Bulk-like electron-phonon coupling in small free sodium clusters

Free size selected Nan+ (n = 16-250) clusters have been studied by femtosecond pump-probe photoelectron and photofragmentation spectroscopy. Thermionic electron emission was used to monitor the cooling of the hot electron gas by energy transfer to the ionic background. The results can be well described by the so-called two-temperature model; even for the smallest size no deviation from such bulk-like, continuum model behavior has been observed. Using this theoretical approach we are able to retrieve the electron-phonon coupling constant as a function of the cluster size. In the studied size range the resulting values can be described by the radius dependent function g(R) = (2.3 + 114 Å2/R2) × 1016 W/m3K.