## 1 Introduction  
Low-frequency Raman spectra (<200 cm\({}^{-1}\)) are interesting for studies of intramolecular vibrational modes of large molecules or heavy functional groups, and for studies of intermolecular force constants. This vibrational region can be accessed with the necessary rejection of elastically scattered light by spontaneous Raman techniques using triple monochromators, which, however, represent an expensive and cumbersome solution with a relatively low signal yield. More recently, special Bragg filters replacing the dispersive systems have been developed to overcome these shortcomings, but they have the disadvantage of being laser excitation wavelength specific. A different approach takes advantage of Raman coherence, in the so-called impulsive stimulated Raman scattering (ISRS) method, which allows excitation of coherent vibrational modes within the bandwidth of ultrashort laser pulses [1] and, accordingly, is capable of producing intense signals. This capability is particularly useful in imaging applications, where signal acquisition speed is an asset, and in characterization of biological, unstable or photodegradable samples, which require low excitation power [2]. In ISRS, different Fourier components contained in the broad spectrum of an ultrafast laser (pump) pulse interact simultaneously with the Raman medium to create a coherent Raman state when their frequency difference matches a vibrational resonance. The associated polarization wave interacts with a subsequent (weak) probe pulse, whose resulting modifications are used to retrieve the sample Raman spectrum. Among other applications, ISRS has proven useful in the detection of intramolecular Raman resonances and of librational and diffusive modes of molecules in liquids [34]. In recent papers [5; 6; 7; 8; 9], several methods to detect ISRS signals were proposed in setups using a single low-energy femtosecond oscillator, thus avoiding the use of amplified systems, which, besides being expensive, are generally not suitable for characterization of biological or unstable samples. The common experimental arrangement is based on Fourier-transform (FT) degenerate pump-probe setups, where an ultrashort laser pulse is split to derive both the pump and the probe pulses. Spectral and refractive (phase) changes of the latter, induced by the interference with the sample Raman polarization, are then monitored as a function of the pump-probe delay, and the sample Raman spectrum is finally obtained from the Fourier transform of the delay-dependent data. The tiny modifications induced on the probe beam can be measured for each delay point using lock-in techniques by chopping the pump beam (delay averaging); alternatively, the delay-dependent signal can be obtained by averaging the time-dependent signal produced by continuous periodic scanning of the delay (scan averaging), avoiding the necessity of pump modulation. Advantages and shortcomings of the two averaging methods have been discussed in the literature [10]; in particular, scan averaging systems have been shown to be more effective in reducing the data noise and increasing the data acquisition speed, and also in decreasing the amount of spurious effects otherwise produced by the pump modulation associated with delay averaging methods [5]. Here, we make use of the scan averaging technique as reported in [5]. Recently, there has been a surge in interest in the use of different detection techniques suitable to reveal the spectral and phase modifications of the probe pulse. While spectral information relies on stimulated Raman gain and loss effects, phase information depends on the Raman-induced Kerr effect (RIKE), due to the electronic polarizability coupling with the nuclear coordinates and producing refractive effects governed by the Raman modes of the material [11; 12].