PHASE DEPENDENCE OF INTENSE FIELD-IONIZATION - A STUDY USING 2 COLORS

We present the first measurement of the ionization rate from above-thr eshold ionization (ATI) in an optical field whose phase dependence is known and controllable. Our method uses a harmonic two-color light fie ld with a calibrated phase difference between the 1 omega and 2 omega fields. Our measurements confirm many of the predictions of recent sem iclassical theories, but show an important discrepancy: An asymmetry i n the ATI rates relative to the sign of the two-color phase difference supports recently suggested modifications to the models to include el ectron-core rescattering during ionization.