Robert L. Sharpless

Chemiluminescent Reactions Involving Atomic Oxygen and Nitrogen

The emission excited in the association of atomic oxygen and nitrogen has been studied in a fast‐flow low‐pressure system by photoelectric and spectroscopic methods. The absolute rates of excitation of the products of these associative processes have been measured, and the effects of added quenching gases have been investigated. Specific excitation processes consistent with the present observations are discussed.

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.

Measurements of vibrationally excited molecules by Raman scattering. II. Surface deactivation of vibrationally excited N2

Wall de‐excitation of N2(v=1) was studied on a variety of different solid surfaces. Using 4880 A laser radiation, we used the intensity of the Q branch of the anti‐Stokes Raman scattering at 4382 A to monitor the N2(v=1) concentration. The vibrationally excited nitrogen was produced by a thermal source and by a microwave discharge. The results were interpreted in terms of the two‐dimensional diffusion equation with Poiseuille flow. The two sources of N2(v=1) gave somewhat different values for the wall deactivation coefficient γ. Furthermore, the results with the microwave source depended on the length of exposure of the surface to the afterglow. The observed differences are probably related to the fact that the microwave source also produces atoms and the thermal source does not. The lowest values of γ were recorded for quartz and Pyrex after 24 h of exposure to the afterglow. The results are interpreted in terms of a mechanism of heterogeneous vibrational de‐excitation.

Collision‐induced emission from O(1S) by He, Ar, N2, H2, Kr, and Xe

Collision‐induced emission from S(1S) has been studied for collisions with He, Ar, N2, H2, Kr, and Xe. The S(1S) was made by OCS photodissociation at 1610 A. The emitted intensity increased linearly with the added gas pressure. This behavior can be described by rate coefficients for induced emission on the 1S→1D transition of (5.6±0.9) ×10−20 cm3 molecule−1⋅sec−1 for helium, (4.2±0.3) ×10−18 for argon, (3.3±0.2) ×10−18 for nitrogen, (1.73±0.15) ×10−18 for hydrogen, (1.5±0.1) ×10−17 for krypton, and (1.1±0.05) ×10−16 for xenon. Xenon was also found to enhance the 1S→3P emission intensity with a rate coefficient of (5.5±1.0) ×10−19 cm3 molecule−1⋅sec−1. Induced emission is a major (and possibly exclusive) path for deactivation of S(1S) by all these gases except hydrogen.

XeF2 photodissociation studies. I. Quantum yields and kinetics of XeF(B) and XeF(C)

Photodissociation of XeF2 with synchrotron light pulses (0.3 ns duration) has been used as the source of the XeF(B, C, and D) excited states. The time‐resolved profiles of the intensity of the resulting fluorescence have been recorded and partially analyzed. Most of the measurements were made in the strong XeF2 absorption band between 145 and 175 nm. The absorption cross section was redetermined out to 210 nm, with a maximum value of (5.9±0.5)×10−17 cm2 at 158 nm. By comparison with O(1S) signals from N2O photodissociation, quantum yields for XeF B, C, and D state production were determined. Radiative lifetimes of (14±1) and (100±10) ns were found for the B and C states. Rate coefficients for quenching by XeF2 are reported as are those for converting B to C by collision with Ne, Ar, and N2, along with upper limits for quenching of the C state by these gases.

Measurements of vibrationally excited molecules by Raman scattering. I The yield of vibrationally excited nitrogen in the reaction N+NO → N2+O

The reaction of nitrogen atoms with nitric oxide leads to vibrationally excited nitrogen. In the presence of molecular nitrogen, V‐V exchange processes rapidly establish a modified distribution for which a measurement of N2(ν=1) is adequate to determine the total vibrational energy yield of the reaction. The measurements of N2 (ν=1) have been made by Raman spectroscopy using the intensity of the Q branch of the anti‐Stokes line at 4382 A (using 4880 A Ar+ laser radiation). The system sensitivity is determined using heated nitrogen. The measurements have established that 25 ± 3% of the available energy (3.27 eV) appears as vibrational energy and that the vibrationally excited nitrogen is an initial product and does not result from collisions of translationally hot oxygen atoms with nitrogen.

Surface chemistry of metastable oxygen. II. Destruction of O2(a 1Δg)

The loss rate O2(a 1Δg) on a variety of surfaces has been investigated. At 300 K, the most rapid deactivants are Cu, Ag, and Co, with Fe, Ni, Pt, and Pd being somewhat less active. Numerous metals exhibit very low activity, including Al, Au, and W. Quite different temperature dependences were observed among the metals, with the activity of Ag decreasing markedly with increasing temperature over the range 220–470 K, that of Ni increasing, and that of Pd showing little change. The loss‐rate–temperature profiles of Fe, Co, and Ni are similar, paralleling the behavior observed in a separate study on these three metals for higher‐lying electronically excited states O2.

Catalyzed Dissociation of N2 in Microwave Discharges. I

Atomic nitrogen, produced from very pure nitrogen in a microwave discharge and detected by means of electron paramagnetic resonance, was increased from 2.0×1013 atoms/cm3 to 1.2×1015 atoms/cm3 by the addition of ≈5×1012 molecules/cm3 of SF6 to the gas before the discharge; the pink glow intensity also is increased several orders of magnitude and its time duration extended by ≈10. Nitric oxide and oxygen increase the number of atoms leaving the discharge to the same extent as SF6, but much larger amounts must be added. When nitric oxide is added between the discharge and the subsequent pink glow, comparable increases in atomic nitrogen concentration (≈20 times) are observed without the production of oxygen atoms.

Rate coefficients at 298 K for SO reactions with O2, O3, and NO2

Abstract SO 2 photodissociation at 193 nm (ArF excimer laser) has been used to create SO radicals; the chemiluminescence from the SO + O 3 reaction has been used to follow the SO radical decay. Rate coefficients at 298 K of (1.07 ± 0.16) × 10 −16 , (1.06 ± 0.16) × 10 −13 , and (1.48 ± 0.20) × 10 −11 cm 3 molecule −1 s −1 were measured for the reactions of SO with O 2 , O 3 , and NO 2 , respectively.