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

Abstract

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 strongfield 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. C an we find universal concepts to understand and control the response of atoms and molecules in interactions with strong laser fields? This question is at the heart of a vast number of experiments in time-resolved spectroscopy (1-30). The wide range of light sources spanning the spectral range from the X-ray (e.g., free-electron laser sources, synchrotrons) over the visible (conventional laser systems) to the far-infrared regime and covering the temporal range from nanosecond down to attosecond time scales created a wealth of new physics insight into quantum mechanisms, however mostly of simple systems in the gas phase (1-4). In chemistry, the generation of femtosecond laser pulses enabled the investigation of wave packet dynamics in molecules, as the induced vibrations occur on these time scales. Experiments focusing on, for instance, dissociation reactions, atom transfer, isomerization, or solvation dynamics have led to a deeper understanding of chemical bonds and their breakage dynamics and have opened and established the field of femtochemistry (5, 6). The aim is not only to study the light−matter interaction, but to use the obtained understanding of the processes to control the dynamics in complex molecules and, in the future, even to be able to control chemical reactions (7-10). Shaping the amplitude and phase of femtosecond laser pulses has been used, for example, to control the shape of wavefunctions in atomic systems (11) or to control and optimize the single-photon and multiphoton fluorescence in atoms such as cesium (12) and complex systems, e.g....

Nonlinear resonant Auger spectroscopy in CO using an x-ray pump-control scheme

“…is complex from the aspect of Autler-Townes splitting, it manifests clearly an asymmetric lineshape with the changes of t, similar to the Fano profile resulting from the interference of the transition to the bound state embedded in the continuum [32,34,[52][53][54]. As was recently approved, the laser control allows us to manipulate the interference and as the results of changing the lineshape of the transient absorption spectra from the Lorentz to Fano profile [31,55,56]. In the present case of CO, the final bound state F , with a large shift of the PEC equilibrium from the ground state and core-excited state and a shallow PEC with the vibrational frequency much smaller than the bandwidth of the control pulse, can be considered as a quasicontinuum, similar to the molecular dissociative continuum, where the Fano profile for the populations dynamics was observed earlier [57].…”

Ultrafast dynamic evolution of multilevel systems in medium-strength laser fields

“…When the probe-pulse intensity is less than 3.7×10 10 W cm −2 , a 1 21 is larger than a 2 21 , and a 1 31 is almost equal to a 2 31 in the probe-pump scenario. As the probe-pulse intensity increases further, a 1 31 becomes smaller than a 2 31 , and a 1 21 and a 2 21 have similar values. The results show that the pump-pulse intensity affects the response of state 5p 2 P 3/2 at low probe intensities and the response of state 5p 2 P 1/2 at higher probe intensities in the probe-pump scenario.…”

“…The measured absorption spectra reveal that both the absorption line shape at fixed pump-probe time delays and the time-dependent spectra at transition energies change with the probe-pulse intensity. The absorption spectra at transition energies of ω 21 and ω 31 measured at varying probepulse intensities are displayed in figures 5(a), (b) and (c), (d), respectively. The black lines associated with the maxima of the absorption spectra in each panel display the phase shift as a function of the probe-pulse intensity.…”

“…Recently, it was demonstrated that the phase of a sample's polarization induced by a resonant laser field in an atomic system can be modified by a strong nonresonant laser field owing to the AC Stark effect [17][18][19][20]. This model is known as the dipole phase control theory and has been extended to multilevel and complex molecular systems by using medium-strength pump and weak probe pulses with the same frequency [21][22][23]. Actually, both the phase and amplitude of a sample's response in the time domain will be significantly modified by the AC stark effect and Rabi oscillation, respectively, regardless of whether the probe pulse precedes or follows the pump pulse.…”

Observation and quantification of the quantum dynamics of a strong-field excited multi-level system

“…The possibility to control the absorption properties of an atomic system by an induced phase shift due to a laser pulse is a promising result. However, it is still an open question how one can transfer this control concept to fully understand and manipulate the spectral response of a multi-electron atom2324 or molecule25262728 interacting with strong fields.…”