Single-shot measurement of laser-induced double step ionization of helium

The spatio-temporal evolution of the free electron density induced in a helium gas jet by an intense femtosecond pulse is measured with ~10 fs resolution in a ~1 ps temporal window. The double-stepionization feature is observed. In these measurements, we use the technique of single-shot supercontinuum spectral interferometry. It is demonstrated that finite laser-gas interaction lengths can strongly affect the interpretation of such measurements.