Abstract
We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is monitored in dilute helium, dense helium, and sulfur hexafluoride (SF6) molecules. We observe and quantify the time-dependence of various transient phenomena in helium atoms,includinglaser-inducedphase(LIP),time-varying(AC)Starkshift,quantumpathinterference, and laser-induced continuum structure. In the case of dense helium targets, we discuss nonlinear macroscopic propagation effects pertaining to LIP and resonant pulse propagation, which accoun tfor the appearance of new spectral features in transient lineshapes. We then use tunable NIR photons to demonstrate the wavelength dependence of the transient laser induced effects. In the case of molecular polarization experiment in SF6, we show suppression of XUV photoabsorption corresponding to inter-valence transitions in the presence of a strong NIR field. In each case, the temporal evolution of transient absorption spectra allows us to observe and understand the transient laser induced modifications of the electronic structure of atoms and molecules.
Highlights
The quantum mechanical motion of electrons and their interaction with light is at the heart of most photophysical and photochemical processes
As the natural electronic timescale lies in the range of a few femtoseconds (10−15 s) to attoseconds (10−18 s), ultrafast light sources are needed to probe these process in real time
It should be mentioned that the reported XUV spectral ranges and the relative intensities are based on our femtosecond near infrared (NIR) pulse parameters, as well as our focusing and interaction geometry in the form of a waveguide or an semi-infinite gas cell (SIGC)
Summary
The quantum mechanical motion of electrons and their interaction with light is at the heart of most photophysical and photochemical processes. One of the common approaches is to use a time-delayed near-infrared (NIR) or visible laser pulse to conduct temporal and energetically resolved spectroscopy of excited electronic states by measuring photofragments, photoabsorption, or photoemission. Among these approaches, the attosecond transient absorption spectroscopy (ATAS) is an emergent all-optical technique that can be applied to study many interesting coherent laser induced phenomena such as Autler–Townes splitting, electromagnetically induced transparency, light induced virtual states, quantum beats, spectral line shape manipulations, and resonant pulse propagation, etc.
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