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Absolute emission intensity studies on the halogen afterglows and excited molecular oxygen

University of British Columbia

The bromine afterglow emission was studied using the discharge-flow technique. The spectrum of discharged bromine was observed to extend from 6000Å to 22000Å and has been attributed to the Br₂ (₃Π₀⁺u→¹Ε⁺g) and Br₂(³Π₁u→Σ⁺g) transitions. The dependence of the emission intensity on atom concentration was observed to vary between I ∝ [Br]²•º ±º•¹ at short wavelengths, and I∝[Br]¹•⁵±º•¹ in the long wavelength region of the spectrum. In the pressure range studied (O.5 to 2 torr), the intensity was found to be independent of the molecular bromine concentration. By measuring the absolute emission intensity between 6000Å and 12000Å, values of the apparent rate constant, defined as kapp=I/[Br]²[Br₂], were measured. These rate constants were found to depend on atom concentration and pressure and varied between kapp =5.4 x 10¹³ and 1.3 x 10¹⁵ cc²mole⁻² sec⁻¹. A mechanism of the emission reaction is discussed and it is suggested that as much as 15% of the total recombination of bromine atoms may be proceeding via excited states. In a similar study of the chlorine afterglow, all of the emission was attributed to the Cl₂ (³Π₀ +u —>¹Σ⁺g) transition. A study of the emission intensity in narrow bands revealed that I∝[CI]²•⁰±º•¹ at short wavelengths, corresponding to low vibrational levels of the ¹Π₀+u state, while I ∝ [Cl]¹•⁰±º‧¹ at long wavelengths and higher vibrational levels. Similarly, the pressure dependence of the intensity changed from I ∝ [Cl₂]⁰•⁵± º•² at long wavelengths to I ∝ [Cl₂]⁰•⁵±⁰•² at short wavelengths. Absolute emission intensity measurements were made in the region from 5000Å to 12000Å and the rate constants, defined by kapp = I/[Cl]²[Cl₂], were found. Extrapolation of these values of kapp to zero atom concentration yields kapp = 1.8 x 10¹⁴ cc² mole⁻² sec⁻¹. A mechanism for the formation and relaxation of the excited state is discussed. The study of absolute emission intensities was extended to measurements on excited oxygen molecules in a flow system. Observations on the 7619Å band of the O₂(¹Σ⁺g→³Σ⁻) transition yielded [0₂ (¹Σ⁺g)] = 1.77 x 10⁻⁹ moles 1.⁻¹, thereby confirming that this species is a minor constituent in the products of discharged oxygen. Absolute intensity studies on the 6340Å and 7030Å bands of the (0₂ (Δg))₂→(0₂(³Σ⁻))₂ transition indicate that the half life of the (O₂ (¹Δg))₂ collision complex is around 0.1 seconds. The emission spectrum produced when Cl₂ is reacted with H₂O₂ in solution was observed to originate in various transitions involving excited molecular oxygen. A yield of 0₂(¹Δg)of 10% from this reaction was estimated.

Text

2011-06-18

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http://hdl.handle.net/2429/35584

Chemistry

University of British Columbia

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