RESUMEN
The valence photoelectron spectra of water clusters are studied experimentally and by ab initio calculations. The size dependence of the vertical ionization energy of the outermost orbitals is explicitly shown. A shift toward lower values is observed. For small cluster sizes, it can be rationalized as an effect of charge delocalization as the system is becoming more extended. Ionization energies of larger clusters decrease linearly with inverse cluster radius and asymptotically approach the value of liquid water. In the calculations, we apply a reflection principle approach based on sampling a quantum mechanical distribution of different initial-state geometries to clusters. An excellent agreement of peak shapes calculated thus with measured ones is shown. Using additional polarization fields, the extension of this approach to the photoionization of liquid water is demonstrated. Upon deuteration of the water clusters, we experimentally and theoretically find slightly larger absolute values of the vertical ionization energies. We suggest that the measurement of electron ionization energies can be used as an alternative means to characterize water cluster sizes, which can complement the use of scaling laws.
RESUMEN
Because of inversion symmetry and particle exchange, all constituents of homonuclear diatomic molecules are in a quantum mechanically non-local coherent state; this includes the nuclei and deep-lying core electrons. Hence, the molecular photoemission can be regarded as a natural double-slit experiment: coherent electron emission originates from two identical sites, and should give rise to characteristic interference patterns. However, the quantum coherence is obscured if the two possible symmetry states of the electronic wavefunction ('gerade' and 'ungerade') are degenerate; the sum of the two exactly resembles the distinguishable, incoherent emission from two localized core sites. Here we observe the coherence of core electrons in N(2) through a direct measurement of the interference exhibited in their emission. We also explore the gradual transition to a symmetry-broken system of localized electrons by comparing different isotope-substituted species--a phenomenon analogous to the acquisition of partial 'which-way' information in macroscopic double-slit experiments.
RESUMEN
The inner-shell photoionization of unoriented camphor molecules by circularly polarized light has been investigated from threshold to a photoelectron kinetic energy of approximately 65 eV. Photoelectron spectra of the carbonyl C 1s orbital, recorded at the magic angle of 54.7 degrees with respect to the light propagation direction, show an asymmetry of up to 6% on change of either the photon helicity or molecular enantiomer. These observations reveal a circular dichroism in the angle resolved emission with an asymmetry between forward and backward scattering (i.e., 0 degrees and 180 degrees to the light beam) which can exceed 12%. Since the initial state is an atomiclike spherically symmetric orbital, this strongly suggests that the asymmetry is caused by final-state effects dependent on the chiral geometry of the molecule. These findings are confirmed by electron multiple scattering calculations of the photoionization dynamics in the electric-dipole approximation.
RESUMEN
We have observed the direct L(2,3)MMM double Auger transition after photoionization of the 2p shell of argon by angle-resolved electron-electron coincidence spectroscopy. The process is responsible for about 20% of the observed Auger electron intensity. In contrast to the normal Auger lines, the spectra in double Auger decay show a continuous intensity distribution. The energy and angular distributions of the emitted electrons allow one to obtain information on the electron correlations giving rise to the double Auger process as well as the symmetry of the associated two-electron continuum state.