SPECTRAL AND TEMPORAL STRUCTURE IN HIGH HARMONIC EMISSION FROM IONIZING ATOMIC GASES

We show that high harmonic generation in ionizing atomic gases should be accompanied by an intensity-dependent harmonic phase that under realistic experimental conditions will lead to a readily observable blue shift and line splitting in the midplateau harmonics. The origins of the observed blue shift and line splitting are detailed. We also show that midplateau harmonics with a duration of only a few femtoseconds can be generated by combining the intensity dependences of the atomic nonlinear polarizability and the free-electron-induced phase mismatch.