Electronic-vibrational Energy Transfer Research Articles

Molecular infrared laser with Br*→CO2electronic–vibrational energy transfer as the pump mechanism

Results are presented of an experimental investigation of an infrared laser utilizing IBr–CO2 and Br2–CO2 mixtures pumped by electronic-vibrational collisional excitation transfer from electronically excited Br*(42P1/2) atoms to Raman vibrational states [(1001)–(0201)] of the CO2 molecule. It is shown that, under conditions of broad-band flash photolysis, the IBr molecule is a more efficient supplier of Br*(42P1/2) atoms than Br2As a result, simultaneous three-frequency laser action at wavelengths of 2.7 μ (42P1/2→42P3/2 transition in atomic bromine), 4.3, and 10.6 μ (vibrational–rotational transitions in the CO2 molecule) was obtained from an IBr–CO2 mixture. On the other hand, laser action was only obtained at 10.6 μ from a Br2–CO2 mixture. A qualitative laser model developed showed satisfactory agreement with the experiment.

Resonance effects in the semiclassical theory of electronically nonadiabatic collision processes

Significant advances in the theory of electronically nonadiabatic collision processes have been made in recent years by the advent of models that treat all the ’’heavy particle’’ degrees of freedom—i.e., translation, vibration, and rotation—by classical mechanics; only electronic degrees of freedom are treated quantum mechanically. The ’’surface hopping’’ model of Tully and Preston and the generalized Stuckelberg model of Miller and George are examples of this type of approach. There have, however, been questions as to whether or not such models are capable of describing resonance effects in electronic–vibrational energy transfer, e.g., A*+BC(v=0) →A+BC(v=1), with ΔEA?h/ωBC. This paper shows that these resonance effects are the result of interference of amplitudes for different classical trajectories that contribute to the transition. The Miller–George model, which incorporates interference and tunneling within the framework of classical S-matrix theory, thus describes resonance behavior, while the Tully–Preston model, which adds probabilities (rather than amplitudes) for the various trajectories, does not.

H2O, NO, and N2O infrared lasers pumped directly and indirectly by electronic-vibrational energy transfer

Pulsed infrared molecular lasers are reported in which pumping is via electronic-vibrational energy transfer from Br(42P1/2). H2O and NO lasers are pumped directly by E-V transfer and operate on a variety of transitions not previosuly seen in stimulated emission. The N2O laser operates near 10.9 μm and is pumped by a two-step process involving E-V transfer to an intermediate molecule and subsequent V-V transfer from that molecule to N2O. This latter technique extends the applicability of E-V pumping to molecules which do not interact directly with the electronically excited species.

A modified model for quenching and electronic—vibrational energy transfer

A new computational procedure and a new transition probability expression have been incorporated into the Bauer, Fisher, and Gilmore (BFG) model for quenching and E—V energy transfer. The new transition probability expression takes into account trajectories that are classically allowed on the lower adiabatic potential energy surface, but are classically forbidden on the diabatic surfaces. The calculated quenching cross section reported are in substantially better agreement with experiment than those calculated according to the original BFG model. Further modifications to the BFG model are proposed that could have major effects on the calculated cross sections.

Electronic–vibrational energy transfer from Br(4 2P1/2) to HCN, and deactivation of HCN (001)

A pulsed dye laser has been used to photolytically produce electronically excited bromine atoms in the 4 2P1/2 state in gas mixtures containing HCN. By monitoring the time resolved fluorescence from the (001) state of HCN, it was possible to determine the rate coefficient for electronic–vibrational (E–V) energy transfer from Br(4 2P1/2) to HCN. Rate coefficients for the deactivation of HCN(001) were also measured for the collision partners HCN, Br2, and Ar.

Collisional deactivation of Br(4 2P1/2) by hydrogen isotopes

The quenching of electronically excited bromine atoms Br(4 2P1/2) upon collision with H2, HD, and D2 has been studied at 300 °K. The temporal profile of the Br(4 2P1/2) concentration was monitored by observation of the time-resolved attenuation of bromine resonance radiation at 153.2 nm. The probabilities of deactivation by the hydrogen isotopes are presented and compared to theoretical treatments based either upon long-range coupling via mulitpolar interactions or explicit treatment of the nonadiabatic crossing probabilities computed using semiempirical potential surfaces. These results are then examined with a view toward assessing the importance of near-resonant electronic–vibrational energy transfer in such an elementary system.

Infrared molecular lasers pumped by electronic-vibrational energy transfer from Br(42P1/2): CO2, N2O, HCN, and C2H2

Infrared lasers pumped by electronic-vibrational (E-V) energy transfer are reported. Gas mixtures containing electronically excited Br atoms (2P1/2) are prepared by flash photolyzing Br2 in the presence of a polyatomic molecule. The ensuing E-V process is selective and pumps the polyatomic molecule into specific energy states. In addition to obtaining gain and/or stimulated emission at CO2 and N2O laser frequencies, we have obtained stimulated emission from HCN at 3.85, 7.25, and 8.48 μm, and from C2H2 in the region 7–8 μm. Extension of the principles involved to other polyatomic molecules is straightforward, and a number of applications are suggested by our results to date.

Electron vibrational energy transfer under MHD combustion generator conditions

An analysis of the energy transfer rate between an electron gas and the vibrational modes of diatomic gases has been made. The electron vibrational energy transfer rate may be characterized by δ v , the electron vibrational energy loss factor. For MHD combustion generator conditions, the electron vibrational energy transfer rate dominates and δ v is approximately equal to δ, the total electron energy loss factor. Detailed numerical predictions have been obtained for N 2 and CO. These results indicate that the energy transfer rate is strongly dependent upon the electron velocity distribution function. In particular, for a Maxwellian electron distribution function (which prevails in MHD combustion generators) δ v has been found to exceed published experimental values of δ by a factor as large as 50 for N 2. Consideration of the conditions under which the experimental values of δ were obtained shows that the electron distribution function was non-Maxwellian. An analytical re-determination of δ v and δ, using appropriate non-Maxwellian distribution functions, resolves the discrepancy between the calculated values of δ v and the measured values of δ.

Hydrogen abstraction by triplet ketones: Applicability of the electronic-vibrational energy transfer model

A critical comparison is made between a theoretical model for photochemical reactions based on their analogy with radiationless transitions and the experimental data on hydrogen abstraction by excited ketones in solution. It is concluded that rapid vibrational energy redistribution tends to rule out mechanisms based on energy transfer to a particular vibrational mode, except in low-pressure gases.

Electronic-vibrational energy transfer in the reaction of O( 1D) atoms with molecular oxygen

The UV flash photolysis of 16O 3/ 18O 2 mixtures has shown that vibrationally-excited molecular oxygen in its electronic ground-state (O 2 ≠) is produced in quantum levels ν″ = 13 and 14 during the reaction of O( 1D) atoms with O 2. Flash photolysis of 16O 3/ 16O 2 mixtures in which the O 3:O 2 pressure ratio was varied showed that the efficiency (α ν) of O 2 ≠ production in the above levels during this reaction lies in the range 0.3 ⩽ α ν ⩽ 0.5. The latter experiments also indicated that the efficiency (γ) of O 2 ≠ formation in the above levels during the reaction of O( 1D) with O 3 lies in the range 0.06 ⩽ γ ⩽ 0.1.

Infrared Emission Arising from Electronic—Vibrational Energy Transfer: Hg*+CO

Infrared Emission Arising from Electronic—Vibrational Energy Transfer: Hg*+CO