D. J. Eckstrom

Characteristics of electron‐beam‐excited Xe*2 at low pressures as a vacuum ultraviolet source

The performance of Xe*2 as a 172‐nm fluorescence or laser source when pumped by a low‐current, long‐pulse electron beam was determined. The fluorescence efficiency of Xe*2 is near the theoretical limit of ∼50% at modest pressures over a range of pump rates up to 106 W/cm. The laser efficiency is limited to values <1% by a very strong medium absorption that is probably due to Xe*2 photoionization. Laser performance is further degraded by early pulse termination that appears related to mirror degradation. An improved kinetics and extraction code was developed to model the performance of the Xe*2 system. A key component of the model is a more detailed treatment of the interactions between secondary electrons and excited atomic and molecular xenon states. Rates for these processes were derived as described herein. With this model, good absolute agreement was obtained between experiments and calculated parameters at pressures as low as 0.5 atm.

Chemiluminescence studies. IV. Pressure‐dependent photon yields for Ba, Sm, and Eu reactions with N2O, O3, O2, F2, and NF3

Chemiluminescence spectra and photon yields are presented for reactions of Ba, Sm, and Eu with N2O, O3, O2, F2, and NF3 for the pressure range 0.5–20 torr. Peak yields range from 2.5% to 70%, with the reactions of Sm with NF3 and F2 having the highest yields. These latter reactions put from 12% to 16% of their available thermal energy into luminous output. The pressure dependence of the photon yields deduced from these and other recent measurements suggests that the initial exothermic reaction primarily populates high vibrational levels of the ground electronic state of the newly formed diatomic. Radiating states would then be populated mainly by collision‐induced vibrational‐to‐electronic internal conversion.

Observations of strong microwave absorption in collisional plasmas with gradual density gradients

Strong microwave absorption was observed when a 10 GHz source illuminated an underdense collisional plasma that had a density gradient scale length several wavelengths long. Significant reductions in angular scattering and cross‐polarized components were also observed. These experiments confirm that absorption was the dominant process. The plasma was created by the photoionization of tetrakisdimethylaminoethylene molecules seeded into atmospheric pressure helium. Sparkboard arrays provided the intense vacuum ultraviolet ionizing radiation. Plasma density profiles were measured using transverse scans of 9.7 GHz probe microwaves and were found to approximate an Epstein profile. The absorption at 10 GHz by this plasma was as large as 28 dB in direct backscattering and 15–20 dB when orthogonally polarized microwaves were launched and detected. The peak absorption scales with sparkboard energy in a way that suggests that electron‐ion recombination is the dominant electron‐loss mechanism at high plasma densities.

Photolytically pumped XeF (C→A) laser studies

Properties of the visible XeF (C→A) photolytically pumped laser system have been investigated in detail. The research covers four main areas: (1) absolute fluorescence intensities from the C and B states, (2) C→A fluorescence spectra as a function of pressure and buffer gas, (3) gain/absorption measurements from 400–650 nm, and (4) demonstration of tunable laser operation over the 454–525‐nm range. All results reported here indicate that the XeF (C→A) laser could be a highly efficient visible laser system.

Vibrational relaxation of shock‐heated N2 by atomic oxygen using the ir tracer method

We have studied the effects of oxygen atoms on the vibrational relaxation of shock‐heated N2 over the temperature range from 1200 to 3000 °K, using the CO tracer technique. The measured relaxation times pτN2–O vary from 0.8 to 2 atm · μsec in this range with an uncertainty of ± 50%, in excellent agreement with existing high‐temperature shock tube and low‐temperature flow tube results. The weak temperature dependence of all these results is in marked contrast to theoretical predictions. Some limited measurements of the relaxation of CO by oxygen atoms yield values of pτCO–O of approximately 0.04 atm · μsec, in good agreement with other recent measurements. This rate is approximately 25 times as fast as the N2–O rate and, coincidentally, is approximately the same as that for the relaxation of O2 by O atoms. The greater rapidity of the CO–O and O2–O relaxation, compared with that of N2–O, may be the result of atom‐exchange processes in the former cases.

Emission spectra and potentials of the 0+u and 1u states of Xe2

An extensive series of time‐resolved measurements of Xe*2 second continuum emission spectra has been completed to reveal the spectra of the relaxed individual emitting states. Optical pumping of high‐pressure Xe by a short‐pulse F2 laser at 157.6 nm was used together with a gated optical multichannel analyzer detector or monochromator and boxcar integrator to time resolve the emission bands of the 0+u and 1u states. The bands are broader than expected and have center wavelengths close together. Curve fits to the data yield the parameters for the fully relaxed states: 1u:λc =171.4±0.6 nm, Δλ=12.2±0.2 nm and 0+u: λc =170.4±0.6 nm, Δλ=13.1±0.3. Temperature‐dependent measurements from 246 to 413 K show a rapidly broadening band width with temperature. The results provide the first definitive measurement of the individual excimer emissions from the Xe2(0+u) and Xe2(1u) states. From these measurements and the known ground‐state potential, we have derived 0+u and 1u potentials that reproduce the emission bands a...

Heat‐pipe‐oven reactor (HPOR): I. A new device for flame studies; photon yields in the reaction of Na with CCl4 and N2O

A new device based on the heat‐pipe oven has been demonstrated to have a unique capability for studying reactions of metal atoms with oxidizers. This device allows spherical diffusion flame studies under known, uniform, and easily adjustable, metal atom concentration. low gas pumping rate, clean window operation, and minimized problems of reactive solid disposal. The reactor was used for the study of chemiluminescence of the reactions of sodium vapors with CCl4, with N2O, and with both CCl4 and N2O. The emission from these reactions in the range 2000–9000 A was identified to be from various excited atomic levels of sodium and from excited C2, mostly in the Swan bands. Even though the emission was very intense, especially in the case of Na+N2O+CCl4 (photon yields of about 10%), it was found that the Na excited‐state populations had a Boltzmann distribution corresponding to an electron temperature of 2260 °K. This suggests that the emission of sodium radiation from both the Na+CCl4 and Na+CCl4+N2O flames re...

Multijoule performance of the photolytically pumped XeF(C&#8594;A) laser

An XeF( C \rightarrow A ) laser with output up to 5.8 J/pulse has been demonstrated. The photolytic pumping scheme begins with e -beam excitation of xenon to produce Xe* 2 fluorescence at 172 nm. This VUV radiation is transmitted through an array of CaF 2 windows into the laser cell, where it photodissociates XeF 2 to produce primarily XeF( B\frac{1}{2} ). Collisions with N 2 buffer gas relax the excited states to XeF( C\frac{3}{2} ), which lases on a transition centered at 481 nm and continuously tunable over more than ±35 nm. Typical values of the experimental parameters were as follows. The 420 kV, 1 m e -beam source delivered an average current of 10 A/cm2over an aperture 14 × 100 cm for pulse lengths up to 1 μs. Total e -beam energy available was 3.5 kJ, of which 2.4 kJ was deposited in the xenon. The total VUV energy radiated was 720 J, of which 115 J was coupled into the laser cell. This produced 32 J of available XeF* energy, of which up to 18 percent was extracted as laser energy. The total system efficiency was 0.2 percent. Optimized designs should achieve better than 1 percent efficiency.

Quantitative aspects of benzene photoionization at 248 nm

Abstract We have carefully remeasured the mass distribution of ions resulting from photoionization of benzene at 248 nm. The fragmentation occurs at lower intensities than previously indicated, beginning below 2 MW/cm2. At 17 MW/cm2, the parent ion constitutes only 12.5% of the total, while at 110 MW/cm2, C+ constitutes 24% of the yield. The functional dependence of the absolute photoionization yield as a function of laser power cannot be described by a simple four-level kinetic model successfully used to describe the absolute MPI yield at low power. Instead, a highly excited neutral intermediate is postulated that can either autoionize or photodissociate upon absorption of an additional photon.

Chemiluminescence studies. V. Production of electronically excited alkali atoms in reactions of Li, Na, and Cs with NF3 and of Cs with F2

The magnitudes and distributions of alkali atom emission have been measured for Li+NF3, Na+NF3, Cs+NF3, and Cs+F2 flames. The production of excited states in all cases follows a gi(Eion−Ei)3 dependence, where gi and Ei are the degeneracy and energy of the state, and Eion is the ionization energy of the alkali atom. This suggests that the primary excitation mechanism involves electron–ion recombination. Examination of other data shows that this energy dependence of excitation may hold in a variety of reaction systems and experimental conditions. Measured photon yields for atomic emission ranged from 0.2% to 3%; considerably higher values would be expected in the absence of radiation trapping. There is no evidence for substantial population inversions between any of the atomic states.