Hyper-Rayleigh scattering spectrum of liquid nitromethane.

The vertical vertical (VV), horizontal vertical (HV), and vertical horizontal (VH) hyper-Rayleigh scattering (HRS) spectra were measured for liquid CH3NO2 at T= 300 K. The main HRS spectral component has a width upsilon1=1.28 +/- 0.04 cm(-1), which gives an orientation relaxation time pi=4.1 +/- 0.1 ps in good agreement with other experiments. However, the VH spectrum also contains a previously unobserved strong narrow peak at zero-frequency shift, absent from the VV and HV spectra, which is due to a slowly relaxing longitudinal orientation mode. The upper bound on the width of this peak is 5 MHz, which corresponds to a relaxation time pi > 30 ns.