George A. Fisk

Laser‐based and thermal studies of stress corrosion in vitreous silica

Crack‐propagation rates in vitreous silica were monitored as specimens were heated thermally or irradiated by a laser beam. The laser was tuned to the OH bond vibration to determine the role of OH stretching in the dissociative chemisorption mechanism for stress corrosion of silica in moist environments. Laser radiation heated the silica near the crack tip but did not generate any bond‐specific changes in crack velocity. For samples maintained in moist air, the propagation velocity decreases as temperature increases. The temperature dependence of crack velocity indicates that physisorption is an important step in stress corrosion, and a simple model of the process is proposed.

A study of the 0.634 μm dimol emission from excited molecular oxygen1

Abstract The rate constant, k d , for the process 20 2 ( 1 Δ) - 20 2 ( 3 Σ) + hv was determined for radiation at 0. 634 μm. It was found that at room temperature k d = (4.4 ± 1.3) × 10 −23 cm 3 molecule −1 s −1 and that k d is independent of pressure for oxygen pressures up to 4.5 Torr and argon pressures up to 77 Torr.

A classical trajectory study of inelastic collisions between highly vibrationally excited KBr and Ar

We present a study of inelastic collisions between highly vibrationally excited KBr and Ar based on three‐dimensional classical trajectory calculations. Calculations are performed for three closely related potential energy surfaces chosen to allow determination of the importance of an attractive well. Comparison with experiment indicates that both depth and shape of an attractive well are important for calculating detailed scattering distributions. Analysis of selected trajectories indicates that double impact collisions play an important role in the overall vibrational deactivation mechanism. Double impact collisions occur frequently for all three surfaces and, unlike the situation in one dimension, transfer energy very efficiently.

A classical trajectory study of intramolecular vibrational relaxation and unimolecular decomposition in methyl hydroperoxide

This paper presents a quasiclassical trajectory study of the energy flow that occurs consequent to high‐overtone excitations of either a CH or an OH local mode in methyl hydroperoxide, CH3 OOH. The potential energy surface employed is an empirical one based on available spectroscopic, thermodynamic, and theoretical data. Energy initially localized in a CH stretch transfers irreversibly on the time scale of the calculations into the methyl bending modes within 0.2 ps. Transfer of energy out of the methyl group to the rest of the molecule occurs more slowly. An initially excited OH bond retains energy longer than does a CH bond and, unlike the energy transfer for an excited CH stretch, partial recurrences in the energy content of the OH mode occur for some excitations. Vibrational resonances are important in determining the rates and pathways of energy flow in the molecule. At total energies near 104 kcal/mol the rate of the O–O bond scission is twice as fast for OH excitation as it is for CH excitation.

Multiple photon induced chemistry in mixtures of C2F6 with H2 or C6H14

CO2 laser induced chemistry is demonstrated to occur efficiently in mixtures of C2F6 with H2 or C6H14 at a fluence of 6 J/cm2 and for a 30 cm-1 red-shift from the C2F6 absorption maximum. H2 or C6H14 pressure may be used to control the distribution of product species.

Amplified spontaneous emission and gain‐saturation nonlinearity in high‐gain optical amplifiers: The biased amplifier

The systematics of amplified spontaneous emission and gain‐saturation nonlinearity in high‐gain optical amplifiers are explored. It is shown that of all the variations of the several dimensionless parameters that occur, only a reduction of the small‐signal gain‐length product can simultaneously minimize these two problems. Furthermore, of the various possible changes of the dimensional quantities that make up the gain‐length product, increasing the saturation intensity achieves the highest overall gain for a given measure of distortion, and the least distortion for a given value of overall gain. This leads to the concept of the biased amplifier, one version of which is then treated in detail numerically.

Digital image processing of velocity‐interferometer data obtained from laser‐driven shock experiments

This paper discusses digital image processing techniques suitable for use with the velocity interferometric records obtained from shock‐wave experiments using ORVIS (optically recorded velocity interferometer system). An ORVIS system is capable of measuring shock events with subnanosecond time resolution, the data consisting of fringe motion (proportional to the impacted material velocity) recorded with a streak camera. Two image processing methods are described which permit the determination of accurate velocity‐time data from the records. The first, based on global analysis of the records, requires no operator intervention during reduction of the data, but breaks down when records show very rapid acceleration of the target under observation. The second, based on a thinning algorithm, can be used to treat rapid acceleration data and data in which the fringes are less than ideal. It requires operator intervention in some cases. The two techniques are shown to agree well when applied to the same data.

Particle Velocity Measurements Of Laser-Induced Shock Waves Using ORVIS

This paper discusses experiments in which 2 to 200-pm-thick aluminum foils were irradiated with Nd/YAG laser pulses at 3-4 GW/cm2 for 10 nsec. An optically recording velocity interferometer system (ORVIS) was used to record the resulting particle-velocity histories with nanosecond resolution. Results show that foils suspended in air are accelerated, over a period about three times the length of the laser pulse, to a final velocity inversely proportional to foil thickness. A 12-pm-thick foil attains a peak velocity of 0.2 km/sec. Foils confined by water undergo most of their acceleration during the laser pulse and attain surface velocities three times greater than do air-suspended foils of the same thickness.

Effects of chemical kinetics of the performance of the atomic iodine laser system

Model calculations show that chemical reactions which take place in the active medium of a photolytically pumped iodine laser limit the efficiency with which pump photons are utilized and convert significant amounts of the starting material RI to the unwanted by‐products R2 and I2. Laser‐ and rf‐discharge‐based methods for regenerating starting materials from by‐products are evaluated experimentally. For economical operation of large iodine laser systems, CF3I is presently the best starting material, and a pulsed rf‐discharge technique is presently the best one for chemical regeneration. The absorbed energy required to regenerate one CF3I molecule using pulsed rf‐discharge techniques is 5.8 eV.

Saturable absorption and rotational relaxation in CO2

Experimental data demonstrating saturated optical absorption of HF P2(4) laser radiation by CO2 is presented and analyzed in terms of a simple kinetic model. Within the framework considered, relatively slow rotational relaxation in the (00°0) vibrational level of CO2 is shown to be responsible for saturated absorption. The relation between the rotational relaxation rate constant appropriate to these experiments and that determined in thermal experiments is discussed.