Devendra K. Sharma

Photophysics of a Zn(Cd)S phosphor on an extended time scale

We have studied the photophysics of a Zn(Cd)S:Ag phosphor, in powder form under ambient conditions, over a time domain extending from picoseconds to milliseconds. Pulsed laser luminescence techniques and time‐resolved dielectric loss spectrometry have been employed. We propose mechanisms to interpret the results and complement the proposals with a computer simulation. We discuss the implications of the results for energy conversion schemes involving photoassisted catalysis.

Photophysical and photochemical primary events in semiconductor particulate systems. Colloidal CdS with methylviologen

Abstract The photogenerated electron/hole pairs in excited CdS, produced by irradiation (355 nm) from a Nd:YAG mode-locked laser, yield a luminescence which decays in ≤ 30 ps. Time-resolved change-in-absorbance spectra of CdS in the presence of methylviologen MV 2+ (degassed) show formation of MV + and its dimer (MV + ) 2 (λ max ∼ 530 nm) in ∼ 1 ns.

OPTICAL LIMITING CHARACTERISTICS AND MECHANISM OF SILVER BROMIDE NANOSOLS

Optical limiting behavior has been observed in nanosols comprising ∼60 A particles of silver bromide. Switching times, τ, are consistently in the ns regime, and values of I1/2;, the pulse laser exposure which elicits a 50% decrease in transmittance of the medium, can be less than 20 mJ/cm2. Nonlinearity of response, I1/2, and response time all increase with decreasing pAg [Eq. (2)] of the nanosols. The materials can be cycled apparently indefinitely, provided they are subjected only to very short (sub‐ns) laser pulses. Optical switching can be spectrally sensitized; comparable response parameters are obtained under these conditions. The nonlinear character of the response is, however, very different: a true response threshold, I0=∼12 mJ/cm2, is observed. Analysis of the optical limiting mechanism suggests that iodide doping, which is adventitious in the present case, is critical to the observed photophysical behavior of the nanosols.

Reduction of acceptor relay species by conduction band electrons of colloidal titanium dioxide: light-induced charge separation in the picosecond time domain

Reference LPI-ARTICLE-1987-008doi:10.1016/0009-2614(87)87296-2View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Fluorescence lifetime of chlorophyll a in pure and mixed langmuir-blodgett films

Abstract Fluorescence lifetimes of pure chlorophyll a monolayers were determined by single-photon counting and by picosecond spectroscopy. Neither technique could resolve its very fast decay rate, attributed to a high aggregate concentration and rapid intermolecular energy transfer. The same experiments were performed on monolayers of chlorophyll a diluted with phospholipids at a molar ratio of 1 to 100. From the single-photon counting decay curves, two fluorescence lifetimes were deconvoluted, i.e. 4.9 and 2.3 ns, indicating the presence of monomeric and aggregated chlorophyll a, respectively. They differ somewhat from the picosecond spectroscopic result where only one lifetime at 3.3 ns was calculated. This is indicative of an exciton annihilation process since it is smaller than the average of the single-photon counting lifetimes. Fluorescence spectroscopy was also used to better understand the photophysics of the chlorophyll a monolayer.

Photophysics of rigidized 7-aminocoumarin laser dyes: efficient intersystem crossing from higher excited state☆

Abstract An unusual slow rise (over ≈ 100 ps) of fluorescence intensity from the julolidylcoumarins, coumarins 102, 314 and 334, is observed under conditions of high-intensity, pulsed laser (355 nm) excitation. The basis of this phenomenon was explored in photoquenching, time-resolved transient absorption and amplified stimulated emission studies of coumarin 314. The slow rise is shown to be the consequence of two photon excitation and the fluorescence rise time associated with the rate of S n —S 1 decay. Principal fate of S n is intersystem crossing, and the fluorescence rise time is thought to be a measure of that rate, i.e. ≈ 10 10 s −1 .

PHOTOPHYSICS OF A WELL CHARACTERIZED HUMIC SUBSTANCE

Abstract— The transient absorbance observed after flash excitation of a well characterized humic substance (“Armadale” fulvic acid) may be analyzed into three components. Two arise promptly (< 20 ps) and are assigned to a solvated electron (on the basis of decay and quenching) and its corresponding radical cation (on the basis of common origin). The third is a broad featureless band which arises only after a few ns and remains for long times. It is suggested to include triplets that may sensitize singlet oxygen.

Fluorescence lifetimes of the well characterized humic substance, armdale fulvic acid

Abstract Decay of luminescence from the well characterized humic substance, Armdale fulvic acid, has been measured with both nanosecond and picosecond flash excitation. In both aerated and deaerated solutions at pH between 2 and 7, three characteristic times are required to account for the decay envelope and these times vary over only a narrow range. The three are in the vicinity of 200 picosecond, 2 nanosecond, and 7 nanosecond respectively. Cu(II) complexation led to static quenching.

Picosecond laser spectroscopy of chromium(III) complexes. Probing the photophysics of 4T2 excited states of Cr(bpy)33+ and Cr(4,7-Me2phen)33+ ions in aqueous solution

Abstract Picosecond laser techniques have been used to probe the photophysics of the short-lived 4 T 2 excited states of Cr(bpy) 3 3+ and Cr(4,7-Me 2 phen) 3 3+ complexes (bpy is 2,2′-bipyridine and 4,7-Me 2 phen is 4,7- dimethylphenanthroline). Contrary to the previously reported value of the lifetime of ( 4 T 2 )Cr(bpy) 3 3+ (≈10 ps) obtained from resonance Ramans studies(3.2 ns laser pulse), luminescence and time-resolved absorption spectroscopy using mode-locked Nd : YAG laser pulses of 30 ps (fwhm) indicate that the lifetime of these quartet states is ≈10–20 ns.

Electron transfer quenching of rhodamine‐6G in polymer films

Electron transfer quenching of the fluorescence of rhodamine‐6G in polyvinyl acetate and poly‐α‐methylstyrene films by aromatic amine quenchers has been examined, using both steady‐state and time‐resolved methods. Observed quencher concentration dependences do not conform to expectations from either the simple Perrin or Stern–Volmer models. The steady‐state quenching results are consistent with long‐range electron (tunnel) transfer, with collective quenching at higher quencher concentration, but existing models of this process do not quantitatively rationalize our results. Some potential difficulties with these models are accordingly discussed. Time‐resolved (streak camera) experiments confirm that all the quenching events are of dynamic origin, to the exclusion of the involvement of a fractional population of fluorescence‐inactive absorbers owing to ground‐state complex formation.