Frederick H. Long

Femtosecond studies of electron photodetachment from an iodide ion in solution: The trapped electron

Abstract We have performed femtosecond studies of electron photodetachment from a simple halide ion in aqueous solution. After photoexcitation, the electron is trapped in the deep potential well formed by the orientational polarization of the water molecules around the anion. This trapped state of the electron is a precursor of the wet electron and in the case of an aqueous iodide ion has a strong absorption in the visible. We hypothesize that the electron recombines with the neutral halogen atom or crosses onto the potential surface of the wet electron by a non-adiabatic electron transfer process. We find that analogous to the case of photoionization of a neutral water molecule, the wet to solvated electron transition in the vicinity of the halogen atom can be described as two-state in character.

Femtosecond studies of electron photodetachment of simple ions in liquid water: Solvation and geminate recombination dynamics

Femtosecond studies of electron photodetachment in aqueous solutions of NaCl and NaOH have been performed. The decays observed in the solvated electron absorption signal we believe to be the first direct observation of geminate electron–atom recombination. The solvation dynamics of the electron originating from a Cl− ion are slower, 500 fs, than the electron originating from a neutral water molecule, 350 fs. The recombination and solvation dynamics strongly suggest that short‐range molecular effects are important.

Intensity dependent geminate recombination in water

Abstract We have observed an intensity dependent geminate recombination of electrons in neat water. Since the recombination dynamics occurs on a time scale comparable to the solvation dynamics, the kinetics characteristic of an isosbestic wavelength are obscured. We have hypothesized that this intensity-dependent geminate recombination is due to the existence of two mechanisms for electron production with different characteristic thermalization distances. These results clarify the discrepancy between experiments at higher intensities and those at lower intensities.

Femtosecond studies of electron-cation dynamics in neat water: the effects of isotope substitution

Abstract Femtosecond photoionization studies in neat protio and deuterio water at room temperature have been performed. The rate of appearance of the solvated electron is slower in D 2 O than in H 2 O, consistent with the predictions of continuum theories. The observed geminate recombination dynamics of the cation-electron pair is also isotope dependent. The recombination time is slower in D 2 O and the fraction of solvated electrons that escape recombination is greater in D 2 O than in H 2 O. After 165 ps, 54±1% (±one standard deviation) of the solvated electrons escape geminate recombination in H 2 O and 62±1% escape in D 2 O. The increased electron escape yield is attributed to an energy transfer effect on the electron thermalization distance.

Optical Studies of Polyphenylenevinylene and Derivatives: Raman and Luminescence Spectroscopy

Abstract We have done Raman and luminescence measurements on a variety of PPV derivatives and oriented polymer blends. Changes in the observed Raman spectra for the polymers studied are consistent with molecular level changes in the polymer structure. Low frequency modes, which may be torsional modes of the polymer, have been observed for the first time. By comparison to site-selective luminescence spectra, it is found that at least two different phonons contribute to the vibrationally resolved luminescence spectra.

Luminescence Excitation Spectroscopy in Poly(p-Phenylene-Vinylene) Derivatives: Implications for a Semiconductor Band VS. Exciton Model in Conjugated Polymers

Abstract High-resolution luminescence excitation spectroscopy (site-selective flourescence) results are presented for two soluble derivatives of poly(p phenylene-vinylene): poly(2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene) (MEH-PPV), and poly(2,5-bis(cholestanoxy)-p-phenylene vinylene) (BCHA-PPV). Measurements have been performed on both thin films of these polymers, and dilute blends in polyethylene oriented by tensile drawing. By examining the effects of oxygen exposure in cast films, and the energy- dependence of the luminescence anisotropy in the oriented blends, we provide evidence that the electronic states in the absorption tail arise from localized states in an amorphous semiconductor.

Experimental Evidence for the Role of Zero-Point Motion in a One-Dimensional Material: Optical Studies of an MX Solid

Abstract Luminescence and resonance Raman spectra have been measured for the quasi-one-dimensional charge-density-wave material [Pt(en)2] [Pt(en)2Cl2] (ClO4)4; en=l, 2- diaminoethane, PtCl. While the luminescence experiments show the existence of tail states at low temperature in the band gap region, the Raman measurements conclusively demonstrate that this tail does not arise from ordinary static structural disorder. These results can be explained by considering the zero-point motion of the lattice.

Quantum lattice fluctuations in a one-dimensional charge density wave material: luminescence and resonance Raman studies of an MX solid

Luminescence spectra, both emission and excitation, and the excitation dependence of the resonance Raman (RR) spectra have been measured for a 1-dimensional charge-density-wave solid, [Pt(L)2Cl2][Pt(L)2](ClO4)4; L equals 1,2- diaminoethane. The luminescence experiments support the existence of tail states in the band gap region, which indicate the presence of disorder. In contrast, the RR measurements conclusively demonstrate that the effects of static structural disorder on the vibrational spectroscopy can be neglected. This apparently paradoxical result can be explained by considering the zero-point motion of the lattice. Our experimental results are compared to recent theoretical models.