Vibrational Effect In Tunneling Ionization Of Simple Molecules

The interaction of ultrafast intense laser fields with atoms and molecules is oneof the frontier research fields in atomic and molecular physics. With the rapiddevelopment in the past20years, the studies in this field have revealed many newphysical phenomena and mechanisms and brought a series of application prospect.Compared to the case of atoms, ionization of molecules is a complex many-bodyproblem due to molecular nuclear motion, even for the most simple molecule or undersingle active electron (SAE) approximation. From aspect of theory, it is still difficultto handle the interactions inclusing nucleus-nucleus, nucleus-electron, and electron-electron. In principal, in intense laser fields, molecules can be excited bothelectronically and vibrationally. Vibration is one of inherent molecular properties, andthe effect of vibration on molecular tunneling ionization is an important subject butvery complicated many-body problem and not yet fully known, therefore, furtherinvestigation is required. For instance, in previous studies the total ionization rates ofmolecule were taken as indicators of the vibration effect in tunneling ionization, withignored for orientation on ionization in vibrational excitation states. In this thesis, wedevelop appropriate models to deal with molecular vibration theoretically. Based onthe weighted averaging method, MO-ADK theory is applied to study the vibrationaleffect on strong field tunneling ionization of some simple molecules such as N2, O2, I2and CS2.The main content of this thesis includes three parts:First, the development of the strong field molecular theory and description ofmolecular tunneling ionization is introduced in detail, and the calculation method andproceedure for tunneling ionization rates based on ADK/MO-ADK theory areparticularly given. The theoretical method of molecular tunneling ionization forhandling different vibrational excitation is formulated for diatomic molecule.Computational approach is put forward too for acquiring more accurate molecularionization energy in terms of the change with varing vibration parameters.In the second part of the paper, detailed calculations are carried out to study the effect of vibration on tunneling ionization for N2, O2and I2moecules by using themethod developed based on MO-ADK theory. Firstly, tunneling ionization of theneutral molecules N2and O2for the ground electronic state are studied, and theconfiguration parameters, tunneling ionization rates and ionization signals areobtained. The results show that our results are in good agreement with those from theexisting theories and experiments, indicating the reliability of the method andcalculation we used. By using the wavefunctions obtained, the vibration effects on thetunneling ionization of diatomic molecular N2, O2and I2are investigated. Sincetunneling ionization rate depends critically upon molecular ionization energy, whichvaries as a function of the internuclear bond length R in the case of vibration, it isvery important to obtain more accurate potential energy curve. By comparison inMorse Function, Murrell-Sorbie Function and CCSD(T) method, the potentials fromCCSD(T) method are finally selected to obtain the molecular ionization energy. Withthis ionization energy we calculate the angle-dependent ionization rates of N2, O2andI2molecules, respectively, from HOMO orbital for different internuclear distanceswithin the frame of MO-ADK and SAE approximation. The angle-dependentionization rates for different vibrational excitation states（νn，n=0,1,2,3）of N2, O2and I2molecules are calculated according to the average of R-dependent rate over theprobability distribution from the nuclear wavefunction, and accordingly, the angle-dependent electron density of the HOMO orbital for different vibrational excitationstates （νn，n=0,1,2,3）of N2, O2and I2molecules are also calculated. The resultsshow that:(1) The angle-dependent tunneling ionization rates are proportional to theelectron density in the asymptotic region and the ionization rates, reflecting theelectron structures and symmetry of molecules.(2) The angle-dependent tunnelingionization rates are influenced by the external laser field polarization effect as thedeviation appears between the tunneling ionization rates and orbital electron densities,espectially at the molecular orientation angle of zero.(3) The vibration effect ontunneling ionization makes the different variation with the vibrational excitation statesfor different molecules. With increasing the vibrational excitation levels, the tunnelingionization rates of N2and O2get lager, in contrast, these of I2at vibrational excitation states decrease compared to that of the ground state.(4) The vibration effect ontunneling ionization is reflected more clearly by the angle-dependent tunnelingionization rates. It is very sensitive to the vibration effect on tunneling ionization formolecular alignment. Therefore, it is belived that the investigation with alignment willprovide a suitable approach for giving insight into the vibration effect on moleculartunneling ionization.In the third part of the paper, taking the triatomic molecule CS2as an example,we explore the vibration effect on tunneling ionization of polyatomic molecules.Different vibrational modes and affects of CS2at different cases are analysed and thesimple estimation based on MO-ADK theory is put forword to calculate the tunnelingionization rares of CS2at different initial C-S bond length and bending angles. Itindicates that there are larger changes in tunneling ionization rates with varing C-Slength, while very little change with the bending angle. Thus, it predicts that there isan important contribution for the vibration mode of symmetric stretching to tunnelingionization of triatomic molecule CS2. The obtained saturation intensity of thetunneling ionization and the changes with bond length through this simple calculationare qualitatively consistent with the experimental observation reported.