Direct picosecond time resolution of unimolecular reactions initiated by local mode excitation

The concept of local mode (LM) states [1] in large molecules raises the possibilty of inducing chemical reactions from a well-defined initial state (bond-selective chemistry). The results of linewidth and energy measurements in gases, [2(a)] and low temperature solids, [2(b)] however, indicate that the relaxation times for such high energy (> 15000 cm^-1) states can be extremely short, < 1ps. Because of the lack of direct time-resolved measurements, the following fundamental questions have not been unequivocally answered: What are the homogeneous linewidths of LM states and what are the rates of energy relaxation or reaction out of these states? Over the past five years we have made several attempts to observe the picosecond dynamics of LM states. Due to the inherent difficulties associated with making these measurements, such as the very small oscillator strength (σ < 10^-23 cm^2), an extremely sensitive probing technique becomes imperative

Electromagnetic shock waves from transmission lines

We have observed subpicosecond electrical pulses to propagate on 5-pm coplanar transmission lines at velocities faster than the phase velocity in the underlying dielectric. This situation produces an electromagnetic shock wave in a manner similar to Cherenkov radiation and electro-optic Cherenkov radiation. Using time-domain spectroscopy, we have measured the strong frequency-dependent loss of energy in the propagating electrical pulse due to this radiation.Peer reviewedElectrical and Computer Engineerin

Carrier dynamics of electrons and holes in moderately doped silicon

A time-domain spectroscopic technique, based on the generation and detection of a collimated beam of subpicosecond broadband terahertz pulses, is used to measure the absorption and dispersion of n- and p-type silicon, with resistivities of 0.1, 1, and 10 Ohm-cm in the submillimeter range of 0.1-2 THz. From the transmission measurements performed at room temperature and at 80 K, the absorption and dispersion, and concomitantly the full complex conductivity, of the doped silicon could be obtained. The results provide an accurate view on the dynamics of the electrons and the holes. Although the simple Drude model, with an energy-independent relaxation time, gives a surprisingly accurate description of the observed carrier dynamics, the measurements do show that some refinements are needed. An extended model, with an energy-dependent carrier-relaxation rate, can explain most of the observed deviations from the simple Drude model.Peer reviewedElectrical and Computer Engineerin

Measurement of ultrafast hot-carrier relaxation in silicon by thin-film-enhanced, time-resolved reflectivity

Time-resolved reflectivity measurements within ~100 fs resolution have revealed an initial 350 fs relaxation time in silicon, believed to be the time it takes hot, photoinjected carriers to relax to the band edge. The measurements were made at low carrier densities (~10^17 cm^-3) for which carrier-carrier processes are negligible, and were facilitated by the greater than order of magnitude enhancement of the change in reflectivity signals that can be produced by the use of thin films.Peer reviewedElectrical and Computer Engineerin

DIRECT OBSERVATION OF INTERMOLECULAR ENERGY REDISTRIBUTION IN p-DIFLUOROBENZENE BY PICOSECOND GATING

Author Institution: Department of Chemistry, University of Pennsylvania PhiladelphiaResults are presented on the picosecond time evolution of the fluorescence spectrum of p-difluorobenzene under collision free circumstances. Time-resolved single vibronic level emission spectra were obtained by the technique of frequency down-coversion by non-linear mixing of the molecular fluorescence with a picosecond laser pulse in a non-linear gating crystal. High resolution down-coverted spectra were obtained by using a monochromator and photo-multiplier tube; lower resolution down-converted spectra were obtained by using a spectrograph and a reticon array-based optical multichannel analyzer system. The time gated spectra consisted of structured emission corresponding to vibronic transitions superimposed on a diffuse background. The diffuse background displayed a time evolution that differed from that of the structured region of the spectrum. The observed kinetics might mean that intramolecular rovibrational energy redistribution is occuring on a timescale of ca. 6 ps

Ultrafast Molecular Relaxation of Isolated Stllbene: Measurements by Picosecond Pump-Probe Techniques

In this Letter, we report measurements of the picosecond relaxation time of trans-stilbene in S1 under collisionless conditions. By acurate determination of the zero-time in our picosecond pump-probe experiments we obtained r = 3 ± 0.5 ps, which is entirely consistent with our earlier measurements of IVR rates of stilbene in bulbs and beams. This establishes the molecular nature of the decay in these picosecond pump-probe multiphoton ionization (or mass spectrometry) experiments

Ultrafast Molecular Relaxation of Isolated Stllbene: Measurements by Picosecond Pump-Probe Techniques

In this Letter, we report measurements of the picosecond relaxation time of trans-stilbene in S1 under collisionless conditions. By acurate determination of the zero-time in our picosecond pump-probe experiments we obtained r = 3 ± 0.5 ps, which is entirely consistent with our earlier measurements of IVR rates of stilbene in bulbs and beams. This establishes the molecular nature of the decay in these picosecond pump-probe multiphoton ionization (or mass spectrometry) experiments