Vibrational branching ratios in the photoelectron spectra of N2 and CO: interference and diffraction effects

Abstract

Esta es la versión de autor del artículo publicado en: This is an author produced version of a paper published in: We present a detailed account of existing theoretical methods specially designed to provide vibrationally resolved photoionization cross sections of simple molecules within the Born-Oppenheimer approximation, with emphasis on newly developed methods based on density functional theory. The performance of these methods is shown for the case of N 2 and CO photoionization. Particular attention is paid to the region of high photon energies, where the electron wavelength is comparable to the bond length and, therefore, two-center interferences and diffraction are expected to occur. As shown in a recent work [Canton et al., Proc. Natl. Acad. Sci., 2011, 108, 73027306], the main experimental difficulty, which is to extract the relatively small diffraction features from the rapidly decreasing cross section, can be easily overcome by determining ratios of vibrationally resolved photoelectron spectra and existing theoretical calculations. From these ratios, one can thus get direct information about the molecular geometry. In this work, results obtained in a wide range of photon energies and for many different molecular orbitals of N 2 and CO are discussed and compared with the available experimental measurements. From this comparison, limitations and further possible improvements of the existing theoretical methods are discussed. The new results presented in the manuscript confirm that the conclusions reported in the above reference are of general validity.

Vibrationally resolved K-shell photoionization cross sections of methane

“…No relative variations of the vibrational components in the vibrationally resolved absolute cross sections are observed, which is consistent with the fact that CH 4 does not exhibit shape resonances. As discussed in [21], these relative variations are better observed in the ratios of vibrationally resolved cross section (the so-called -ratios); however, apart from a pronounced dip around 320-340 eV [45], whose origin is still unknown, such ratios do not exhibit pronounced structures in the low photon energy range considered in figure 3. In contrast, pronounced oscillation of the -ratios as a function of photon energy have been predicted and observed [3].…”

“…The theoretical method is the same as that described in references [3,21]. Therefore, here we only summarize its basic ingredients.…”

“…For each value of the C-H distance, the initial Kohn-Sham orbitals have been generated by running a standard LCAO-DFT calculation with the program Amsterdam Density Functional (ADF) using a Double ZetaPolarization (DZP) basis set. The resulting density is then used to build the hamiltonian matrix in a new basis set of B-spline functions and real spherical harmonics [3,21]. We use both a one-center expansion (OCE) centered on the C atom, which allows us to correctly describe the long range oscillatory behavior of the continuum wave function, and four identical off-center expansions centered on the H atoms, which are used to accurately describe the cusps of the bound state wave functions at the nucleus positions.…”

“…The method presented here and explained in more details in [21] is based on the Born-Oppenheimer approximation which permits to treat separately electronic and nuclear degrees of freedom. The electronic structure is calculated using the so-called static-exchange DFT method developed during the last 15 years by Decleva et al [22][23][24][25].…”

Angular Momentum Sensitive Two-Center Interference

“…The equation implies that the molecular system absorbs a photon from both possible sites of electron excitation. Recent studies of the molecular double-slit phenomenon in the vibrational branching ratios of molecular photoionization gave the first experimental evidence that the basic model of Cohen and Fano is indeed valid for the valence states [9][10][11]. However, the oscillation of the partial cross sections was never directly observed.…”