Velocity of a Molecule Evaporated from a Water Nanodroplet: Maxwell–Boltzmann Statistics versus Non‐Ergodic Events

Abstract

The velocity of a molecule evaporated from a mass-selected protonated water nanodroplet is measured by velocity map imaging in combination with a recently developed mass spectrometry technique. The measured velocity distributions allow probing statistical energy redistribution in ultimately small water nanodroplets after ultrafast electronic excitation. As the droplet size increases, the velocity distribution rapidly approaches the behavior expected for macroscopic droplets. However, a distinct high-velocity contribution provides evidence of molecular evaporation before complete energy redistribution, corresponding to non-ergodic events.

Impact of a hydrophobic ion on the early stage of atmospheric aerosol formation

“…The velocity distributions measured for the evaporation of 1 water molecule from (PyH + )(H 2 O) n (shown in Fig. 1 A for n = 4) exhibit 2 distinct components, as observed previously for pure water clusters (26). However, in contrast to the latter case, the low-velocity part of the distribution presently measured for single-molecule evaporation cannot be satisfactorily fitted by a single MB distribution.…”

“…The measured velocity distributions of the molecules evaporated from (H 3 O + )(H 2 O) n protonated water clusters for n = 1 to 10 exhibit 2 contributions, namely a low-velocity part that can be well described by MB statistics and a high-velocity part corresponding to nonergodic (NE) events caused by molecules that evaporate before complete redistribution of the excitation energy within the cluster. Through dedicated atomistic modeling based on statistical molecular dynamics (SMD) simulations (26, 27), the statistical nature of the main contribution at low velocities and the sequential evaporation of several molecules were confirmed by the ability to reproduce the experimental velocity distributions (28). Moreover, the contribution of NE events observed at high velocities was analyzed and interpreted as being due to the evaporation of molecules before complete redistribution of the excitation energy.…”

“…In small water clusters with few degrees of freedom, evaporation competes more strongly with energy redistribution upon excitation after condensation and different velocity distributions may arise. To elucidate this phenomenon on a microscopic level, molecular evaporation from ultimately small protonated water clusters containing less than 10 molecules was investigated using a recently developed experimental technique (25, 26). After a femtosecond excitation induced by a single high-velocity collision with an argon atom, the out-of-equilibrium water cluster ion relaxes via the evaporation of 1 or several molecules.…”

Sequential water molecule binding enthalpies for aqueous nanodrops containing a mono-, di- or trivalent ion and between 20 and 500 water molecules

“…49)) and depend only slightly on the cluster size. These results and recent velocity map imaging experiments 50 are consistent with the idea that the absorbed photon energy is fully redistributed into the internal modes of the entire cluster for n > 10, resulting in sequential water molecule evaporation, although ion fluorescence resulting in fewer water molecules that are lost can also occur. 47,51 …”

From atoms to aerosols: probing clusters and nanoparticles with synchrotron based mass spectrometry and X-ray spectroscopy

“…For instance recent innovative experiments used collisions of water cluster ions with an argon beam, to measure energy distributions and subsequent evaporation dynamics. 115 The timescales of reaction (2) was only recently clocked with femtosecond X-ray spectroscopy 116 at an FEL in which the proton transfer between the water cation and neutral water leading to the elimination of the hydroxyl radical and formation of protonated water was mapped out in liquid water.…”