Atomic Cluster Au10+ Is a Strong Broadband Midinfrared Chromophore

Abstract

We report an intense broadband mid infrared absorption band in the Au10 + cluster in a region in which only molecular vibrations would normally be expected. Observed in the infrared multiple photon dissociation spectra of Au10Ar + , Au10(N2O) + and Au10(OCS) + , the smooth feature stretches 700-3400 cm -1 (=14-2.9 µm). Calculations confirm unusually low-energy allowed electronic excitations consistent with the observed spectra. In Au10(OCS) + , IR absorption throughout the band drives OCS decomposition resulting in CO loss, providing an alternative method of bond activation/breaking. TOC GRAPHICSAu10 + exhibits a remarkable, broad absorption feature in the mid infrared where only molecular vibrational bands would normally be expected.

Near‐Infrared Spectrum of the First Excited State of Au2+

“…Most previous computations of Au n + clusters rely merely on the calculation of vertical electronic excitations. [ 6 , 7 , 15 ] In our particular case, the predicted NIR transition shifts from 1320 to 1610 nm upon geometry optimization. Ar complexation increases the oscillator strength to f =0.0017 and has only a minor stabilizing impact on the Au−Au bond in the à state (Δ r e =−7.7 mÅ, Δ ω 2 =+12 cm −1 ).…”

“…This band has been attributed to three overlapping LUMO←SOMO electronic transitions, and its large width has been rationalized by spectral congestion from unresolved vibronic excitation, vibronic coupling of the Jahn‐Teller distorted tetrahedral structure, and/or lifetime broadening. [7] In contrast to the larger and more complex Au 10 + cluster, the lowest‐energy NIR excitation of Au 2 + observed at 0.71 eV arises from a single and well‐isolated SOMO←HOMO‐1 transition resulting in a regular well‐resolved electronic spectrum, with a long‐lived excited state ( τ ≥1 ps). Significantly, the Au 2 + Ar spectrum allows the determination of all vibrational frequencies, thereby providing very detailed information about the Au−Au and Au−Ar bonds as a function of electronic excitation.…”

“…The lowest excited states of Au n + clusters show a strong even‐odd alternation, and the open‐shell n =odd clusters have predicted transitions in the near‐infrared (NIR) range. [7] In this respect, Au 10 + exhibits a particularly low and broad transition centered at around 0.55 eV, which extends down into the vibrational domain of the ground electronic state. For Au 2 + , calculations predict an optically active and well isolated A 2 Σ u + state around 0.8 eV above the X 2 Σ g + ground state arising from 6 s←5d excitation of an electron from the antibonding σ u * orbital (HOMO‐1) into the bonding σ g (s) orbital (SOMO), as shown in Figure 1 .…”

“… [6] Thus, in addition to reducing the temperature of Au 2 + , the Ar tag drastically reduces the effective dissociation energy of the considered cation, thereby enabling single‐photon dissociation from the vibronic ground state. [ 7 , 8 , 9 ]…”

Thermal radiative cooling of carbon cluster cations C$_N^+$, $N = 9, 11,12, 17-27$

“…[20] provides a review of the status of the laboratory studies of the subject until 2019. In a recent development, the reverse process of absorption has been observed for both the cationic gold decamer cluster [21], and very recently spectroscopic experiments on the same clusters that have been studied here have shown absorption for wavelengths from UV over visible to near infrared wavelengths [22].…”