## 2 Materials and Methods  
The experimental apparatus is an FT ISRS spectrometer based on a Michelson interferometer with orthogonally polarized beams in the two arms acting as pump and probe, see Figure 1.  
The two beams are produced from a single polarized laser source by means of a half-wave plate and a polarizing beam splitter; the half-wave plate is also used to control the pump/probe energy ratio, which is set to 10:3 in the present experiments. The laser beam is provided by a cavity-dumped femtosecond oscillator (Coherent MIRA 900F, Santa Clara, CA, USA; plus PulseSwitch, APE GmbH, Berlin, Germany) operating at 780 nm with typical pulse energy equal to about 18 nJ, 40 kHz repetition rate and 12 nm bandwidth. The 12 nm bandwidth allows for impulsive excitation of Raman modes below approximately 200 cm\({}^{-1}\). Before the interferometer, the beam is pre-treated by two compressors, a chirped-mirrors one (DCMP 175, Thorlabs GmbH Bergkirchen, Germany) and a double-pass prism one, to compensate for the laser chirp and for the layout optics dispersion, allowing to obtain a minimum autocorrelator pulse width at the sample position equal to 106 fs, corresponding to a Gaussian 1/e pulse width \(\tau_{G}\cong 44\) fs. One of the interferometer arms (the pump beam one) is equipped with a retroreflector mounted on a rapid scanning actuator (a loudspeaker) capable of producing a periodically modulated delay of more than 12 ps with a frequency of few tens of Hertz. In the experiments, the delay is modulated by a triangular waveform whose amplitude and frequency were experimentally optimized for extended delay range and linear loudspeaker excursion; moreover, the loudspeaker rest position is adjusted so that the modulation causes the pump pulses to arrive from slight delay to substantial advance with respect to the probe pulse. The fast periodic delay modulation is necessary to implement the signal averaging by delay-scan [18; 28], a noise reduction technique that was demonstrated to be advantageous over fixed-point averaging. The pump and probe beams recombine at the beam splitter and are sent to the sample by a 2.5 cm focal length lens; then, beams are recollimated by a second lens with the same focal length after the sample. The two lenses are mounted as windows of a homemade stainless-steel cell with a removable cap allowing access to the main lens focus position. For calibration purposes, a 0.1 cm quartz cuvette containing a liquid reference sample can be inserted in the cell in the focal position of the lenses, while for gas-phase measurements, the cell is evacuated and filled with pure N\({}_{2}\) at a controlled pressure. After the sample, the probe beam is singled out by a Glan-laser polarizer (P3) and is sent to the detection assembly. Note that to minimize the pump leakage on the probe beam, the polarization purity of the two beams in the interferometer is enforced by using a Glan laser polarizer in each arm (P1-P2). As widely discussed in the literature [12; 17; 19], the probe beam is subjected to refractive and spectral effects due to the Raman coherent oscillations in the sample; in this experiment, we detect the ISRS-induced spectral shifts using the standard arrangement [11; 19] consisting of an edge filter (LP02-780 RU-25 Semrock, IDEX Health & Science, LLC, Rochester, NY, USA), tuned at the center wavelength of the probe, placed in front of one of the channels of a balanced photodiode detector (Thorlabs PBD210/A). To ensure  
Figure 1: Experimental layout. tuning at the actual center of the probe spectrum and thus maximize the spectral detection sensitivity [19], the edge filter is tilted in order to transmit approximately 50% of the probe beam intensity.