Continuum Probe Research Articles

Coulomb enhancement of above-band-gap pump-continuum probe spectroscopy.

The effect of Coulomb attraction between a photoexcited electron-hole pair on femtosecond pump - continuum probe nonlinear absorption for above-band-gap photoexcitation is discussed. It is shown that in pump-continuum probe experiments the enhancement of the optical matrix element resulting from the Coulomb interaction is strongest near the band gap for the split-off transition and explains the experimentally observed large change in absorption at 1.78 eV. The enhancement factor summed over all transitions must be included to correctly match the experimental results

Subpicosecond study of band-edge absorption in Al0.25Ga0.75As

Differential transmission measurements have been performed on a 200 nm thick Al0.25Ga0.75As layer at 300 K. The transient photoinduced transmission is observed in the vicinity of the fundamental absorption edge using tunable 150 fs pump pulses with 11 meV and 75 meV excess energy and white light continuum probe pulses. Comparison of the experimental data with a theoretical model, including excitonic as well as plasma effects reveals the dominant influence of Coulomb screening on the band-edge-absorption kinetics. Agreement with the experimental data could only be achieved by using dynamical instead of static screening of all possible scattering processes.

Femtosecond spectral evolution of the excited state of bacterial reaction centers at 10 K.

The femtosecond spectral evolution of reaction centers of Rhodobacter sphaeroides R-26 was studied at 10 K. Transient spectra in the near infrared region, obtained with 45-fs pulses (pump pulses centered at 870 nm and continuum probe pulses), were analyzed with associated kinetics at specific wavelengths. The t = 0-fs transient spectrum is very rich in structure; it contains separate induced bands at 807 and 796 nm and a bleaching near 760 nm, reflecting strong changes in interaction between all pigments upon formation of the excited state. A complex spectral evolution in the 800-nm region, most notably the bleaching of the 796-nm band, takes place within a few hundred femtosecond--i.e., on a time scale much faster than electron transfer from the primary donor P to the bacteriopheophytin acceptor HL. The remarkable initial spectral features and their evolution are presumably related to the presence of HL, as they were not observed in the DLL mutant of Rhodobacter capsulatus, which lacks this pigment. A simple linear reaction scheme with an intermediate state cannot account for our data; the initial spectral evolution must reflect relaxation processes within the excited state. The importance for primary photochemistry of long distance interactions in the reaction center is discussed.

A reliable pump-probe, broadband spectrometer for subpicosecond transient absorption

A pump-and-probe spectrometer is described that employs a pump pulse at 308 nm, with up to 1.4 mJ in 250 fs, and a continuum probe pulse in the spectral range 360–750 nm, of 80-fs pulse duration. The corresponding laser system is based on a colliding-pulse-mode dye laser operating at 616 nm, and a fs dye laser amplifier pumped by a XeCl* excimer laser. The amplifier has been designed specifically for pump-and-probe experiments, by multiplexing with horizontally and vertically polarized pulses for the pump and probe beam, respectively. Near-diffraction-limited output beams are achieved using a novel, capillary prismatic dye amplifier cell. The reliability of the apparatus is demonstrated by examining the photodissociation of bis-(p-aminophenyl) disulfide. The absorption band of the resulting p-aminophenyl-thiyl radicals shows a biexponential frequency shift in the course of several ps. This behavior is correlated with that of dielectric relaxation of the solvent.

Classical model of femtosecond time-resolved absorption spectra of dissociating molecules

We present a simple classical model for understanding time-resolved absorption spectra of molecules that are in the process of dissociating. The model applies to absorption spectra that are obtained by measuring the spectral power density of an ultrafast, continuum probe pulse after transmission through the sample. We show that the classical model can yield results in good agreement with quantum-mechanical wave packet propagation calculations. In a close analogy with collisional line broadening, the time-resolved absorption spectra are shown to have an impact region near the separated-atom transition frequency and a far-wing region. The impact region is due to radiation emitted after the molecule has separated into atomic fragments, and the far-wing region is due to radiation emitted during the time of strong molecular interaction. The spectrum in the impact region depends upon an effective phase shift for a ‘‘partial’’ collision, which begins at the time that the probe pulse sweeps through the molecular transition frequency. For narrow wave packets, this phase shift can be directly measured, and the molecular transition frequency can be recovered as a function of time along the path of dissociation. For very broad wave packets, the time-resolved absorption spectra approach a statistical limit, in which the absorption line shape becomes an image in frequency space of the probability density in configuration space at the time of excitation by the probe pulse. In all cases, the frequency-integrated absorption is proportional to the net population of molecules that are excited by the probe pulse. In principle, this result can be used to obtain the strength of the transition dipole moment as a function of internuclear separation. We also consider fluorescence induced by a short optical probe pulse, as in the experiments of Zewail and co-workers. Fluorescence measurements are shown to be fundamentally different from measurements of the transmitted spectral power density: fluorescence depends upon the net population excited by the probe pulse, whereas the transmitted spectral power density depends upon interference between the incident probe field and the polarization field. Thus these two experimental techniques are sensitive to different aspects of the dissociation process.

Femtosecond dynamics of the n=2 image-potential state on Ag(100).

We report the first time-resolved studies of higher-order image-potential states on Ag(100). Two-photon photoemission measurements are performed with femtosecond resolution using pump and continuum probe techniques. We measure the lifetime of the n=2 image state on Ag(100) to be \ensuremath{\sim}180 fs, substantially longer than that of the n=1 state. These results have implications for theoretical models used for predicting the lifetimes of these states.

Femtosecond studies of intervalley scattering in GaAs and Al xGa 1-xAs

Results are presented from ensemble Monte Carlo simulations of the relaxation of photoexcited electrons and holes. The results are compared directly with three types of femtosecond optical experiments: transient absorption saturation, pump and continuum probe, and tunable pump-probe experiments. For these experiments we find that intervalley scattering has a dominant effect for the first several hundred femtoseconds, and that electron-electron scattering is only important at later times.

Femtosecond hot carrier energy redistribution in GaAs and AlGaAs

Excited carrier dynamics in GaAs and Al .2Ga .8As are investigated using femtosecond pump and continuum probe techniques. Absorption saturation measurements provide evidence for transient spectral hole burning due to split-off as well as heavy and light hole valence to conduction band transitions. The initial nonthermal carrier distribution thermalizes and assumes a broad energy distribution on a femtosecond time scale.

Femtosecond absorption saturation studies of hot carriers in GaAs and AlGaAs

Femtosecond carrier dynamics in GaAs and AlGaAs thin films are studied by time-resolved pump-probe spectroscopy. Measurements are performed using identical pulses as well as a continuum probe. Observations of the ultrafast absorption saturation response are related to the excitation of the carriers into specific nonthermal distributions, to their rapid scattering out of these states, and to subsequent cooling of the quasi-thermal carrier distribution to the temperature of the lattice. A strong dependence of the initial scattering rates on the mole fraction of Al and the excess electron energy is observed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Femtosecond hot-carrier energy relaxation in GaAs

Excited carrier dynamics in GaAs and Al0.2Ga0.8As are investigated using femtosecond pump and continuum probe techniques. We observe absorption spectral hole burning arising from excited carriers generated by transitions from the split-off band as well as the heavy- and light-hole bands. Transient absorption saturation measurements indicate that the initial nonthermal carrier distribution thermalizes on a time scale of several tens of femtoseconds.

Femtosecond studies of nonequilibrium electronic processes in metals.

High-intensity femtosecond laser pulses are used to induce non- equilibrium electron heating in gold metal. The thermal relaxation of the electronic distribution is studied through pump and continuum probe measurements of transient reflectivity. Measurements are performed for different heating-pulse fluences and probe-photon energies. The observed reflectivity line shape demonstrates the generation of nonequilibrium electron temperatures which cool to the lattice on a 2--3 ps time scale.

Excited singlet absorption in blue laser dyes: Measurement by picosecond flash photolysis

Absorption from the lowest relaxed excited singlet state in the laser dyes POPOP, carbostyril 165, 4-methylumbelliferone, 7-diethylamino-4-trifluoromethylcoumarin, and coumarin 30 has been measured over the range, approximately, from 350 to 650 nm. In addition, rhodamine 6G was remeasured to permit comparison of our procedures with previous work and to help promote the emergence of a standard substance which all laboratories may use to test and refine their procedures. For comparison, 9,10-diphenylanthracene was investigated as an example of an "unsuccessful laser dye." Measurements were made using excitation pulses of 40 ps duration at 347 nm and a continuum probe, of the same duration, delayed by 200 ps.

Ion-current fluctuations recorded with a cylindrical electrostatic probe passing premixed flames

The ion-current fluctuation recorded with a cylindrical, electrostatic probe traversing a premixed flame has been studied. The experiments were performed by moving the probe across a rectangular or circular-nozzle burner flame. Effects of the ion-denesity distribution on the ion-current fluctuation were estimated by analyzing the phenomena near the probe on the basis of the continuum probe theory, and the deviation of the ion current was shown to increase with the ion-density gradient. By comparing the maximum ion currents to the probe traversing the flame in different directions, the deviation was found to decrease with increasing probe velocity. It was estimated to be less than 15% if the probe was used in the range of probe velocities higher than 3 m/sec. An analysis was made for inferring the ion-current fluctuations to be recorded with the probe being passed through flames of known configuration. The results showed that the ion-current fluctuation of an inclined or curved flame could be inferred fairly well on the basis of the fluctuation derived from a plane flame parallel to the probe axis.

Studies of the Large Sheath Concept for the Continuum Probe Theory

Studies of the Large Sheath Concept for the Continuum Probe Theory

Ion current to a spherical probe in a flowing high-pressure plasma under thin-sheath conditions

The current to a large (radius = 2.4 mm) negatively biased spherical probe in a flowing plasma (a propane/air flame) has been measured over wide ranges of plasma-ionisation density (3 × 1015 to 1018 m?3), probe bias (10?400V) and probe/plasma relative velocity (3?40m/s). Under these conditions, the sheath surrounding the probe is thin and the probe current is a function of the probe/plasma velocity. In consequence, the static continuum probe theory of Su and Lam, and the motionally dependent thick-sheath theory of Clemenets and Smy are both found to be in error by at least an order of magnitude. A thin relationship has been derived for the spherical probe which shows excellent agreement with the experimental data: I=12.13(??0)1/4(neevf)3/4V1/2rp5/4 where I = probe current, ? = ionic mobility, ?0 = permittivity of free space, ne = ionisation density, e = electron charge, vf = flow velocity, V = probe bias and rp = probe radius (all in MKS units)