Formative period in the x-ray-induced photodissociation of organic molecules

Absorption of x-ray photons by atomic inner shells of light-element organics and biomolecules often leads to formation of dicationic electronic states and to molecular fragmentation. We investigated the x-ray-induced dissociation landscape of a representative medium-sized organic molecule, thiophene, by femtosecond x-ray pulses from the Super Photon Ring-8 GeV (SPring-8) Angstrom Compact Free-Electron Laser (SACLA). Holes, created in the sulfur $2p$ orbital by photoemission, were filled by the Auger process that created dicationic molecular states within a broad range of internal energies—a starting point particular to x-ray-induced dynamics. The evolution of the ionized molecules was monitored by a pump-probe experiment using a near-infrared (800 nm) laser pulse. Ion-ion coincidence and ion momentum analysis reveals enhanced yields of ionic fragments from multibody breakup of the ring, attributed to additional ionization of the highly excited fraction of the dicationic parent molecular states. The transient nature of the enhancement and its decay with about a 160-fs time constant indicate formation of an open-ring parent geometry and the statistical survival time of the parent species before the dissociation events. By probing specific Auger final states of transient, highly excited nature by near-infrared light, we demonstrate how pump-probe signatures can be related to the key features in dynamics during the early period of the x-ray-induced damage of organic molecules and biomolecules.

Figure 1

Schematic of the experiment.

Figure 2

Schematic depiction of the processes involved in the x-ray-pump–NIR-probe experiment of thiophene following S $2p$ core ionization. The x-ray-induced transitions and evolution of the system on the dicationic ($M^{2+}$) potential-energy surfaces are marked in yellow, while the absorption of the NIR radiation lifting the dynamics onto the ($M^{3+}$) surfaces is shown in red. The evolution of molecular geometry is shown at a few points in time. The actual number of the surfaces is much larger than the few sketched.

Figure 3

Photoion-photoion coincidence (PIPICO) map of the dissociation of the thiophene dication following the S $2p$ core ionization. The time-of-flight ranges of ionic species are indicated by the row and column of labels near the corresponding axis. The regions surrounded by the red line indicate the ion pairs for which strong pump-probe effects were observed. The region within the dashed blue line covers the two-body processes.

Figure 4

Population of the transient linear geometry of the parent dication during molecular dynamics according to the SCC-DFTB simulations shown as a function of time and dependent on the internal energy of the molecule. Red curves mark the times of reaching the half-maximum population.

Figure 5

(a) Measured ion-pair yield (${\mathrm{CSH}}^{+}$, ${\mathrm{C}}_3{\mathrm{H}}_3^{+}$) from the strongest two-body pathway and (b) multibody fragmentation ion-pair yield (${\mathrm{S}}^{+}$, ${\mathrm{C}}_2{\mathrm{H}}_n^{+}$), per 1000 pump-probe events. (c) Total multibody ion-pair yield from the areas encircled by the red lines in Fig. 3. Dashed lines are the yield values from a FEL-only measurement, and the blue curves are fits of the model function of (3) to the data. The shaded area in (c) shows the internal-energy-averaged population of the transient linear geometry from the SCC-DFTB simulations, convoluted by the temporal instrument function. Data error bars are based on Poisson statistics of ion-pair counting.

Figure 6

Top: momentum distribution of ${\mathrm{S}}^{+}$ ions as a function of the NIR delay, after subtracting the averaged NIR-early momentum distribution ($\text{NIR}\text{delay}<-300$ fs). The color scale is set relative to the peak value of the distribution function over the shown range of ion momenta and pulse delays. Bottom: momentum distribution of the ${\mathrm{S}}^{+}$ ions in the transient (between the red lines in the top panel) and off-transient regions of the NIR delay. Dots with a fitted Gaussian function are the difference of the two curves. The ion kinetic energies corresponding to the peak maxima are marked.

Figure 7

Momentum distribution of $S^{+}$ ions in various NIR-delay ranges and in the FEL-only measurement (black line). The range of the transient effects was set to the 20-to-190 fs FEL-NIR-pulse delay (red curve) and also the pre-transient (cyan curve), after-transient (dashed pink) and long FEL-delay (dashed yellow) ranges are shown.

Figure 8

Combined ion-pair yield of all two-body channels, as a function of NIR-delay. Dashed line is the yield value from a FEL-only measurement.

Figure 9

Ion-pair yields of multi-body fragmentation events, as a function of NIR-delay. Dashed lines are yield values from a FEL-only measurement.

Figure 10

Ion triplet yields of some prominent multi-body channels, as a function of NIR-delays. Dashed lines are the yield values from a FEL-only measurement.