The physics of extreme ultraviolet attosecond pulse trains generated during the interaction between an intense laser pulse and a gas medium is presented, including a simple model based on the solution of the classical equations of motion of an electron in an oscillating laser field. The reconstruction of attosecond beating by the interference of a two-photon transition is described and used to determine the pulse duration of the emitted attosecond pulses. This application is the basis of a laboratory practical for a graduate atomic physics course.

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The reason why it is only the odd and not the even harmonics that are produced is that there is a phase-shift of π between the photons emitted at consecutive events (separated in time by T2).

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