| Dissociative double ionization of molecules in strong laser fields has been a subject of current interest.Several mechanisms such as sequential multiphoton double ionization(?) non-sequential double ionization by the electron recollision,etc.,have been explored.We report here an investigation of strong-field effects on dissociative double ionization of molecular formic acid.The sample molecule was doubly ionized using 800 nm or 400 nm, 100 fs laser pulses from a Ti:sapphire laser system in the intensity region of 30～300 TW/cm~2.The doubly charged molecular ion decomposes into ion pairs.The ion pairs are detected in coincidence with the ion velocity imaging time of flight(TOF) spectrometer[1].This spectrometer is equipped with a position sensitive detector with multi-hit capability and allows us to extract 3D momentum of the individual ions based on the information of their hit positions and TOF.According to the photoion-photoion coincidence map,the correlation between the ion pair was found and it indicated that the ion pair come from the two body dissociation process of doubly ionized molecule. Another point is that the resulted fragments have the high kinetic energy, which implies the Coulomb explosion happened. The measured ion-ion coincidence results indicate that doubly charged formic acid ion decomposes from different two body dissociation channels into COH~+-OH~+,COH~+-O~+,CO~+-OH~+,and CO~+-O~+,and among them,the COH~+-OH~+ pair formation is the dominant channel.The ion pair counts from different channels were extracted from the phtoion-phtoion coincidence map,the ratio of ion pair count and total parent count was calculated.The ion yield ratio for different channels indicates that CO~+-O~+ ion pair increase much faster than others and the H dissociate much easier for high laser intensity.We can guess the CO~+-O~+ channel will be the dominant channel at much higher laser intensity.For investigating the mechanism of this double ionization process, circularly polarization was used in our experiment.It is reported that the ratio of the double to single ionization rates will be suppressed by the circularly polarized laser pulses if the double ionization process is non-sequential mechanism.The rate observed is not suppressed in comparison with that by the linearly polarized laser pulses at the same laser field strength?.The same results are observed using higher laser intensity.This indicates that sequential multiphoton double ionization is dominant in the present conditions of the laser fields for formic acid.The kinetic energy distributions for COH~+ and OH~+ detected in coincidence are shown in the.The COH~+ peaked at～1.8 eV and the OH~+ peaked at～3.0 eV,respectively.The kinetic energy ratio for COH~+ and OH~+ is accordant well with mass ratio of OH~+ and COH~+.The kinetic energy of the ion pair nearly does not change as a function of laser power density in a range of our experiment.The distance,R,between the ion pair before the Coulomb explosion can be estimated using the momentum release values.The kinetic energy release of the dissociating ions is inversely proportional to the internuclear separation.The value of R is about 5.66 au estimated by this simple coulomb explosion model. The angular distribution of the COH~+-OH~+ dissociation axis relative to the laser polarization axis is obtained.The distribution does not depend much on the laser power density in our energy range and mainly along with the laser polarization axis.We attribute this phenomenon to the alignment effect of polarized laser.Effusion beam was used in our experiments,so the geometric alignment should be dominated.Another possibility is the dynamics alignment of the single charge ions before double ionization happened.In summary,dissociative double ionization of formic acid in intense femtosecond laser was researched in this thesis.We first found the double ionization and two body dissociation of this molecule,and got the information about different dissociate channels,momentum angular distribution,kinetic energy.
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