Multiple Ionization of Atoms by 25 and 7 fs Laser Pulses

Double, triple and fourfold ionization of rare gas atoms has been studied using a “reaction microscope”. We show that ionization dynamics drastically depends on atomic species and present the first data obtained with few-cycle pulses. Since the first experimental observation of doubly charged ions produced by intense laser field [1], many-electron dynamics in laser-induced ionization has been the subject of numerous experimental and theoretical studies. It was soon found out that the yields of second and higher charge states produced by intense linearly polarized laser pulses exceed significantly those expected for independent successive removal of two or more electrons. The origin of this enhancement (which was called “non-sequential” ionization) remained controversial until 2000 when first measurements of recoil ion momentum distributions [2,3] along with the results of earlier experiments using circularly polarized light [4] provided convincing evidences in favour of the so-called “recollision” model. However, the details of the double ionization dynamics are still far from being completely understood. In particular, one of the most intriguing questions is whether and how atomic structure can influence ionization dynamics [5]. For the case when more than two electrons are ionized, there is much less experimental results available. Intensity dependence of ion yield was measured for different rare gases (see, e.g. [6]). Ion momentum distribution obtained in [3] for Ne at 1.3 PW/cm remains, to the best of our knowledge, the only differential data published up to now. Here we report on the systematic experimental study of multiple (up to fourfold) ionization of Ar and Ne by intense laser field and present the first results obtained with the laser pulses as short as 7 fs. The experiments were performed using a new “reaction microscope” [7] designed to meet the specific requirement of the experiments with high-intensity lasers. We used linearly polarized radiation of a Kerr-lens mode locked Ti:sapphire laser at 795 nm wavelength with 25 fs pulse width (FWHM). To generate few-cycle pulses they are spectrally broadened in a gas-filled hollow fiber and then compressed to 6-7 fs (FWHM) by chirped mirrors and a prism compressor. Fig. 1 shows momentum distributions of Ne ions along the laser polarization direction. All spectra exhibit a clear double peak structure which is a signature of the recollision process In the momentum distributions of Ne and Ne ions this structure is even more pronounced than for double ionization, with almost no ions produced with zero momentum. Momentum distribution of Ne ions created by 7 fs laser pulse (solid line in Fig.1a) closely follows the results obtained with 25 fs pulses, indicating that momentum distribution is mainly defined by the recollision within one or two cycles after the removal of the first electron. The widths of the spectra are in good agreement with kinematical constraints following from the classical consideration [8]. In general, confirming the results of [3] we can state that ion momentum distributions of Ne ions agree well with the model assuming direct impact ionization by rescattered electron via (e,ne) process. Fig. 1. Longitudinal momentum distributions of Ne ions. Arrows indicate ion