Evaluation of photodissociation spectroscopy as a structure elucidation tool for isolated clusters: a case study of Ag4+ and Au4+

Abstract

Resolving the structure of clusters in the gas phase often requires the comparison of experimental data to quantum chemical calculations. Herein, we present the variation of a straightforward approach, in which photodissociation spectra of isolated clusters are compared to optical response calculations in order to elucidate cluster structures. Our absorption spectra were measured using a newly built longitudinal beam depletion spectroscopy apparatus and recorded in the photon energy range ħω = 1.9-3.5 eV. Cluster geometries were obtained using the unbiased Birmingham Cluster Genetic Algorithm coupled with density functional theory, while the optical response was calculated in the framework of time-dependent density functional theory. Experiments and excited state calculations are in excellent agreement using long-range corrected exchange correlation functionals for both ground and excited state calculations. Our methodology indicates a contribution of Y shaped Au4(+) whereas for Ag4(+) only the ground state isomer has to be considered to explain the experimental absorption spectrum. Our extended methodology shows two nearly degenerate isomers of Au4(+) probably being present in the molecular beam and therefore shows promise for the further structure determination of pure and binary transition-metal clusters.

Application of a parallel genetic algorithm to the global optimization of medium-sized Au–Pd sub-nanometre clusters

“…However, even the optimisation of gas-phase structures is not a trivial task. There is usually little energy separation between many competitive isomers of metallic clusters [25], which may explain the challenges that face experimentalists in determining the preferred isomer [26,27]. In addition, charge-neutral clusters are more difficult to investigate experimentally compared with anions and cations, as most characterizations rely on mass spectrometry measurements, which creates a clear difficulty in separating and probing different size of neutral clusters [28,29].…”

Communication: Global minimum search of ${\rm Ag}^+_{10}$ Ag 10+ with molecular beam optical spectroscopy

“…32 The long-range corrected (LC) exchange correlation (xc) functional, LCωPBEh, 33 is used throughout our whole analysis. It has been shown to perform well for the calculation of ground and excited state properties of gold, 24,34 silver, 24 and mixed silver gold clusters, 35 where it leads to the reliable prediction of optical absorption spectra.…”

“…41 The long-range corrected xc functional LC-ωPBEh 33 is used in order to accurately recover the asymptotic behaviour of the exchange correlation potential, since this has proven to reliably reproduce vertical electronic excitation spectra. 4,6,8,24,33,34 The theoretical description of optical properties of clusters based on TDDFT calculations is easily applicable and widely used, in particular due to the low computational costs associated with the single-reference character. 42 In minimizations the BP-86 43,44 and M06-L 45 functionals are also studied for comparison purposes.…”

“…17 However, especially at small sizes, a quantum chemical treatment becomes important since the electronic structure of the sub-nanometer particles becomes more discrete. In order to probe the optical response of small silver clusters, photodissociation spectroscopy [18][19][20][21][22][23][24] and rare-gas matrix spectroscopy 14,16,25 have been reported previously. Very recently, Ito et al presented optical spectra of Ag + n (n = 8-14) clusters in the range¯ω = 3.7-4.4 eV.…”

“…24 Briefly, cluster cations are produced by pulsed laser vaporization and separated with time-of-flight mass spectrometry. The optical response is probed by longitudinal photodissociation spectroscopy with a tunable ns-laser pulse from an optic parametric oscillator, pumped by the third harmonic of an Nd:YAG laser.…”