Following a chemical reaction using high-harmonic interferometry

Abstract

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High-Harmonic Spectroscopy: From Femtosecond to Attosecond Molecular Dynamics

“…The undiffracted high-harmonic signal is similar to a measurement carried out in collinear excitation with the same spatially averaged excitation fraction (see ref. [23] and [24]). The diffracted highharmonic intensity is proportional to r 2 /4|d e exp(if e ) − d g exp(if g )| 2 , and thus represents a background-free measurement of the excited state dynamics through 1:1 interference with the ground state emission.…”

“…E 1,1 can be used as a static reference to measure dynamical changes in E 2,2 . This detection scheme is highly sensitive to weak emission and simultaneously probes its amplitude and phase [23,24] which are conveniently separated using the transient grating method, [26,27] illustrated in Fig. 1a.…”

“…The periodic modulation of the excitation fraction leads to a spatial modulation of the amplitude and phase of harmonic emission in the generating medium and results in diffraction along the direction of the modulation. [23] Writing the emission from the ground and excited states in terms of the excitation fraction r = |c 2 | 2 and high-harmonic amplitudes and phases, we obtain…”

“…We now extend the formalism to describe a photochemical reaction. [23,24] A weak femtosecond pump pulse prepares the molecule in a coherent superposition of the ground state and an excited electronic state through one-photon absorption…”

“…In addition, these contributions are also proportional to the overlap of the nuclear wave functions in the two electronic states, which vanishes quickly in the case of photodissociation. [23] The high-harmonic emission from a photoexcited molecule can thus be understood from the coherent addition of the electromagnetic fields emitted by the ground and excited electronic states. In addition to Eqn.…”