Proton Ejection from Molecular Hydride Clusters Exposed to Strong x-Ray Pulses

Clusters consisting of small molecules containing hydrogen do eject fast protons when illuminated by short x-ray pulses. A suitable overall charging of the cluster controlled by the x-ray intensity induces electron migration from the surface to the bulk leading to efficient segregation of the protons and to a globally hindered explosion of the heavy atoms even outside the screened volume. We investigate this peculiar effect systematically along the isoelectronic sequence of methane over ammonia and water to the atomic limit of neon as a reference. In contrast to core-shell systems where the outer shell is sacrificed to reduce radiation damage, the intricate proton dynamics of hydride clusters allows one to keep the entire backbone of heavy atoms intact.

Figure 1

Time evolution for a molecular (solid lines) and an atomic (dashed lines) cluster. The light-gray (yellow) shaded region marks the x-ray pulse ($I={10}^{18}\mathrm{W}/{\mathrm{cm}}^2$) with a duration of 10 fs (FWHM) centered at $t=0$. (a) Charge inside the sphere defined by the outermost carbon ion. (b) Radii in units of the initial radii, with the proton part shown separately [upper (red) solid line]. (c) Average kinetic energy of trapped electrons (temperature of the trapped plasma). The reduction of carbon charging, carbon Coulomb explosion and energy of trapped electrons due to proton ejection is marked by gray arrows.

Figure 2

Kinetic energy of the fastest ion $E_{\max }$, 0.5 ps after the peak of the pulse ($T=10\mathrm{fs}$), versus the average number of photons absorbed per atom/molecule $n_{\omega }=N_{\omega }/N$. Cluster size is $N=689$. The kinetic energies are normalized with the total energy $E_{\mathrm{tot}}=N_{\omega }\hslash \omega$ absorbed by the cluster.

Figure 3

Ratio $K$ of the average kinetic energy of protons and heavy atoms according to Eq. (3) for $X=\mathrm{O},\mathrm{N},\mathrm{C}$ as a function of the x-ray intensity $I$. Same parameters as in Fig. 2.

Figure 4

Ion kinetic energies of a heterogeneous cluster model with $N={10}^4$ particles in a sphere of radius $R$ as a function of the total charge $Q/N$ of the system, with scenarios A, B, and C as discussed in the text. (a) Ratio of average energies of heavy and light ions, cf. Eq. (3) and Fig. 3. Energies resolved for the initial position of the light (b) and heavy (c) ions, where $(r/R{)}^3$ measures the distance from the cluster center. Energies are given in terms of $E^{\star }=Q/R$ for the light and $E^{\star }=Q/4R$ for the heavy component, respectively. The solids lines indicate the part which is expected to explode if the charge would be concentrated in a shell at the surface.

Figure 5

Charge-state distribution for carbon ions ${\mathrm{C}}^{q+}$ from $({\mathrm{CH}}_4{)}_{689}$ and ${\mathrm{C}}_{689}$ for various x-ray intensities $I$ indicated in the respective panel.