## 3 Results and Discussion  
### Time-of-Flight Mass Spectrum and DC-Sliced Images  
Figure 1 shows the TOF mass spectra of molecule CF\({}_{2}\)Br\({}_{2}\) irradiated by the 800 nm (a) and 400 nm (b) laser fields with intensities of 2.5 \(\times\) 10\({}^{14}\) and 4.0 \(\times\) 10\({}^{13}\) W/cm\({}^{2}\), respectively. Both in Figure 1 a,b, the ions C\({}^{+}\), F\({}^{+}\), CF\({}^{+}\), Ar\({}^{+}\) (carrier gas), CF\({}_{2}\)\({}^{+}\), Br\({}^{+}\) and CF\({}_{2}\)Br\({}^{+}\) can be observed distinctly. However, the signals of ion Br\({}_{2}\)\({}^{+}\) and CF\({}_{2}\)Br\({}_{2}\)\({}^{+}\) are very weak and only appear in the 400 nm spectrum. The parent ion CF\({}_{2}\)Br\({}_{2}\)\({}^{+}\) in Figure 1b indicates that parent ions CF\({}_{2}\)Br\({}_{2}\)\({}^{+}\) are prone to dissociate into fragments in the 800 nm rather than 400 nm laser field. The ions Br\({}_{2}\)\({}^{+}\) in Figure 1b indicates that a new reaction channel is involved in our experimental conditions (See Supplementary Materials, Figure S2), and the details will be discussed in the following section. On the other hand, the CF\({}_{2}\)Br\({}^{+}\), Br\({}^{+}\), CF\({}_{2}\)\({}^{+}\) and CF\({}^{+}\) ions yields are much higher in both cases. This suggest that the major dissociation processes originate from the cleavage of C-Br bond. To support this assumption, the relaxed force constants of the singly charged ion CF\({}_{2}\)Br\({}_{2}\)\({}^{+}\) have been calculated. In the compliance constants matrix, the higher is the value of diagonal element, the weaker is the corresponding chemical bond. Namely, the reciprocals of these diagonal elements, which are called relaxed force constants, can represent the bond strength (See Supplementary Materials, Table S1 and Figure S3a). The C-F bond in parent ion CF\({}_{2}\)Br\({}_{2}\)\({}^{+}\) is much stronger than C-Br bond. Therefore, it is reasonable that the cleavage of C-Br bond is dominant in the dissociation processes. In next section, we focus on the C-Br bond cleavage.  
In order to accurately assign the dissociation channels, the raw dc-sliced images were obtained, and the corresponding kinetic energy releases were also extracted. As shown in  
Figure 1: Time-of-flight (TOF) mass spectra of molecular CF\({}_{2}\)Br\({}_{2}\) at 2.5 \(\times\) 10\({}^{14}\) W/cm\({}^{2}\) in 800 nm laser field (**a**) and 4.0 \(\times\) 10\({}^{13}\) W/cm\({}^{2}\) in 400 nm laser field (**b**), respectively. The inset shows the signal of Br\({}_{2}\)\({}^{+}\) and CF\({}_{2}\)Br\({}^{+}\) ions magnified thirty times.  
Figure 2, the velocity distribution of fragment ions is similar to the mountain, and each peak represents the most probable velocity distribution of fragments from the dissociation process. We used Gaussian function to fit the distribution, and the peak values of the each fitted function were obtained. According to the Equation (1), the velocities were converted into kinetic energy (E\({}_{\rm{K}}\)) of fragments, which were labelled in units of eV, where m and V represent the mass and velocity of fragments.  
\[\rm{E_{k}}=\frac{1}{2}mV^{2} \tag{1}\]  
In Figure 2a, the velocity distribution of ion \(\rm{CF_{2}}^{+}\) in the 800 nm laser field exhibits two peaks with the kinetic energy values at 0.11 and 2.89 eV, and in 400 nm laser field, the kinetic energy peaks are 0.02 and 0.76 eV. In Figure 2b, the Br\({}^{+}\) ions exhibit two peaks in 800 nm and 400 nm laser fields, which locate at (0.13, 2.36) eV and (0.14, 2.53) eV, respectively. In Figure 2c, the only kinetic energy peak of \(\rm{CF_{2}Br^{+}}\) ion in the 800 nm is at 0.01 eV, however, there are two peaks at 0.02 and 0.75 eV in the 400 nm laser field, respectively. Interestingly, the fragment Br\({}_{2}\)\({}^{+}\) ion appears only in the presence of the 400 nm pulses, and the kinetic energy peak is located at 0.02 eV (Figure 2d). The existence of the fragment Br\({}_{2}\)\({}^{+}\) ion indicates that laser wavelength has influence on the reaction pathway. We will explain these peaks in the following section detailedly.