Pathway of D + in Sequential Double Ionization of D$_{2}$ in an Intense Laser Pulse We show the details of the pathway for dissociative ionization process of ground electronic state of aligned D + 2 due to first ionization of D$_{2}$ in short ($_{∼}$100 fs) and intense (4 . 0 × 10 14 W cm − $^{2}$) 480 nm laser pulses. The initial vibrational state of D + 2 comes from the vertical transformation of the ground state of D$_{2}$. The initial wavepacket in the ground electronic state of D + 2 is outgoing through dissociation-ionization channel accompanied by a strong coupling between 1 sσ$_{g}$ and 2 pσ$_{u}$ electronic states. We show explicitly that the transition from the coupling states 1 sσ$_{g}$ and 2 pσ$_{u}$ to the ionization state is not a direct transition but takes place through other intermediate states with some dissociation energy that results in the internuclear distribution of the ionization to move considerably to larger internuclear distances. I. INTRODUCTION PACS numbers: 33.80.Rv, 33.80.Gj, 42.50.Hz arXiv:1001.0301v1 [physics.atom-ph] 2 Jan 2010 When molecules are exposed to an intense laser pulse (even for the simplest molecules, i.e. H$_{2}$ and H + 2 ), a wealth of fascinating phenomena may be observed. A few examples are: above-threshold or tunneling ioniza- tion, charge resonance enhanced ionization, dissociative- ionization, above threshold dissociation, bond softening and hardening, high order harmonic generation, etc. [1]. An interesting subject in studying of H$_{2}$ (D$_{2}$) in the in- tense laser pulse is double ionization [2]. The double ionization of H$_{2}$ (D$_{2}$) exposed to an intense laser field can proceed through either non-sequential or sequential mechanisms, depending on the intense laser pulse param- eters. Sequential double ionization (SDI) is the dominant mechanism at higher intensities and also for circularly polarized fields. Another interesting new phenomenon has been ob- served in a recent experimental study of D$_{2}$ exposed to an intense laser field that revealed a new high-energy band for the kinetic energy release (KER) spectrum at short wavelength in the SDI [3]. In the experiments of Litvinuk et. al., the linearly polarized laser pulses of different peak intensities with duration of $_{∼}$100 fs were focused into a well-collimated supersonic jet of target D$_{2}$ molecules in- side the uniform-electric-field ion imaging spectrometer. They found a new high-energy band for the KER spec- trum in an 480 nm laser pulse with peak intensity 4 × 10 14 W cm − $^{2}$. The emergence of this new band depend on the wavelength, i.e. the energy of this band is decreasing with increasing the wavelength until this band merges with the enhanced ionization band for 800 nm and longer wave- length [3]. The position of this high-energy band seems to be independent of the intensity for the various wave- lengths. To explain this phenomenon, Litvinyuk and et. al. proposed some possible pathways to emerge of this high- band energy (Fig. 1) as follow. After the neutral molecule is singly ionized, the vibrational wavepacket propagates on the field-dressed electronic potential energy surfaces of D + 2 , staying mostly in the lowest two electronic states $^{∗}$Electronic address: mo˙vafaee@sbu.ac.ir $^{†}$Electronic address: B-Shokri@sbu.ac.ir FIG. 1: We suppose that at a definite time, τ$_{on}$ , H$_{2}$ (D$_{2}$) is ionized vertically from its ground state to the ground elec- tronic state of H + 2 (D + 2 ). Ionization of H + 2 (D + 2 ) practically occurs through the DIC. The nuclear component in this chan- nel (DIC) posses both dissociation energy (DE) and Coulomb explosion energy (CEE). The green arrows are related to the results of the present simulation. Mohsen Vafaee ∗ and Babak Shokri † Laser-Plasma Research Institute, Shahid Beheshti University, G. C., Evin, Tehran 19839-63113, Iran