Below Band Gap Formation of Solvated Electrons in Neutral Water Clusters?

Abstract

Below band gap formation of solvated electrons in neutral water clusters using pump–probe photoelectron imaging is compared with recent data for liquid water and with above band gap excitation studies in liquid and clusters. Similar relaxation times on the order of 200 fs and 1–2 ps are retrieved for below and above band gap excitation, in both clusters and liquid. The independence of the relaxation times from the generation process indicates that these times are dominated by the solvent response, which is significantly slower than the various solvated electron formation processes. The analysis of the temporal evolution of the vertical electron binding energy and the electron binding energy at half-maximum suggests a dependence of the solvation time on the binding energy.

Size-Resolved Electron Solvation in Neutral Water Clusters

“…The high yield within the IRF time mainly results from direct (1 + 1′) two-photon ionization. The systematic decrease of the ion yield at longer Δ t represents the decrease in the solvated electron signal with Δ t , as previously observed by photoelectron spectroscopy ( Figure 3 and ref ( 51 )). In addition to the general loss of ion signal, the shape of the size distribution also changes with increasing Δ t : the ion yield of smaller clusters decreases more rapidly than that of larger clusters with increasing Δ t .…”

“…An intriguing result is the apparent independence of the hydrated electron signatures on the system size. Similar solvation dynamics, vertical binding energies (VBE), and survival probabilities are reported for both neutral clusters (average sizes ranging from 300 to 500 water molecules 50 − 52 ) and bulk liquid (liquid microjet 53 ) (see Table 1 in ref ( 51 )). Likewise, the solvation dynamics and VBEs in neutral clusters and bulk liquid are also found to be largely independent of the excitation energy from below to above band gap in the region from 7.7 to 15.5 eV.…”

“… (a) Time-dependent cluster ion yield I tot (Δ t ) (circles) with fit results (black line) for a sequential kinetic model with two decaying (blue and green dashed lines) and an impulsive (yellow dashed-dotted line) signal component. (b) Time-dependent total ion yield (thick black line) and total electron signal 51 (blue dashed line) after subtraction of the impulsive signal component and scaling to the same maxima. In both panels, the abscissa is linear up to 0.9 ps (vertical line) and logarithmic for longer times.…”

“…The experimental setup was previously used and described in refs ( 50 ), ( 51 ), and ( 62 ), with the major difference here being the use of time-of-flight mass spectrometric (TOF-MS) detection instead of photoelectron detection. Neutral water clusters were generated in a pulsed supersonic expansion through an Even-Lavie valve 63 with Ne as carrier gas.…”

“…The results are compared with our previous TRPES study performed with the same pump–probe excitation scheme. 51 The time-resolved TOF-MS provides complementary cluster-size-resolved information about hydrated electron formation and loss processes.…”