Laser Induced Chemistry 1968-1976
By the early seventies, the mission of the Atomic Energy Commission was beginning to broaden considerably from its original boundaries. At the same time, The western world was beginning to notice its dependence on imported oil. This combination generated new interest in research toward new sources of energy, both nuclear and otherwise.
Lasers were now sufficiently powerful that their coherence and narrow wavelength output meant that they might be used to induce photochemical reactions with product specificity previously impossible using incoherent sources. It was anticipated that products of specific isotopic composition could be generated for the purpose of enriching isotopes as potential fission fuels. Therefore, programs pursuing basic research investigating the mechanisms enhancing or hampering laser isotope separation appeared. Two programs in the Chemistry Department performed research in this field: one group evolved naturally from the group previously studying isotope effects; the second group represented growth in the Chemistry Department in a new direction.
Laser Induced Reactions for Isotope Separation
With funding initially from the Atomic Energy Commission through Yeshiva University, and with collaborators from Columbia University, one program began to study the prospects for selective laser photochemistry involving oxygen, sulfur and chlorine isotopes and the molecular energy processes that degrade selectivity.
One proposed scheme for laser isotope separation involved conventional laser-based visible/ultraviolet electronic photochemical excitation of a molecule such as SO2 or O3, combined with isotopically selective vibrational excitation using an infrared laser. This method demands that energy sufficient to induce dissociation absorbed by a single isotopomer not spread to other isotopomers before dissociation. Laser-induced fluorescence was used to measure the rates of collisionally-induced energy transfer between 3P oxygen atoms and the vibrational modes of SO2. It was discovered that energy transfer among SO2 vibrational modes induced by collisions with O-atoms involving the SO2 stretching modes was very efficient, approximately 30-40 times the rates for energy transfer induced by rare gases, and that O-atom-induced energy transfer between the SO2 symmetric stretching mode and the bending mode was much less efficient than that induced by rare gas atoms.1 A study of the rate of disappearance of vibrationally excited ozone in the presence of 3P atoms revealed that at most 30% of the observed rate was due to reaction enhanced by vibrational excitation of the ozone.2
When the discovery of isotope separation by infrared multiphoton dissociation was published, studies were initiated using this process. It was found that IR laser irradiation of sulfur hexafluoride led to the selective disappearance of sulfur hexafluoride containing a specific sulfur isotope. This process was shown to proceed by dissociation into SF5 and a fluorine atom, which then reacted with a hydrogen-containing molecule to form hydrogen fluoride. This product was detected by IR emission. It was discovered that when halogen atoms were produced by infrared multiphoton dissociation of sulfur hexafluoride, hydrogen halide molecules produced by reacting the halogen atoms with H2, D2 or C2H6 were generated with a vibrational distribution which suggested that the halogen atoms had little excess translational energy.3
1. "Deactivation of Vibrationally Excited SO2 by O(3P) Atoms" G.A. West, R.E. Weston, Jr. and G.W. Flynn, J. Chem. Phys. 67 4873 (1977).
2."The Influence of Reactant Vibrational Excitation on the O(3P) + O3† Bimolecular Reaction Rate" G.A. West, R.E. Weston, Jr. and G.W. Flynn, Chem. Phys. Lett. 56 429 (1978).
3. "Infrared Chemiluminescence From Hydrogen Halides Produced in IR Photodissociation" J.M. Preses, R.E. Weston, Jr. and G.W. Flynn, Chem. Phys. Lett. 48 425 (1977).
Selective Laser Induced-Chemistry
The second group involved in this field investigated the IR multiphoton dissociation (IRMPD) process from a physical organic approach, studying the factors influencing reaction channels induced by IRMPD: large vs. small molecules, collisions, energy distributions, and thermal vs. nonthermal processes. If one intended to use IRMPD to induce "new" chemistry, i.e. to observe either chemical channels that are not open in conventional thermally induced reactions or classical photochemistry, or to alter product distributions away from those characteristic of reactions due to a thermal distribution of activated molecules, it is important to determine the irradiation conditions leading to thermal or nonthermal products. Ethyl vinyl ether can decompose by two different pathways. It had been shown elsewhere that the product distribution from CO2 laser-induced IRMPD of ethyl vinyl ether was characteristic of a thermal distribution of decomposing molecules. Experiments at BNL on the product yields and the branching ratio of the two channels, varying laser fluence and pulse duration suggested that nonthermal effects were present.4 Other studies of the IR multiphoton absorption process in propyne under collisionless conditions suggested that the vibrational energy distribution produced by IR multiphoton absorption was wavelength dependent and not well correlated with the total energy deposited.5
4. "Infrared Multiphoton-Induced Chemistry of Ethyl Vinyl Ether: Dependence of Branching Ratio on Laser Pulse Duration" D. M. Brenner, Chem. Phys. Lett. 57, 357 (1978).
5. "Collisionless IR Multiphoton Absorption in Propynal" D. M. Brenner, K. Brezinsky and P. M. Curtis, Chem. Phys. Lett. 72 202 (1980).
Last Modified: June 28, 2012