## 3 Results and Discussion
### The Excited States and Angular Distribution of Fragment Ions  
Besides the information about the kinetic energy for fragments, the angular distribution is another important factor to analyze the reaction channels. The angular distribution for fragment ions was obtained from the sliced images by integrating the signal over the pixel with the same kinetic energy at different angles in polar coordinate (See Figure 6). According to the above estimated channels, the precursor ions CF\({}_{2}\)Br\({}_{2}\)\({}^{+}\) should dissociate into the fragment ion CF\({}_{2}\)Br\({}^{+}\) or Br\({}^{+}\), as shown in channels one or two. It is likely that similar angular distribution structure appears in both cases, however, there are remarkable differences on the angular distribution structure between fragment ion CF\({}_{2}\)Br\({}^{+}\) and Br\({}^{+}\). The angular distribution of ion CF\({}_{2}\)Br\({}^{+}\) is isotropic, instead, the angular distribution for Br\({}^{+}\) ion is anisotropic along laser polarization vector direction. \begin{table}
\begin{tabular}{c c c c c} \hline  & **C-Br(4)** & **C-Br(5)** & **Br-Br \({}^{*}\)** & \(\angle\)**Br-C-Br** \\ \hline CF\({}_{2}\)Br\({}_{2}\) & 1.96 & 1.96 & 3.24 & 112.0 \\ CF\({}_{2}\)Br\({}_{2}\)\({}^{*}\) & 1.99 & 1.99 & 2.86 & 92.0 \\ TS\({}_{1}\) & 2.57 & 2.57 & 2.30 & 53.2 \\ TS\({}_{2}\) & 2.62 & 2.62 & 2.34 & 53.5 \\ IS & 2.28 & 3.45 & 2.32 & 41.9 \\ \hline \end{tabular} \({}^{*}\) The equilibrium internuclear distance of molecular Br\({}_{2}\) is 2.32 Å (\(G4\) theory).  
\end{table} 
Table 3: The bond lengths of the two C-Br bonds, the distance of the two Br atoms and bond angle of \(\angle\)Br-C-Br for the related channels. To further explore the above reason, the potential energy surfaces (PESs) along the C-Br bond were calculated at the EOMCCSD/cc-pVTZ level. In Figure 7, the G\({}_{0}\), S\({}_{0}\) and S\({}_{1-3}\) represent the ground state and excited states of molecule CF\({}_{2}\)Br\({}_{2}\) and ion CF\({}_{2}\)Br\({}_{2}\)\({}^{+}\), respectively. It is reasonable that the non-adiabatic excitation (the vertical excitation) dominates the ionization process in our experiment (70 fs FWHM). The equilibrium internuclear distance of the C-Br bond in the ground state of molecule CF\({}_{2}\)Br\({}_{2}\) is 1.95 A. Following to the Franck-Condon principle, the molecule CF\({}_{2}\)Br\({}_{2}\) is excited to the high-lying excited states at this internuclear distance. Then the evolution of the vibrational wave packets undergoes on the corresponding PES of the excited states, and eventually dissociates into fragments with kinetic energies. Figure 6: The angular distribution of Br\({}^{+}\) and CF\({}_{2}\)Br\({}^{+}\) ions, in which the black double headed arrow represents the laser polarization direction. Figure 7: Potential energy surfaces (PESs) of ground state of neutral molecule CF\({}_{2}\)Br\({}_{2}\), parent ion CF\({}_{2}\)Br\({}_{2}\)\({}^{+}\) and the excited states of ion CF\({}_{2}\)Br\({}_{2}\)\({}^{+}\), which were calculated at the equation of motion coupled cluster singles-and-doubles (EOMCCSD)/cc-pVTZ level. For channel one, the excitation of neutral molecule CF\({}_{2}\)Br\({}_{2}\) can follow G\({}_{0}\)\(\rightarrow\) S\({}_{1}\) or G\({}_{0}\)\(\rightarrow\) S\({}_{2}\) transition by multiphoton absorption. Because the energy gap (0.4 eV) between the excited state S\({}_{1}\) and S\({}_{2}\) is less than one photon energy (1.55 eV@800 nm). Obviously, the excited state S\({}_{2}\) is a bound state, so the parent ions hardly dissociate along this potential energy surface and the related vibrational wave packet still oscillates in the potential well. Once the molecule is excited to the state S\({}_{1}\), it will dissociate on a short time scale, on the picosecond to sub-picosecond time scale [9]. Further, the rotation period for the precursor state parent ions estimates in tens of picoseconds [29]. Hence, the angular distribution of produced ions CF\({}_{2}\)Br\({}^{+}\) is anisotropic.