Metastable Excited Molecules Research Articles

Determination of collisional quenching rate coefficient of by H2O

The measurement of the effective lifetime of in mixtures of nitrogen and small amounts of H2O (10.2 and 103 ppm) was carried out while observing the transient ionization current after turning off the ultraviolet (UV) light illuminating the cathode in a non-self-sustained Townsend discharge region. The transient current was formed by the emitted current sustained by the γm action of metastable excited molecules , which were produced by the impact of electrons with nitrogen molecules in the gap, and returned to the cathode by diffusion. Then, the collisional quenching rate coefficient of by H2O, the diffusion coefficient of , and the reflection coefficient of at the cathode surface were determined from the observed effective lifetime of by an analytical procedure based on solving the diffusion equation. The obtained collisional quenching rate coefficient of by H2O was (5.7 ± 0.6) × 10−13 cm3 s−1.

Detection of metastable excited molecules N2(A3Σu+) in an atmospheric pressure nitrogen discharge by Raman scattering

AbstractRaman scattering from metastable excited nitrogen molecules N2(A3Σu+) created by an impulse discharge in nitrogen at atmospheric pressure was detected. A pulsed Nd:YAG laser at a wavelength of 266 nm was used as the light source, and Raman scattering from N2(A3Σu+) at a wavelength of 277 nm was detected using an interference filter and photomultiplier tube. The filter had sufficient rejection of Rayleigh scattering of laser light at a wavelength of 266 nm and of Raman scattering from ground‐state nitrogen molecules N2(X1Σg+) at a wavelength of 284 nm. The temporal variation of the signal intensity of Raman scattering from N2(A3Σu+) was measured by transmitting the laser light at different time delays relative to sparkover. The results showed that the signal intensity decayed with a time constant of about 200µs. © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(6): 34–40, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10205

Active nitrogen species dependence on radiofrequency plasma source operating parameters and their role in GaN growth

Radiofrequency (RF) nitrogen plasma sources are commonly used for the growth of III-nitrides by plasma-assisted molecular beam epitaxy. Their output mainly consists of a mixture of atomic nitrogen and metastable excited molecules. The relative concentration of these species as a function of RF power and nitrogen gas flow rate has been investigated by plasma optical emission spectroscopy. It is shown that the concentration of excited molecular nitrogen species is monotonically increasing with both parameters while the concentration of nitrogen atoms is almost independent of the nitrogen gas flow rate and increases monotonically with the coupled RF power. As a result of this functional dependence, the plasma source can be tuned to produce mainly excited nitrogen molecules or a mixture of excited molecules and atomic nitrogen. Comparison of the growth rates of gallium nitride films grown under different operating conditions of the RF source with the optical spectroscopy data shows that the metastable excited nitrogen molecules component of the source is the predominant factor contributing in the III-nitrides growth.

Electron acceleration in gas by impulse electric field and its application to selective promotion of an electron–molecule reaction

We have simulated electron acceleration under short intense impulse electric fields of 0.1–1 kV cm−1 strength and ∼1 ns duration in N2 at 65.5 Pa. The electron acceleration is completed in a practically collisionless condition when the impulse duration is comparable with or shorter than the electron mean free time. On the basis of this feature, we have proposed a way to produce a group of monoenergetic electrons. The electron energy after acceleration is controllable by the impulse strength and duration, and we have demonstrated selective promotion of the excitation producing N2(A 3Σu+) metastable excited molecules. The selectivity has been quantified as the efficiency of the electron energy utilized for the target reaction, and efficiencies of 20–30% have been achieved in this simulation. These values are higher than those of electron swarms in DC equilibrium, in which the efficiency is at most 10%. The electron behaviour under impulse electric fields and the dependence of the energy efficiency on the impulse profile have been investigated.

Diffusion coefficients of two metastable states of the nitrogen molecule

Abstract Previous experiments of Ernest et al. (1986) showed the existence of a metastable state of the nitrogen molecule with a coefficient for diffusion through ground-state nitrogen of about one quarter of that of the well-known A 3 Σ u + state. Both states were populated by pre-breakdown discharges through pure nitrogen. This paper presents further results on the diffusion of metastable excited molecules through nitrogen. The effects of varying background gas number density N and the reduced electric field E N were explored, and data were taken in two ionization chambers with two different cathode materials. Consistent results are obtained with the different chambers and cathodes. The characteristic times for the decay of metastable population density are independent of E N but scale with 1/N, consistent with the existence of two slowly diffusing metastable states of nitrogen. The values at 295 ± 4 K of the product DmN, where Dm is the metastable diffusion coefficient, are 551 ± 25 am−1s−1 ((5.51 ± 0.25) × 1016cm−1s−1) for the first slowly diffusing state and 158 ± 17 am−1s−1 for the second. Both states show very small quenching in collisions with ground-state nitrogen molecules. The first state is identified as the A 3Σu+ state. Far from having a small effect, the action of molecules in the second slowly diffusing state can dominate the ionization growth near breakdown.

CHEMICAL SCAVENGER PROBE STUDIES OF ATOM AND EXCITED MOLECULE REACTIVITY IN ACTIVE NITROGEN FROM A SUPERSONIC STREAM

A quartz chemical scavenger probe has been developed to study the local composition of supersonic electrically discharged gas streams. The probe samples the central portion of a nonequilibrium jet and allows direct comparison with other local measurement techniques (e.g. differential catalytic detectors) for determining active species concentrations. Active nitrogen from a Mach 3 stream was sampled and reacted inside the probe with one of the scavenger gases NO, NH3, or C2H4at 18.8 mm Hg and at an average temperature of 500 °K. Limiting values of the NO destruction rate and the HCN production rate were observed; however, NH3destruction exhibited no plateau. The observed maximum rate of NO destruction was 2.1 times as large as the NO flow rate at the light titration end-point. This difference is attributed to a reaction of NO, added in excess of the titration end-point flow, with metastable electronically excited molecules formed within the discharge zone. The converging-diverging supersonic nozzle-glow discharge source used in these experiments apparently delivers metastable excited molecules to the reaction zone in a higher relative concentration than do the more conventional subsonic electrical discharge flow systems used for most previous active nitrogen studies.