Eunsook S. Hwang

Temperature dependence of the collisional removal of O2(b 1Σg+, v=1 and 2) at 110–260 K, and atmospheric applications

The temperature dependence of the collisional removal of O2 in the v=1 and 2 vibrational levels of the b 1Σg+ state by O2, N2, and CO2 has been investigated over the 110–260 K temperature range. For the v=1 level, O2 removes energy more effectively than N2 or CO2 at all measured temperatures. For v=2, the difference in effectiveness between colliders is not as great, and the removal by O2 exhibits a substantial positive activation energy; at 125 K O2 in the v=2 level is removed about 40 times more slowly than v=1. Even with this large difference, in the terrestrial atmosphere removal via collisions with O2 controls the lifetime of the vibrationally excited species. Recent atmospheric nightglow measurements performed at the W. M. Keck telescope on Mauna Kea, Hawaii, confirm the presence of O2 in these and higher levels through observation of the naturally occurring b 1Σg+–X 3Σg− emission.

Two-Photon REMPI Spectra from a 1 Δ g and b 1 Σ + g to d 1 Π g in O 2

Abstract Well-calibrated, resonance-enhanced multiphoton-ionization (REMPI) spectra of eight bands of d 1 Π g ←a 1 Δ g and four bands of d 1 Π g ←b 1 Σ g + in O2 have been recorded and analyzed. These data provide detailed characterizations of the complex rotational structures in the five lowest vibrational levels (v=0–4) of the d 1 Π g state. The observed complexities in the rovibronic structure of d 1 Π g are the result of strong perturbations between the d 1 Π g Rydberg and the II 1 Π g valence states. Weaker perturbations resulting from interactions between d and the II 1 Δ g valence state also are observed.

Temperature Dependence of the Collisional Removal of O2(A³Σu+, υ=9) with O2 and N2

The temperature dependence of the collisional removal of O 2 molecules in the υ = 9 level of the A 3 Σ u + electronic state has been studied for the colliders O 2 and N 2 , over the temperature range 150 to 300 K. In a cooled flow cell, the output of a pulsed dye laser excites the O 2 to the υ =9 level of the A 3 Σ u + state, and the output of a time-delayed second laser monitors the temporal evolution of this level via a resonance-enhanced ionization. We find the thermally averaged removal cross section for O 2 collisions is constant (∼10 A 2 ) between room temperature and 200 K, then increases rapidly with decreasing temperature, doubling by 150 K. In contrast, the N 2 cross section at 225 K is ∼ 8 % smaller and gradually increases to a value at 150 K that is - 60% larger than the room temperature value. The difference between the temperature dependence of the O 2 and N 2 collision cross section implies that the removal by oxygen becomes more important at the lower temperatures found in the mesosphere, but removal by N 2 still dominates.

Collisional removal of NO (B 2Π,v=2 and 3) at 230 K

Removal rate coefficients for NO(B 2Π) in the v=2 and 3 levels are measured at 230 K for seven colliders: NO, N2O, CO2, O2, N2, Ar, and He. These measurements are the first below room temperature and are compared to earlier 295 K measurements. These NO(B 2Π) vibrational levels differ from each other in that the v=2 level is unperturbed, and the v=3 level is significantly perturbed by the v=12 level of the a 4Π state. Although there are large variations in removal rate coefficients between the two B 2Π vibrational levels, the effect of reducing the temperature on the removal rate coefficients is modest, the largest effects occurring with the least effective colliders, He and Ar.

Upper limit for the generation of N2Ofrom reaction ofO2(A3Σu+)and N2

Evidence from several sources suggest possible in situ production of N 2 O in the stratosphere. Considering that solar photoabsorption provides a large stratospheric source of O 2 (A 3 Σ u + ), and since vibrational levels of v6 are primarily removed by N 2 , the O 2 (A 3 Σ u + )+N 2 system is studied to determine whether it is an atmospherically significant N 2 O source. Using 243–250 nm photoexcitation to produce vibrationally excited O 2 (A 3 Σ u + , v=7–10), and frequency modulation diode laser spectroscopy as the detector of N 2 O, we examine the products generated in a closed cell. We thereby set an upper limit of 0.002% on the N 2 O yield for the process, and conclude that stratospheric N 2 O production by this route is not significant compared to existing ground-based sources. The stability of N 2 O in an N 2 O–O 3 –N 2 mixture subjected to prolonged 245 nm radiation is also studied. For low levels of O 3 (10 ppm) and N 2 O (40–90 ppb), no loss of N 2 O is observed.

Laboratory studies of CO2(ν2)-O vibrational energy transfer

For altitudes above about 80 km, oxygen molecules are increasingly dissociated by solar vacuum ultraviolet absorption, and O atoms, together with N2, become a principal constituent of the atmosphere. Through collisions with the ambient O atoms, the ground vibrational state of CO2 is efficiently excited to its lowest excited vibrational state, with one quantum of energy in the ν2 bending mode. In the near-space environment, a sizable fraction of this population relaxes via 15-μm spontaneous infrared emission, which effectively converts ambient kinetic energy into radiative energy that passes into space. This process is the principal upper atmospheric cooling mechanism in the 75-120 km altitude range. Despite the importance of this mechanism, current estimates of the CO2(ν2)-O vibrational relaxation rate constant kO(ν2) vary over a factor of six, with the laboratory measurements clustering in the 1-1.5 × 10-12 cm3s-1 range, and the aeronomical estimates in the 3-6 × 10-12 cm3s-1 range. We are currently pursuing vibrational relaxation measurements on the CO2(ν2)-O system in the laboratory, using the temperature jump method together with transient diode laser absorption spectroscopy detection of the CO2 vibrational level populations. We will present the current state of progress of the experimental effort, as well as possible future directions.

Laboratory Studies of Vibrational Excitation in O2(a1Δg, v) Involving O2, N2, and CO2

Collisional removal of electronic energy from O2 in the low-lying a1Δg state is typically an extremely slow process for the v = 0 level. In this study, we report results on the deactivation of O2( a1Δg, v = 1-3) in collisions with O2 and CO2. Ozone photodissociation in the 200-310 nm Hartley band is the source of O2( a, v), and resonance-enhanced multiphoton ionization is used to probe the vibrational-level populations. Deactivation of the a( v = 1-3) levels in collisions with O2 at 300 K is fast, with rate coefficients of (5.6 ± 1.1) × 10-11, (3.6 ± 0.4) × 10-11, and (1.9 ± 0.4) × 10-11 cm3 s-1 (2σ) for v = 1, 2, and 3, respectively. The relaxation process appears to involve a near-resonant electronic energy transfer pathway analogous to that observed in vibrationally excited O2( b1Σg+). With CO2 collider gas, the removal rate coefficient at 300 K is (1.8 ± 0.4) × 10-14 and (4.4 ± 0.6) × 10-14 cm3 s-1 (2σ) for v = 1 and 2, respectively. Despite the small mole fraction of O2 in the atmospheres of Mars and Venus, O2 is at least as important as CO2 in the final stages of collisional relaxation within the O2 vibrational-level manifold.

Room temperature measurements of CO 2 (v 2 )-O vibrational energy transfer

In the Earths upper atmosphere, collisions between ground state CO2 molecules and translationally excited O atoms effectively populate the bending (v2) vibrational modes of CO2. Subsequent relaxation of the v2 modes occurs through spontaneous or stimulated emission of 15-μm radiation. Much of this radiation escapes into space, thereby removing ambient kinetic energy from the atmosphere. This cooling mechanism is especially important at altitudes between 75 and 120 km where the O atom density is relatively high and the conditions are optically thin. We have performed laboratory measurements to better characterize the vibrational energy transfer efficiency for this system. Several improvements to the experiment have been made since our preliminary manuscript on this topic. The temperature-jump method is used to form vibrationally excited CO2, and transient diode laser absorption spectroscopy is used to monitor the vibrational level populations following collisions with atomic oxygen. Using this approach, the room-temperature vibrational relaxation rate coefficient, kO(v2), has been measured to be (2.0±0.3)x10-12 cm3s-1. This value is slightly higher than previous laboratory measurements, which have clustered in the (1-1.5)x10-12 cm3s-1 range, and on the low end of aeronomical estimates of (2-6)x10-12 cm3s-1.