Control of Giant Breathing Motion in ${\mathrm{C}}_{60}$ with Temporally Shaped Laser Pulses

Femtosecond laser pulses tailored with closed-loop, optimal control feedback were used to excite oscillations in ${\mathrm{C}}_{60}$ with large amplitude by coherent heating of nuclear motion. A characteristic pulse sequence results in significant enhancement of ${\mathrm{C}}_2$ evaporation, a typical energy loss channel of vibrationally hot ${\mathrm{C}}_{60}$. The separation between subsequent pulses in combination with complementary two-color pump-probe data and time-dependent density functional theory calculations give direct information on the multielectron excitation via the $t_{1g}$ resonance followed by efficient coupling to the radial symmetric $a_g(1)$ breathing mode.

Figure 1

(a) ${\mathrm{C}}_{50}^{+}$ signal as a function of generation of the evolutionary algorithm. The inset shows the XFROG trace of the optimal solution. On the right, mass spectra recorded with optimal (b), stretched to 340 fs (c), and unshaped pulses of 31 fs FWHM are plotted. The ratios $R={\mathrm{C}}_{50}^{+}/{\mathrm{C}}_{60}^{+}$ are given.

Figure 2

(a) Metastable ${\mathrm{C}}_{48}^{3+}$ ion signal as a function of time delay between the blue pump and red probe pulse. An exponential decay is fitted to the data. (b) A modulation is found for all large fragments ${\mathrm{C}}_{60-2m}^{3+}$ by subtracting the fits from the measured transients. (c) Optimized temporal shape with $220\mu \mathrm{J}$ laser pulses and (d) $280\mu \mathrm{J}$ pulses.

Figure 3

(a) Period of the $a_g(1)$ breathing mode (circles) and number of excited electrons (diamonds) as a function of deposited energy derived from NA-QMD simulations (laser parameters: $\lambda =370\mathrm{nm}$, $\tau =27\mathrm{fs}$). (b) Vibrational energy of normal modes after laser excitation ($I=3.3\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$). The kinetic energy is mainly stored in the $a_g(1)$ mode.

Figure 4

(a) Absorbed energy (circles) and number of excited electrons (squares) of ${\mathrm{C}}_{60}$ after excitation with pump $\lambda =370\mathrm{nm}$, $I=3.3\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$, $\tau =27\mathrm{fs}$) and probe pulses ($\lambda =800\mathrm{nm}$, $I=7.3\times {10}^{13}\mathrm{W}/{\mathrm{cm}}^2$, $\tau =27\mathrm{fs}$) as a function of time delay. The horizontal line indicates the absorbed energy after the pump pulse alone (292 eV). (b) ${\mathrm{C}}_{60}$ radius at the maximum of the probe pulse as a function of delay time.