# A Reaction Microscope for AMO Science at Shanghai Soft X-ray Free-Electron Laser Facility
## 3 Result and Discussion
### Coulomb Explosion Experiment of Oxygen Molecules  
In the oxygen Coulomb explosion experiment, a femtosecond laser with \(1.0\times 10^{15}\,\mathrm{W/cm^{2}}\)-intensity and a polarization direction along the y axis interacted with the oxygen molecules. Due to the larger momentum from the Coulomb explosion, the accelerating electric field was set at 15 V/cm to ensure that all ions could be detected by the detector. The ion fragments produced from both supersonic jet and the background gas were detected and the TOF spectra are shown in Figure 5, which shows that the composition of background gas was mainly water and hydrogen molecules. In contrast to the peaks of \(H_{2}O^{+}\), \(H_{2}^{+}\) and \(H^{+}\) originating from the background, the \(O_{2}^{+}\) peak is obviously sharper due to the different thermal properties between the warm residual gas and the cold jet. The \(O^{2+}\) peak is wider than \(O_{2}^{+}\) peak, which indicates that a Coulomb explosion resulting in a larger kinetic energy occurred. At the TOF position, corresponding to a charge/mass ratio of 16 a.m.u./e, a narrow peak and a wide peak, which correspond to the \(O_{2}^{2+}\) ions from the ionization processes and the \(O^{+}\) ions from the Coulomb explosion processes, respectively, are overlapped. Figure 4: **(a)** TOF spectrum of a photo-ion for single ionization of oxygen; **(b)** TOF correlation spectrum of electron and \(O_{2}^{+}\) ion ; **(c)** sum of momentum of electron and \(O_{2}^{+}\) ion; **(d)** radius as a function of TOF for electrons. Figure 6a presents the TOF correlation between the first ion and the second ion and five bright curves correspond to five different Coulomb explosion channels coming from two-to-five-fold charged oxygen ions. It can be seen from Figure 6b that most of the reaction products were within the range of the detector, and several distinctly separated rings appeared as a fingerprint of the Coulomb explosion, occurring in the polarization direction mostly. By calculating the momentum of all kinds of ions and sifting through the momentum conservation conditions, five Coulomb explosion channels were obtained including (1,1), (1,2), (1,3), (2,2) and (2,3). After the ion matching and data selection, more information on specific channel could be further analyzed. The released momenta were converted to their released kinetic energies. Figure 7 presents the peak distributions of kinetic energy releases for the four most distinct channels of (1,1), (1,2), (2,2) and (2,3), the peak positions of which were 11.3 eV, 20.5 eV, 35.1 eV and 49.6 eV, respectively. Compared with the previous results listed in Table 2, the measured KERs were generally smaller than those from electron and ion impact experiments, due to the stretched molecular bond length in the laser field, which is also consistent with other laser-induced fragmentation experiments [46; 47]. On the other hand, [PERSON] et al. found that the KERs become larger with increasing laser intensity [47]. Figure 5: TOF spectrum of recoil ions. Figure 6: (**a**) Photoion–photoion coincidence (PIPICO) spectrum. The curve (m,n) represents the Coulomb explosion channels \(O_{2}^{(m+n)+}\to O^{m+}+O^{n+}\). (**b**) TOF versus ion position in the y direction (the direction of the laser’s polarization). The larger KER values indicated a higher laser intensity in our experiment. For example, the measured KER of 11.3 eV from the channel (1,1) was relatively larger compared with most KER peaks obtained in the 8-fs laser experiment [48] and smaller compared with the results from the electron impact experiment using Doppler-free spectroscopy [49]. For the Coulomb explosion process of \(O_{2}^{2+}\), extensive studies have been performed, both theoretically and experimentally [46; 54]. Explosions of the parent dications are generally considered to occur near the equilibrium internuclear distance of the neutral molecule [50].