E. F. Hilinski

A picosecond bleaching study of quantum‐confined cadmium sulfide microcrystallites in a polymer film

We report a picosecond pump–probe study of 55 A cadmium sulfide microcrystallites embedded in polymer films. Large negative absorbance changes at wavelengths corresponding to energies near the band gap are observed. This absorption bleaching and the associated changes in refractive index are mainly responsible for the large nonlinearity observed in degenerate four‐wave mixing experiments. Based on photoluminescence data, the known electron‐trapping cross section of defects, and these pump–probe experiments, we show that the conventional carrier density‐dependent band‐filling mechanism cannot account for the data, and the absorption bleaching is due to the saturation of the excitonic transition. We further show that the phase‐space filling and exchange effects from exciton–exciton and exciton‐free carrier interactions fail to account for the observed data. Instead, we propose that the exciton‐trapped carrier interaction is mainly responsible for the observed bleaching of the excitonic absorption. This inte...

Optical transient bleaching of quantum‐confined CdS clusters: The effects of surface‐trapped electron–hole pairs

We studied the optical transient bleaching of ∼40 A, ammonia‐passivated CdS clusters in a polymer with nanosecond and picosecond pump‐probe techniques. The transient bleaching spectra behave differently in different time regimes. Within the 30‐ps pump laser pulse width, we tentatively attribute the bleaching to the exciton‐exciton interaction, and the magnitude can be enhanced by surface passivation. On time scales of tens of picoseconds and longer following the pump pulse, when only trapped electron‐hole pairs remain from the pump excitation, the bleaching is due to the interaction between such a trapped electron‐hole pair and a bound exciton produced by the probe light. Experimentally we determined that roughly one trapped electron‐hole pair can bleach the excitonic absorption of the whole CdS cluster. We developed a theoretical model which considers the effects of the trapped electron‐hole pair on the energy of the exciton transition and its oscillator strength. We found that, when a trapped electron a...

Transient laser‐induced voltages in room‐temperature films of YBa2Cu3O7−x

Transient laser‐induced electrical signals with peak voltages of ∼50 mV and <20 ns duration (full width half‐maximum) have been observed in room‐temperature thin films of YBa2Cu3O7−x in the absence of an applied current. It is demonstrated that the signal polarity is reversed when the films are illuminated through the substrate rather than at the air/film interface. The mechanism for this effect has yet to be elucidated.

Biological applications of picosecond spectroscopy

Technological advances in picosecond spectroscopy have permitted the mechanisms of various chemical, physical and biological processes to be elucidated and understood to a greater degree than ever before. By means of picosecond emission, absorption and Raman spectroscopy, one can probe and measure directly the transient intermediates and kinetics of primary events in complex biological processes. A description of two current types of laser systems—solid-state and synchronously pumped dye lasers—and their application to determining the primary events in the biological processes of dissociation of oxy- and carboxymyoglobin, excited-state relaxation of porphyrins and visual transduction, illustrate the power of picosecond spectroscopy.

Electron donor-acceptor complexes of polyoxometalates with organic molecules. Picosecond spectroscopy of [(N-methylpyrrolidinone)2H+PW 12O403−

The photochemistry of the electron donor-acceptor complex formed between N-methylpyrrolidinone and H3PW12O40 was investigated by means of picosecond absorption spectroscopy. Excitation at 355 nm generates PW12O404− within ≈90 ps after excitation. Evidence exists for the production of a species within the time duration of the excitation laser pulse which either gives rise to or exists with PW12O404−.

Solvent effects on the relaxation mechanism of Cu(II) protoporphyrin

Abstract The effect of solvent(s) on the energy dissipation mechanism and rate of a Cu(II) porphyrin has been studied by means of picosecond absorption spectroscopy. The relaxation rates of Cu(II) protoporphyrin and Cu(II) protoporphyrin IX dimethyl ester were measured in various solvents, including pyridine, glacial acetic acid and benzene. The time constant for repopulation of the ground state varies by over an order of magnitude from non-polar solvents (benzene, 400 ps) to highly polar solvents (water, 15 ps). The data suggest that the solvent polarity and/or the number of axial ligands are dominant factors in determining the electronic relaxation rate of the porphyrins studied.

Concurrent pump-probe and streak camera measurements with a single mode-locked Nd:YAG laser: a picosecond absorption/emission spectrometer

A description is presented of a laser system that can be used for two concurrent and independent types of picosecond spectroscopic measurements. Two data‐collection sybsystems, (1) a picosecond pump‐probe transient absorption/emission spectrometer and (2) a streak camera system for time‐dependent measurements of absorption and emission, have been developed as independent work stations within an integrated system based on a single mode‐locked Nd:YAG laser oscillator which concurrently supplies each subsystem with picosecond pulses. The electronic links among the two work stations and the laser are discussed. In addition to applications involving pump‐probe absorption and pulsed laser‐induced emission measurements, this system can be used for picosecond‐pulsed preparation‐pump‐probe absorption and preparation‐pump emission experiments. Examples of data collected with each subsystem are presented to illustrate the complementary nature of the information that can be obtained with this versatile laser system.

Picosecond spectroscopy of charge-transfer processes. Photochemistry of anthracene-tetranitromethane EDA complexes

Abstract The temporal sequence of events that follow 532 nm excitation of electron donor-acceptor, EDA, complexes of several substituted anthracenes with tetranitromethane is monitored by means of picosecond spectroscopy. Excitation of the charge-transfer band of these EDA complexes produces high yields of 1 : 1 adducts. Absorption spectra and kinetics of the transient species involved in these photochemical reactions provide the basis for elucidation of the reaction mechanism following charge-transfer excitation to the ion pairs.

Time‐resolved spectroscopy of intramolecular energy transfer in a rigid spiran

The kinetics of intramolecular energy transfer has been studied in the rigid spiran, spiro[9,10‐dihydro‐9‐oxoanthracene‐10,2′‐5′,6′‐benzindan], using picosecond time‐resolved spectroscopy. The rate constant for triplet–triplet energy transfer from the donor (anthrone) to the acceptor (naphthalene) chromophore is found to be ∼3×1010 s−1. Low temperature microwave‐induced delayed phosphorescence measurements on the spiran show that triplet–triplet energy transfer from the anthrone populates the sublevels of the naphthalene acceptor state randomly, rather than the Tz (z=long axis) sublevel selectively. The latter prediction arises from the known sublevel selectivity for intersystem crossing in anthrone and the geometrical arrangement of the chromophores in the rigid spiran. The randomness of the triplet–triplet energy transfer process is consistent with the rate constant for energy transfer which limits the donor lifetime to a value below the spin precession period in the donor triplet state. The results are...

Angle‐dependent laser‐induced voltages in room‐temperature polycrystalline wafers of YBa2Cu3O7−x

Room‐temperature laser‐induced voltages were observed in unsupported, polycrystalline wafers of YBa2Cu3O7−x in the absence of a transport current. Peak voltages of ∼1 V were detected in response to 40 mJ pulses of 532 nm light. The rise and fall times for the signals were detector limited. The half widths of the signals were instrument response limited (laser pulsed limited for a 10 ns laser pulse and digitizer limited for a 30 ps laser pulse). At fixed pulse energy, the induced peak voltage scales as the sine of the angle of incidence of the laser beam and is nulled at normal incidence. The magnitude of the signal as a function of sample thickness, preparation, and laser excitation wavelength is discussed.