UV Photoionization of Sodium‐Doped Formic Acid Clusters

Abstract

The processes involved in the photoionization of sodium-doped clusters are complex, not fully understood for many systems and still strongly debated, especially because of the discrepancy between experimental results and predicted cluster structures. We have performed a study on sodium doped formic acid clusters based on UV photoionization spectroscopy and DFT/TDDFT calculations. Apart from the monomer, all the predicted structures show vertical ionization potential values higher than those obtained by the photoionization measurements. We have calculated the absorption spectra and found many Rydberg-like states near the adiabatic ionization potentials and, crucially, in the UV range where the clusters appearance energies fall. This finding supports the hypothesis of adiabatic contributions in the measured ionization potentials for these clusters.

Photoionization, Structures, and Energetics of Na‐Doped Formic Acid–Water Clusters

“…In the supersonic expansion in the vacuum, carboxylic acid vapors easily aggregate forming pure acid or mixed acid‐water clusters detected in our apparatus as Na‐doped clusters by laser UV ionization [13–15] . In particular, in ref [14] we demonstrated that the ion signals of Na−(FA) n are produced by one‐photon ionization, so the cluster ionization potentials were correctly assigned in the laser UV range.…”

“…We have already found appearance energies in the UV range for similar classes of clusters. These appearance energies have been attributed to clusters characterized by adiabatic ionization energies in the same UV energy range of the PIE spectra [14] . In fact, the adiabatic photoionization implies a significant rearrangement of the ionic core and formation of electronic Rydberg‐type excited states that could give rise to autoionization during orbital relaxing.…”

“…As results, the mass spectra are simplified, pure (FA) n or mixed (FA) m (H 2 O) n clusters are not formed, but the Na−FA(H 2 O) n are detected as low intensity peaks. Nevertheless, it was still possible to evaluate the appearance energy of these clusters although with a signal/noise ratio lower than that of Na−(FA) n or Na−(H 2 O) n clusters [14,17] …”

“…Each spectrum has been normalized to its maximum and the appearance energies have been retrieved from the wavelength corresponding to the 10 % of the maximum. The PIE spectra of smaller clusters (n≤2) have been acquired on the beam front as their signals are more intense [14] . From the figure it is clear a shift of the spectrum onset from n=1 to n=2.…”

“…The experimental setup is described in details in [13,14,23] . Basically, it was constituted by a molecular beam, a homemade Wiley–McLaren time of flight spectrometer (TOF), and an Optical Parametric Oscillator (OPO) laser source.…”