Calculation of photoionization cross sections of Na2–8 and K2–8 clusters

Abstract

Analysis of free metal clusters studied with photoionization mass spectrometry or photoelectron spectroscopy requires theoretical predictions of the photoionization cross sections to gain a deeper physical understanding. Calculated energy-dependent photoionization cross sections of Na2_ 8 and K2_ 8 clusters are presented in this study. The ground state electronic structure of the clusters are calculated using the Local Spin Density method (LSD) which is also the starting point for the cross section calculation with the continuum multiple scattering method. A basic analysis of the photoionization process is given within the independent electron picture. Strong resonances are predicted in the UV cross sections (5-10 eV) of K3_ 8 but not for Na3_s, interpreted as shape resonances, i.e. quasibound states in which electrons are trapped by a potential barrier. Unfortunately experimental data are only known close to the ionization threshold and a comparison between our values and experimental data in a broad energy range is not possible.

Analysis of the odd-even alternation in simple metal clusters

“…The method has previously been described in detail [6,20], and only the main features are sketched here. Atomic numerical wave functions have been used as basis functions.…”

“…These were generated for the corresponding spherical symmetric atomic problem using an SCF code. To allow for a larger variational freedom, we used, in addition to the occupied wavefunctions for the neutral atom, polarization wavefunctions (np) and the corresponding valence wavefunctions, (ns, np) for the doubly ionized atom [6]. The LCAO description, in combination with the Rayleigh-Ritz variational principle reformulates the Kohn-Sham equations into a matrix problem, which was solved using standard numerical matrix techniques.…”

Ionization energies and stabilities of Na n ,n<25: shell structure from measurements on cold clusters

“…In the case of sodium, the large decline in qS, predicted to occur after n = 18 and 20 were actually deduced, on the basis of assigned threshold energies, to be entirely absent or inverted (see Table t below), and the interpretation of the predicted large drop from n = 8 to 9 was left uncertain because of the long thermal tails present in the photoionization spectra [6]. In the case of n = 9, two values had been recommended, the first being plotted most often, E~, +) values from [5]; although for some sizes dimer loss is the predominant fragmentation channel, for these sizes (n = 8,9,[18][19][20][21] We have recently described a new low-temperature source of soft-metal clusters [11]. In the case of n = 9, two values had been recommended, the first being plotted most often, E~, +) values from [5]; although for some sizes dimer loss is the predominant fragmentation channel, for these sizes (n = 8,9,[18][19][20][21] We have recently described a new low-temperature source of soft-metal clusters [11].…”

Photoionization of alkali metal clusters

“…where the initial and final states are characterized by the respective energies E in (7) and by different phases in (8).…”

“…Similarly, theoretical studies of photoionization have been limited to a few eV above the threshold [7,8]. The theoretical description of photoionization of clusters -although fundamentally very simple and straightforward -poses some problems which are conceptually hard to overcome.…”