Sub-optical-Cycle Attosecond Control of Molecular Ionization by Using Multicolor Fourier-synthesized Laser Fields

We report that intense (~1013 W/cm2) phase-controlled laser fields consisting of a fundamental pulse and its second-harmonic pulse induce directionally asymmetric tunneling ionization and resultant orientation-selective molecular ionization in the gas phase. The mechanism is discussed in connection with the shape of the highest occupied molecular orbital. This method is robust, being free of constraints such as the laser wavelength, pulse duration, polarity, and weight of molecules, and thus can be applied to a wide range of molecules. Furthermore, we have found four-mode multi-selection with a combination of positive/negative orientation-selected and yield-enhanced/suppressed molecular tunneling ionization by dual-phase control of three-color Fourier-synthesized laser pulses. Positive/negative orientation-selected molecular ionization is induced by the ω + 2ω laser fields, and yield-enhanced/suppressed ionization is induced mainly by ω + 3ω laser fields. The Fourier-synthesized ω + 2ω + 3ω laser fields concurrently enhance both positive/negative orientation-selected and yield-enhanced/suppressed molecular tunneling ionization. The mechanism is discussed in connection with the sub-optical-cycle interference control of the laser waveforms.