We have extended the tunneling ionization model of Ammosov-Delone-Krainov ͑ADK͒ for atoms to diatomic molecules by considering the symmetry property and the asymptotic behavior of the molecular electronic wave function. The structure parameters of several molecules needed for calculating the ionization rates using this molecular ADK model have been obtained. The theory is applied to calculate the ratios of ionization signals for diatomic molecules with their companion atoms that have nearly identical binding energies. The origin of ionization suppression for some molecules has been identified. The predicted ratios for pairs with suppression (D 2 :Ar, O 2 :Xe) and pairs without suppression (N 2 :Ar, CO:Kr͒ are in good agreement with the measurements. However, the theory predicts suppression for F 2 :Ar, which is in disagreement with the experiment. The ionization signals of NO, S 2 , and of SO have also been derived from the experimental data, and the results are also shown to be in agreement with the prediction of the present molecular ADK theory.

The orientation-dependent strong-field ionization of CO molecules is investigated using the fully propagated three-dimensional time-dependent Hartree-Fock theory. The full ionization results are in good agreement with recent experiments. The comparisons between the full method and the single active orbital method show that although the core electrons are generally more tightly bound and contribute little to the total ionization yields, their dynamics cannot be ignored, which effectively modifies the behavior of electrons in the highest occupied molecular orbital. By incorporating it into the single active orbital method, we identify that the dynamic core polarization plays an important role in the strong-field tunneling ionization of CO molecules, which is helpful for the future development of the tunneling ionization theory of molecules beyond the single active electron approximation.

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The kinetic energy distribution of D + ions resulting from the interaction of a femtosecond laser pulse with D2 molecules is calculated based on the rescattering model. From analyzing the molecular dynamics, it is shown that the recollision time between the ionized electron and the D + 2 ion can be read from the D + kinetic energy peaks to attosecond accuracy. We further suggest that more precise reading of the clock can be achieved by using shorter fs laser pulses (about 15fs).PACS numbers: 34.50. Rk, 31.70.Hq, 95.55.Sh Human experience shows that new areas of science and technology open up with the ability to make measurements at increasingly shorter time regime. With the advent of femtosecond (fs) lasers, femtochemistry became possible where chemical reaction dynamics can be probed at the atomic scale [1]. Clearly, fs lasers cannot be used directly to probe electron dynamics which is in the attosecond (as) regime. While a substantial effort is being dedicated to developing single attosecond pulses [2,3,4,5], presently few laboratories have such lasers available.An ingenious suggestion for performing measurements at attosecond resolution with fs lasers was proposed by Corkum and his group recently. Their results were reported in two recent publications, here to be called I [6] and II [7], respectively. In their experiment, a 40 fs pulse, with mean wavelength ranging from 800 nm to 1850 nm, and peak intensity of about 1.5 × 10 14 W/cm 2 , was used to ionize a D 2 molecule to produce D + ion. It was assumed that D 2 was first ionized near the peak of the laser pulse to create a correlated electronic and nuclear wave packet. Within a single optical cycle, the electron was driven back to collide with D + 2 and to excite it to the excited σ u electronic state which subsequently dissociated to produce D + . The kinetic energy of the D + ion reflects the internuclear distance, as well as the time when the rescattering occurs. With proper laser intensity, both the initial ionization and the rescattering are found to occur at time interval of far less than one optical cycle, thus providing attosecond temporal resolution, irrespective of the femtosecond pulse duration of the laser. In II, by changing the wavelength of the fs laser, they concluded that the dissociation dynamics of D + 2 can be used as a molecular clock and the clock can be read with attosecond resolution.To read the molecular clock accurately, the rescatter- * Contact: xmtong@phys.ksu.edu ing mechanism which leads to the measurable D + kinetic energy distribution has to be understood in details. In this Letter we report the main conclusion of our careful analysis of the rescattering mechanism. In contradiction to I and II, our analysis shows that the D + ions are not produced by the dissociation of the excited D + produced from Coulomb explosion vs. from dissociation was determined. Our analysis also shows that the dominant peak of the D + kinetic energy distribution is from the third return of the rescattering process, rather than from the first retu...

We perform a joint measurement of terahertz waves and high-harmonics generated from argon atoms driven by a fundamental laser pulse and its second harmonic. By correlating their dependence on the phase delay between the two pulses, we determine the generation of THz waves in tens of attoseconds precision. Compared with simulations and models, we find that the laser-assisted soft collision of the electron wave packet with the atomic core plays a key role. It is demonstrated that the rescattering process, being indispensable in high-harmonic generation processes, dominates THz wave generation as well in a more elaborate way. The new finding might be helpful for the full characterization of the rescattering dynamics.