(Color online) (A) Mass spectrum of generated by the pulsed electron impact source. The traces of , , and clusters are indicated by the asterisk. An unidentified ion peak at and its water clusters are indicated by a question mark (see text for details). (B) Metastable species detected by the linear reflectron time-of-flight mass spectrometer (without a laser pulse). The detection threshold in the experiment was lower than for other clusters due to a lower signal level. (C) Photofragments generated by irradiating with the pump pulse . For (B) and (C), the vertical axis corresponds to the fragment ion intensity and the enhancement of the ion intensity, respectively, normalized to the intensity of the stable parent ion (without laser).
Photoelectron spectra of , where , obtained by irradiating with the probe pulse .
(Color) (A) Time-dependent photoelectron spectra (TD-PE) of plotted on the electron time-of-flight axis. The label in each panel is the time delay in femtoseconds. Each spectrum is constructed by subtracting a reference ( at ) from a total signal ( at ). In order to illustrate the change in shape, each panel, except for the bottom right, displays the TD-PE spectrum at the time delay indicated with intensity adjusted to match that of TD-PE at (red trace). In the first and second panels, PE signals of and a vibrationally excited form of are overlaid with the TD-PE spectra of (see text). The bottom right panel contains a few TD-PE spectra in the true intensity scale as determined from the transient of Fig. 4. (B) TD-PE spectra of . Same as (A), except that PE spectra of and vibrationally excited are shown in green and the intensities in the bottom right panel are determined from the transient of Fig. 4. (C) Simulated (red) and experimental (black) TD-PE spectra of (top panel) and (bottom three panels) at . Second panel: simulation with and . Third panel: simulation with , , and . Bottom panel: simulation with , , and (see text).
Femtosecond transients obtained by monitoring the spectral window indicated in Figs. 3(A) and 3(B) for the excitation of , where . The transient of is fitted to a biexponential rise function with two time constants ( and ), while for the other clusters, an additional rise is included. The fits are shown by solid lines. The inset in the top panel shows an experimental magic angle scan, while all other transients are constructed magic angle scans (see text). Transients of , where , over long time ranges are presented as insets.
Polarization-dependent transients at . The polarization of the probe pulse was fixed parallel to the photoelectron detection axis, while the polarization of the pump pulse was rotated. shows polarization-dependence opposite to the larger clusters, where , but only is provided for comparison.
Time-dependent anisotropy decay obtained from parallel and perpendicular transient scans normalized to give at long time. The estimated coherence times are indicated (see text). Solid lines are the fits using the hypothesized structures in Fig. 7 as described in the text. The grey shading indicates time for which and are small and becomes erratic.
(Color online) Proposed structures for , where . The midpoints of molecules lie on the plane. The deduced transition dipole moment for the photodetachment process of is along the axis. Angles between and are indicated.
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