Experimental investigation of nondipole effects in photoemission at the advanced light source

Abstract

Breakdowns in the dipole approximation in the soft-X-ray photon-energy range ðhno5 keVÞ were first observed 30 years ago and have been studied theoretically for many years. However, only recently their significance at low energies has been appreciated when advances in gas-phase-photoemission experiments using synchrotron radiation began to highlight numerous examples of significant nondipole effects at photon energies as low as tens of eV. In a previous publication [Hemmers et al., 2004a. Radiat. Phys. Chem. 70, 123-147], we presented a description of the recent advances made in the investigation of nondipole effects in photoionization from an experimental perspective. In this article, we report the results obtained by the X-ray atomic and molecular spectroscopy (XAMS) group at the Advanced Light Source, over the last 10 years, on the limits of the dipole approximation, probed by angle-resolved photoelectron spectroscopy for atoms and molecules in gas phase. r

Non-dipole effects in the angular distribution of photoelectrons in sequential two-photon atomic double ionization

“…For example the absolute value of γ (2) 4 for the 1 S residual ion state is ten and two times larger than for the 1 D and 3 P states, respectively. Note that it is possible to write down the expression for δ (2) in the product form (24), where dipole and quadrupole amplitudes are separated in different factors. The dipole factor (…”

X-ray photoelectron spectroscopy: From origins to future directions

“…the much-used TPP-2 M formula [14,15]; y is the electron emission angle with respect to the surface and O(hn, x, y), which is equivalent once integrated over x and y to the spectrometer intensity response function, can be determined from reference-sample calibration measurements [16]. For excitation energies in the keV regime, if not even below this, non-dipole or retardation corrections are needed for a fully accurate description of ds Qn'j ðhn;Þ=dO, as first demonstrated by Krause [17], and discussed in much more detail elsewhere [18]. Illustration of a typical experimental configuration for photoelectron spectroscopy experiments, together with the various types of measurements possible, including: (a) simple spectra or energy-distribution curves, (b) core-level photoelectron diffraction, (c) valence-band mapping or energy vsk plots, (d) spin-resolved spectra, (e) exciting with incident X-rays such that there is total reflection and/or a standing wave in the sample, (f) using higher or much lower photon energies than have been typical in the past, (g) taking advantage of space and/or time resolution, and (h) surrounding the sample with high ambient sample pressures of several torr.…”

“…22. Note that, as the photon energy is increased, one can no longer neglect the momentum of the photoñ k hn in conserving wave vector, one manifestation of non-dipole effects in the excitation [17,18,79]. Determiningk f inside the surface from a measurement ofk f outside the surface (which may be slightly different fromk f inside due to crossing the inner potential at the surface) and then the set ofg n vectors which projectk f back into the reduced Brillouin zone (BZ) in which the band structure is usually described thus permits directly measuring E binding ðk i Þ ¼ E i ðk i Þ, the band structure, or if final-state screening and many-electron excitations are taken into account, more properly the spectral function as calculated from some sort of many-electron theory [5].…”

Symmetry breakdown of electron emission in extreme ultraviolet photoionization of argon

“…The momentum transfer can lead to a symmetry breakdown in terms of a forward-backward asymmetry in the photoelectron angular distribution (PAD). Generally, the importance of this nondipole effect and its strong influence on various phenomena were demonstrated in numerous studies ranging from fundamental research in atoms and molecules 1 , 2 , over applications in condensed matter 3 and realization of high-harmonic generation lasers 4 to investigations of astrophysical interest 5 . With the rapid evolution of ultrabright X-ray light sources such as free-electron lasers (FELs), the relevance of nondipole effects for photoionization of ionic targets in nonlinear and time-resolved studies gains further importance.…”