Photoelectron Momentum Spectra And Dissociative Ionization Of Molecules In Intense Femtosecond Laser Field

The interactions between molecules and ultrafast intense laser field manifest manyinteresting phenomena, such as multiphoton ionization, above-threshold ionization(ATI), double ionization or multiple ionization. Laser-induced photoelectronmomentum spectra provids a powerful tool for exploring various ionization processes inintense laser field. The coincidence momentum imaging method offers a great potential forinvestigating the dissociative double ionization of molecule. The first part of thisdissertation explores the two dimensional photoelectron momentum spectra of C₂H₄, C₂H₆,C₆H₆and H2O in intense femtosecond field. Tthe second part, using the ion-ioncoincidence momentum imaging method, is devoted to an investigation of the dissociativeionization process.1. Measurement of the high-order ATI photoelectron momentum spectra(so calledRescattering Photoelectron Spectroscopy, RPS) of C₂H₄, C₂H₆and C₆H₆. We measure theangle resolved RPS spectra with a264mm long linear time-of-flight spectrometer. Thepolarization direction of the light is rotated using a/2plate. Basing on the quantitativerescattering(QRS) theory, we can extract from the experimental RPS the large-angleelastic differential cross sections(DCSs) for scattering of free electrons by the partiallyaligned C₂H₄⁺, C₂H₆⁺and C₆H₆⁺. For the returning electron, the DCSs of C₂H₄⁺werefound to exhibit the rather flat distribution in the momentum range of0.55a.u⁰.8a.u.,except at the higher momentum region(0.8a.u.¹.05a.u.), the DCSs show a clearmaximum at180degree which is parallel to the laser polarization. The theoretical DCSsare calculated by the ab initio method for free electrons. The calculation is performedusing the Schwinger configuration interaction method. The experimental and theoretical DCSs agree well which confirms the validity of the extraction procedure in the experiment.The sensitivity to the partially alignment in the signal results from the angle-dependentionization rate. The calculated Molecular Frame DCSs(MFDCSs) are used to explainthe modulation of the DCSs in the low collision momentum region. We extracted the ion-electron elastic DCSs of C₂H₆⁺of the momentum range0.55a.u.¹.15a.u.. Strikingly,the DCSs have a very clear maximum at180degree for all momentum in this range, whichis different compared to that of C₂H₄⁺. The highest occupied molecular orbitals(HOMOand HOMO-1) of C₂H₆are orbitals of degenerate type and type. Qualitatively, webelieve that both orbitals simultaneously contribute to the RPS, which explain thecharacteristic DCSs distribution of C₂H₆⁺. For first time we extended our measurement ofRPS to the cyclic molecule C₆H₆. As increasing the momentum from0.65a.u. to0.9a.u.,the DCSs of C₆H₆⁺have a minimum at180degree for0.65a.u., which changes to110degree for0.9a.u.. Since C₆H₆has type HOMO orbital, the destructive interference maylead to this minimum evolution in the DCSs. In summary, these results show that ourextraction method can be applied to polyatomic molecules and hence pave the way forapplication of RPS as a tool to probe the structure of more complex molecular systems.In addition, we also measured the low-order ATI photoelectron momentum spectra forC₂H₄, C₂H₆and H2O. We focused on the so called jets-like structures which arecharacterized by appearing the maximum and minimum alternately in the angulardistribution of each ATI peak. The significant jets-like structures were observed in firstthree ATI peaks for C₂H₄. We believe that resonance ionization(e.g.,(8+1) photonresonance ionization), intermediating with the high Rydberg state leads this jets-likedistribution. A channel switching phenomenon turned out at a higher peak laser intensity.No clear jets-like structures have been seen for C₂H₆, this is because two resonancechannels contribute to this ATI process for same laser intenstiy and the energy differencebetween them is quite small. We found two discrete series of ATI peaks to appear in the photoelectron momentum spectra of H₂O. The clear jets-like structures were seen for eachATI peak. We extracted the number of maxima(jets number) in each jets-like ofstructure. The jets number increases one when one more photon is absorbed for both series.The jets number for first ATI peak was found to differ between the two series. We believethat these two series are born out different resonance channels intermediate with differenthigh Rydberg states. These two Rydberg states carry different quantum number.2. The dissociative single ionization and double ionization of formic acid in intenselaser field also were explored. Our results obtainded with the coincidence momentumimaging method, confirm that the enhanced sequential double ionization at Rc playsimportant roll, and securely identify that the hydrogen migration occur in the dissociativedouble ionization of formic acid. We explore the intensity dependent ion yields of OH⁺and HCO⁺originating from singly charged formic acid. The results indicate that bothfragments dissociate from electronical excited state of parent ion. The fragments born outfrom singly and doubly charged formic acid both show anisotropic angualr distributions. Aqualitative explanation for those anisotropic distributions is for the angle-dependentionization rates.3. The design and major parameter of new reaction-microscope were described. Theimages of ions and electrons producing from the ionization of rare gas were shown. Thereasonable images prove that the reaction-microscop work smoothly and satisfy therequirement of our research. The dissociative ionization of ethane was explored usingion-electron coincidence method. The main two-body Coulomb explosion channel hasbeen identified and the kinetic energy release of this channel was used to normalize the ionenergy spectrum.