Isotope Effects In The Dynamics Of Diatomic Molecule Ionization Using Laser Pulses

The experiments in this work are conducted using the reaction microscopes(REMI), and the intense laser beam is from femtosecond laser system. REMI uses position sensitive MCP detectors to measure recoil ions and electrons simultaneously, and the 3D momentum of the charged particles is deduced from the time and position signals. A supersonic jet system is also integrated in REMI system, and very high momentum resolution can be achieved. So far, REMI has been used in studies of laser ionization of atoms and molecules. The femtosecond laser system consists a Ti:sap laser, hollow-fiber-compressor, and carrier-envelope phase(CEP) stabilization system.Firstly, this work systematically investigated the isotope effect in the tunneling ionization rate of H2and D2 in laser field and the ionization/dissociation in two-color laser field using a REMI setup. The results are summarized in three aspects as follow.(1) Experimental measurement of the isotopes effects in tunneling ionization of H2 and D2. Taking advantage of the multi-hit detection ability and 4Pi detection ability of REMI, we use the 800 nm, 28 fs circularly polarized pulses to ionize a supersonic target of 50:50 binary hydrogen and deuterium mixture. The reaction products are recorded by the REMI system and the kinetic energy spectra of proton and deuteron are used to separate the ions from different channels. The yield of tunneling ionztion of H2(or D2) is deduced by taking into account the undissociated H2+ and H+(D2+ and D+)of different channels. The measured ratio of the ionization yields of H2 and D2 agrees quantitatively with the prediction of the generalized weak-field asymptotic theory, and to our knowledge, this is the first experiment conform the prediction. According to the measurement and theory, the tunneling ionization rate of H2 is higher than that of D2 for the same field strength, and the ionization rate of H2 is about 70% higher than that of H2. The ratio decease monotonously with increase of laser field Faccrording to exp[(1- 1/?2)0.0017/F2].(2) Experimental measurement of the isotope effect in the angular distribution of ionization. We measure the molecular frame electron angular distribution in the H2(or D2) ionization by circularly polarized laser beam using the photo-electron and photoion coincidence measurement method. The angular distributions are similar with the ionization rate when the molecular parrel to the laser is 1.4 higer than that of perpendicular case, the asymmetry of H2 is marginally larger than that of D2. The results consist with the published works, where the experiments of H2 and D2 were performed by different groups with different experimental conditions. In our experiment the probrom has been tested using a 50:50 H2 and D2 mixture target, which ruled out the uncertainty from the experimental condions, such as laser parameters, thus the result is more persuasive.(3) Experimental studies of isotope effect in the molecule ionization/dissociation with two color(800nm + 400nm) field. Manipulating the ionization/dissociation process by a two color field consisting fundermantal wave and second harmonic wave is an important technique. In this experiment, we use a linearly polarized fundermantal wave and second harmonic wave with the same polarization in the same polarization plane to generate a two-color field, and vary the relative phase of two compoments to control the ioinization and dissociation processes. Firstly, comparing the measured kinetic energy spectra of H+ and D+ reveals that there is a significant shoulder structure in the spectrum of H+ at the low energy(<0.3 e V) area, whereas there is a slope in the spectrum of D+; The number of high energy(>3 e V) D+ from recoil ionization or dissociation is higher than that of H+. Secondly, we focus on the up-down asymmetry in the dissociation of H2+/D2+ as a function of relative phase and kinetic energy of H+/D+. Asymmetry of H+ is larger than that of D+, and there is no phase offset between the asymmetries of H+and D+, which has been observed in a CEP controlled H2+/D2+ dissocation experiment. Lastly, we compare the yields of H+and D+in two different energy regimes(bond softening and above threshould ionizaiton), and observe the modulation of the yield in bond softening area for both H+and D+, the amplitude of H+is larger than that of D+. This is the first experimental study of the isotope effect in the dynamics of H2/D2 dissociation and ionization with two-color laser pulses.Secondly, this work theoretically and experimentally investiaged Ar ionization by elliptically polarized few-cycle laser pulses. We observed the significant discrepancy between the ionization time for parallel and anti-parallel electron emission numerically for the first time and the impact of CEP on the emission of electrons in Ar double ionization. Using the CEP stabilized 5.8 fs pulses, we measured the sub-optical cycle ionization process of Ar with attocolck technology.Last but not least, we propose a novel electron acceleration approach with two overlapping linearly polarized laser pulses in vacuum. By our simulation, the energy gain and monochromaticity can be greatly increased while the space spreads can be improved as the same time comparing with the acceleration with only one laser pulse. Using numerical simulation, analytical criteria for optimal regimes of our acceleration scheme is found.