Ultrafast Extreme Ultraviolet Induced Isomerization of Acetylene Cations

Ultrafast isomerization of acetylene cations (inverted right perpendicular HC = CH inverted left perpendicular(+)) 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 C(+) + 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 +/- 15 fs in a kinetic energy release (KER) window of 5.8 < KER < 8 eV, providing clear evidence for the existence of a fast, nonradiative decay channel.