Nearest-Neighbor-Atom Core-Hole Transfer in Isolated Molecules

Abstract

A new phenomenon sensitive only to next-door-neighbor atoms in isolated molecules is demonstrated using angle-resolved photoemission of site-selective core electrons. Evidence for this interatomic coreto-core electron interaction is observable only by measuring nondipolar angular distributions of photoelectrons. In essence, the phenomenon acts as a very fine atomic-scale sensor of nearest-neighbor elemental identity. DOI: 10.1103/PhysRevLett.92.223002 PACS numbers: 33.60.Fy, 33.80.Eh For decades, photoelectron spectroscopy (PES) has been an established method for probing the electronic and chemical structure of matter in both gaseous and condensed phases [1]. Coupled with a tunable light source, such as synchrotron radiation, PES is often done resonantly; e.g., direct photoemission intensity from an outer (valence) orbital or energy level is modified in a narrow wavelength range by interference with resonant excitation of an electron in a deeper-lying (core) orbital. Recently, a new phenomenon, multiatom resonant photoemission (MARPE), was reported in condensed phase MnO [2 -4] in which the core-level photoemission intensity, or cross section, from one element (O) is enhanced upon resonant excitation of a core electron from a different element (Mn) in the solid. The unprecedented core-to-core interaction represented by MARPE is explained as a collective resonant effect from several nearby atoms and has been proposed as a new tool for identifying near-neighbor atoms (within 2 nm) in solids. Reports of MARPE have engendered both interest and skepticism in the photoemission community [5,6]. From an isolated-molecule point of view, MARPE is an unusual form of resonant-Auger decay and can be understood as an interatomic coupling between direct core-electron photoemission from one atom and resonant core-electron excitation of a different atom in the same molecule. Up to now, attempts to find experimental evidence for this MARPE-like effect in small gas-phase molecules by looking for variations in photoemission intensities, whether integrated (cross sections) or differential (angular-distribution parameters), have been in vain.In this Letter we report the first evidence of an interatomic coupling effect in molecular photoemission solely involving core levels. To distinguish it from the relatively longer-range MARPE effect, the inherently short-range (i.e., nearest-neighbor-only) effect in isolated molecules will be referred to as nearest-neighbor-atom core-hole transfer, or NACHT. While solid-state MARPE affects photoemission cross sections, no such intensity variations are observable in the molecular analog; the NACHTeffect is measurable only in the differential cross section, which describes the angular distributions of photoelectrons and is sensitive to the phases of the photoelectrons' continuum wave functions, as well as their magnitudes. Moreover, it is necessary to consider differential-cross-section effects beyond the usual dipole approximation (DA) for interactions between radiation and matter. The electri...

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

“…The same nearest-neighbor Q-NACHT effect was observed (Guillemin et al, 2004). The same nearest-neighbor Q-NACHT effect was observed (Guillemin et al, 2004).…”

“…To search for NACHT effects, direct photoemission from the two inequivalent N 1s core levels in N 2 O was probed in the vicinity of the resonant excitation of an O 1s electron to the lowest unoccupied 3p n molecular orbital (Guillemin et al, 2004). The O 1s ionization threshold lies about 130 eV above the nitrogen thresholds, at 541.4 eV.…”

Interatomic resonant Auger effects in core-level photoemission from NO and CS2molecules

“…Therefore, we could not find any signature of the IARA effect in the LF-PADs. In addition, there was no significant difference in the non-dipole parameters between on-and off-resonant LF-PADs, whilst non-dipolar angular distributions of core-level photoemissions were a measure of the IARA in the free N 2 O and OCS molecules [19]. In contrast to this, the geometrydependent MF-PADs, or equivalently the symmetry selected MF-PADs, can provide the direct evidence of the IARA, as reported in our previous paper [20].…”

“…One promising approach to observing such an interatomic effect is differential crosssection measurements which provide detailed information about photoionization dynamics. The first attempt to observe the IARA in isolated molecules has been made by measuring the nondipole contributions of core-level photoelectrons from free N 2 O and OCS molecules [19]. Later, a fixed-molecule experiment based on photoelectron-fragment-ion-fragmention coincidence momentum spectroscopy was applied to the IARA of NO [20], which found that the molecular frame N 1s photoelectron angular distributions (PADs) are slightly affected by the O 1s → π * resonance.…”

Observation and resonant x-ray optical interpretation of multi-atom resonant photoemission effects in O1semission from NiO

“…However, this study underestimates the relative intensity of such effects in considering the squares of the direct and resonant matrix elements involved, rather than the matrix elements themselves, since it is the interference between the two processes that leads to MARPE 4,10 . Finally, more subtle effects on the non-dipole angular distributions parameters in core emission from small gasphase molecules have been observed, in particular for N 1s emission from N 2 O with two inequivalent N atoms as the O 1s resonance is scanned, and C 1s and S 2p emission from OCS, again as the O 1s resonance is scanned 18 . These effects have been termed near-atom core-hole transfer (NACHT), but they also represent a manifestation of MARPE, albeit a more subtle one.…”