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 rea listic experimental conditions will lead to a readily observable blue shift and line splitting in the midplateau harmonics. The origins of t he observed blue shift and line splitting are detailed. We also show t hat midplateau harmonics with a duration of only a few femtoseconds ca n be generated by combining the intensity dependences of the atomic no nlinear polarizability and the free-electron-induced phase mismatch.