Detailed instantaneous ionization rate of H$_2^+$ in intense laser field

Component instantaneous ionization rate (IIR) is introduced and the approach of its calculation is formulated. The component IIR's and the overall (time-averaged) component ionization rates are calculated for H$_2^+$ at different values of inter-nuclear distance in a linearly polarized laser field with $1.0 \times10^{14}$W cm$^{-2}$ intensity and $\lambda \sim 1064$nm wavelength by direct numerical solution of the fixed-nuclei full dimensional time-dependent Schr \"odinger equation. The exact overall component ionization rates calculated by time-averaging of the component IIR are compared with those calculated approximately via the virtual detector method (VD). Details of the time dependent behavior of the outgoing and incoming electron wavepackets of the H$_2^+$ system in intense laser field at sub-femtosecond time scale are studied based on the calculated component IIR. It is shown clearly that the positive (outgoing electron wavepacket) signals of the IIR and its z component are strong and sharp but the negative (returning electron wavepacket) signals of the IIR are smooth and weak. The structure of the $\rho$ component of the IIR has smooth structure. Relation between the R-dependent ionization rate and duration of the ramp of the laser pulse is studied and it is explicitly shown that for internuclear distance R<5.6, when the laser pulse is turned on without a ramp, the first peak of R-dependent ionization rates moves towards the peak of the lower time dependent Floquet quasi-energy state (QES).