## 3 Results and Discussion  
### Time-of-Flight Mass Spectrum and DC-Sliced Images  
Figure 1 shows the time-of-flight (TOF) mass spectrum of CHBrCl\({}_{2}\) molecules irradiated by the 800 nm femtosecond laser pulses with the laser intensity of 6.0 \(\times\) 10\({}^{13}\) (a) and 1.5 \(\times\) 10\({}^{14}\) W/cm\({}^{2}\) (b), respectively. The background ions H\({}^{+}\), H\({}_{2}\)O\({}^{+}\), N\({}_{2}\)+, and Ar\({}^{+}\) (seeding air) were ignored, and the ions C\({}^{+}\), Cl\({}^{+}\), CHCl\({}^{+}\), Br\({}^{+}\), CHCl\({}_{2}\)+, CHBr\({}^{+}\), and CHBrCl\({}^{+}\) and molecular ion CHBrCl\({}_{2}\)+ have been observed. Due to the isotopes of \({}^{35}\)Cl/\({}^{37}\)Cl and \({}^{79}\)Br/\({}^{81}\)Br, all the fragment ions containing Cl and Br atoms exhibited multi-peak structures. Considering the laser pulse duration (FWHM = 70 fs) in our experiment, these fragmentations are mainly attributed to the dissociation of parent ions. The weak signal of CHBrCl\({}_{2}\)+ demonstrates that its chemical property is unstable. Thus, it is prone to be photolyzed in the intense laser field. We focused on the rupture of the C-Br and C-Cl bonds. Hence, the relationship between ions' yields and laser power was analyzed, which is shown in Figure 2. The yield ratio of Cl\({}^{+}\)/CHBrCl\({}^{+}\) increased from 0.03 to 2, but that of Br\({}^{+}\)/CHCl\({}_{2}\)+ increased slowly and remained constant at 1. This indicates that laser intensity influences the above dissociation pathways. The related discussion is carried out in the next section.  
To further investigate the related dissociation processes, the dc-sliced imaging technology was utilized to measure the kinetic energy releases (KERs) and angular distributions of fragment ions. Figure 3 presents the pseudo-color sliced images of the observed ions: (a) Cl\({}^{+}\), (b) Br\({}^{+}\), (c) CHBrCl\({}^{+}\), and (d) CHCl\({}_{2}\)+, with the laser intensity of 1.5 \(\times\) 10\({}^{14}\) W/cm\({}^{2}\). The corresponding velocity distributions were extracted, where the experimental data were fitted with the multi-peak Gaussian function, and the related KER values were labeled in eV. In Figure 3a, the kinetic energy distribution of Cl\({}^{+}\) ions exhibited two peaks, which were located at 0.25 and 2.7 eV, respectively. In Figure 3b, the fragment Br\({}^{+}\) ion exhibited two peaks, with the corresponding values at 0.18 and 1.45 eV. However, the high kinetic energy peak of CHBrCl\({}^{+}\) ions almost vanished, and there was one peak at 0.13 eV in Figure 3c. In Figure 3d, the fragment CHCl\({}_{2}\)+ ion had two peaks at 0.17 and 1.39 eV, respectively.  
Figure 1: TOF mass spectrum of CHBrCl\({}_{2}\) molecules with the laser intensities of **(a)** 6.0 \(\times\) 10\({}^{13}\) (blue solid line) and **(b)** 1.5 \(\times\) 10\({}^{14}\) W/cm\({}^{2}\) (red solid line), respectively.  
Figure 3: The velocity distributions of ions (**a**) Cl\({}^{+}\), (**b**) Br\({}^{+}\), (**c**) CHBrCl\({}^{+}\), and (**d**) CHCl\({}_{2}\)\({}^{+}\). The black circles are experimental data. The green dashed lines are the fitting peaks, and the red solid lines are simulated results with multipeak Gaussian functions. The insert figure is the sliced images of the relevant fragment ions at the intensity of \(1.5\times 10^{14}\) W/cm\({}^{2}\), where the black double-headed arrow represents the laser polarization direction.  
Figure 2: The ions’ yield ratio of Cl\({}^{+}\)/CHBrCl\({}^{+}\) (blue solid block dotted line) and Br\({}^{+}\)/CHCl\({}_{2}\)\({}^{+}\) (red hollow block dotted line) versus laser power.