Effect Of Pulse Length Research Articles

Long-pulse, broadband-Xe*2 -fluorescence excitation of OCSe: Initiation of the Se(1S→3P) laser and implications for large-system design

The formation and loss of Se(1S) excited states have been examined using long-pulse (0.7–1.0 μsec), broadband (13 nm FWHM)-Xe*2- fluorescence photodissociation of carbonyl selenide. Experimental data are compared to results of a computer code that models the Xe*2 radiation transport and Se(1S) kinetics. The effective quantum yield for Se(1S) production over the wavelength interval 160–180 nm has been found to be near 0.6. The observed Se(1S) decay rates have been used in conjunction with the code to predict the effects of excitation pulse length on the efficiency of the selenium laser system. Laser oscillation has been demonstrated for the Se(1S0→3P1) transition in a long-active-length (54 cm) optical cavity. Oscillator performance was characterized as a function of OCSe pressure. The optimized laser output was 1.3 mJ, a value consistent with the Se(1S) kinetics and laser cavity conditions.

Effect of gas pressure and pulse length on the isotopically selective multiphoton dissociation of CF3Cl under CO2 laser pulses

The multiphoton dissociation of CF3Cl induced by TEA-CO2 laser pulses has been studied in a focused beam geometry. TheR(10) [00°1–02°0] (\(\bar v = 1071.9cm^{ - 1} \)) laser line was used, so as to dissociate preferentially the minor isotopic component13CF3Cl. The isotopic selectivityS and the dissociation probability per pulse ω were measured in the pressure range between 0.25 and 8 Torr. With short laser pulses (90 ns FWHM),S is found to increase slightly with gas pressure up to 2 Torr, and ω, to increase almost linearly over the whole pressure range studied. A schematic model is proposed which satisfactorily explains these results if the transition rates across the energy level spectrum of the CF3Cl molecules are assumed to increase with gas pressure.

Erratum: Effect of pressure and pulse length on the isotopically selective photodissociation of freon CF2Cl2 under CO2 laser pulses

When the 13C species of CF2Cl2 is excited by short TEA‐CO2 laser pulses, the isotopic enrichment of the resultant photodissociation remains constant between 0.25 and 2 Torr CF2Cl2 pressure, whereas excitation of the 12C species yields a monotonous decrease of the enrichment with pressure in the same range. These results are compared to those obtained with longer pulses and with the 32S and 34S species of SF6.

Effect of pressure and pulse length on the isotopically selective photodissociation of freon CF2Cl2 under CO2 laser pulses

When the 13C species of CF2Cl2 is excited by short TEA-CO2 laser pulses, the isotopic enrichment of the resultant photodissociation remains constant between 0.25 and 2 Torr CF2Cl2 pressure, whereas excitation of the 12C species yields a monotonous decrease of the enrichment with pressure in the same range. These results are compared to those obtained with longer pulses and with the 32S and 34S species of SF6.

A high repetition rate ion laser spanning the 195-225 nm spectral region

This paper describes the construction and operation of a water-cooled noble gas ion laser which generates average powers of the order of a milliwatt at 5 strong transitions between 195 and 225 nm. This pulsed laser has been operated at repetition rates up to 200 Hz and has produced average powers up to 5.1 mW at 219.2 nm. The pressure and voltage dependence of output power, along with preionization effects, gain, saturation intensities, and the effects of discharge current pulse length are discussed. The optical pulses can be precisely synchronized with respect to an electrical reference signal, and the laser operates at all wavelengths with Brewster windows, permitting convenient interchanging of resonator mirrors. This device has been designed for use as an ultraviolet illuminator in a photoelectron microscope.

A negative result on the effect of laser pulse length on the Rayleigh scattering cross section in argon

The absolute Rayleigh scattering cross section in argon was measured for ruby laser pulses of 10 ns, 24 ns and 70 ns. The values agree with Rayleigh's formula and show no dependence on the pulse length. This contrasts with other recent work.

Near field transient radiation patterns for circular pistons

The exact impulse response of field parameters for any field point on or off axis for the case where a circular disc radiator face is subjected to a displacement step corresponding to a velocity impulse is reviewed. By convolution, the transient field pattern for any arbitrary motion of the disc can be obtained. The exact response for a half-sine monopulse is computed. An approximate representation of the transient pressure response to the velocity impulse input at the disc is derived, and it is shown to correspond to the replica pulses described previously. The regions of validity of the approximation are quite limited and the replica pulses are displaced in time from the positions formerly attributed to them. The displaced replica approximation is applied to an examination of the structure of the near field for continuous sinusoidal excitation and a plot of positions of extrema is produced. It is shown that this approximation gives good agreement with the exact values and is superior to the previous published approach in this regard. For short sinusoidal pulses the effect of pulse length on the field pattern, and of field point on the time history of a transient wave are shown. When the excitation is a short sinusoidal pulse the effect of the pulse length and field point position on the field pattern and wave shape are demonstrated.

Waveguide Breakdown Effects at High Average Power and Long Pulse Length

Analytical predictions of the power handling capabilities of waveguide systems generally have not considered the effects of high average power and long pulse length. It has experimentally been noted, however, that a substantial reduction in power handling capability below expected levels does occur as average power and pulse length are increased. This reduction can be attributed to the presence of loose particulate matter which is heated by the average power, causing localized rarefication of the dielectric gas fill as well as the expected voltage enhancement. An unstable arcing situation is shown to exist when the arc duration exceeds some critical time. Typical pulse lengths in use today exceed this critical time and may result in continuous arcing. The use of control circuitry to terminate each particle-induced arc prevents continuous arcing and deletes the particle, and is therefore essential to stable operation at long pulse lengths and high average power.

Effects of Rotating Magnetic Fields on Free-Induction Decay Shapes

The behavior of a set of particles in a rotating magnetic field is analyzed to determine how the lengthening of a nuclear magnetic resonance free- induction decay might occur. An experimert to determine the effect of pulse length on the subsequent free-induction decay was carried out for gypsum, calcium fluoride, and ice. A single r-f pulse at the Larmor frequency of the spin system was applied to the sample in equilibrium and the resulting free-induction decay shape was recorded. The r-f pulse length was varied from 1 to 18 mu sec and the magnetic field ranged from 3 to 30 gauss. (C.E.S.)

Cloud chamber study of electron avalanche growth

The growth of avalanches and the transition to a visible discharge has been studied in air, nitrogen, argon, carbon dioxide, hydrogen and oxygen by means of an expansion cloud chamber. The effect of expansion ratio, vapour filling and type of gas on the formation of droplets was investigated. Pulse lengths of 0·11, 0·08 and 0·05 μs and pressures of 400, 580 and 680 mmHg were used in air and agreement with previous mobility measurements found. As the voltage was increased the shape of the avalanche changed, its breadth increased and finally a filamentary visible discharge was produced. The position of the avalanche head at the time of transition has been correlated with the occurrence of a constriction in the visible discharge at the same point. A single avalanche has been found to be sufficient to form a streamer. Streamer thresholds have been measured as a function of pulse length, and vapour filling. The addition of methyl, ethyl or propyl alcohol to dry nitrogen reduced the streamer threshold by about 14%. Reduction of pulse length increased the streamer threshold. Nitrogen was very similar to air in regard to the effect of pulse length, vapour filling and voltage. The streamer constriction could be moved nearer the cathode with increasing voltage and its position ( x er. being distance from cathode) was given by α x er. = 14 to 20 depending on pressure and pulse length. The X/p value of streamer threshold was unaffected by a change of pd from 150 to 780 mmHg cm. Streamers were produced in argon but overvoltages of up to 200 % were required. Effect of vapour and pulse length was similar to air. Carbon dioxide was difficult to use owing to its dissociation by ultra-violet light, and agreement with mobility measurements in the dry gas is not very good. Streamers were obtained with overvoltages. Results for hydrogen were in reasonable agreement with previous mobility measurements. Velocity measurements in oxygen were less than the previous values, but this is most likely caused by electron attachment which reduced the apparent length of the avalanche. Streamers with bright knobs at the constriction were observed. The position of the avalanche head has been calculated based on the assumption that it coincided with the neck in the streamer, and some agreement with observation particularly at high X/p values was found. Thermal energy derived from avalanche width and velocity differs by as much as a factor of 2 and a possible explanation is put forward.

Some Fundamental Considerations concerning Noise Reduction and Range in Radar and Communication

A general analysis based upon information theory and the mathematical theory of probability is used to investigate the fundamental principles involved in the transmission of signals through a background of random noise. Three general theorems governing the probability relations between signal and noise are proved, and one is applied to investigate the effect of pulse length and repetition rate on radar range. The concept of generalized selectivity is introduced, and it is shown how and why extra bandwidth can be used for noise reduction. It is pointed out that most noise-improvement systems are based upon coherent repetition of the message information either in time or in the frequency spectrum. It is also pointed out why more powerful noise-improvement systems should be possible than have so far been made. The general mechanism of noise-improvement thresholds is discussed, and it is shown how they depend upon the establishment of a coherence standard. The reason for and the limitation of the apparent law that the maximum operating range of a communications system, for a given average power, is independent of the type of modulation used is then explained. General ways in which improvements in range of radar and communication systems may be made are also discussed. The possibility of using extra bandwidth to reduce distortion is pointed out. Finally, some possible relations of this work to biology and psychology are described.