Direct Measurement of the Angular Dependence of the Single-Photon Ionization of Aligned N₂ and CO₂

Abstract

By combining a state-of-the-art high-harmonic ultrafast soft X-ray source with field-free dynamic alignment, we map the angular dependence of molecular photoionization yields for the first time for a nondissociative molecule. The observed modulation in ion yield as a function of molecular alignment is attributed to the molecular frame transition dipole moment of single-photon ionization to the X, A and B states of N2(+) and CO2(+). Our data show that the transition dipoles for single-photon ionization of N2 and CO2 at 43 eV have larger perpendicular components than parallel ones. A direct comparison with published theoretical partial wave ionization cross-sections confirms these experimental observations, which are the first results to allow such comparison with theory for bound cation states. The results provide the first step toward a novel method for measuring molecular frame transition dipole matrix elements.

Semiclassical complex-time method for tunneling ionization: Molecular suppression and orientational dependence

“…Zhao et al [37] suggested that the experiment of Pavičić et al [34] should be reexamined. They compared their CO 2 data with the experiment of Thomann et al [46] which agrees much better with MO-ADK and TDSE calculations. Although there are some experimental issues, we also think • , for three values of intensity.…”

Precise Access to the Molecular-Frame Complex Recombination Dipole through High-Harmonic Spectroscopy

“…Knowledge of molecular orientation in most valence-shell photoionization experiments requires using laser-induced impulsive pre-alignment techniques [6] when the ion does not dissociate, thus necessitating short-pulsed XUV sources for spectroscopy like high-order harmonic generation (HHG) [7], femtosecond sliced synchrotrons [8], or freeelectron lasers [9]. Among these, HHG is unique in that it allows for complete characterization of the complex transition dipole matrix element in both magnitude (the cross section) and phase.…”

Tunnel Ionization of Molecules and Orbital Imaging

“…The physics responsible for this effect is a subject of hot debate (see, e.g., [7,10,11,13]), including the possible contribution of multiphoton resonances ignored in the tunneling picture [10], or multiple ionic states (multiple orbitals) [13]. The extreme sharpness of the peaks in [9], not observed in [14] and questioned in [7,10], might indeed be related to the artifacts in the deconvolution procedure (see [10]), which is required to extract the ionization rates when dealing with imperfect molecular alignment. However, the rotation of the ionization peaks to about 45 was confirmed [15].…”