GaN p–i–n photovoltaic diode arrays were fabricated from epitaxial films deposited on sapphire by molecular beam epitaxy. Peak UV responsivity was 0.11 A/W at 360 nm, corresponding to 48% internal quantum efficiency. Visible rejection over 400–800 nm was 3–4 orders of magnitude. Typical pulsed time response was measured at 8.2 μs. Spectral response modeling was performed to analyze the photocurrent contributions from photogenerated carrier drift in the depletion region and from minority carrier diffusion in the p and n layers. With the model, a maximum internal quantum efficiency of 55% at 360 nm was calculated for the photovoltaic diode structure.
The spectra of vibrationally hot HOI formed in the reaction of alkyl iodides with oxygen atoms are observed by Fourier transform infrared emission spectroscopy. The v=1–3 levels of the OH stretch are observed via the Δv=−1 and Δv=−2 sequence bands. The spectrum of DOI is observed by using 2,2,2-d3-iodoethane as the precursor in the oxygen atom reaction. The v=1–4 levels of the OD stretch are observed in the Δv=−1 sequence band, and the v=1–5 levels of the OD stretch are observed in Δv=−2. Medium resolution spectra (0.031 cm−1 apodized) are recorded and rotationally analyzed for the ν1 fundamental and 2ν1−ν1 hot band of HOI. An estimate of the HOI ground state structure is made by constraining the OH bond length to its value for HOCl and HOBr and calculating the HOI bond angle and the OI bond length by least squares fit to the ground state rotational constants.
The emission spectra of vibrationally excited hydroxyl radical products formed in the reactions of alkyl radicals with O(3P) atoms are detected using a laser photolysis/time-resolved Fourier transform infrared spectroscopy technique. For the reaction between oxygen atoms and ethyl, the radicals are produced simultaneously by the 193 nm photolysis of the precursors SO2 and diethyl ketone, respectively. The observed initial OH(v) product vibrational state distribution for the C2H5+O(3P) reaction is 0.18±0.03, 0.23±0.04, 0.29±0.05, 0.23±0.07, and 0.07±0.04 for v=1 to 5, respectively. The population inversion is best explained by a direct abstraction mechanism for this radical–radical reaction. Vibrationally excited hydroxyl radicals are also observed in the O+ethyl, O+n-propyl, and O+i-propyl reactions when using alkyl iodides as precursors of the alkyl radicals, although quantitative detail is not obtained due to competing reaction processes.
Vibrationally excited hypoiodous acid (HOI) is observed as a product in the reaction of alkyl iodides with O(3P). Fourier transform infrared emission techniques are used to detect the excited ν1, OH, stretch of the HOI product, to determine the mechanism of HOI production, and to measure the vibrational product state distributions. The HOI product is formed by O atom reaction with two-carbon and larger straight or branched chain alkyl iodides and cyclic alkyl iodides, e.g., C2H5I, n-C3H7I, i-C3H7I, (CH3)3CI, n-C6H13I, and c-C6H11I, but not with CH3I. Experiments with selectively deuterated ethyl iodides provide direct evidence that HOI is formed in a beta-elimination mechanism involving a five-membered ring transition state. The O atom attacks the iodine and then abstracts a hydrogen from the beta carbon during the lifetime of the complex. Time-resolved experiments allow the extraction of nascent vibrational state distributions for the ν1 stretch of HOI (v=1:v=2:v=3) using different alkyl iodides and assuming the radiative rates are given by Aν→ν−1=vA1→0: C2H5I, 0.53(4):0.39(3):0.08(3); n-C3H7I, 0.61(6):0.34(5):0.05(2); and i-C3H7I, 0.54(6):0.38(4):0.08(3). These distributions are nonstatistical with the v=2 states having only slightly less population than those with v=1. For product molecules with up to three quanta of ν1 excitation, more than 50% of the reaction exothermicity is deposited into the OH stretch.
An infrared double-resonance laser spectroscopic technique is used to study state-resolved rotational (R–R, R–T) energy transfer in ammonia (14NH3) (self-collisions and between ammonia and foreign gases). NH3 molecules are prepared in selected rovibrational states of the v2=1 level using coincidences between CO2 -laser lines and ν2 fundamental transitions. Measurements of both the total rate of depopulation by collisions, and the rates of transfer into specific final rovibrational states (v,J,K) have been carried out using time-resolved tunable diode laser absorption spectroscopy. For NH3–NH3 collisions, measurements of total depopulation rates of selected JK states in v2=1 and ground-state recovery rates are found to be three and eight times larger, respectively, than the Lennard-Jones collision rate, in accord with theoretical expectations for polar molecules.
A kinetic master-equation analysis of time-resolved level populations yields state-to-state rate constants and propensity rules for NH3–NH3 and NH3–Ar collisions. Individual rotational energy-transfer rates in v2=1 are slower than in the vibrational ground state, but still comparable to the Lennard-Jones collision frequency. Our experiments show that rotational energy transfer in v2=1 is not governed by simple ‘‘dipolelike’’ selection rules. They show fast rotational energy transfer, which can be related to long-range interaction potentials, but at the same time considerable amounts of ΔJ=2 and 3, ΔK=0, and ΔJ=1–4, ΔK=3, transitions, which may be attributed to higher-order terms in the multipole expansion of the intermolecular potential. No pronounced symmetry-state correlation and no preferred pathways were found except the preference for relaxation within a K stack and the expected separate relaxation of different nuclear-spin species, which can be labeled by their K-quantum number. Rates of collision-induced symmetry change (a↔s) in v2=1 are on the order of kas=4 μs−1 torr−1, smaller than kas in the ground state, but over an order of magnitude larger than that recently reported in the literature. Depopulation rates for other collision partners (Ar, H2, N2, and He) can be understood in terms of the intermolecular potentials. Comparisons are made between the relaxation rates measured in this work and infrared pressure-broadening coefficients reported in the literature.
A time-resolved infrared double-resonance technique has been used to measure vibrationally and rotationally inelastic collision rates in ground and vibrational overtone levels of methane. A Raman-shifted Ti:sapphire laser is used to pump J=0 through 7 states in the 2ν3 and ν3+ν4 levels of 12CH4, and a tunable diode laser is used to probe the time-dependent level populations. Vibrational equilibration is observed among the octad, pentad, and dyad levels, with subsequent relaxation to the ground state. State-to-state rotational energy transfer rates are obtained in the ground and ν3+ν4 excited vibrational levels, and compared with theoretical predictions and with pressure-broadening measurements on the corresponding transitions. The probability of molecular reorientation in an inelastic collision is also inferred from the polarization dependence of the relaxation times. Parity-conserving and vibrational angular momentum propensity rules are inferred for the lower rotational levels of methane.
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