Attosecond-Recollision-Controlled Selective Fragmentation of Polyatomic Molecules

Control over various fragmentation reactions of a series of polyatomic molecules (acetylene, ethylene, 1,3-butadiene) by the optical waveform of intense few-cycle laser pulses is demonstrated experimentally. We show both experimentally and theoretically that the responsible mechanism is inelastic ionization from inner-valence molecular orbitals by recolliding electron wave packets, whose recollision energy in few-cycle ionizing laser pulses strongly depends on the optical waveform. Our work demonstrates an efficient and selective way of predetermining fragmentation and isomerization reactions in polyatomic molecules on subfemtosecond time scales.

Figure 1

(a)–(c) Measured fragmentation and ionization yields, normalized to 1 at their respective maxima, as a function of CEP for different fragmentation channels of acetylene (a), ethylene (b), and 1,3-butadiene (c), measured at the laser intensities indicated in the panels. The various fragmentation reactions are ${\mathrm{C}}_2{\mathrm{H}}_2^{2+}\to {\mathrm{CH}}^{+}+{\mathrm{CH}}^{+}$ (red dots) and ${\mathrm{C}}_2{\mathrm{H}}_2^{2+}\to {\mathrm{H}}^{+}+{\mathrm{C}}_2{\mathrm{H}}^{+}$ (blue squares) for acetylene (a); ${\mathrm{C}}_2{\mathrm{H}}_4^{2+}\to {\mathrm{CH}}_2^{+}+{\mathrm{CH}}_2^{+}$ (red dots), ${\mathrm{C}}_2{\mathrm{H}}_4^{2+}\to {\mathrm{H}}^{+}+{\mathrm{C}}_2{\mathrm{H}}_3^{+}$ (blue squares), and ${\mathrm{C}}_2{\mathrm{H}}_4^{2+}\to {\mathrm{C}}_2{\mathrm{H}}_2^{+}+{\mathrm{H}}_2^{+}$ (green triangles) for ethylene (b); ${\mathrm{C}}_4{\mathrm{H}}_6^{2+}\to {\mathrm{C}}_2{\mathrm{H}}_3^{+}+{\mathrm{C}}_2{\mathrm{H}}_3^{+}$ (red dots) and ${\mathrm{C}}_4{\mathrm{H}}_6^{2+}\to {\mathrm{CH}}_3^{+}+{\mathrm{C}}_3{\mathrm{H}}_3^{+}$ (blue squares) for 1,3-butadiene (c). The ionization yields of the singly and doubly charged molecular ions are denoted by black dots and gray squares, respectively. (d), (e) Cuts through the measured three-dimensional momentum spectra of ${\mathrm{CH}}_2^{+}$ (d) and ${\mathrm{H}}_2^{+}$ (e) associated with two of the three fragmentation channels identified for ethylene shown in (b), in a plane along ($p_z$) and perpendicular ($p_y$) the laser polarization direction, for $|p_x|<10\mathrm{a}.\mathrm{u}$. and integrated over all CEP values. (f) Kinetic energy release (KER) spectrum of the fragmentation reaction ${\mathrm{C}}_2{\mathrm{H}}_4^{2+}\to {\mathrm{CH}}_2^{+}+{\mathrm{CH}}_2^{+}$ as a function of CEP. (g) Mean $p_z$ value (integrated over $p_x$ and $p_y$) of the singly (black dots) and doubly (gray squares) charged molecular ions of acetylene as a function of CEP, in comparison with the $p_z$ sum of the two fragments from the two fragmentation channels shown in the same color and point style as in (a).

Figure 2

(a) Electronic energy levels of the neutral, cation, and dication of acetylene as a function of C-C distance, calculated by ab initio quantum-chemical methods as described in the Supplemental Material [10]. TI, RI, and RE denote tunnel ionization, recollision ionization, and recollision excitation, respectively. (b) Electron recollision energy over the time of ionization calculated for a 4.5 fs (FWHM) long laser pulse with an intensity of $1.5\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ for the three indicated values of the CEP: 0 (red lines), $0.25\pi$ (green lines), and $0.5\pi$ (blue lines). The contribution of each recollision event as determined by the ionization probability is indicated by the color intensity. (c), (d) Fragmentation yields over the CEP of the same channels as in Figs. 1a, 1b (same color and point styles apply), but measured for a slightly higher intensity (as indicated). (e) Measured intensity dependence of the yield of the fragmentation channel ${\mathrm{C}}_4{\mathrm{H}}_6^{2+}\to {\mathrm{CH}}_3^{+}+{\mathrm{C}}_3{\mathrm{H}}_3^{+}$ over CEP. The intensities are indicated in the figure.

Figure 3

Comparison of simulated and measured fragmentation probability of the channel ${\mathrm{C}}_2{\mathrm{H}}_2^{2+}\to {\mathrm{CH}}^{+}+{\mathrm{CH}}^{+}$. (a) Simulated fragmentation probability encoded in color scale as a function of CEP and laser intensity, normalized to its maximum value and calculated as described in the Supplemental Material [10]. The CEP and intensity dependent position of maximum and minimum fragmentation yield is indicated by black upwards and gray downwards pointing triangles, respectively. (b) Modulation depth of the fragmentation probability [as defined in (c)], as a function of laser intensity. (c) Simulated fragmentation probability over CEP (full green line) obtained by averaging of the data in (a) over the intensity range $1.2–1.4\times {10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ to account for the experimental intensity beam profile [as indicated in (a) and (b)], in comparison with the experimental data from Fig. 1a shifted by 60°.