We describe two improvements to an existing tandem mass spectrometer coupled to a laser vaporization cluster ion source suitable for photodissociation spectroscopy: (i) cooling of the cluster source nozzle and (ii) mass selection prior to the photodissociation region via replacing an octupole ion guide by a quadrupole mass spectrometer. The improved sensitivity and transmission enable the production of larger heteroatomic clusters as well as rare gas solvated clusters. We present two examples demonstrating the new capabilities of the improved setup. In the first application, cooling of the cluster source nozzle produces SiAr and SiAr cluster cations with n = 1-25. Magic numbers are extracted from the mass spectrum by applying a transmission function obtained via simulations. In the second example, the vibronic photodissociation spectrum of cold Au cluster ions is recorded with unprecedented detail, resolution, and sensitivity. Such high-resolution optical excitation spectra of metal cluster cations may serve as a benchmark for the performance of Franck-Condon simulations based on quantum chemical calculations for excited states.

Knowledge of the geometric and electronic structure of gold clusters and nanoparticles is vital for understanding their catalytic and photochemical properties at the molecular level. In this study,w er eport the vibronic optical photodissociation spectrum of cold and mass-selected Au 4 + clusters measured at aresolution high enough to allowfor comparison with Franck-Condon simulations of the excited state transitions based on time-dependent density functional theory calculations.T he three vibrational frequencies identified for the lowest-lying optically accessible excited state at 2.17 eV stem from the Y-shaped isomer (C 2v )and not from the rhombic isomer (D 2h )c onsidered to be the ground state structure of Au 4 + .T his study demonstrates that an analysis of lowresolution electronic spectra by calculations of vertical transitions alone is not sufficient for areliable isomer assignment of such metal clusters.

Knowledge of the geometric and electronic structure of gold clusters and nanoparticles is vital for understanding their catalytic and photochemical properties at the molecular level. In this study,w er eport the vibronic optical photodissociation spectrum of cold and mass-selected Au 4 + clusters measured at aresolution high enough to allowfor comparison with Franck-Condon simulations of the excited state transitions based on time-dependent density functional theory calculations.T he three vibrational frequencies identified for the lowest-lying optically accessible excited state at 2.17 eV stem from the Y-shaped isomer (C 2v )and not from the rhombic isomer (D 2h )c onsidered to be the ground state structure of Au 4 + .T his study demonstrates that an analysis of lowresolution electronic spectra by calculations of vertical transitions alone is not sufficient for areliable isomer assignment of such metal clusters.