E. S. Hwang

Vibrational relaxation of NO(υ=1) by oxygen atoms

The rate constant kO(υ=1) for NO(υ=1) vibrational relaxation by O has been measured at room temperature using a laser photolysis-laser probe technique. Vibrationally excited NO and relaxer O atoms were formed using 355 nm laser photolysis of a dilute mixture of NO2 in argon bath gas. The time evolution of both the NO(υ=1) and the O atoms was monitored using laser-induced fluorescence (LIF). The required absolute O-atom densities were obtained through a comparison of O-atom LIF signals from the photolysis source and from a titrated cw microwave source. At early times the O atoms constitute the most important loss mechanism for the nascently produced NO(υ=1). Possible effects from NO(υ=1) vibrational ladder-climbing and from thermal expansion have been shown to be minimal. The rate constant kO(υ=1)=(2.4±0.5)×10−11 cm3 s−1 determined herein is a factor of 2 to 3 lower than the generally accepted value of kO(υ=1) used in thermospheric modeling. The present value for kO(υ=1) is the same, within the error bars,...

Hyperthermal atomic oxygen source for near-space simulation experiments

A hyperthermal atomic oxygen (AO) beam facility has been developed to investigate the collisions of high-velocity AO atoms with vapor-phase counterflow. Application of 4.5 kW, 2.4 GHz microwave power in the source chamber creates a continuous discharge in flowing O(2) gas. The O(2) feedstock is introduced into the source chamber in a vortex flow to constrain the plasma to the center region, with the chamber geometry promoting resonant excitation of the TM(011) mode to localize the energy deposition in the vicinity of the aluminum nitride (AlN) expansion nozzle. The approximately 3500 K environment serves to dissociate the O(2), resulting in an effluent consisting of 40% AO by number density. Downstream of the nozzle, a silicon carbide (SiC) skimmer selects the center portion of the discharge effluent, prior to the expansion reaching the first shock front and rethermalizing, creating a beam with a derived 2.5 km s(-1) velocity. Differential pumping of the skimmer chamber, an optional intermediate chamber and reaction chamber maintains a reaction chamber pressure in the mid-10(-6) to mid-10(-5) Torr range. The beam has been characterized with regard to total AO beam flux, O(2) dissociation fraction, and AO spatial profile using time-of-flight mass spectrometric and Kapton-H erosion measurements. A series of reactions AO+C(n)H(2n) (n=2-4) has been studied under single-collision conditions using mass spectrometric product detection, and at higher background pressure detecting dispersed IR emissions from primary and secondary products using a step-scan Michelson interferometer. In a more recent AO crossed-beam experiment, number densities and predicted IR emission intensities have been modeled using the direct simulation Monte Carlo technique. The results have been used to guide the experimental conditions. IR emission intensity predictions are compared to detected signal levels to estimate absolute reaction cross sections.

The AFRL Scholars Program: a STEM-based summer internship initiative

The Air Force Research Laboratory (AFRL) Scholars Program offers stipend-paid summer internship opportunities to undergraduate- and graduate-level university students as well as upper-level high school students who are pursuing or plan to pursue degrees in science, technology, engineering, and mathematics (STEM). Internships through the AFRL Scholars Program are currently offered through the Directed Energy, Space Vehicles, and Munitions Directorates of AFRL with locations at Kirtland Air Force Base, New Mexico, Eglin Air Force Base, Florida, and Maui, Hawaii. Throughout their internships, AFRL Scholars gain valuable hands-on experience working with full-time AFRL scientists and engineers on cutting-edge research and technology. Overall, the selected interns are able to contribute to unique, research-based projects which often contain a strong emphasis in optics and photonics. This paper celebrates the continued success of the AFRL Scholars Program and shares a statistical overview of its growth over the past few years. In particular, the analysis focuses on how these STEM-related internships will hopefully meet the needs of an aging AFRL workforce in the years to come. This paper also provides an overview of two optics and photonics related internships at the undergraduate and graduate levels, respectively. Both interns received the Outstanding AFRL Scholar Award in their respective categories and are currently pursuing careers in optics and photonics based on their experiences as AFRL Scholars.