Absolute measurement of the transient optical nonlinearity in N 2 , O 2 , N 2 O, and Ar

The absolute time-dependent nonlinear response of O2, N2, N2O, and Ar to intense nonionizing, ultrashort optical pump pulses is measured with single-shot spectral interferometry. The instantaneous and delayed rotational responses are distinguished as a function of pump-pulse duration and probe central wavelength. Our measurements are central to the modeling and understanding of nonlinear propagation of intense ultrashort laser pulses in gases.