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.