Karl W. Holtzclaw

Experimental determination of the Einstein coefficients for the N2(B–A) transition

We have used a branching‐ratio technique to measure the relative variation in the transition‐dipole moment with internuclear separation for the N2(B–A) transition. Our spectral observations cover the range from 500 to 1800 nm, and use several different detectors and excitation sources. The data from different sets are consistent in the regions of spectral overlap. Using well established values for the radiative lifetimes of N2(B,v’≥5) allows the relative dipole‐moment function to be placed on an absolute basis. From the dipole‐moment function and a set of RKR‐based Franck–Condon factors which we have computed, we derive Einstein coefficients covering the range v’=0–12 and v‘=0–20. Our results indicate that currently accepted lifetimes for N2(B,v’=0–2) should be revised upwards by 20% to 40%.

Chemical timing 3. The picosecond dynamics of intramolecular vibrational redistribution from 11 levels in S1p‐difluorobenzene vapor

Collisional quenching of S1–S0 fluorescence (chemical timing) has been used to study the dynamics of intramolecular vibrational redistribution (IVR) from 11 initial vibrational levels in S1 p‐difluorobenzene (pDFB). The levels range in energy from 1615 to 3310 cm−1 with total state densities ranging from about 10 to 2000 per cm−1. The IVR follows intermediate case kinetics of radiationless transition theory for levels below about 2500 cm−1 and statistical case kinetics for higher levels. The IVR involves coupled states spanning an energy width of 0.02–0.3 cm−1. The IVR lifetimes vary from about 15 to 300 ps with much variability among initial levels. A low frequency out‐of‐plane mode appears to be an accelerating mode for IVR. Average matrix elements for the level coupling associated with the IVR are found by analysis of the kinetic data to be in the range 0.01–0.1 cm−1. Vibration‐rotation coupling appears to be an important contributor to IVR, at least for the lower levels.

Analysis of hydroxyl earthlimb airglow emissions: Kinetic model for state‐to‐state dynamics of OH (υ,N)

Detailed spectroscopic analysis of hydroxyl fundamental vibration-rotation and pure rotation emission lines has yielded OH(υ,N) absolute column densities for nighttime earthlimb spectra in the 20 to 110-km tangent height region. High-resolution spectra were obtained in the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A) experiment. Rotationally thermalized populations in υ = 1–9 have been derived from the fundamental bands between 2000 and 4000 cm−1. Highly rotationally excited populations with N ≤ 33 ( ≤ 2.3 eV rotational energy) have been inferred from the pure rotation spectra between 400 and 1000 cm−1. These emissions originate in the airglow region near 85–90 km altitude. Spectral fits of the pure rotation lines imply equal populations in the spinrotation states F1 and F2 but a ratio Π(A′):Π(A″) = 1.8±0.3 for the Λ-doublet populations. A forward predicting, first-principles kinetic model has been developed for the resultant OH(υ,N) limb column densities. The kinetic model incorporates a necessary and sufficient number of processes known to generate and quench OH(υ,N) in the mesopause region and includes recently calculated vibration-rotation Einstein coefficients for the high-N levels. The model reproduces both the thermal and the highly rotationally excited OH(υ,N) column densities. The tangent height dependence of the rotationally excited OH(υ,N) column densities is consistent with two possible formation mechanisms: (1) transfer of vibrational to rotational energy induced by collisions with O atoms or (2) direct chemical production via H + O3 → OH(υ,N) + O2.

Spatial and spectral characterization of laboratory shuttle glow simulations

Laboratory experiments designed to uncover mechanistic information about the spectral and spatial characteristics of shuttle glow were conducted. Pulsed oxygen atoms traveling at orbital velocities were directed toward a substrate which was previously dosed with NO molecules. Heterogeneous recombination of the O and NO species resulted in NO2* exiting the surface of the sample, and an associated emission was found to extend from the sample plane. In the experiments the materials investigated were Z306 Chemglaze® (a common baffle black paint), aluminum, and nickel. The sample temperatures were varied from 300 to 77 K, and the oxygen atom velocity was varied from 5 to 10 km s−1. The experimental results include the measure of (1) an effective NO2* lifetime of 185 μs, (2) complete surface thermal accommodation of the formed NO2*, (3) a large NO2* emission brightness which was inversely related to surface temperature, and (4) a spectral shape which indicates a red shifting to distance from the sample as well as (5) a slight spectral shift which appears to be material related. The preliminary experimental data from this experiment were presented by Caledonia et al. (1990).

Lifetimes of triplet state alkylbenzenes in the vapor phase

Abstract Rate constants for triplet state decay of C 6 H 6 , C 6 D 6 and 15 alkylbenzenes in the vapor phase have been found, using a flash-sensitized biacetyl phosphorescence technique, to range between 800 s −1 and ⩾ 18 000 s −1 . Only benzene has a significant positive activation energy for decay. Kinetic and spectroscopic evidence supports a photoisomerization decay channel in t -butylbenzene. Comparison of lifetimes with molecular size shows that increased density of rotational levels does not account for rapid decay of the triplet state. This contradicts a recent suggestion to explain short lifetimes of triplet state aromatic hydrocarbons in the vapor phase, relative to the long lifetimes of the same molecules in low temperat matrices. Evidence suggests that the dominant decay paths for triplet state alkylbenzenes are different in room temperature vapors and low temperature matrices.

Mechanistic investigations of shuttle glow

A series of laboratory measurements have been performed in order to provide a mechanistic interpretation for the visible shuttle glow. These studies involved interactions of an 8 km/s oxygen atom beam with both contaminant dosed surfaces and gaseous targets. We conclude that visible shuttle glow arises from surface mediated O + NO recombination via a Langmuir-Hinshelwood mechanism and that the gas-phase exchange reaction O + N2 → NO + N provides a viable source of precursor NO above surfaces oriented in the ram direction.

Rotational relaxation of high-N states of OH (X2Π, v=1–3) by O2

We have studied the relaxation of OH(X 2 II, v, N) produced by the reaction O( 1 D) + H 2 → OH * + H. Infrared emission measurements of the fundamental vibration-rotation band of OH were acquired at a temperature of 100 K in a large cryogenic chamber. Trace amounts of ozone were added to mixtures of H 2 , O 2 , and Ar at low (s38 mT) pressures. The ozone was photolyzed to produce O( 1 D). Rapid reaction with H 2 produced OH*. Spectrally and temporally resolved emissions from levels up to the exothermic limit were observed with sufficient spectral resolution to permit kinetic analysis of individual level populations. The production rate was observed to scale as the rotational quantum number except at the highest levels populated. At early times we observed inverted rotational state distributions that subsequently relaxed to form thermal distributions in each vibrational level. Under these conditions, rotational relaxation was rapid in comparison with vibrational relaxation. Rotational relaxation within a given vibrational state could be represented by single quantum collisional exchange at near-gas-kinetic rates. The rotational level dependence of the deduced relaxation was determined.

Ultrasensitive gain diagnostic for chemical lasers

The determination of small signal gain in candidate laser systems, especially low gain chemically pumped lasers, is a challenging problem. In this paper we discuss a new approach to this problem using tunable diode lasers as sensitive probes for optical gain. The gain diagnostic was verified on optically pumped molecular iodine. Small signal gains of 104 were detectable with signal to noise exceeding 10. The gain diagnostic was also applied to a candidate chemically pumped laser system, the oxygen pumped IF laser.

Laser-pumped gas-phase mid-IR lasers

We describe the results of an analytical and experimental program that is investigating the feasibility of developing mid-IR lasers based upon laser pumped gas phase molecules. We present results for lasing on overtone pumped HF, HCl, and DF. In addition, describe several possible excitation sources including diode lasers and alexandrite lasers.

Correction to “Laboratory investigation of shuttle glow mechanisms”

Figure 2. Spectra/response corrected luminescence measured in the present study as a result of 8 km/s O atom interaction with NO doped Z306 Chemglaze