Absorption and free-jet action spectroscopy of states in the region of four quanta of C-H stretching of acetylene (C 2 H 2 ) around 12 676 cm Ϫ1 were studied. Rovibrational excitation combined with 243.135 nm promotion of C 2 H 2 molecules to the upper electronic trans-bent states, Ã 1 A u /B 1 B u , and H photofragment ionization generated action spectra measuring the H yield as a function of the excitation wavelength. The findings show that the (1030 0 0 0 ) IR bright state of the third C-H stretch overtone has a smaller photodissociation cross section than the (1214 0 0 0 ) combination band containing trans-bend mode excitation and lying in its vicinity, due to a favorable Franck-Condon factor for the latter. Certain line pairs accessing similar JЈ levels of the (1030 0 0 0 ) state show anomalous intensities in the action spectrum, with P transitions excessively enhanced over R transitions. This implies enormous dissociation efficacy as a consequence of Coriolis-type local resonances with nearby states of another band and due to the overlap of the P transitions with another band.
The photofragmentation dynamics of acetylene, C 2 H 2 , was explored via vibrationally mediated photodissociation. Direct near infrared (NIR) excitation efficiently prepared rovibrational states in the region of three C-H stretch quanta ($9640 cm À1 ), subsequently $243.1 nm UV 1 photons promoted the pre-excited C 2 H 2 molecules to the A ˜1A u state and dissociated them and finally the ensuing H atoms were probed by UV 2 photons via (2 + 1) resonantly enhanced multiphoton ionization. UV dependent action spectra, monitoring the H photofragment yield vs. the UV 1 dissociating laser wavelength displayed sharp peaks depending on the combined energy and the initially excited rovibrational state. These spectra indicate that the rovibrational transitions belonging to the (1112 0 0 0 ) combination band, containing trans-bend mode excitation, are excessively enhanced over those of the (0030 0 0 0 ) state with three C-H stretch quanta, due to favorable Franck-Condon (FC) factor. The UV absorption from these states sampled particular rovibronic levels of the potential well on the upper A ˜1A u state, disclosing transitions to FC active vibronic modes, involving couplings between torsion and cis-bend with C= =C stretch and trans-bend modes. These results suggest that the initial state preparation samples the bound rather than the purely repulsive region of the A ˜1A u state and that the rovibronic structure is the essential factor in affecting the absorption cross-section. They also indicate that the provided energy is insufficient to overcome the barrier on the A ˜1A u state, leading to the dominant non-adiabatic predissociative photofragmentation into C 2 H (X ~2S + ) + H.
Photoluminescence (PL) is investigated as a function of the excitation intensity and temperature for lattice-matched InGaAs/InAlAs quantum well (QW) structures with well thicknesses of 7 and 15 nm, respectively. At low temperature, interface fluctuations result in the 7-nm QW PL exhibiting a blueshift of 15 meV, a narrowing of the linewidth (full width at half maximum, FWHM) from 20.3 to 10 meV, and a clear transition of the spectral profile with the laser excitation intensity increasing four orders in magnitude. The 7-nm QW PL also has a larger blueshift and FWHM variation than the 15-nm QW as the temperature increases from 10 to ~50 K. Finally, simulations of this system which correlate with the experimental observations indicate that a thin QW must be more affected by interface fluctuations and their resulting potential fluctuations than a thick QW. This work provides useful information on guiding the growth to achieve optimized InGaAs/InAlAs QWs for applications with different QW thicknesses.
A three-dimensional (3D) photonic crystal (PhC) structure consisting of gradient quasidiamond lattices was fabricated using multiphoton photopolymerization nanofabrication technique. The photonic bandgap (PBG) of this 3D PhC was experimentally confirmed by reflection and transmission measurements and simulated with finite-difference time domain calculations. The results indicate that a 3D PhC with gradient lattices could effectively expand the width of the PBG and may be beneficial for developing complete-bandgap PhCs with low refractive index materials for applications in polymer based optoelectronic devices and integrated systems.
A novel pattern recognition method based on Empirical Mode Decomposition (EMD) and extreme gradient boosting (XGBoost) is proposed to recognize the disturbance events in phase sensitive optical time-domain reflectometer (ϕ-OTDR) to reduce nuisance alarm rate (NAR) and improve real-time performance in this paper. Eleven typical eigenvectors are extracted from components obtained by EMD of the disturbance signals and XGBoost is selected as a classifier to identify different type of disturbance signals. Five kinds of disturbance events, including watering, knocking, climbing, pressing and false disturbance event, can be identified, effectively. Experimental results show that NAR is 4.10% and identification time is 0.093 s. The recognition accuracy for the five patterns is 97.96%, 95.90%, 91.10%, 94.84% and 99.69%, respectively. The effectiveness of the proposed method is evaluated by using confusion matrix and decision boundary visualization. Experimental results demonstrate that our proposed pattern recognition method based on XGBoost has better performance in recognition rate and recognition time than other commonly used methods, such as support vector machine (SVM), Gradient Boosting Decision Tree (GBDT), Random Forest (RF) and Adaptive Boosting (Adaboost). INDEX TERMS Phase-sensitive optical time-domain reflectometer (ϕ-OTDR), extreme gradient boosting (XGBoost), nuisance alarm rate (NAR), empirical mode decomposition (EMD), pattern recognition, decision boundary visualization.
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