Dissociative photodetachment studies of O−(H2O)2, OH−(H2O)2, and the deuterated isotopomers: Energetics and three-body dissociation dynamics

Formation of Hydrogen Peroxide from O-(H2O)n Clusters

Hydrogen peroxide (H₂O₂) has recently received much attention as a safe and clean energy carrier for hydrogen molecules. In this study, based on direct ab initio molecular dynamics (AIMD) calculations, we demonstrated that H₂O₂ is directly formed via the photoelectron detachment of O^-(H₂O)_n (n = 1-6) (water clusters of an oxygen radical anion). Three electronic states of oxygen atoms were examined in the calculations: O(X)(H₂O)_n (X = ³P, ¹D, and ¹S states). After the photoelectron detachment of O^-(H₂O)_n (n = 1) to the ¹S state, a complex comprising O(¹S) and H₂O, O(¹S)-OH₂, was formed. A hydrogen atom of H₂O immediately transferred to O(¹S) during an intracluster reaction to form H₂O₂ as the final product. Simulations were run to obtain a total of 33 trajectories for n = 1 that all led to the formation of H₂O₂. The average reaction time of H₂O₂ formation was calculated to be 57.7 fs in the case of n = 1, indicating that the reaction was completed within 100 fs of electron detachment. All the reaction systems O(¹S)(H₂O)_n (n = 1-6) indicated the formation of H₂O₂ by the same mechanism. The reaction times for n = 2-6 were calculated to range between 80 and 180 fs, indicating that the reaction for n = 1 is faster than that of the larger clusters, that is, the larger the cluster size, the slower the reaction is. The reaction dynamics of the triplet O(³P) and singlet O(¹D) potential energy surfaces were calculated for comparison. All calculations yielded the dissociation product O(X)(H₂O)_n → O(X) + (H₂O)_n (X = ³P and ¹D), indicating that the O(¹S) state contributes to the formation of H₂O₂. The reaction mechanism was discussed based on the theoretical results.

Photoelectron Imaging Study of the Effect of Monohydration on O2- Photodetachment

The photodetachment of the O(2)(-).H(2)O cluster anion at 780 and 390 nm is investigated in comparison with O(2)(-) using photoelectron imaging spectroscopy. Despite the pronounced shift in the photoelectron spectra, the monohydration has little effect on the photoelectron angular distributions: for a given wavelength and electron kinetic energy (eKE) range, the O(2)(-).H(2)O angular distributions are quantitatively similar to those for bare O(2)(-). This observation confirms that the excess electron in O(2)(-).H(2)O retains the overall character of the 2ppi(g) HOMO of O(2)(-). The presence of H(2)O does not affect significantly the partial wave composition of the photodetached electrons at a given eKE. An exception is observed for slow electrons, where O(2)(-).H(2)O exhibits a faster rise in the photodetachment signal with increasing eKE, as compared to O(2)(-). The possible causes of this anomaly are (i) the long-range charge-dipole interaction between the departing electron and the neutral O(2).H(2)O skeleton affecting the slow-electron dynamics; and (ii) the s wave contributions to the photodetachment, which are dipole-forbidden for pi(g)(-1) transitions in O(2)(-), but formally allowed in O(2)(-).H(2)O due to lower symmetry of the cluster anion and the corresponding HOMO.

Femtosecond study of Cu(H2O) dynamics

The short-time nuclear dynamics of Cu(H(2)O) is investigated using femtosecond photodetachment-photoionization spectroscopy and time-dependent quantum wave packet calculations. The Cu(H(2)O) dynamics is initiated in the electronic ground state of the complex by electron photodetachment from the Cu(-)(H(2)O) complex, where hydrogen atoms are oriented toward Cu. Several time-resolved resonant multiphoton ionization schemes are used to probe the ensuing reorientation and dissociation. Immediately following photodetachment, the neutral complex is far from its minimum energy geometry and possesses an internal energy comparable to the Cu-H(2)O dissociation energy and undergoes both large-amplitude H(2)O motion and dissociation. Dissociation is observed to occur on three distinct time scales: 0.6, 8, and 100 ps. These results are compared to the results of time-dependent J=0 wave packet calculations, propagating the initial anion vibrational wave functions on the ground-state potential of the neutral complex. An excellent agreement is obtained between the experimental results and the ionization signals derived from the calculated probability amplitudes. Related experiments and calculations are carried out on the Cu(D(2)O) complex, with results very similar to those of Cu(H(2)O).

Electron scattering on OH−(H2O)n clusters (n=0–4)

The cross sections for electron scattering on OH-(H2O)n for n = 0-4 were measured from threshold to approximately 50 eV. All detachment cross sections were found to follow the classical prediction given earlier [Phys. Rev. Lett. 74, 892 (1995)] with a threshold energy for electron-impact detachment that increased upon sequential hydration, yielding values in the range from 4.5 eV +/- 0.2 eV for OH- to 12.10 eV +/- 0.5 eV for OH-(H2O)4. For n > or = 1, we found that approximately 80% of the total reaction events lead to electron detachment plus total dissociation of the clusters into the constituent molecules of OH and H2O. Finally, we observed resonances in the cross sections for OH-(H2O)3 and for OH-(H2O)4. The resonances were located at approximately 15 eV and were ascribed to the formation of dianions in excited states.

Four-body reaction dynamics: complete correlated fragment measurement of the dissociative photodetachment dynamics of O(-)(8)

The four-body dissociative photodetachment (DPD) dynamics of O-8 were studied using photoelectron photofragment coincidence (PPC) spectroscopy. All four neutral photofragments were measured in coincidence with the photodetached electron, yielding a five-body kinematically complete experiment. Velocity and angular correlations for DPD of O(-)(8) are presented and compared to those for O(-)(6). The DPD dynamics and energetics of O(-)(8).are found to be similar to those of O-4 and O-6 implying that the additional solvating O(2) molecules act essentially as spectators, but exhibit inequivalent kinematic behavior implying asymmetric solvation.