“…Properties of solution-phase noble metal clusters in the size regime of 1–3 nm have drawn significant interest in the last few year because of unique properties dependent on the structure, where more than 100 such clusters have been synthesized and characterized so far (ref and references therein). Clusters of noble metal atoms can have light absorption and emission properties useful for applications, including photodynamic therapy, − photovoltaics, − optical memory, − imaging, − sensors, − and biolabels. − Noble metal clusters demonstrate strong luminescence, − tunability in the absorption wavelength, and also a large two-photon absorption (TPA) cross-section. ,, Materials that exhibit TPA have advantages in tighter spatial confinement, as well as in accessing electronic excitation of twice the incident photon energy, and can be particularly useful in photodynamic therapy, − imaging, − and optical memory, , as well as in confocal microscopy , and microfabrication. − The possible applications of noble metal clusters are being expanded by using them as building blocks to form superclusters. , We have investigated one-photon absorption (OPA) and TPA spectra of [Au 25 (SR) 18 ] −1 by applying density functional theory (DFT) and time-dependent DFT (TDDFT). The large TPA cross-section was explained in terms of resonance enhancement .…”