  We generate high-order harmonics in gaseous medium with tunable photon energy
using time domain interferometry of double pulses in a non-collinear generation
geometry. The method is based on the fact that the generated harmonics inherit
certain spectral properties of the driving laser. The two temporally delayed
ultrashort laser pulses, identical in all parameters, are produced by a
custom-made split-and-delay unit utilizing wave front splitting without a
significant energy loss. The arrangement is easy to implement in any attosecond
pulse generation beamline, and and is suitable for the production of an extreme
ultraviolet source with simply and quickly variable central photon energy,
useful for a broad range of applications.
