Control of ${\mathrm{H}}_2$ Dissociative Ionization in the Nonlinear Regime Using Vacuum Ultraviolet Free-Electron Laser Pulses

The role of the nuclear degrees of freedom in nonlinear two-photon single ionization of ${\mathrm{H}}_2$ molecules interacting with short and intense vacuum ultraviolet pulses is investigated, both experimentally and theoretically, by selecting single resonant vibronic intermediate neutral states. This high selectivity relies on the narrow bandwidth and tunability of the pulses generated at the FERMI free-electron laser. A sustained enhancement of dissociative ionization, which even exceeds nondissociative ionization, is observed and controlled as one selects progressively higher vibronic states. With the help of ab initio calculations for increasing pulse durations, the photoelectron and ion energy spectra obtained with velocity map imaging allow us to identify new photoionization pathways. With pulses of the order of 100 fs, the experiment probes a timescale that lies between that of ultrafast dynamical processes and that of steady state excitations.

Figure 1

(a) Schematic of resonant two-photon ionization via the $B{{}^1\mathrm{\Sigma }}_u^{+}$ ($v=9$) neutral intermediate state (12.51 eV). $Q_1$ and $Q_2$ feature the lowest states of ${}^1{\mathrm{\Sigma }}_g^{+}$ symmetry of each series of DES. The gray shaded area highlights the FC region for one-photon absorption from the ${\mathrm{H}}_2(X{}^1{\mathrm{\Sigma }}_g^{+})$ ground state (solid arrow). The dashed arrows depict the range for absorption of the second photon after population of the $v=9$ level. The dotted horizontal lines show the $H_2^{+}({{}^2\mathrm{\Sigma }}_g^{+},v=0)$ ionization threshold (green, 15.43 eV) and the $H^{+}+H(1s)$ dissociation limit (red, 18.08 eV). Inset (b) depicts the measured KER spectrum for DI in resonance with the $B(v=9)$ state.

Figure 2

(a) Experimental and theoretical DI/NDI ratio as a function of the photon energy showing a major enhancement of DI for two-photon ionization compared to the one-photon case recorded at 25.5 eV ($\mathrm{DI}/\mathrm{NDI}\approx 0.02$) [8]. The measured ratio is derived from the PES. Error bars indicate the ratios obtained by shifting the integration limits by $\pm 100\mathrm{meV}$. The blue (black) comb indicates the position of the vibrational levels of the $B{{}^1\mathrm{\Sigma }}_u^{+}$ ($C{{}^1\mathrm{\Pi }}_u$) state. The computed DI/NDI ratio for $v=11$ amounts to 3.25. Inset (b) shows the TOF mass spectrum for a FEL photon energy of 12.76 eV.

Figure 3

(a) Experimental PES for resonant two-photon ionization via the $B(v=9)$ state at 12.51 eV. The top-right comb features the position of the $H_2^{+}(X{{}^2\mathrm{\Sigma }}_g^{+},v^{+})$ vibrational levels. Low-energy electrons (gray shaded area) corresponding to ionization of the intermediate state by one seed laser photon are left out in the determination of the DI/NDI ratio. The inset shows an enlargement of the DI region of the PES together with the $H^{+}$ KER spectrum. The KER scale was converted into the electron energy scale: $E_e=2h\nu -E_D-\mathrm{KER}$. (b) Computed PES for pulse total durations $T=10–40\mathrm{fs}$ (normalized by $T$). The contributions of the two ionization continua for 40-fs pulses are illustrated in the inset. (c) Infinite-time limit spectrum. The gray line depicts a convolution of the theoretical data with a Gaussian function of $\mathrm{FWHM}=150\mathrm{meV}$ (200 meV) for the DI (NDI) part of the spectrum to account for the experimental resolution. (d) Comparison of the infinite-time limit spectra obtained from the complete calculation (full lines) with a truncated computation in which DES are omitted (dotted lines).

Figure 4

Experimental and theoretical photoelectron spectra for $B{{}^1\mathrm{\Sigma }}_u^{+}$ ($v=8–10$). The gray shaded areas depict the FC overlap between the $B(v)$ state with the $H_2^{+}(2p{\sigma }_u{{}^2\mathrm{\Sigma }}_u^{+})$ continuum.