26,098 research outputs found
Observation of high-energy vibrational overtones of molecules in solids: Local modes and intramolecular relaxations
In the last few years, the spectra of vibrational overtones (1) (at ~15-20,000 cm^-1) in large molecules have received considerable attention. The focus is on three problems dealing with the origin of relaxation at such high energies, the association of spectral band positions with the local modes (LM) in molecules, and the relevance of these spectra to possible selectivity in laser-induced chemistry
Thermal Lensing Spectroscopy With Picosecond Pulse Trains and a New Dual Beam Configuration
In this communication, we wish to report on the use of synchronously mode-locked picosecond lasers in a pump-probe configuration for TL spectroscopy. The peak power for these picosecond lasers is very high and, of course, the fundamental of the dye laser (red beam) can be efficiently (~10%) doubled in frequency (U.V. beam) by second harmonic generation in nonlinear crystals. We use this generated U.V. beam as a probe to monitor the very weak absorption of the red beam. An arrangement [4] of the beams involving different waist positions for the pump and probe is used. This arrangement results in an enhancement of sensitivity (at least a factor of three to seven). Also, a different dependence of the signal on the cell position compared to the single beam method is obtained. Finally, we have obtained the ÎÎœ = 5 CH-stretching overtone absorption spectrum of liquid toluene with this method (see Figure 1). Suggestions are made regarding new applications of this picosecond pulse thermal lensing technique
Non-BPS D8-branes and Dynamic Domain Walls in Massive IIA Supergravities
We study the D8-branes of the Romans massive IIA supergravity theory using
the coupled supergravity and worldvolume actions. D8 branes can be regarded as
domain walls with the jump in the extrinsic curvature at the brane given by the
Israel matching conditions. We examine the restrictions that these conditions
place on extreme and non-extreme solutions and find that they rule out some of
the supersymmetric solutions given by Bergshoeff {\em et al}. We consider what
happens when the dilaton varies on the worldvolume of the brane, which implies
that the brane is no longer static. We obtain a family of D8-brane solutions
parametrized by a non-extremality term on each side of the brane and the
asymptotic values of the 10-form field. The non-extremality parameters can be
related to the velocity of the brane. We also study 8-brane solutions of a
massive IIA supergravity theory introduced by Howe, Lambert and West. This
theory also admits a 10-form formulation, but the 10-form is not a R-R sector
field and so these 8-branes are not D-branes.Comment: 23 pages REVTeX, 2 .eps figures. This paper completely replaces and
extends an earlier paper (hep-th/9712112) by Chamblin and Perr
Recommended from our members
Composite drill stem of epoxy fiber glass reinforced with boron filaments and a retrievable core liner/sample return container for the Apollo lunar surface drill
Composite drill stem of epoxy fiber glass and boron filaments and lunar core sampling system for Apollo lunar surface dril
Maternal haemodynamic function differs in preâeclampsia when it is associated with a smallâforâgestationalâage newborn: a prospective cohort study
Objective
To describe maternal haemodynamic differences in gestational hypertension with smallâforâgestationalâage babies (HDP + SGA), gestational hypertension with appropriateâforâgestationalâage babies (HDPâonly) and control pregnancies.
Design
Prospective cohort study.
Setting
Tertiary Hospital, UK.
Population
Women with gestational hypertension and healthy pregnant women.
Methods
Maternal haemodynamic indices were measured using a nonâinvasive Ultrasound Cardiac Output Monitor (USCOMâ1AÂź) and corrected for gestational age and maternal characteristics using deviceâspecific reference ranges.
Main outcome measures
Maternal cardiac output, stroke volume, systemic vascular resistance.
Results
We included 114 HDP + SGA, 202 HDPâonly and 401 control pregnancies at 26â41 weeks of gestation. There was no significant difference in the mean arterial blood pressure (110 versus 107 mmHg, P = 0.445) between the two HDP groups at presentation. Pregnancies complicated by HDP + SGA had significantly lower median heart rate (76 versus 85 bpm versus 83 bpm), lower cardiac output (0.85 versus 0.98 versus 0.97 MoM) and higher systemic vascular resistance (1.4 versus 1.0 versus 1.2 MoM) compared with control and HDPâonly pregnancies, respectively (all P < 0.05).
Conclusion
Women with HDP + SGA present with more severe haemodynamic dysfunction than HDPâonly. Even HDPâonly pregnancies exhibit impaired haemodynamic indices compared with normal pregnancies, supporting a role of the maternal cardiovascular system in gestational hypertension irrespective of fetal size. Central haemodynamic changes may play a role in the pathogenesis of preâeclampsia and should be considered alongside placental aetiology
In-Plane Magnetolumnescence of Modulation-Doped GaAs/AlGaAs Coupled Double Quantum Wells
In-plane magnetic field photoluminescence spectra from a series of
GaAs/AlGaAs coupled double quantum wells show distinctive doublet structures
related to the symmetric and antisymmetric states. The magnetic field behavior
of the upper transition from the antisymmetric state strongly depends on sample
mobility. In lower mobility samples, the transition energy shows an -type kink with fields (namely a maximum followed by a minimum), whereas
higher mobility samples have a linear dependence. The former is due to a
homogeneous broadening of electron and hole states and the results are in good
agreement with theoretical calculations.Comment: 3 pages, 4 figures, submitted to Appl. Phys. Let
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 (Ï < 10^-23 cm^2), an extremely sensitive probing technique becomes imperative
A possible explanation for the inconsistency between the Giotto grain mass distribution and ground-based observations
Giotto measured the in situ Halley dust grain mass distribution with 2 instruments, Particle Impact Analyzer and Dust Impact Detection System (DIDSY), as well as the total intercepted mass from the deceleration of the spacecraft (Giotto Radio-Science Experiment, GRE). Ground based observations made shortly before encounter have fluxes much higher than would be predicted from Giotto data. It is concluded that Giotto DIDSY and GRE data represent observations of dust originating from a narrow track along the nucleus. They are consistent with ground based data, if assumptions are made about the level of activity along this track. The actual size distribution that should be used for modeling of the whole coma should not include the large mass excess actually observed by Giotto. Extrapolation of the small grain data should be used, since for these grains the velocity dispersion is low and temporal changes at the nucleus would not affect the shape of the mass distribution
A finite element based formulation for sensitivity studies of piezoelectric systems
Sensitivity Analysis is a branch of numerical analysis which aims to quantify the affects that variability in the parameters of a numerical model have on the model output. A finite element based sensitivity analysis formulation for piezoelectric media is developed here and implemented to simulate the operational and sensitivity characteristics of a piezoelectric based distributed mode actuator (DMA). The work acts as a starting point for robustness analysis in the DMA technology
- âŠ