Electronic relaxation dynamics in (water)$_{n}^{-}$ ($n$=25-100) and (CH$_{3}$OH)$_{n}^{-}$ ($n\sim $140-530) clusters via femtosecond photoelectron imaging

Large clusters of (H$_{2}$O)$_{n}^{-}$ ($n$=25-50), (D$_{2}$O)$_{n}^{-}$ ($n$=25-100) and (CH$_{3}$OH)$_{n}^{-}$ ($n\sim $140-530) are studied with femtosecond time-resolved photoelectron imaging. For all three systems, the excess electron is promoted to an excited state with a pump laser pulse at 1.55 eV. Subsequent dynamics are monitored by observing photoelectrons detached after a variable delay with a probe pulse at 3.1 eV. For all three systems the excess electron is seen to decay via internal conversion back to the ground state with lifetimes of 190-130 fs for (H$_{2}$O)$_{n}^{-}$, 360-150 fs for (D$_{2}$O)$_{n}^{-}$ and 260-170 fs for (CH$_{3}$OH)$_{n}^{-}$. For all three systems, lifetime of the excited state decreases with increasing cluster size and is found to vary linearly with 1/$n$. Extrapolation to the bulk yields lifetimes of 54$\pm $30 fs for H$_{2}$O, 72$\pm $22 fs for D$_{2}$O and $\sim $150 fs for CH$_{3}$OH.