Richard A. Keller

Galvanic detection of optical absorptions in a gas discharge

An electrical signal, resulting from discrete optical absorptions, has been observed for a variety of elements, including several for which such an effect had not been previously reported. In the present case, the effect is observed as a change in the voltage across a gas discharge tube produced by irradiation with a laser tuned to the wavelength of a transition of a species in the discharge. This signal may be used—without optical detection apparatus—for spectroscopic investigations or analytical determinations of materials in the discharge. Signals were obtained for transitions of lithium, sodium, calcium, barium, uranium, neon, and helium, in commercial hollow cathode lamps, and neon and helium in conventional discharge tubes.

Enhancement of absorption spectra by dye-laser quenching, III: Quantitative aspects and a comparison of flash-lamp-pumped and cw systems under high resolution

The enhancement of the detectability for trace absorptions by placing samples inside the laser cavity was found to be a factor of 100 for a flash-lamp-pumped dye laser and one thousand for a cw dye laser. High-resolution spectra showed that the holes in the laser output were as narrow as the absorptions that caused them. An approximately linear relationship (rather than the step-function behavior often associated with threshold phenomena) exists between pressures necessary to produce visually identical absorption spectra from samples placed inside and outside of the laser cavity. If such a relationship is of general occurrence, it will greatly facilitate the use of intracavity absorption for quantitative analysis, determination of relative absorption cross sections, and for the study of the kinetics of appearance and disappearance of transient species.

Extinction Coefficients of Triplet↔Triplet Transitions in Aromatic Compounds

The extinction coefficients and oscillator strengths were measured for the first strong triplet↔triplet transition in naphthalene, phenanthrene, and triphenylene. The results are: naphthalene, emax=14 000 liter/mole·cm, f=0.05; phenanthrene, emax=27 000 liter/mole·cm, f=0.11; triphenylene, emax=7000 liter/mole·cm, f=0.05.

Laser induced photochemical enrichment of chlorine isotopes

Abstract Chlorine isotopic abundances were significantly altered by laser induced, selective excitation of particular isotopic species of thiophosgene followed by a chemical reaction between the electronically excited thiophosgene and diethoxyethylene. The concentration of 35 Cl in thiophosgene was changed from 75% to 64% or 80% depending upon the isotopic species initially excited.

Excited triplet — singlet intersystem crossing

Abstract The quantum yield for intersystem crossing from excited triplet states into the singlet manifold was estimated for several molecules by observing the fluorescence emission from the lowest excited singlet state which resuleted from photo-excitation of metastable triplet states. In all cases these quantum yields were very small.

Detection of sodium trace contamination in furnace atmospheres at 1000 °C

Free sodium atoms were detected by resonance fluorescence in an open contaminated quartz tube heated to 1000 °C. The quartz tube and furnace were similar to those used in semiconductor device processing. Fluorescence was excited by a cw dye laser tuned to the sodium D1 or D2 transition and directed along the axis of the furnace. Fluorescence from the sodium D2 line emitted in the axial direction was collected by a telescopic system and focused onto a photomultiplier tube. The estimated minimum detectable sodium density in the furnace is 5×105 atoms/cm3. No free sodium was detectable in a processing tube that had not been intentionally contaminated.

Flash Photolysis of Sulfur Dioxide

The body of information presented in this paper is directed to photochemists and air pollution scientists interested in species which result from the interaction of SO2 and light. When SO2 at low pressures is subjected to an intense photolysis flash, the characteristic, very structured SO2absorption spectrum disappears immediately after the flash and is replaced by a continuous absorption. The continuous absorption gradually decays and the normal SO2 absorption spectrum returns. The initial absorbance of the continuous absorption is proportional to the square of the SO2 pressure and the square of the flash irradiance. From these facts we propose the formation of a metastable dimer of SO2 formed by the collision of two excited molecules. Some properties of this dimer are: natural lifetime = 2 sec; energy above separated monomers = 4 kcal; lifetime at atmospheric pressure = 1 sec (quenching coefficients with several foreign gases = 10-20 cm3/sec molecule); absorption of ultraviolet light results in photode-...

Quantitative determination of gaseous nitrogen dioxide concentrations over long path lengths by selective absorption of argon ion laser emission

An argon ion laser emits several laser lines in the visible region of the optical spectrum. The absorption coefficients of NO/sub 2/ at these laser emissions were measured in a multiple pass absorption cell. A differential technique, in which the ratio of the transmitted intensities of the argon laser emissions is measured, is described to determine the concentration of NO/sub 2/ in a polluted atmosphere over path lengths of several kilometers. Measurement of ratios eliminates interferences from particle scattering and thermal index gradients. Evaluation of the data taken in the 48 meter multipass cell indicates that concentrations of NO/sub 2/ less than one part per million could be determined in a 1 km optical path. (auth)

Isotopic enrichment in laser photochemistry

Laser technology has permitted more efficient study of the chemistry of molecules in excited states. The extent to which isotopic specificity is preserved in going from initial excitation to final product is a valuable diagnostic for excited state chemistry. This report summarizes initial results from several areas of investigation, each of which suggests that laser stimulation may offer more than simple rate enhancement.