Subpicosecond Range Research Articles

On the quantum character of molecular motion in liquids at standard conditions

Abstract Rototranslational motion in simple liquids (like CH 3 CN at 300 K or Ar at 80 K ) is analyzed from the viewpoint of quantum theory. Usually it is assumed that the molecular or atomic motion in the sub-picosecond time range can be treated within classical mechanics, e.g. with the aid of molecular dynamics (MD) simulators. A well known dynamical criterion concerning the applicability of classical mechanics in the present context is discussed. Simple numerical estimations show that the above criterion is violated in the case under consideration. Further analysis based on the quantum mechanical uncertainty relations reveals that the classical-mechanical trajectories cannot be considered to represent physically meaningful concepts in the sub-picosecond time scale. Nevertheless, the success of MD simulations does indicate that some fundamental connection between the quantum and classical treatment of microdynamical processes in liquids may exist. It is argued that in the considered physical context: (i) the “standard” justifications of the applicability of classical mechanics appear to be in contrast to the validity of the uncertainty relations; (ii) the Ehrenfest theorem or Hidden Variables theories cannot support the physical significance of classical trajectories; (iii) Reductionism offers a conceptual basis on which the aforementioned connection can be established.

Nonlinear effects in molecular dynamics of water: Large sample computer simulation of rise and fall transients

In one of the largest computer simulations to data of liquid water, the effect of a strong, external, uniaxial electric field on an equilibrium sample of 1372 molecules has been investigated. The orientational rise transient from field-off to field-on equilibrium shows a complex pattern of oscillations in the sub-picosecond range which are neither predicted theoretically nor seen in previous computer simulations. On the other hand, the phenomenon of fall transient acceleration first discovered by computer simulation in a sample of only 108 molecules is confirmed in the present work, indicating that the acceleration effect is independent of sample size, and is a fundamental physical property of liquids, including water.

Laser studies of gas-phase kinetics and photochemistry

A brief review is presented of the advantages (and of some of the avoidable pitfalls) in the use of lasers in gas-phase kinetics and photochemistry. Commercially available laser sources for both photolysis and detection are listed, and the laser-based techniques of fluorescence, multiphoton ionisation and absorption are discussed. Ways of dealing with the common difficulties encountered with these methods, such as scattered light and saturation, are briefly described. Examples are given of the exploitation of characteristics of laser light in kinetic and photochemical studies, and these include high intensities, narrow bandwidths, polarisation and the use of short pulses in the sub-picosecond range.

Far infrared spectroscopy with subpicosecond electrical pulses on transmission lines

Optically generated and detected electrical pulses on transmission lines in the subpicosecond range have frequencies extending up to 1 THz, thereby covering the far infrared region of the spectrum from 0 to 30 cm−1. We have studied the propagation of these short pulses through a section of the transmission line covered with erbium iron garnet which shows distinct absorption lines in the far infrared at low temperatures (2–30 K). The absorption and dispersion of the garnet modify the shape of the pulse, and the absorption spectrum is obtained by Fourier transforming the propagated pulse shape.

Nonlinear excitations and nonlinear phenomena in conductive polymers

Semiconductor polymers such as polyacetylene and polythiophene have experimentally demonstrated nonlinear optical processes with characteristic time scales in the sub-picosecond range. Fast transient photoconductivity measurements on trans -(CH) x as a function of temperature and photon energy indicate a relatively high quantum efficiency for the photoproduction of mobile, charged, nonlinear excitations, consistent with the Su-Schrieffer mechanism for the photogeneration of charged solitons. The major shifts in oscillator strength due to these nonlinear photoexcitations lead to relatively large resonant third-order nonlinear optical processes ( χ (3)) on time scales of order 10 −13 s. A direct measurement of χ (3) in polyacetylene has been carried out by third harmonic generation. The measured nonresonant value of χ (3)(3 ω = ω+ ω+ ω) = 4 × 10 −10 esu. The implied value for χ ‖ (3) is an order of magnitude greater than the corresponding value for polydiacetylene.

Time-dependent ensemble Monte Carlo simulation for planar-doped GaAs structures

A time-dependent ensemble Monte Carlo simulation has been developed which includes the computation of the self-consistent electric field and electron concentration. The Monte Carlo code is used to study transient and steady-state transport in planar-doped-barrier (PDB) GaAs diodes and transistors. Results are presented for the temporal and spatial evolution of electronic transport through the PDB. We show that (i) average velocities achievable with a PDB are significantly above the steady-state velocities by about a factor of 3, (ii) the rise time of the electric current in the PDB is in the subpicosecond range for diode length below 2000 Å, (iii) a pronounced displacement current dominates the transient electronics at onset of injection, and (iv) polar optical phonon absorption contributes significantly to the thermionic-emission current over the barriers.

Framing camera tube for subnanosecond imaging applications

A dissector/restorer-type framing camera tube has been characterized with a Nd:YAG mode-locked laser. The three-frame format tube, referenced to the photocathode, has a 5×5-mm field of view and 0.3-ns duration frames with better than 5 lp/mm spatial resolution. A scan-rate matching technique is described that can increase the framing speed into the subpicosecond range for this type of camera tube.

Tunneling hot electron transfer amplifiers (theta): Amplifiers operating up to the infrared

A family of novel three-terminal devices which relies on the transfer of a quasi-monoenergetic hot electron beam through a thin base is described. The devices are similar in principle to the proposed tunneling amplifier by Mead in the early sixties (“Cold Cathode” or “Metal Base” amplifiers). Results are reviewed and the probable reasons for the poor performances are pointed out. It is predicted that, with a proper choice of parameters, metal-base amplifiers can operate as switches, negative resistance devices and continuous amplifiers in the subpicosecond range. Two subclasses are described: The tunneling emitter (THETA), in the major part of the work, and the nontunneling emitter (BHETA) amplifiers. In the THETA family the metal-oxide-metal-oxide-metal (MOMOM), the MOM-semiconductor (MOMS), and the heterojunctions devices are described. Members of the BHETA family generate quasi-monoenergetic electron beams by injecting electrons by an n + n − or a metal- n − junctions, and include a variety of metals and semiconductor combinations. Very thin films are required in these devices (oxides ∼15 Å, metals ∼100 Å, semiconductors ∼100 Å). The molecular beam epitaxy technique and lattice matching considerations are required for pinhole free semiconductors and metal films with minimum interface states. Sputter-oxidation methods are needed for thin oxide growth. Systems which combine these features with availability of microfabrication make these devices feasible today.

Effect of photon synchronization in intensity interference: New effect and new possibilities

A new optical photon-correlation effect is predicted, which appears when short light pulses are subjected to a narrow-band spectral filtration. The effect allows to study the temporal form and photon statistics of short optical pulses and to measure their length in nanosecond to subpicosecond range without use of fast photodetectors.