Abstract
Transient absorption is a very powerful observable in attosecond experiments on atoms, molecules and solids and is frequently used in experiments employing phase-locked few-cycle infrared and XUV laser pulses derived from high harmonic generation. We show numerically and analytically that in non-centrosymmetric systems, such as many polyatomic molecules, which-way interference enabled by the lack of parity conservation leads to new spectral absorption features, which directly reveal the laser electric field. The extension of attosecond transient absorption spectroscopy (ATAS) to such targets hence becomes sensitive to global and local inversion symmetry. We anticipate that ATAS will find new applications in non-centrosymmetric systems, in which the carrier-to-envelope phase of the infrared pulse becomes a relevant parameter and in which the orientation of the sample and the electronic symmetry of the molecule can be addressed.
Highlights
Attosecond transient absorption spectroscopy (ATAS) [1, 2] is emerging as one of the most potent techniques in attosecond science, since it takes advantage of both the appealing temporal and spectral properties of attosecond XUV pulses
We anticipate that attosecond transient absorption spectroscopy (ATAS) will find new applications in non-centrosymmetric systems, in which the carrier-to-envelope phase of the infrared pulse becomes a relevant parameter and in which the orientation of the sample and the electronic symmetry of the molecule can be addressed
Badankó et al [19] investigated the importance of the orientation of the transition dipole moment in non-adiabatic molecular dynamics, and Rørstad et al [20] studied ATAS of polar molecules, discovering light-induced structures (LIS) near bright rather than dark states and a ladder structure in the spectra that is spaced by the infrared (IR) photon energy
Summary
Attosecond transient absorption spectroscopy (ATAS) [1, 2] is emerging as one of the most potent techniques in attosecond science, since it takes advantage of both the appealing temporal and spectral properties of attosecond XUV pulses. In atoms all states have a well-defined parity and the excitation of a particular state by both an even number of photons (e.g. the combination of an XUV photon and an IR photon) and an odd number of photons (e.g. an XUV photon only) is not possible This is a direct consequence of the Laporte rule [24], which states that parity has to change in a dipole-allowed electronic transition. The parameter d02 depends on the symmetry of the model system, i.e. d02 = 0 for the centrosymmetric He atom and d02 = 0 for the non-centrosymmetric model system that we will consider here In the latter case, we make the arbitrary choice to set d02 = d01 = 0.33 a.u. to ensure an equal population of both excited states by the XUV pulse
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