Delayed fragmentation of ethylene and allene induced by electron impact

We experimentally investigated the delayed fragmentation processes of ethylene and allene dications induced by 300-eV electrons with the cold target recoil ion momentum spectroscopy in an observation window up to ∼2 μs. For each molecule, several distinct pathways involving prompt or delayed fragmentations were observed and relative branching ratios among them were analyzed. According to the time-of-flight coincidence measurements of corresponding recoil ionic fragments, the survival times and thus lifetimes from nanoseconds to microseconds of different delayed-dissociative precursor dications were retrieved. Besides the two-body channels, i.e., ${\mathrm{H}}^{+}+{\mathrm{C}}_2{{\mathrm{H}}_3}^{+}$ and ${\mathrm{H}}^{+}+{\mathrm{C}}_3{{\mathrm{H}}_3}^{+}$, the delayed deprotonation was also observed in many-body channels, i.e., $\mathrm{H}+{\mathrm{H}}^{+}+{\mathrm{C}}_n{{\mathrm{H}}_2}^{+}$ ($n=2$, 3) and $2\mathrm{H}/{\mathrm{H}}_2+{\mathrm{H}}^{+}+{\mathrm{C}}_3{\mathrm{H}}^{+}$. The lifetimes of the metastable intermediate diacations ${\mathrm{C}}_n{{\mathrm{H}}_3}^{2+}$ and ${\mathrm{C}}_3{{\mathrm{H}}_2}^{2+}$ were estimated.

Figure 1

Schematic of the fly traces of ionic fragments in the TOF windows. $l$ represents the flight distance between collision center and PSD. The acceleration region and drift region are 100 and 200 mm long, respectively. $y$ axis is defined by the electron beam, and $x$ axis by the molecular beam, $z$ corresponds to the TOF ($t$) axis.

Figure 2

The measured coincidence time-of-flight spectra with (a) ${\mathrm{C}}_2{\mathrm{H}}_4$ and (b) ${\mathrm{C}}_3{\mathrm{H}}_4$, respectively. The black, red, and yellow dots represent events with KER = 0 and delay times of $n$ 200 ns with $n=1$, 2, 3…. Panels (c) and (d) are the simulated coincidence time-of-flight spectra of various delayed fragmentation channel for ${\mathrm{C}}_2{\mathrm{H}}_4$ and ${\mathrm{C}}_3{\mathrm{H}}_4$, respectively.

Figure 3

Semilogarithmic histogram of the survival time of delayed dissociation decay of metastable dication for ethylene induced by 300-eV electrons. Corresponding exponential fits are shown by red solid lines. “(M)” is the abbreviation for “metastable,” which represents the metastable parent ion corresponding to the delayed dissociation.

Figure 4

Semilogarithmic histogram of the survival time of delayed dissociation decay of metastable dication for allene induced by 300-eV electrons. Corresponding exponential fits are shown by red solid lines.

Figure 5

Histogram diagram of branching ratios for prompt and delayed fragmentation channels of ethylene and allene dications, respectively.