupied. Hence, the order parameter $ \Delta ( t) $ does not oscillate, instead it decreases monotonically towards the asymptotic value $\Delta_{\infty}$ \[Fig. \[FigM2\](c)\]. Correspondingly, the pump-probe signal $\sigma(\delta t,\omega)$ does not exhibit any oscillations, neither as a function of delay time nor of frequency \[Fig. \[FigM2\](a)\]. As in Fig. \[FigM2\](a), $\textrm{Re} [ \sigma(\delta t,\omega)] $ has a sharp edge at the frequency $\omega_{\Delta_{\infty}}$ corresponding to twice the energy of the asymptotic gap value $\Delta_{\infty}$. However, otherwise, it is almost featureless, except for a small dip just below the pump frequency $\omega_{\textrm{p}}$; see Fig. \[FigM2\](b). This reduced absorption in the vicinity of $\omega_{\textrm{p}}$ is due to Pauli blocking which leads to a saturation in the narrowly peaked quasiparticle distributions [@Papen07; @footnote1].

![ \[FigM3\] \[plot:sigma-neg\] (Color online) (a) Absorption spectrum $\textrm{Re} [ \sigma(\delta t,\omega)]$ versus $\omega$ and $\delta t<0$ for the nonadiabatic regime \[$\tau_{\textrm{p}}=0.5$ ps, $|\textbf{A}_{\textrm{p}} |=13\cdot10^{-8}$ Js/(Cm)\] in the absence of phonons. (b) Fourier transform of the data in panel (a), i.e., $\textrm{Re} ( \sigma ) $ as a function of Fourier frequency $\omega_{\delta t}$ and absorption frequency $\omega$. (c), (d) Time dependence of $| \Delta (t)  |$ and spectral distribution of the gap oscillation, respectively, for 