Ultrafast Extreme Ultraviolet Induced Isomerization of Acetylene Cations

Ultrafast isomerization of acetylene cations ($[\mathrm{HC}=\mathrm{CH}{]}^{+}$) in the low-lying excited $A{}^2\Sigma _g{}^{+}$ state, populated by the absorption of extreme ultraviolet (XUV) photons (38 eV), has been observed at the Free Electron Laser in Hamburg, (FLASH). Recording coincident fragments ${\mathrm{C}}^{+}+\mathrm{CH}_2{}^{+}$ as a function of time between XUV-pump and -probe pulses, generated by a split-mirror device, we find an isomerization time of $52\pm 15\mathrm{fs}$ in a kinetic energy release (KER) window of $5.8<\mathrm{KER}<8\mathrm{eV}$, providing clear evidence for the existence of a fast, nonradiative decay channel.

Figure 1

Energy level diagrams for the acetylene (left) and vinylidene (right) cations as well as sketches of the expected isomerization-dissociation processes (middle). Numbers indicate possible time-sequenced stages of neutral acetylene (1), acetylene cation (2), transient states to vinylidene (3), ionization of the vinylidene cation (4), and Coulomb explosion of the vinylidene dication (5), respectively. Vertical arrows indicate photon absorptions. Energy levels were taken from Refs. 13, 18, 23.

Figure 2

Time of flight (TOF) of the first fragment versus TOF of second one. The data are integrated over all time delays $|\Delta t|<360\mathrm{fs}$.

Figure 3

Kinetic energy release (KER) and time-delay spectra for coincident $\mathrm{CH}_2{}^{+}+{\mathrm{C}}^{+}$ fragments. (a) KER distribution integrated at all delay times. Full circles, present data; solid line, strong IR laser pulses [26]. (b) KER spectra at different delay times integrated over $\pm 50\mathrm{fs}$. (c) Yield of $\mathrm{CH}_2{}^{+}+{\mathrm{C}}^{+}$ coincident fragments (solid red line) for $5.8<E_{\mathrm{KER}}<8\mathrm{eV}$ in (a). Dashed green line, fit of exponential function; dotted blue line, autocorrelation trace for coincident ${\mathrm{C}}^{2+}+{\mathrm{C}}^{2+}$ fragments (right $y$ axis).