P. Davidovits

Ultraviolet Absorption Cross Sections for the Alkali Halide Vapors

The ultraviolet‐absorption cross sections for the alkali halide vapors were measured in the region from 2000 to 4000 A. Potential curves were constructed from these data and are compared to some previous work. From the cross sections the radiative lifetime of the excited molecular states is calculated to be on the order of 10−8 sec.

Ultraviolet Absorption Cross Sections for the Thallium Halide and Silver Halide Vapors

The ultraviolet cross sections for the thallium and silver halide vapors were measured in the region 2200–4500 A. In the region of the main continua the cross sections are between 5 × 10−17 cm2 and 1 × 10−17 cm2. For the thallium halides the data revealed a new process consisting of two closely spaced absorptions. It is shown that this process may be due to transitions into states formed by interactions between excited atomic and ionic potentials. For the silver halides there is a significant continuum absorption on the long‐wavelength side of the band structure. This indicates that there is a repulsive state below the bound excited state of the molecule.

Cross Sections for the Alkali‐Metal‐Halogen Molecule Reactions: Na, K, Rb, and Cs with I2

The reaction cross sections of alkali‐metal atoms with I2 molecules were obtained by direct measurements of alkali‐atom decay rates. The alkali atoms were produced in the presence of a known amount of I2 molecules, by photodissociating the alkali iodide salt with a short pulse of uv light. As the alkali atoms reacted with the I2 molecules their decay was monitored by observing the transmission of alkali atom resonance light through the vapor. The cross sections were computed from the decay rates. They are: Na: 97 A2; K: 127 A2; Rb: 167 A2; Cs: 195 A2. These results are in good agreement with the cross sections computed from a modified orbiting theory.

Cross sections for the alkali atom‐Br2 reactions

The reaction cross sections of alkali metal atoms with bromine molecules have been obtained by direct measurements of alkali atom decay rates. The alkali atoms were produced in the presence of a known amount of bromine molecules by photodissociating the bromide of the particular alkali atom with a short pulse of uv light. As the alkali atoms reacted with bromine molecules their decay rate was monitored by observing the transmission of alkali atom resonance light through the vapor. The cross sections were computed from the decay rates. They are Na, 116 A2; K, 151 A2, Rb, 197 A2, and Cs, 204 A2. These results are accurate to about 15%. The results are compared with theoretical calculations.

Collisional De‐excitation of the Thallium 62 P3/2 State

The collisional de‐excitation cross sections of the 62 P3/2 metastable state of thallium have been measured for the following atoms and molecules: He, Ne, Ar, TlI, TlBr, TlCl, O2, NO, H2, CO, CH4, C2H4, C2H6, C3H6, C3H8, CO2, and N2. Metastable thallium was produced in a quartz cell by photodissociating thallium halide vapor with a pulse of ultraviolet radiation. The density of the metastable thallium decays due to collisions with the gas added to the cell. The decay is exponential with a time constant governed by the density of the added gas. The lifetime of the metastable state was measured by monitoring the absorption of the thallium 5350 A line from a beam of resonance radiation passing through the cell. Since the density of the added gas is known, the cross section can be computed from the measured lifetime. Our results are accurate to about 15%.

Cross Sections for the Alkali‐Metal–Halogen Molecule Reactions: Cs with I2

The reaction cross section of alkali‐metal atoms with halogen molecules is obtained by a direct measurement of the alkali‐atom decay rates. The alkali atoms are produced in the presence of a known amount of halogen molecules, by photodissociating an alkali halide salt with a short pulse of uv light. The alkali atoms react with the halogen molecules and the decay is monitored by observing the transmission of alkali‐atom resonance light through the vapor. Since the halogen density is known, the reaction cross section can be computed from the decay rate. Using this method we studied the reaction Cs+I2→CsI+I. The cross section for this process is 180 ± 25 A2.

Cross Sections for the Collision of Thallium (62 P1/2 and 62 P3/2) with I2

The inelastic collision cross sections with I2 have been measured for the ground state (62P1/2) and the metastable state (62P3/2) of thallium. The thallium atoms were produced in the presence of a known amount of I2 by photodissociating TlI salt vapor with a short pulse of uv light. The cross sections were computed from the decay rate of the thallium density. We have observed that the collision of the metastable state with I2 results in the de‐excitation of the state rather than in chemical reaction. The cross section for the process Tl(62P1/2)+I2→ TlI+I is 105 A2 and for the process Tl(62P3/2)+I2→Tl(62P1/2)+I2 is 159 A2. these results are in good agreement with theory.

Thermal diffusion of Br2 and Cl2 in the noble gases

We have measured the thermal diffusion factors of Br2 and Cl2 in He, Ne, Ar, and Kr. The halogen‐noble gas mixture is contained in a closed two bulb apparatus, the ends of which are maintained at different temperatures. The halogen density at the cold end is determined by the equilibrium vapor pressure above the temperature controlled halogen reservoir. The halogen density at the hot end is determined by measuring the total reaction rate; γ of sodium atoms with halogen molecules. Since the cross sections for the reactions of Na atoms with Br2 and Cl2 are known, measurement of γ yields directly the halogen density. The thermal diffusion factors computed from the transport equations are compared with the first Kihara approximation to the Chapman‐Enskog theory. The agreement is within experimental error.

Thermal Diffusion of I2 in the Noble Gases

The thermal diffusion factors for I2 in He, Ne, Ar, Kr, and Xe have been measured using a two‐bulb apparatus. The iodine noble‐gas mixture is contained in a closed system the two ends of which are maintained at different temperatures. The I2 density at the cold end is determined by the equilibrium vapor pressure above a temperature‐controlled reservoir of solid iodine. The I2 density at the hotter end is obtained by measuring the reaction rate of cesium and thallium atoms with I2. Since the cross sections for the Cs—I2 and Tl—I2 reactions are known, the I2 density in this region can be obtained. The thermal diffusion factors computed from the transport equations are in good agreement with the Kihara approximation to the Chapman—Enskog theory.