Photon-number-resolved asymmetric dissociative single ionization of ${\mathrm{H}}_2$

The electron-nuclear joint energy spectrum allows one to unambiguously count the total number of photons absorbed by the electrons and nuclei of a molecule. Driven by phase-controlled, linearly polarized two-color femtosecond laser pulses, we experimentally demonstrate that the asymmetric bond breaking of a singly ionized ${\mathrm{H}}_2$ depends on the total number of photons absorbed by the molecule in the ionization and dissociation processes. The accessibilities of different dissociation pathways and their interference-induced asymmetric electron localization as a function of the absorbed photons are retrieved. Our results strengthen the understanding of the directional bond breaking of a molecule from the aspect of the correlated electron-nuclear dynamics.

Figure 1

Schematic diagram of the experimental apparatus. The red and blue arrows stand for the polarization of the FW and SH components of the two-color field. HWP: half-wave plate, DM: dichroic mirror, NDF: neutral density filter. The inset illustrates that the remaining electron can localize at one of the nuclei depending on the phase of the two-color field in the dissociative single ionization of ${\mathrm{H}}_2$.

Figure 2

(a) Schematic illustration of the multiphoton dissociative single ionization of ${\mathrm{H}}_2$ driven by a two-color laser pulse. A NWP on the $1s{{\sigma }_{\mathrm{g}}}^{+}$ state of ${{\mathrm{H}}_2}^{+}$ is launched by releasing one electron from ${\mathrm{H}}_2$ in the ionization step. The created NWP afterwards propagates on the potential curves of ${{\mathrm{H}}_2}^{+}$ and asymmetrically dissociates into a neutral H and an ionic ${\mathrm{H}}^{+}$ after conclusion of the two-color laser pulse, assisted by photon-coupled transitions among the $1s{{\sigma }_{\mathrm{g}}}^{+}$ and $2p{{\sigma }_{\mathrm{u}}}^{+}$ states. The schematic potential curves of neutral ${\mathrm{H}}_2$ and ionic ${{\mathrm{H}}_2}^{+}$ are adopted from Refs. [51] and [52]. (b) Measured two-dimensional spectrum of the asymmetry parameter as a function of the laser phase ${\varphi }_{\mathrm{L}}$ and the kinetic energy of the nuclei $E_{\mathrm{N}}$. (c) Measured phase-averaged $E_{\mathrm{N}}$ spectrum. (d) Asymmetry parameters of the low-$E_{\mathrm{N}}$ ($0.4\mathrm{eV}<E_{\mathrm{N}}<0.9\mathrm{eV}$) and high-$E_{\mathrm{N}}$ ($1.1\mathrm{eV}<E_{\mathrm{N}}<1.6\mathrm{eV}$) regions as indicated between the white dashed lines in (b). The solid sinusoidal curves are the numerical fits of the measured data.

Figure 3

(a) Measured electron-nuclear JES of the ${\mathrm{H}}_2(1,0)$ channel. (b) The corresponding sum energy $E_{\mathrm{sum}}$ of the ejected electron and nuclear fragments from a single molecule. Each peak in the $E_{\mathrm{sum}}$ spectrum stands for a diagonal line in the JES spectrum in (a) as indicated by the numbers. (c) The normalized $E_{\mathrm{N}}$ distributions integrated over $E_{\mathrm{e}}$ for the first four diagonal energy conservation lines in (a). The expected location of the $E_{\mathrm{N}}$ for four dissociation pathways mentioned in the text are marked by the dashed lines.

Figure 4

(a) Fitted asymmetry amplitude $A_0$ of the ${\mathrm{H}}_2(1,0)$ channel in low-$E_{\mathrm{N}}$ (red circles) and high-$E_{\mathrm{N}}$ (blue squares) regions as a function of the ATI order of the $E_{\mathrm{sum}}$ spectrum. (b) Deduced relative weight ($R$) of various dissociation pathways as a function of ATI order of the $E_{\mathrm{sum}}$ spectrum.