Hydrogen and fluorine migration in photo-double-ionization of 1,1-difluoroethylene (1,1-${\mathrm{C}}_2$${\mathrm{H}}_2$${\mathrm{F}}_2$) near and above threshold

We have studied the nondissociative and dissociative photo-double-ionization of 1,1-difluoroethylene using single photons of energies ranging from 40 to 70 eV. Applying a coincident electron-ion three-dimensional momentum imaging technique, kinematically complete measurements have been achieved. We present the branching ratios of the six reaction channels identified in the experiment. Electron-ion energy maps and relative electron emission angles are used to distinguish between direct and indirect photo-double-ionization mechanisms at a few different photon energies. The influence of selection and propensity rules is discussed. Threshold energies of double ionization are extracted from the sum of the kinetic energies of the electrons, which hint to the involvement of different manifolds of states. The dissociative ionization channels with two ionic fragments are explored in detail by measuring the kinetic energy release of the fragment ions, sum of the kinetic energies, as well as the energy sharing of the two emitted electrons. We investigate the migration of hydrogen and fluorine atoms and compare the experimental results to the photo-double-ionization of centrosymmetric linear and planar hydrocarbons (${\mathrm{C}}_2$${\mathrm{H}}_2$ and ${\mathrm{C}}_2$${\mathrm{H}}_4$) whenever possible.

Figure 1

Photoion-photoion coincidence (PIPICO) spectrum used to identify and separate the different breakup channels of the PDI of 1,1-${\mathrm{C}}_2$${\mathrm{H}}_2$${\mathrm{F}}_2$ using single photons at 60 eV. The PIPICO spectrum shows the ion-pair yield as function of TOF of the first and the second recoil ion. Channels with two ionic fragments for which we have the kinematically complete information are labeled in the upper half. The breakup channels producing two protons and a neutral fragment (H${}^{+}$ + H${}^{+}$ + ${\mathrm{C}}_2$${\mathrm{F}}_2$) or two or more neutral or charged ions which are lost in our detection scheme (H${}^{+}$ + H${}^{+}$ + L) are marked in the lower half. The dominant many-body breakup channels are also marked as CH${}_2^{+}$ + CF${}^{+}$ + F, CH${}^{+}$ + CHF${}^{+}$ + F, and C${}^{+}$ + C${\mathrm{H}}_2$F${}^{+}$ + F.

Figure 2

Branching ratios of different channels from the photo-double-ionization of the 1,1-${\mathrm{C}}_2$${\mathrm{H}}_2$${\mathrm{F}}_2$ molecules using linearly polarized photons of 40- to 70-eV energy. (a) Branching ratio of different two ionic fragments channels. (b) Sum of the branching ratio of the channels involving the cleavage of the central C=C bond (black solid squares) and rest of the dissociative ionization channels (red solid circles). (c) Branching ratio of the long-lived dications (black solid circles).

Figure 3

Photo-double-ionization yield as a function of threshold energy (defined as $E_{\mathrm{sum}}$ subtracted from the photon energy $E_{\gamma }$) of the different channels of 1,1-${\mathrm{C}}_2$${\mathrm{H}}_2$${\mathrm{F}}_2$ using 40- (open symbols) and 60- eV (solid symbols) photons. The error bars indicate the statistical uncertainty in the data. The 40-eV spectra are scaled to match the 60-eV spectra at the threshold energy where the first one peaks. For branching ratio of the channels, see Table 1.

Figure 4

Yield as a function of cosine of the angle ${\theta }_{12}$ between the momenta of the expelled electrons associated with NDI and different DI channels measured for the PDI of 1,1-${\mathrm{C}}_2$${\mathrm{H}}_2$${\mathrm{F}}_2$ at 60-eV photon energy. The lines (whose colors match the symbols' color of the corresponding channels) are just to guide the eye.

Figure 5

(a) Density plot of the kinetic energies of the two electrons measured in coincidence with the dications (${\mathrm{C}}_2$${\mathrm{H}}_2$F${}_2^{2+}$). (b) Yield as a function of the cosine of the relative angle ${\theta }_{12}$ between the two electrons' momenta and the energy sharing. Both (a) and (b) were measured using a photon energy of 40 eV. (c), (d) Same as (a) and (b), but for 60-eV photon energy. The regions A, B, and C in panel (c) are used later on to identify the different ionization processes.

Figure 6

Schematic drawing of the Auger decay following the PDI of 1,1-${\mathrm{C}}_2$${\mathrm{H}}_2$${\mathrm{F}}_2$ at a photon energy of 60 eV. (a) Photoionization of the lowest-lying $\tilde{I}$ state of the neutral molecule (adapted from [29]). (b) Recombination of the vacancy and emission of the Auger electron in the cation. (c) Resulting excited dication. Note that the $\tilde{I}$ state represents a band of lines with different intensities that have different single IPs. The lines corresponding to different ionic states in (b) and (c) are shifted down (by an unknown value) compared to that of the neutral molecule in (a) due to the change of the effective potential after the ionization of the first electron.

Figure 7

Yield as a function of cosine of the angle ${\theta }_{12}$ between the momenta of two emitted electrons, with equal energy sharing, measured in coincidence with the dications (${\mathrm{C}}_2$${\mathrm{H}}_2$F${}_2^{2+}$) from the nondissociative ionization of 1,1-${\mathrm{C}}_2$${\mathrm{H}}_2$${\mathrm{F}}_2$ for different photon energies (40–70 eV).

Figure 8

Top row: Polar angle ($\theta$) of electrons, with respect to the polarization of light, measured in coincidence with the dications (${\mathrm{C}}_2$${\mathrm{H}}_2$F${}_2^{2+}$) from the nondissociative ionization of 1,1-${\mathrm{C}}_2$${\mathrm{H}}_2$${\mathrm{F}}_2$ using 60-eV photon energy for (a) Auger electron (i.e., the electrons with low kinetic energy), (b) photoelectron (i.e., the electrons with high kinetic energy) both from region A in Fig. 5, and (c) photoelectrons from region B in Fig. 5 (i.e., both of the electrons have similar range of kinetic energy). Bottom rows: Plot of the cosine of the relative angle ($\theta$${}_{12}$) between the momenta of two electrons for (d) indirect double ionization [i.e., molecular Auger decay region A marked in the energy correlation map in Fig. 5, i.e., islands between 18 and 25 eV], (e) direct double ionization [region B along the major diagonal in the Fig. 5], (f) direct double ionization with equal energy sharing [i.e. a narrow region in the middle of the diagonal in Fig. 5], and (g) mix of indirect and direct double ionization with asymmetric energy sharing between electrons [islands between 25 and 31 eV from region C in Fig. 5]. The first electron always goes to the right as indicated with the arrow. The lengths of the arrows correspond to a very asymmetric electron energy sharing in panels (d) and (g), and a rather equal energy sharing in the (e) and (f) panels.

Figure 9

Electron energy correlation maps, shown as density plots as a function of kinetic energy of two emitted electrons ($E_1$, $E_2$) for DI channels using 40-, 50-, and 60-eV photon energies, from top to bottom row, respectively: first column CF${}^{+}$ + C${\mathrm{H}}_2$F${}^{+}$, second column CH${}_2^{+}$ + CF${}_2^{+}$, third column H${}^{+}$ + ${\mathrm{C}}_2$HF${}_2^{+}$, and fourth column HF${}^{+}$ + ${\mathrm{C}}_2$HF${}^{+}$ channels.

Figure 10

Yield as a function of KER and energy sharing ($E_1$/$E_{\mathrm{sum}}$) for the major DI channels using 40-, 50-, and 60-eV photon energies, from top to bottom row, respectively: first column CF${}^{+}$ + C${\mathrm{H}}_2$F${}^{+}$, second column CH${}_2^{+}$ + CF${}_2^{+}$, third column H${}^{+}$ + ${\mathrm{C}}_2$HF${}_2^{+}$, and fourth column HF${}^{+}$ + ${\mathrm{C}}_2$HF${}^{+}$ channels.

Figure 11

Electron-ion energy maps of the PDI of 1,1-${\mathrm{C}}_2$${\mathrm{H}}_2$${\mathrm{F}}_2$ shown as a density plot as a function of KER and $E_{\mathrm{sum}}$ for DI channels using 60-eV photon energy: (a) CF${}^{+}$ + C${\mathrm{H}}_2$F${}^{+}$, (b) CH${}_2^{+}$ + CF${}_2^{+}$, (c) H${}^{+}$ + ${\mathrm{C}}_2$HF${}_2^{+}$, and (d) HF${}^{+}$ + ${\mathrm{C}}_2$HF${}^{+}$.

Figure 12

(a) Electron energy correlation map (similar to Fig. 9), (b) PDI yield as a function of KER and electron energy sharing (similar to Fig. 10), and (c) PDI yield as a function of KER and $E_{\mathrm{sum}}$ (similar to Fig. 11), all panels for the H${}_2^{+}$ + ${\mathrm{C}}_2$F${}_2^{+}$ channel at 60-eV photon energy.