Explosion, ion acceleration, and molecular fragmentation of methane clusters in the pulsed beam of a free-electron laser

X-ray lasers offer new possibilities for creating and probing extreme states of matter. We used intense and short x-ray pulses from the FLASH soft x-ray laser to trigger the explosions of CH${}_4$ and CD${}_4$ molecules and their clusters. The results show that the explosion dynamics depends on cluster size and indicate a transition from Coulomb explosion to hydrodynamic expansion in larger clusters. The explosion of CH${}_4$ and CD${}_4$ clusters shows a strong isotope effect: The heavier deuterons acquire higher kinetic energies than the lighter protons. This may be due to an extended inertial confinement of deuterons vs. protons near a rapidly charging cluster core during exposure.

Figure 1

Experimental setup and mass over charge spectra for methane molecules and clusters irradiated by single laser pulses. (a) Clusters produced through supersonic expansion of gas are exposed to 15-fs-long soft x-ray pulses at $\lambda =13.5$ nm, with an average intensity of $5\times {10}^{14}$ W/cm${}^2$. Positively charged ions from cluster fragmentation are accelerated by an electric field between a repeller plate $V_1=1000$ V and the grounded field-free drift tube, and are detected by a multichannel plate (MCP) detector $V_2=-2600$ V. (b) The spectrum for CH${}_4$ molecules shows a weak H${}^{+}$ signal, signature peaks for typical fragments CH${}_n{}^{+}$, and the water background. The insert shows a zoom into the region from $m$/$q$ $=$ 12 to 18. (c) and (d) Spectra of methane clusters show a significant increase in the H${}^{+}$ and C${}^{+}$ peaks. The signal of CH${}_5{}^{+}$, a sign of molecular recombination, reaches a maximum for $\langle N\rangle =20000$ molecules. Note the different scales on the vertical axes.

Figure 2

Ion yields from fragmented methane clusters as a function of backing pressure and cluster size. (a) Ion yields for H${}^{+}$ and D${}^{+}$ from pure and deuterated methane are increasing as a function of gas jet pressure and number of molecules in the cluster. The FEL intensity is $5\times {10}^{14}$ W/cm${}^2$. (b) Under the same conditions, ion yields for the molecular fragments CH${}_5{}^{+}$ and CD${}_5{}^{+}$ reach a maximum at the cluster size of $\langle N\rangle =20000$ molecules (radius of 6.7 nm).

Figure 3

Normalized ion signals as a function of cluster size. (a) Contribution of different ion yields, normalized to the total ion yield is shown for the CH${}_4$ experiment. The CH${}_n{}^{+}$ and C${}_2$H${}_n{}^{+}$ data contain all adducts with one or two carbon atoms (except for CH${}_5{}^{+}$ shown separately). The data points at close to zero size correspond to the single-molecule (no clustering) experiment. Higher charge states of carbon have a contribution below 20$\%$ of the single ionized carbon. (b) Ion yield ratio for higher charge states of carbon from pure methane clusters (C${}^{2+}$, C${}^{3+}$, C${}^{4+}$) to the singly charged carbon ion (C${}^{+}$). The carbon yields from CD${}_4$ clusters show similar behavior.

Figure 4

Highest detected kinetic energy of hydrogen and deuterium ions from molecular clusters. (a) The maximum detected kinetic energy, with a confidence level (CL) of 95$\%$, for light ions (H${}^{+}$ and D${}^{+}$) from CH${}_4$ and CD${}_4$ clusters, is shown as a function of cluster size. The average pulse energy is 43 $\mu$J (intensity $5\times {10}^{14}$ W/cm${}^2$). (b), (c), and (d) Kinetic energy of H${}^{+}$ and D${}^{+}$ ions from pure and deuterated methane as a function of pulse energy for fixed cluster sizes of (b) 10 000 molecules, (c) 30 000 molecules, and (d) 80 000 molecules. Deuterium ions from larger clusters (6.7-nm radius and higher) tend to be more energetic than hydrogen ions. The panel on the right shows the shape of the H${}^{+}$ and D${}^{+}$ peaks in the TOF data. The highest signal values of the two peaks were aligned by moving them to a reference time $t=0$. Pulse intensity is $5\times {10}^{14}$ W/cm${}^2$, and the arrows show the time where signal is detected above background at 95$\%$ confidence level. The kinetic energy of the particles scales with ($\Delta$t)${}^2$/mass.