Observation of photon-nucleus angular-momentum transfer in the strong-field breaking of molecules

The photon-electron angular-momentum transfer is intrinsic and overwhelmingly important in light-molecule interaction. However, the photon-nuclei angular-momentum transfer is seldom observed because of the very large inertial moment and the very small dipole of nuclei as compared to those of the electron. Here, we report the theoretical and experimental observation of the transfer of the spin angular momentum of the photon into the nuclear orbital angular momentum of a molecule. This spin angular-momentum transfer is revealed by carefully examining the emission direction of the ejected proton from the breaking ${\mathrm{H}}_2$ molecule, which deviates from the initial orientation of the molecular axis. The deviation of the emission direction of the nuclear fragments from the molecular axis depends on its kinetic energy and the ellipticity of the driving laser field, which is confirmed in our numerical simulations.

Figure 1

Schematic diagram of SAMT of left-hand circularly polarized laser pulse with spin angular momentum $\sigma =–1\left(S_z=–\mathrm{\hslash }\right)$ via the single-photon absorption from the $1s{\sigma }_g$ to $2p{\sigma }_u$ states in ${{\mathrm{H}}_2}^{+}$. The inset plot shows the scaled kinetic energy release of ${\mathrm{H}}^{+}$ generated in 790 nm (red line) and 395 nm (blue), respectively.

Figure 2

(a) Sketch of the electron- and proton-momentum measurement from the dissociative ionization of ${\mathrm{H}}_2$ by a pump pulse (395 nm, blue areas) and a 300-fs time delayed probe pulse (790 nm, red areas). (b) Experimentally measured electron-momentum distribution under the photoionization of pump and probe pulses. The regions enclosed by the gray dashed lines present the independent photoionization from the pump pulse (395 nm). (c) The emission-angle-resolved KER of ${\mathrm{H}}^{+}$. The dissociative fragments induced by the pump pulse and probe pulse are located in the upper and lower half spaces divided by the gray dashed line.

Figure 3

(a) Numerically simulated momentum distribution of emitted protons driven by a left-hand elliptically polarized laser field with ellipticity of 0.6. (b) Differentially normalized momentum distribution $\beta \left(p_x,p_y\right)$ of protons driven by left- and right-hand elliptically polarized laser fields with ellipticity of ±0.6. (c) Calculated angular momentum ${\langle \vec{M}\rangle }_z$ and deviation angle $\delta {\theta }_{\mathrm{H}+}$ of ${\mathrm{H}}^{+}$ as a function of laser ellipticity.

Figure 4

(a,b) Experimentally measured proton emission direction ${\varphi }_{\mathrm{H}+}$ as a function of $E_{\mathrm{H}+}$ in the $x\text{−}y$ plane under the clockwise elliptically polarized probe laser field with ellipticity of (a) ${\varepsilon }_{\mathrm{probe}}=0.6$ and (b) ${\varepsilon }_{\mathrm{probe}}=0.4$, respectively. (c,d) The same as in (a,b) but for differential normalization $\beta \left(p_x,p_y\right)$.

Figure 5

Experimentally measured and theoretically simulated proton emission angle as a function of the kinetic energy of the proton $\left(E_{\mathrm{H}+}\right)$ for different ellipticities of (a) ${\varepsilon }_{\mathrm{probe}}=0.6$ and (b) ${\varepsilon }_{\mathrm{probe}}=0.4$, where ${\varphi }_{\mathrm{H}+}$ and $d{\theta }_{\mathrm{H}+}$ indicate the directly measured emission angle in the x-y plane and the deviation angle from the major axis of the dissociation laser field, respectively. The gray dashed lines and green and orange solid lines are extracted from classical rotor model and SBOA quantum simulation, respectively.