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Electron energy distributions of singly and doubly ionized helium in an intense 390 nm laser field have been measured at two intensities (0:8 PW=cm 2 and 1:1 PW=cm 2 , where PW 10 15 W=cm 2 ). Numerical solutions of the full-dimensional time-dependent helium Schrödinger equation show excellent agreement with the experimental measurements. The high-energy portion of the two-electron energy distributions reveals an unexpected 5U p cutoff for the double ionization (DI) process and leads to a proposed model for DI below the quasiclassical threshold. DOI: 10.1103/PhysRevLett.96.133001 PACS numbers: 32.80.Fb, 31.90.+s, 32.80.Rm In an intense low-frequency laser field, the mechanism for double ionization of helium varies with intensity. At the highest intensities, the process can be accurately described by a sequence of independent, single-electron ionization steps [dubbed sequential double ionization (SDI) [1]], in which the atom is singly ionized first, followed by photoionization of the residual He ion. This process produces uncorrelated pairs of electrons. The rate at which this process occurs is limited by the slower of the two steps, typically the ionization of the residual He ion. In helium, at visible and near-infrared (IR) wavelengths, SDI becomes negligible at intensities 4PW=cm 2 . Below this intensity, He is produced predominantly via nonsequential double ionization (NSDI), a process in which correlated electron pairs are ejected near simultaneously (1 optical cycle). In the near IR, NSDI is well described by a quasiclassical rescattering model [2,3], in which a single electron is ejected at or near the peak of the electric field, but returns to the atom as the field changes direction, and does so with sufficient energy to free the remaining electron of He in an inelastic (e; 2e) collision. The maximium energy the rescattered electron can bring to the He ion is 3:2U p , where U p / I 2 is the ponderomotive energy. The threshold intensity at which the rescattered electron has sufficient energy to directly ionize the He 1s is the intensity at which 3:2U p equals the ionization potential of He :2 au. We label this intensity I t1 . Below I t1 the rescattered electron has insufficient energy to directly ionize He 1s, but if the intensity is above I t2 0:75I t1 , it does have sufficient energy to collisionally excite the first excited state of He , which can then ionize rapidly, absorbing energy from the laser. Although this is a plausible mechanism for NSDI in the region I t2 < I < I t1 it has not been established if it is the only mechanism, or whether it is the dominant NSDI process. Below the I t2 threshold the physical origins of NSDI have remained puzzling [4]. Recently, experimental progress [5] has been reported in this intensity limit, but for higher-Z inert gases excited by 800 nm light.In this Letter we report on an experimental and theoretical double ionization study of helium atoms in the intensity range I < I t2 and I t2 < I < I t1 using intense 390 nm light. We propose a possible mechanism for ...

We present calculations of intense-field multiphoton ionization processes in helium at XUV wavelengths. The calculations are obtained from a full-dimensional integration of the two-electron time-dependent Schrödinger equation. A momentum-space analysis of the ionizing two-electron wavepacket reveals the existence of double-electron above threshold ionization (DATI). In momentum-space two distinct forms of DATI are resolved, namely non-sequential and sequential. In non-sequential DATI correlated electrons resonantly absorb and share energy in integer units of ℏωlaser.

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