Steering Proton Migration in Hydrocarbons Using Intense Few-Cycle Laser Fields

Proton migration is a ubiquitous process in chemical reactions related to biology, combustion, and catalysis. Thus, the ability to manipulate the movement of nuclei with tailored light within a hydrocarbon molecule holds promise for far-reaching applications. Here, we demonstrate the steering of hydrogen migration in simple hydrocarbons, namely, acetylene and allene, using waveform-controlled, few-cycle laser pulses. The rearrangement dynamics is monitored using coincident 3D momentum imaging spectroscopy and described with a widely applicable quantum-dynamical model. Our observations reveal that the underlying control mechanism is due to the manipulation of the phases in a vibrational wave packet by the intense off-resonant laser field.

Figure 1

Strong-field steering of hydrogen migration in acetylene. (a) Measured CEP-averaged momentum distribution for ${\mathrm{C}}^{+}$ ions emitted from vinylidene dications following hydrogen migration in acetylene induced by 4-fs laser pulses with an intensity $I\approx 1.3\times {10}^{14}{\mathrm{W}/\mathrm{cm}}^2$. The shaded area is cut out for technical reasons (see text). The red arrow indicates the axis of laser polarization. (b) CEP-dependent asymmetry parameter for the ${\mathrm{C}}^{+}$ yield shown in (a), evaluated for a cone of 80° around the polarization axis of the laser. Since the absolute value of the CEP is not known from the experiment, the phase is given relative to the asymmetry parameter recorded for ${\mathrm{C}}_2{\mathrm{H}}_2^{+}$ recoil ions (see text). The solid line depicts the calculated asymmetry curve for an intensity of $1.5\times {10}^{14}{\mathrm{W}/\mathrm{cm}}^2$, shifted along the CEP axis for best agreement with the experiment.

Figure 2

Strong-field steering of hydrogen migration in allene. (a) CEP-averaged momentum distribution and (b) CEP-dependent asymmetry parameter for ${\mathrm{H}}_3^{+}$ ions emitted from propyne dications following hydrogen migration in allene. The red arrow in (a) indicates the axis of laser polarization. The CEP dependence of the ${\mathrm{H}}_3^{+}$ emission is evaluated for a cone of 80° around the polarization axis of the laser and at a peak intensity of $\approx 2.9\times {10}^{14}{\mathrm{W}/\mathrm{cm}}^2$. The phase is given relative to the asymmetry parameter recorded for ${\mathrm{C}}_3{\mathrm{H}}_4^{+}$ recoil ions (see text). The measured asymmetry curve (symbols) is compared to the one calculated (solid line) for an intensity of $3.0\times {10}^{14}{\mathrm{W}/\mathrm{cm}}^2$, shifted along the CEP axis by the same value as the theory curve in Fig. 1.

Figure 3

Reactive coordinates and normal modes used in the theoretical description of the hydrogen migration reactions in acetylene and allene. The wave packet formation in acetylene (allene) is calculated on two- (three-) dimensional potential energy surfaces (PES) along the normal modes shown in panel (a) [(b)]. The reactive coordinates (${\xi }_1$, ${\xi }_2$) represent the CCH bond angles (shown in orange) and are used to describe the wave packet propagation leading to isomerization by hydrogen migration from the left to the right or from the right to the left. (c) The PES of the $A^3\mathrm{\Pi }$ state of the acetylene dication, on which isomerization occurs. The binding energy is plotted as a function of the two reactive coordinates. The initial form of the molecule and the two isomerized configurations are indicated. (d) Same as (c) for the $B^3\mathrm{\Pi }$ state of the allene dication relevant for the isomerization reaction which leads to ${\mathrm{H}}_3^{+}$ formation.

Figure 4

Schematic of the control mechanism for hydrogen migration in acetylene. Vibrational wave packets of the ${\mathrm{\Psi }}_{\text{basic}}(0)=|01⟩+|10⟩$ (top row) and the ${\mathrm{\Psi }}_{\text{basic}}(\pi )=|01⟩-|10⟩$ (bottom row) superposition (see text) are propagated on the PES of the first excited state of the acetylene dication along two reactive coordinates. The contour lines display snapshots of the calculated nuclear wave packet which leads to isomerization of the acetylene dication. The CEP of the few-cycle pulse determines the sign in the superposition and thereby influences whether the left proton moves to the right (top row) or the right proton to the left (bottom row).