G. Black

Vacuum‐Ultraviolet Photolysis of N2O. IV. Deactivation of N(2D)

The NO(β) emission (B 2II→X 2II) observed during photolysis of N2O at 1470 A is due to a chemiluminescent reaction between N(2D) and N2O. Transient measurements of this emission are analyzed to give the rate coefficients for removal of N(2D) by several simple molecules. Photolysis at 1236 A also produces NO(B 2Π), and the decay curves can be analyzed to give the same rate coefficients as observed at 1470 A, although there is evidence that an additional NO(B 2Π) production mechanism is involved. Quenching coefficients are also tabulated for N2(A 3Σ) and O(1S), two other species investigated by the same technique.

Reaction and Deactivation of O(1D)

Relative rates for quenching of O(1D) to O(3P), and for removal of O(1D) by reaction, have been measured for various gases. O(1D) was produced by photolysis of O2 at 1470 A. The rates were determined by measuring the variation of the production rate of O(3P) atoms as a function of the amounts of quenching and reactive gases added to the system. From limits set by the lack of detection of the 6300‐A line of the O(1D) → O(3P) transition, and the maximum rates of two‐body reactions, the rate coefficients were put on an absolute basis. Except for Ar and He, all gases measured interact with O(1D) with rate coefficients ≥ 4 × 10−11 cm3 molecule−1·sec−1.

Excited‐State Formation and Destruction in Mixtures of Atomic Oxygen and Nitrogen

Absolute measurements were made of the intensity of emission from atomic oxygen (5577 A), molecular oxygen (the Herzberg bands below 4000 A, the atmospheric band at 7618 A), nitric oxide (the β bands below 4000 A), and molecular nitrogen (the first positive bands from 5000 A to 6000 A) when excited in a low‐pressure gas atmosphere containing atomic nitrogen and/or oxygen in absolutely measured quantities. The relationship between light emission and atomic concentration, pressure, gas mixture, and quenching gas concentration was obtained. With these data as bases, excitation and de‐excitation processes are discussed and rate coefficients of several elementary processes deduced. These have been applied to nightglow phenomena of the earths upper atmosphere.

A New Laboratory Source of Ozone and Its Potential Atmospheric Implications

Although 248-nanometer radiation falls 0.12 electron volt short of the energy needed to dissociate O2 large densities of ozone (O3) can be produced from unfocused 248-nanometer KrF excimer laser irradiation of pure O2. The process is initiated in some undefined manner, possibly through weak two-photon O2 dissociation, which results in a small amount of O3 being generated. As soon as any O3 is present, it strongly absorbs the 248-nanometer radiation and dissociates to vibrationally excited ground state O2 (among other products), with a quantum yield of 0.1 to 0.15. During the laser pulse, a portion of these molecules absorb a photon and dissociate, which results in the production of three oxygen atoms for one O3 molecule destroyed. Recombination then converts these atoms to O3, and thus O3 production in the system is autocatalytic. A deficiency exists in current models of O3 photochemistry in the upper stratosphere and mesosphere, in that more O3 iS found than can be explained. A detailed analysis of the system as it applies to the upper atmosphere is not yet possible, but with reasonable assumptions about O2 vibrational distributions resulting from O3 photodissociation and about relaxation rates of vibrationally excited O2 a case can be made for the importance of incuding this mechanism in the models.

Photodissociative channels at 1216 Å for H2O, NH3, and CH4

H(2S) and O(1D, 3P) yields have been measured from photolysis of H2O, NH3, and CH4 at 1216 A. From H2O, it is confirmed that the H2+O(1D) channel occurs with a yield of 0.1, but in addition there is a yield of 0.12 for the channel giving 2H(2S)+O(3P). The remaining process is H+OH production. For NH3, the NH+2H(2S) channel accounts for at least 90% of the dissociation. For CH4, combination of our data with earlier work on H2 and CH production at 1236 A leads to the conclusion that the two major dissociative channels generate 1CH2+H2 and CH2+2H(2S), with comparable probabilities. CH and CH3 production is at most minor. The yields at 1216 A for the three‐fragment processes O+2H, NH+2H, and CH2+2H, exhibit a dependence on the available excess energy.

Vacuum‐Ultraviolet Photolysis of N2O. II. Deactivation of N2(A 3Σu+*rpar; and N2(B 3Πg)

Quenching of N2(A 3Σu+), produced during the photodissociation of N2O with 1470‐A radiation, was studied using the NO γ‐band emission excited by the process N2(A 3Σu+) + NO → N2 + NO(A 2Σ+) as a monitor of N2(A 3Σu+). The relative quenching efficiencies of NH3, C2H2, C2H4, NO, C2N2, N2O, O2, CO, CO2, H2 CH4, N2, Ar, and He are 2.6, 2.3, 2.2, 1, 0.8, 0.091, 0.054, 0.036, < 4 × 10−4,≤ 10−4, < 10−4, < 10−4, < 10−4, and < 10−4. At 1236 A the B 3Πg state of N2 is produced in N2O photolysis and the variation of the intensity of N2O first positive emission with N20 pressure was used to evaluate the quenching rate of N2(B 3Πg) by N2O. The value obtained was 1.6 × 10−10 cm3 molecule−1·sec−1. Quenching efficiencies for other gases were determined relative to N2O and hence absolutely. The values obtained for NO, CO2, O2, CO, H2, N2, Ar, and He are ∼2.4 × 10−10, 1.5 × 10−10, 1.1 × 10−10, 8.5 × 10−11, 4.6 × 10−11, 2.7 × 10−11, 1.6 × 10−12, and 8 × 10−13 cm3 molecule−1·sec−1.

Quantum yields for the production of O(1D) from photodissociation of O2 at 1160–1770 Å

The quantum yield for the production of O(1D) by photodissociation of O2 was measured in the 1160–1770 A wavelength region. For wavelengths longer than 1390 A, the quantum yields are unity and constant, with a sharp cutoff at about 1750 A. For wavelengths shorter than 1390 A, the O(1D) quantum yields depend strongly on wavelength. The positions of many of the structures correspond to Rydberg states identified by various authors, and the data show by which of the two principal dissociative channels, O(3P)+O(3P) or O(1D)+O(3P), the excited molecules predissociate. The total oxygen atom yields were also measured and clearly show that all photon absorption leads to dissociation in the spectral region studied. Possible identification of absorption to the 3Πu valence state has been made, with a peak at 1356 A (9.14 eV).

Quantum yields for the production of O(1S), N(2D), and N2(A 3Σ+u) from the vacuum uv photolysis of N2O

Relative quantum yields have been measured for O(1S), N(2D), and N2(A 3Σ+u) production from N2O over the wavelength range 1100–1500 A. The measurements of O(1S) were made by observing the 1S0 → 1D2 emission at 5577 A. N(2D) was measured via the intensity of NO β bands generated by N(2D)+N2O → N2+NO(B 2Πr) followed by NO(B 2Πr) → NO(X 2Πr) % +hν (NO β bands). The N2(A 3Σ+u) was measured by the intensity of NO γ bands generated by N2(A 3Σ+u)+NO → N2+NO(A 2Σ+) followed by NO(A 2Σ+) → NO+hν (NO γ bands). The O(1S) quantum yield is close to unity over the 1280–1380 A wavelength range. N(2D) exhibits a large yield for λ?1200 A. The quantum yield of N2(A 3Σ+u) is ? 0.2 over the entire 1100–1500 A region.

Electronic‐to‐vibrational energy transfer efficiency in the O(1D)–N2 and O(1D)–CO systems

With the aid of a molecular resonance fluorescence technique, which utilizes optical pumping from the v=1 level of the ground state of CO by A1Π‐X1Σ+ radiation, we have investigated the efficiency of E‐V transfer from O(1D) to CO. O(1D) is generated at a known rate by O2 photodissociation at 1470 A in an intermittent mode, and the small modulation of the fluorescent signal associated with CO(v=1) above the normal thermal background is interpreted in terms of the E‐V transfer efficiency. The CO(v=1) lifetime in this system is determined mainly by resonance trapping of their fundamental band, and is found to be up to ten times longer than the natural radiative lifetime. For CO, (40 ± 8)% of the O(1D) energy is converted into vibrational energy. By observing the effect of N2 on the CO(v=1) fluorescent intensity and lifetime, it is possible to obtain the E‐V transfer efficiency for the system O(1D)–N2 relative to that for O(1D)–CO. The results indicate that the efficiency for N2 is (83 ± 10)% of that for CO. ...

Deactivation of O(1D)

Emission at 7618 A from O2(b 1Σg+) produced in the energy‐transfer reaction O(1D)+O2→O2(b 1Σg+)+O(3P), whose rate coefficient is greater than 10−11 cm3/sec, was measured as a function of [O2] and [He]. The O(1D) was produced by photolysis of O2 at 1470 A. The decrease of O2(b 1Σg+) when N2, CO, CO2, NO, and N2O were added was measured. It was also found that O2 perturbs O2(b 1Σg+) during a collision and increases its rate of emission. The rate coefficients of these processes are reported.Emission at 7618 A from O2(b 1Σg+) produced in the energy‐transfer reaction O(1D)+O2→O2(b 1Σg+)+O(3P), whose rate coefficient is greater than 10−11 cm3/sec, was measured as a function of [O2] and [He]. The O(1D) was produced by photolysis of O2 at 1470 A. The decrease of O2(b 1Σg+) when N2, CO, CO2, NO, and N2O were added was measured. It was also found that O2 perturbs O2(b 1Σg+) during a collision and increases its rate of emission. The rate coefficients of these processes are reported.