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.

Differential cross sections for H + D2 → HD(v' = 2, j' = 0,3,6,9) + D at center-of-mass collision energies of 1.25, 1.61, and 1.97 eV.

We have measured differential cross sections (DCSs) for the reaction H + D(2) → HD(v' = 2,j' = 0,3,6,9) + D at center-of-mass collision energies E(coll) of 1.25, 1.61, and 1.97 eV using the photoloc technique. The DCSs show a strong dependence on the product rotational quantum number. For the HD(v' = 2,j' = 0) product, the DCS is bimodal but becomes oscillatory as the collision energy is increased. For the other product states, they are dominated by a single peak, which shifts from back to sideward scattering as j' increases, and they are in general less sensitive to changes in the collision energy. The experimental results are compared to quantum mechanical calculations and show good, but not fully quantitative agreement.

Time-dependent depolarization of aligned D(2) caused by hyperfine coupling.

Molecular deuterium is prepared in the J = 2, M = 0 sublevel of ν = 1 by stimulated Raman pumping of the ν = 0 S(0) line. Following optical excitation, the degree of alignment of the rotational angular momentum J oscillates in time caused by the coupling of J to the total nuclear spin angular momentum I(T). This coupling is of two kinds, the interaction of J with the magnetic moments and the quadrupole fields of the two I = 1 deuterium nuclei. The alignment is monitored via the O(2) line of the E,F(1)Σ(g)(+)-X(1)Σ(g)(+) (0,1) band using [2+1] resonance enhanced multiphoton ionization for pump-probe delays from 0 to 20 µs. Using the hyperfine coupling constants found previously for the ν = 0 state (R. F. Code and N. F. Ramsey, Phys. Rev. A, 1971, 4, 1945), we are able to fit the time dependence essentially within our experimental error, but this requires that the presence of both I(T) = 0 and I(T) = 2 nuclear spin states for this o-deuterium level is properly weighted and taken into account.

Search for Br* production in the D+DBr reaction.

Deuterium bromide (DBr) is expanded from a pulsed jet into a vacuum and a synchronized pulsed laser causes photodissociation of some of the DBr molecules to produce primarily (approximately 85%) ground-state bromine atoms ((2)P(3/2)) and fast D atoms. The latter collide with the cold DBr molecules and react to produce molecular deuterium (D(2)) via two possible channels, the adiabatic channel D(2)+Br((2)P(3/2)) and the nonadiabatic channel D(2)+Br*((2)P(1/2)), which are asymptotically separated in energy by the spin-orbit splitting (0.457 eV) of the bromine atom. Ion images are recorded for D(2)(v'=1, J'=16, 18-21), D(2)(v'=2, J'=6,7, 10-12, 14-16), and D(2)(v'=3, J'=2-5) for various collision energies. For the nonadiabatic production of spin-orbit-excited Br* in the D+DBr reaction for the conditions studied we estimate that this channel contributes 1% or less.

Time-dependent depolarization of aligned HD molecules.

An aligned sample of HD(v = 1, J = 2, M(J) = 0) molecules is prepared under collision-free conditions using the S(0) stimulated Raman pumping transition. Subsequent coupling to the spins of the deuteron I(D) and the proton I(H) causes the initial degree of alignment to oscillate and decrease as monitored over the time range from 0-13 mus via the O2 line of the [2 + 1] REMPI E,F(1)Sigma-X(1)Sigma (0,1) band. The time dependence of the rotational alignment is also calculated using both a hierarchical coupling scheme in which the rotational angular momentum J is regarded first to couple to I(D), and then the resultant F(i) to couple to I(H), to form the total angular momentum F and a non-hierarchical coupling scheme in which the HD energy level structure is not assumed to be diagonal in the |I(H)(JI(D))F(i)FM(F)> basis set. The experimental data is in good agreement with the non-hierarchical calculation but not with the hierarchical calculation, as expected for this system. Additionally, we calculate the time dependence of the H and D nuclear spin polarizations.

Preparation of oriented and aligned H(2) and HD by stimulated Raman pumping.

Stimulated Raman pumping has been used to prepare oriented and aligned samples of H(2)(nu=1,J=1,2,3) and HD(nu=1,J=2) under collision-free conditions using the (1,0) S(0), S(1), Q(1), Q(2), and O(3) lines. The M-sublevel anisotropies were interrogated by polarized [2+1] resonance-enhanced multiphoton ionization via the (0,1) O(2), O(3), and S(1) lines of the E,F (1)Sigma(g) (+)-X (1)Sigma(g) (+) system. The optical excitation schemes employed in this study generate highly oriented and aligned molecular ensembles. We show that the H(2)(nu=1,J=2,M=0) and H(2)(nu=1,J=2,M=2) samples retain their initial polarization for greater than 100 ns and are therefore suitable candidates for targets or projectiles in future scattering experiments.