Photoabsorption of silver cluster cations in an ion trap: nonlinear action spectra via multi-photon dissociation vs. directly measured linear absorption spectra

We report photodissociation processes and spectral measurements upon photoabsorption of size-selected cationic silver clusters, Ag_N⁺, stored in an ion trap. The experiment shows that small clusters (N ≲ 15) dissociate upon one-photon absorption, whereas larger ones require multiple photons up to five in the present study. The emergence of multi-photon processes is attributed to collisional cooling in the presence of a buffer helium gas in the trap, which competes with size-dependent dissociation rates. These observations are explained by simulations that consider the two competing effects, where the statistical Rice-Ramsperger-Kassel (RRK) theory is employed to evaluate dissociation rates. Action spectra of photodissociation are compared with linear absorption spectra directly measured by cavity-ring-down-type high-sensitivity spectroscopy, revealing that the profiles of the action spectra are sharpened by the nonlinear effects in the multi-photon regime. This observation demonstrates the importance of the linear absorption measurement to obtain both spectral profiles and cross sections for large clusters that exhibit multi-photon dissociation.