Crossed Beam Photodissociation Imaging of ${\mathrm{HeH}}^{+}$ with Vacuum Ultraviolet Free-Electron Laser Pulses

Molecular photofragmentation has been studied by event imaging on ${\mathrm{HeH}}^{+}$ ions at 32 nm (38.7 eV) in a fast ion beam crossed with the free-electron laser in Hamburg (FLASH), analyzing neutral He product directions and energies. Fragmentation into $\mathrm{He}(1snl,n\ge 2)+{\mathrm{H}}^{+}$ was observed to yield significant photodissociation at 32 nm with an absolute cross section of $(1.4\pm 0.7)\times {10}^{-18}{\mathrm{cm}}^2$, releasing energies of 10–20 eV. A clear dominance of photodissociation perpendicular to the laser polarization was found in contrast to the excitation paths so far emphasized in theoretical studies.

Figure 1

Excited potential energy curves, atomic final-state limits, and sample kinetic energy release $E_k$ for ${\mathrm{HeH}}^{+}$ photodissociation at 32 nm. The lowest vibrational wave function and the Franck-Condon region for high vibrational excitation are shown schematically for the $X{}^1\Sigma ^{+}$ ground potential [3] with excited ${}^1\Sigma ^{+}$ [14] (dashed lines) and ${}^1\Pi$ states [7] (full lines), the gray scale (colors) indicating the (adiabatic) correlation to the final levels. The upper thick gray lines (green) traces the ${\mathrm{He}}^{+}+{\mathrm{H}}^{+}$ Coulomb potential.

Figure 2

Schematic of the crossed-beam imaging setup at the FLASH beam line.

Figure 3

Photodissociation image of neutral fragments from ${\mathrm{HeH}}^{+}$ at 32 nm measured (a) with $\sim 1.2\times {10}^6$ photon pulses of typically $10\mu \mathrm{J}$ in the crossing region and (b) between photon pulses (background). Counts for given radial position and event time are shown for a nominal ion energy of ${\tilde{E}}_0=7200\mathrm{eV}$ after adding several runs as described in the text. Lines indicate constant $E_k$ for He [gray (red) curve] and H (white line) fragments and $E_k=8$ and 20 eV, respectively.

Figure 4

Photofragment distributions for ${\mathrm{HeH}}^{+}$ at 32 nm. (a) Kinetic energy release ($E_k$) distribution from imaging of the He fragments, compared with $E_k$ for $\mathrm{He}(1snl)$ final levels. (b),(c) ${\mathrm{HeH}}^{+}$ dissociation angle distributions relative to the FEL polarization for $E_k=8–13\mathrm{eV}$ and 13–20 eV, respectively. Full black lines show fitted distributions composed of a dominant $\Sigma -\Pi$ contribution [full gray (blue) curves] plus additional $\Sigma -\Sigma$ contributions (dashed lines).