Curve-crossing collisions of excited lead atoms in flames

1981 ◽  
Vol 376 (1767) ◽  
pp. 545-564 ◽  
Keyword(s):  
Rate Constant ◽  
Atomic Hydrogen ◽  
Negative Temperature Coefficient ◽  
Negative Temperature ◽  
Model Systems ◽  
Attractive Potential ◽  
Hydrogen Atoms ◽  
Lennard Jones ◽  
Increasing Temperature ◽  
Curve Crossing

Lead atoms, present as a trace additive in a series of premixed H 2 –N 2 –O 2 flames, were excited to the 7 3 P o 1 state by 405.8 nm radiation from a nitrogen-pumped dye laser. Rate constants for spin-orbit relaxation to the 7 3 P o 0 state were obtained separately for collisions with atomic hydrogen and for collisions with the bulk flame gas, by measuring the relative intensities of fluorescence at 364.0 and 368.3 nm as a function of distance from the reaction zone in each flame. For hydrogen atoms the rate constant is typically 1 x 10 -9 cm 3 molecule -1 s -1 , decreasing with increasing temperature; for the bulk flame gas the rate constant is typically 1 x 10 -11 cm 3 molecule -1 s -1 , increasing with increasing temperature. Numerical calculations for model systems, with the use of Morse and Lennard-Jones potentials to describe the interaction of the colliding species, show that the negative temperature coefficient found for atomic hydrogen can be attributed to the crossing of attractive potential curves, corresponding to bound excited states of PbH.

Shock-wave observations of rate constants for atomic hydrogen recombination from 2500 to 7000 °K: collisional stabilization by exchange of hydrogen atoms

1969 ◽  
Vol 310 (1501) ◽  
pp. 253-276 ◽  
Keyword(s):  
Shock Waves ◽  
Rate Constant ◽  
Rate Constants ◽  
Atomic Hydrogen ◽  
Functional Form ◽  
Kcal Mole ◽  
Hydrogen Atoms ◽  
Hydrogen Molecules ◽  
Hydrogen Recombination ◽  
Equilibrium Dissociation

Rate constants for the recombination of atomic hydrogen with hydrogen molecules, hydrogen atoms, and argon atoms as the third bodies are presented in functional form for the range of temperatures from about 2500 to 7000 °K and are critically compared with the results of other workers. The rate constants are evaluated from detailed analyses of spectrum-line reversal measurements of the fall in temperature accompanying dissociation behind shock waves in gas mixtures containing 20, 40, 50 and 60% of hydrogen in argon. The rate constants for recombination with hydrogen molecules ( k -1 ) and argon atoms ( k -3 ) fit the equations log 10 k -1 = 15.243 - 1.95 x 10 -4 T cm 6 mole -2 s -1 , log 10 k -3 = 15.787 - 2.75 x 10 -4 T cm 6 mole -2 s -1 , with a standard deviation of 0.193 in log 10 k -1 . The rate constant for recombination with hydrogen atoms is about ten times larger than these at 3000 °K and shows a steep inverse dependence on temperature ( ~ T -6 ) above 4000 °K. Below this temperature the power of this dependence decreases rapidly and there is strong evidence that the value of this rate constant has a maximum around 3000 °K. This behaviour is interpreted on the basis of a process of collisional stabilization by atom exchange, requiring an activation energy around 8 kcal mole -1 and taking place under conditions of vibrational adiabaticity. The over-all results indicate that the assumption of equality between the equilibrium constant and the ratio of the rate constants for dissociation and recombination is valid throughout the region of non-equilibrium dissociation and at all temperatures in the shock waves examined.

scholarly journals Atomic hydrogen - I—The calorimetry of hydrogen atoms

1937 ◽  
Vol 163 (914) ◽  
pp. 404-414 ◽  
Keyword(s):  
Heterogeneous Catalysis ◽  
Glow Discharge ◽  
Atomic Hydrogen ◽  
Hydrogen Molecule ◽  
Considerable Extent ◽  
Hydrogen Atoms ◽  
Discharge Tubes ◽  
Method Of Production ◽  
Increasing Temperature ◽  
Gas Pressures

Since the discovery of Wood (1920, 1921, 1922 a , b ) that hydrogen can be dissociated to a considerable extent into free atoms in the glow discharge, many workers have used this method of production of atomic hydrogen for the study of its chemistry. Few attempts have been made, however, to determine the optimum discharge conditions for large yields of atoms, although the consensus of opinion has favoured the use of long discharge tubes, operated with fairly heavy currents at gas pressures of about 0·3-1mm. The work described in this series of papers was undertaken to deter­mine and explain the factors controlling yield. Atomic hydrogen has been estimated by several different methods. The calorimetry of the heterogeneous catalysis reaction, H+H+ M → H 2 + Q + M ( Q being the dissociation energy of the hydrogen molecule, and M a suitable catalyst) has been the method employed by a number of workers. Bonhoeffer (I924) Bichowsky and Copeland (1928), v. Wartenberg and Schultze (1930), Dixon (1932), and Smallwood (1929) have all devised calorimeters based on this equation. But with none of these has it been possible to obtain a good degree of accuracy, coupled with reasonably rapid operation and the possibility of making consecutive readings. Probably the method of Small­ wood, also used by Amdur and Robinson (1933), is the best of those cited; but this involved corrections for conduction losses amounting to 40 or 45% of the observed rise of temperature; also about 20min. were required for each determination, and consecutive readings were not possible owing to the increasing temperature.

Electrical Properties of Mn-Co-Ni Oxides Prepared at Low Temperatures

2007 ◽  
Vol 29-30 ◽  
pp. 359-362 ◽  
Author(s):  
T. Yokoyama ◽  
Takeshi Meguro ◽  
S. Okazaki ◽  
H. Fujikawa ◽  
T. Ishikawa ◽  
...  
Keyword(s):  
Electrical Properties ◽  
Electrical Resistance ◽  
Negative Temperature Coefficient ◽  
Negative Temperature ◽  
B Value ◽  
Spinel Oxide ◽  
Practical Application ◽  
Spinel Type ◽  
Molar Ratios ◽  
Increasing Temperature

Changes in crystal phase and electrical properties of oxides composed of Mn, Co, and Ni with the molar ratios of 5:2:1 and 3:3:1 were investigated. Starting oxides were fired from 250°C to 800°C, then maintained at prescribed temperatures for 3 h in air. In the 5:2:1 specimen, a monophase consisting of a cubic spinel oxide that is important to electrical conductivity was obtained at firing temperature of 800°C. In the 3:3:1 specimen, the monophase was obtained at temperatures ranging from 600°C to 800°C. Electrical resistance decreased exponentially with increasing temperature for all specimens fired at temperatures ranging from 250°C to 800°C, indicating that the oxides have intrinsic thermistor characteristics with negative temperature coefficient (NTC). The temperature dependence of the thermistor constant (B value) necessary for practical application was considered to be related to the existence ratio of cubic spinel-type and ilmenite-type structures and the lattice constant of the cubic spinel-type structure. The electrical conduction was stabilized by annealing at prescribed temperatures for more than 720 minutes.

THE REACTION OF OXYGEN ATOMS WITH NO

1959 ◽  
Vol 37 (10) ◽  
pp. 1690-1695 ◽  
Author(s):  
E. A. Ogryzlo ◽  
H. I. Schiff
Keyword(s):  
Rate Constant ◽  
Negative Temperature Coefficient ◽  
Negative Temperature ◽  
Order Rate Constant ◽  
Third Body ◽  
Kcal Mole ◽  
Arrhenius Activation Energy ◽  
Third Order ◽  
First Order ◽  
Calorimetric Technique

In the reaction between NO and O-atoms, the concentration of NO remains essentially unaltered. The reaction can therefore be considered as a NO-catalyzed recombination of O-atoms. Its rate can be conveniently studied by following the disappearance of O-atoms by an isothermal calorimetric technique. The reaction was found to be third order, first order in the concentrations of O, NO, and M, where M is some third body. The third-order rate constant was found to be 1.85 × 1016 cc2 mole−2 sec−1 when M = O2, A, or He and 2.0 × 1016 cc2 mole−2 sec−1 when M = CO2 The rate constant was found to have a slight negative temperature coefficient which corresponded to a negative Arrhenius activation energy of about 0.2 kcal/mole. A detailed mechanism for the reaction has been proposed.

Structural, optical and electrical properties of thermal evaporated nano sized In2Te3 thin films

2018 ◽  
Vol 1 (1) ◽  
pp. 21-25
Author(s):  
R Revathi ◽  
R Karunathan
Keyword(s):  
Thin Films ◽  
Negative Temperature Coefficient ◽  
Thermal Evaporation ◽  
Structural Parameters ◽  
Negative Temperature ◽  
Resistivity Measurements ◽  
Allowed Transition ◽  
Evaporation Technique ◽  
Indium Telluride ◽  
Made In

Indium Telluride thin films were prepared by thermal evaporation technique. Films were annealed at 573K under vacuum for an hour. Both as-deposited and annealed films were used for characterization. The structural parameters were discussed on the basis of annealing effect for a film of thickness 1500 Å. Optical analysis was carried out on films of different thicknesses for both as - deposited and annealed samples. Both the as- deposited and annealed films exhibit direct and allowed transition. Electrical resistivity measurements were made in the temperature range of 303-473 K using Four-probe method. The calculated resistivity value is of the order of 10-6 ohm meter. The activation energy value decreases with increasing film thickness. The negative temperature coefficient indicates the semiconducting nature of the film.

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