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The time-resolved photodynamics of the methyl iodide cation (CH _3 I ^+ ) are investigated by means of femtosecond XUV–IR pump–probe spectroscopy. A time-delay-compensated XUV monochromator is employed to isolate a specific harmonic, the 9th harmonic of the fundamental 800 nm (13.95 eV, 88.89 nm), which is used as a pump pulse to prepare the cation in several electronic states. A time-delayed IR probe pulse is used to probe the dissociative dynamics on the first excited $\tilde {A}\enspace {}^{2}\mathrm{A}_{1}$ state potential energy surface. Photoelectrons and photofragment ions— ${\mathrm{C}\mathrm{H}}_{3}^{+}$ and I ^+ —are detected by velocity map imaging. The experimental results are complemented with high level ab initio calculations for the potential energy curves of the electronic states of CH _3 I ^+ as well as with full dimension on-the-fly trajectory calculations on the first electronically excited state $\tilde {A}\enspace {}^{2}\mathrm{A}_{1}$ , considering the presence of the IR pulse. The ${\mathrm{C}\mathrm{H}}_{3}^{+}$ and I ^+ pump–probe transients reflect the role of the IR pulse in controlling the photodynamics of CH _3 I ^+ in the $\tilde {A}\enspace {}^{2}\mathrm{A}_{1}$ state, mainly through the coupling to the ground state $\tilde {X}\enspace {}^{2}\mathrm{E}_{3/2,1/2}$ and to the excited $\tilde {B}\enspace {}^{2}\mathrm{E}$ state manifold. Oscillatory features are observed and attributed to a vibrational wave packet prepared in the $\tilde {A}\enspace {}^{2}\mathrm{A}_{1}$ state. The IR probe pulse induces a coupling between electronic states leading to a slow depletion of ${\mathrm{C}\mathrm{H}}_{3}^{+}$ fragments after the cation is transferred to the ground $\tilde {X}\enspace {}^{2}\mathrm{E}_{3/2,1/2}$ states and an enhancement of I ^+ fragments by absorption of IR photons yielding dissociative photoionization.

The development of schemes for coherent nonlinear time-domain spectroscopy in the extreme-ultraviolet regime (XUV) has so far been impeded by experimental difficulties that arise at these short wavelengths. In this work we present a novel experimental approach, which facilitates the timing control and phase cycling of XUV pulse sequences produced by harmonic generation in rare gases. The method is demonstrated for the generation and high spectral resolution characterization of narrow-bandwidth harmonics (≈14 eV) in argon and krypton. Our technique simultaneously provides high phase stability and a pathway-selective detection scheme for nonlinear signals—both necessary prerequisites for all types of coherent nonlinear spectroscopy.