Fig. 1: Electric field induced Stark effect and experimental concept.

Electric field induced Stark shift due to a difference in ground- and excited state dipole (a), i.e. linear Stark effect, and polarizability (b), i.e. quadratic Stark effect. For an isotropic distribution of molecules the ground state absorption band versus energy A(E) broadens in the case of the linear Stark effect with the difference signal ΔA(E) resembling the second order derivative of the absorption band. In the case of the quadratic Stark effect the ground state absorption band shifts resulting in a difference signal ΔA(E) that is proportional to the first order derivative of absorption band. c Schematic representation of the experimental setup. The femtosecond supercontinuum probe pulse is scanned in time across the collinear single-cycle THz pulses and its spectrum is recorded by a spectrometer.