Photoionization and excitation energies of an Al atom in Ar_N clusters

Photoionization measurements on Al·Ar_N clusters (N < 150) in a cold cluster beam have been used to determine ionization threshold energies E_{l, th}(N) of the metal atom in a variable-size dielectric medium. The E_l,th(N) values descend from the atomic value [5.99 eV (1 eV ≈ 1.602 × 10^-19 J)] to below 5.5 eV near N ≈ 55. The E_{l, th}(N) functional form is compared to dielectric continuum predictions and to N-specific Monte Carlo calculations incorporating polarization and model-potential interactions. Satisfactory agreement is obtained when a strong neutral Al-Ar interaction is assumed. Measurements of the slow fragmentation probability indicate that near-threshold ionization of smaller clusters (N < 25) produces negligible fragmentation, resulting in N-specific threshold measurements. Comparison of neutral cluster distributions, measured by threshold photoionization to the corresponding cold Al⁺·Ar_N distribution (produced in the source) supports a predominant charge-invariance of the icosahedral-based structures, also found in Monte Carlo simulations. By contrast, photo-absorption spectra in bound states, as exemplified by R2PI spectra of the Al 3p → 4s transition, exhibit only a small fraction (ca. 0.12 eV) of the bulk shift (ca. 0.6 eV). This result may be partly explicable in terms of the distinction between sudden and adiabatic polarization energies in the two experiments.