We propose a new algorithm for deconvolution of electrospray ionization mass spectra based on direct assignment of charge to the measured signal at each mass-to-charge ratio (m/z). We investigate two heuristics for charge assignment: the entropy-based heuristic is adapted from a deconvolution algorithm by Reinhold and Reinhold;10 the multiplicative-correlation heuristic is adapted from the multiplicative-correlation deconvolution algorithm of Hagen and Monnig.6 The entropy-based heuristic is insensitive to overestimates of z(max), the maximum ion charge. We test the deconvolution algorithm on two single-component samples: the measured spectrum of human beta-endorphin has two prominent and one very weak line whereas myoglobin has a well-developed quasi-gaussian envelope of 17 peaks. In both cases, the deconvolution algorithm gives a clean deconvoluted spectrum with one dominant peak and very few artefacts. The relative heights of the peaks due to the parent molecules in the deconvoluted spectrum of a mixture of two peptides, which are expected to ionize with equal efficiency, give an accurate measure of their relative concentration in the sample.
Time domain spectroscopy has been used to measure the room temperature transmission of highly doped GaAs in the frequency range from 0.2 to above 3 THz. We studied n- and p-type layers, with carrier densities between 1016 and 2×1018 cm−3, which had been grown on undoped GaAs substrates. Transmission spectra could be fitted within experimental error by using a Drude model for the conductivity. Fitted carrier densities for both carrier types and mobilities for p-type GaAs were in good agreement with the results of Hall measurements. In the case of n-GaAs, the optically determined mobility appeared to underestimate slightly the Hall mobility.
The emission and detection efficiencies of photoconducting THz transmitters and receivers are found to be sensitive to the polarization of the optical gating pulse. Signal amplitudes from GaAs coplanar stripline transmitters and silicon-on-sapphire dipole receivers vary by factors of up to 4 and 3, respectively, with rotation of the exciting pulse polarization. In both cases, maximum signal is obtained when the polarization of the normally incident light is perpendicular to the edge of the metal electrodes. This polarization sensitivity, which appears to arise from differences in the spatial distribution of photoexcited carriers in the semiconductor, needs to be considered when optimizing the signal-to-noise ratio in coherent THz spectroscopy.
We have demonstrated the excitation and control of coherent cyclotron emission from a semiconductor using a THz beam containing pairs of sub-picosecond electric field pulses closely spaced in time. The source of THz radiation in these experiments is a biased coplanar stripline fabricated on semi-insulating GaAs and edge illuminated with pairs of temporally and spatially separated 70 fs pulses of near infrared light. A GaAs/AlGaAs two-dimensional electron gas was used in the experiments because of its long intraband phase relaxation time. Changes in the amplitude and phase of the cyclotron emission are observed when varying the interpulse delay and are well described within the theoretical framework of a two level system.
Impulsive interband optical excitation of the lowest two conduction subbands of a suitably engineered GaAs/AlGaAs double quantum well can lead to coherent THz emission. We demonstrate that, contrary to previous expectations, the dominant emission mechanism can involve the beating of either continuum or exciton states, depending on excitation conditions. The coherence of the continuum beats persists for several picoseconds even for excitation an optical phonon energy above the band edge. We attribute this to the small energy difference between the component eigenstates, which substantially reduces the number of relevant scattering events.
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