Victor H. Vilchiz

The ejection distribution of solvated electrons generated by the one-photon photodetachment of aqueous I− and two-photon ionization of the solvent

The ultrafast dynamics following one-photon UV photodetachment of I− ions in aqueous solution are compared with those following two-photon ionization of the solvent. Ultrafast pump–probe experiments employing 50 fs ultraviolet pulses reveal similar and very rapid time scales for electron ejection. However, the electron ejection process from water pumped into the conduction band and from iodide ions detached at threshold are readily distinguishable. The observed picosecond timescale geminate recombination and electron escape dynamics are reconstructed using two different models, a diffusion-limited return of the electron from ∼15 A to its parent and a competing kinetics model governed by the reverse electron transfer rate. We conclude that the “ejected” electron in the halide detachment is merely separated from the halogen atom within the same solvent shell. The assignment of detachment into a contact pair is based on the recombination profile rather than by the postulate of any new spectral absorption due...

Femtosecond dynamics of photodetachment of the iodide anion in solution: resonant excitation into the charge-transfer-to-solvent state

Abstract The photodetachment of iodide ions in solution is probed via ultrafast spectroscopy with ∼50 fs time resolution. Excitation of I − is achieved with a single photon at 255 nm, populating the quasi-bound charge-transfer-to-solvent (CTTS) state. The detached electron is trapped by the solvent within 200 fs. Electron–iodine recombination is observed to take place on ∼25 ps timescale, but a reasonable fraction of electrons escape geminate recombination beyond 400 ps. The dynamics of electrons generated from the I − CTTS state are compared with those arising from two-photon ionization of the neat solvents.

Time-resolved scavenging and recombination dynamics from I:e− caged pairs

The competition between geminate recombination of electrons with their parent radicals and electron scavenging with H+ is directly time resolved with ∼100 fs resolution at several acid concentrations. Electrons were produced from iodide photodetachment or two-photon ionization of H2O. With regards to those produced from iodide photodetachment, the separation between primary and secondary I:e− recombination is established using a full numerical solution to the diffusion equation. Electron ejection is found to be short range and a potential well of ∼3kbT depth stabilizing the solvent caged pair is required to yield a satisfactory fit to experiment. From time-resolved scavenging data up to 5 M HCl, it is shown that the electron can be scavenged both inside and outside the caged pair by H+ with nearly equal efficiency. The steady-state scavenging yield as a function of scavenger concentration is then predicted based on the determined time-dependent recombination function. Reassessment of several benchmark sca...

Electron photodetachment from Fe(CN)6]4- : photoelectron relaxation and geminate recombination

Abstract The relaxation of photoelectrons detached from aqueous [Fe(CN)6]4− is captured via 50 fs pump-multicolor probe spectroscopy. The recovered relaxation (570 fs, 0.53 eV exponential spectral shifting due to solvation) is very similar to the electron dynamics subsequent to ionization of water, however the trapping time is slower (∼ 310 fs ). The observed geminate recombination varies strongly with the effective charge on the photodetached species. The influence of the Coulomb potential is successfully unraveled from the recombination dynamics while extracting the average electron photoejection distance. In contrast to threshold detachment of monovalent anions, electrons are ejected to ∼ 15 A .

Mechanisms for Photodetachment in Water

The effect of excitation energy and solvent deuterium substitution is explored for photodetachment of aqueous iodide. The results are explained in part by a new ab initio characterization of the vertically excited state. Threshold electron ejection processes, such as photodetachment via optical charge-transfer-to-solvent (CTTS) excitation, provide very simple models for understanding solvent-controlled electron transfer (ET). In previous work, we have demonstrated that ejection from the prototypical CTTS system, the aqueous iodide anion, is to short range into a caged pair [1]. The electron subsequently either diffuses out of the cage, an activated process due to the attractive interaction between the nascent electron and the polarizable iodine neutral, or undergoes reverse electron transfer from the cage to reform I [2]. In the current experiments, our goal was to gain additional insight into the molecular mechanism of ejection by exploring the effect of a large increase in the excitation energy and by isotopic substitution of the solvent. The results are rather unexpected and an ab initio characterization of the vertically excited bulk CTTS wavefunction turns out to be particularly useful in rationalizing the dynamics of the electron. Aqueous iodide ions are excited at the red-most edge (250 nm) or at the band center (225 nm) of their CTTS absorption. 250 nm pulses of ~ 50 fs are generated by doubling the signal output of a 390 nm pumped OPA and 225 nm pulses (~ 150 fs) are produced by sum frequency mixing of the OPA signal with residual 390 nm. The ejected electrons once trapped are detectable by their absorption across the visible and near IR. Mapping the spectral evolution of this absorption at several probe wavelengths reports on the subsequent solvent response to the trapped electron [3]. Results are shown here for a 700 and 1000 nm probe selected by an interference filter from a variably delayed white light continuum. Fig. 1 shows the delayed appearance of the electron at earliest times (longer than instrument response) and a tens-of-picoseconds population decay due to geminate recombination at two different excitation energies. The fraction remaining has escaped iodine and any slow population decay after ~200 ps is due to secondary recombination [2]. The temporal signature of the geminate recombination directly relates the location of the newly formed electron with respect to its geminate partner (and parent) [1]. It is observed that there is no significant difference in the traces at any phase of the ejection and recombination dynamics. This suggests that ejection proceeds essentially unaltered despite the large increase in available energy that one might have predicted would be available for longer range, one would see a slower population decay phase and a higher escape fraction, as is Fig. 1. Femtosecond transient absorption of aqueous I at 250 and 224 nm (5.0 and 5.5 eV). observed in the detachment of Na [4] and in the multiphoton ionization of liquid water above 9 eV [5]. Figure 2 demonstrates the effect on the ejection mechanism on changing the solvent from H2O to D2O. It is observed that neither the appearance, as judged by the rising edge of the 1000 nm data, or the spectral evolution is significantly altered on isotopic substitution. One might expect that the dominant response of the water molecules in the first shell would be inertial libration of the adjacent solvent (giving a large deuterium effect) in response to the change in local charge density as observed for dye solvation dynamics in water. It appears that translation of the immediate solvent (which gives a small deuterium effect due only to the change in overall mass) dominates [6] and facilitates the budding of the electron into an adjacent solvent cavity. Interestingly, there is a large effect on the geminate recombination on solvent deuteration. This cannot be explained simply by the higher viscosity of D 2O. The fits shown in Fig. 2 are numerical diffusion simulations [2] for the encounter pair which include a reaction probability for nonadiabatic electron transfer if the electron and iodine are in contact. It is found that the effective rate constant for return ET must be a factor of 0.55 slower in D 2O to fit the data. To consider these results it is useful to compare with benchmark mixed quantum classical molecular dynamics simulations for halide CTTS detachment [7]. These suggest that electron separation is solvent driven, but that the CTTS band is made up of both pÆs and pÆd electron promotions with the detachment timescale controlled by transitions between different angular momentum waves. Thus, tuning the pump over the absorption band should vary the angular momentum composition of the initially prepared state again predicting different ejection dynamics [7]. We have performed our own ab initio calculations in an attempt to get an improved description for the vertically-prepared wavefunction [8]. These calculations provide an explanation for the results in Fig. 1. Inclusion of all iodine valence electrons reveals that promotions into d character orbitals are at much higher energy and do not contribute to the CTTS band. Instead, exciting at varying photon energies selects from different non-degenerate occupied iodide 0 1 2 3 I in H 2 O

Femtosecond Study of Electron Photodetachment from Complex Anions: Fe(CN)64- and CuBr2- in H20

The ultrafast dynamics of photoelectron detachment from the molecular anions, Fe(CN)6 4- and, for the first time CuBr2 -. are studied in detail by means of pump-multicolor probe spectroscopy and compared to the prototypical I- system.

Ultrafast thermalization dynamics of hot photoelectrons injected into water

We present results on the mechanism of solvated electron production by photoionization of water and photodetachment of iodide with 50fs deep-UV pulses. The relaxation of photoejected electrons from the two systems differs markedly, consistent with the suggestion that iodide detachment leads to an electron:iodine caged pair.