Angular Distribution of MolecularK-Shell Auger Electrons: Spectroscopy of Photoabsorption Anisotropy

Abstract

The angular distribution of Auger electrons emitted in the decay of molecular iC-shell vacancies created by photoabsorption is predicted to be a direct probe of the anisotropy of molecular photoabsorption. The cr -*-n discrete absorption and the cr -*• cr /-wave shape resonance in N 2 and CO are given as examples.PACS numbers: 33.80.Eh Atomic #-shell vacancies are isotropic and hence electrons emitted by Auger decay of such vacancies have an isotropic angular distribution. 1 " 4 Molecules are different, however, for two reasons. First, molecular photoabsorption is highly anisotropic, both in the discrete, because excited states have definite symmetries but generally different energies, and in the continuum, where shape resonances can enhance photoabsorption along characteristic directions of the molecule. 5 " 9 As a result, molecular photoabsorption produces neutral-molecular excited states with definite, characteristic orientations and shape-resonant photoionization produces molecular ions with energy-dependent orientations characteristic of each resonance. Second, subsequent decay of the K-shell vacancy is a fast process compared with molecular rotation, so that the orientation of the molecular excited state/ ion will be reflected in the angular distribution of the emitted Auger electrons. For example, in , CO and N 2 there is a very intense discrete absorption of 7r symmetry about 1 Ry below the K-shell ionization threshold and a broad a-symmetry shape resonance centered about 1 Ry above threshold. 6 ' 7 ' 10 The photoabsorption to the 77 excited states preferentially creates excited molecules perpendicular to the electric vector of the light while photoionization within the a shape resonance yields molecular ions preferentially parallel. The angular distribution of a particular Auger electron will mirror these alternative orientations as one scans through the photoabsorption spectrum, Thus, molecular Auger-electron angular distributions, as a function of photoexcitation energy, provide us with a spectroscopy of the anisotropy of molecular photoabsorption. This spectroscopy permits an identification of the symmetry of discrete excited states as well as a symmetry decomposition of continuum states c For randomly oriented molecules in the gas phase, the molecular orientation induced by photoabsorption is given by Eq 0 (13) of Ref. 11 asin terms of the azimuthal and polar angles Q, m = (0 m , <p m ) of the molecule with respect to the laboratory coordinates, with z axis along the electric vector for linearly polarized light (ra^O) or along the propagation direction for left-and rightcircularly polarized light (m p =±l). The maximum harmonic K=2 reflects the dipole character of the photoabsorption. The dynamics of the molecu-1 lar orientation is contained in the coefficients Z, which depend on the photon energy hv. Explicit expressions for them in terms of photoabsorption dipole amplitudes are given in Ref. 11. If we denote by d 2 P a /dk a dQ m the probability of Auger emission along the direction k a , with the ...

Core-level photoelectron and Auger shape-resonance phenomena in CO, CO2, CF4, and OCS

“…The ground-state electronic configuration of C02 can be written 2 2 The results for the co 2 C(KVV) asymmetry parameter are also presented in Fig. 10.…”

“…They in turn can be determined by measuring da(hv,e)/dQ at two known angles e. Given the close relationship between hv and E, these parameters are often listed as a(E) and a(E) for photoelectrons. The notations aA (hv) and aA(hv) are used here to denote the properties of Auger transition is given by (2) where am{hv) is the orientation parameter and A is a photon-energy independent quantity specific to each Auger-decay process. There are six sections to follow.…”

Recoil-ion and electron momentum spectroscopy: reaction-microscopes

“…One might ask, if for innershell ionisation the two steps of photoelectron emission and the subsequent Auger electron emission can be treated independently (two-step model) (Dill et al 1980;Kuznetsov and Cherepkov 1996;Guillemin et al 2001). The validity of the two-step model has recently been confirmed in a study of the decay channel and molecular frame angular distribution of the Auger electron from Carbon K-shell photoionised CO .…”

Auger Electron Emission from Fixed-in-Space CO

“…These have, as we show below, a very rich structure. It has been emphasized from the theory side that the angular distributions from fixed in space molecules are a key to deeper understanding of the molecular Auger process [3,7,8]. Zähringer et al have shown that the Auger electron angular distribution can be understood as resulting from two processes acting together.…”

“…Similar to photoelectrons [3,4] the Auger electron will be multiply scattered in the molecule hence modifying Auger rates and angular distributions. A third challenge was posed recently by the claim [1] that even off-resonance the creation of a core hole by photo-ionization and its subsequent Auger decay cannot be treated as two independent steps (two-step model), as has been commonly assumed [3,5].In the present letter we address these three challenges by reporting an experiment on the Auger decay of carbon K-shell ionized CO + . We have measured the Auger electron energy and angle in coincidence with the energy and angle of both fragment ions of the CO 2+ .…”