Signatures and control of strong-field dynamics in a complex system

Significance Using intense lasers to control complex molecules is a long-held dream in science. In this article, we develop a physics concept for measuring and controlling the quantum states of complex molecules by strong laser fields. We show that, in particular, the quantum-mechanical phase of excited molecular states can be manipulated by the intense laser, a key quantity for full (amplitude and phase) control of molecular quantum states. With the help of time- and intensity-resolved absorption spectroscopy experiments, we apply this idea to the dynamics of a large dye molecule in solution. The demonstrated phase-control concept thus represents a major leap toward the ultimate goal of laser chemistry. Controlling chemical reactions by light, i.e., the selective making and breaking of chemical bonds in a desired way with strong-field lasers, is a long-held dream in science. An essential step toward achieving this goal is to understand the interactions of atomic and molecular systems with intense laser light. The main focus of experiments that were performed thus far was on quantum-state population changes. Phase-shaped laser pulses were used to control the population of final states, also, by making use of quantum interference of different pathways. However, the quantum-mechanical phase of these final states, governing the system’s response and thus the subsequent temporal evolution and dynamics of the system, was not systematically analyzed. Here, we demonstrate a generalized phase-control concept for complex systems in the liquid phase. In this scheme, the intensity of a control laser pulse acts as a control knob to manipulate the quantum-mechanical phase evolution of excited states. This control manifests itself in the phase of the molecule’s dipole response accessible via its absorption spectrum. As reported here, the shape of a broad molecular absorption band is significantly modified for laser pulse intensities ranging from the weak perturbative to the strong-field regime. This generalized phase-control concept provides a powerful tool to interpret and understand the strong-field dynamics and control of large molecules in external pulsed laser fields.