Investigation Of Angular Dependence Of Strong-field Tunneling Ionization For Linear Molecules

A molecule exposed to an intense laser fields can be ionized by differentionization mechanisms: multiphoton ionization (MPI), above-threshold ionization(ATI), tunneling ionization (TI), over-the barrier ionization, and non-sequentialdouble ionization(NSDI). When the Kelydsh parameter1, multiphoton ionizationwill dominant; if1, tunneling ionization (or over-the barrier ionization) willprevail. With the development of ultra-short and super-intense laser technology, theinteraction between intense femtosecond laser fields and molecules has attracted agreat deal of interests. This is because not only the similar phenomena can happenwhen a molecule interacts with an intense laser field compared to the atom case, someinteresting and complex physical process can be observed. Of course, investigation ofstrong-field phenomena of molecules are still challenged due to its complexity. Suchas bond-softening, bond-hardening, above-threshold dissociation,charge-resonance-enhanced ionization, Coulomb explosion，alignment-dependentionization probability and ionization suppression compared to their companion atomset al.. In principle, these phenomena can be theoretically studied by solvingtime-dependent Schrodinger equation of molecules in an intense laser fields. In thisdissertation, we study the angular dependence of tunneling ionization rates ofmolecules with the molecular tunneling ionization theory (i.e., MO-ADK theory). Themain advantages of the MO-ADK theory is the simple analytical formulism ofionization rate and the less computational demand.The major investigations of this dissertation can be separated into two sections asfollowing:Firstly, We determine the structure parameters for the asymmetric heteronucleardiatomic molecule HeH2+at several internuclear distances with the molecularwavefunctions obtained by solving the time-independent Schr dinger equation withB-spline functions based on the single-center expansion approach. Then the angulardependence of strong-field ionization rates of HeH2+is investigated with themolecular tunneling ionization theory. Our results show that the shape of severallowly excited states (i.e.2pσ，2pπ，3dδ) for HeH2+are reflected in the orientationdependent ionization rates very well, however, the angle-dependent ionization rate fails to follow the angular distribution of the asymptotic electron density for theground state1sσ. We found that the internuclear distance dependent ionizationprobabilities are in a good agreement with the more accurate result obtained from thenumerical solution of the time-dependent Schr dinger equation.Secondly, we also determine the structure parameters for the asymmetricheteronuclear diatomic molecule LiH3+、BeH4+at several internuclear distances. Byintroducing the Stark shift of molecular levels for the polar molecules to theMO-ADK theory, we studied the influence of the Stark effects on the orientationdependent ionization rates. We found that the Stark effect play a minor role on theangular dependence of the ionization rate, however, the ground state (1sσ) of HeH2+and the highest occupied molecular orbital (5σ)(HOMO) of CO are exceptions. Theorientation dependent ionization rates from the ground state of HeH2+and the HOMOof CO molecule using the Stark-shift corrected MO-ADK theory favors the oppositedirection with respect to those from the original MO-ADK theory.