[MOS] March 26, 2013

[Zina M Queen]

Modern Optics and Spectroscopy

 

Using multiple probes to characterize multi-channel chemical reactions

 

William Green,

Massachusetts Institute of Technology

Tuesday, March 26, 2013

 

12:00 – 1:00 p.m.

Most molecules can undergo several different reactions leading to different products, each with different rate coefficients. Most experiments measure either the timescale of the conversion (in effect measuring the sum of these rate coefficients) or the final products (which are related to the ratio of the rate coefficients). Neither approach is sufficient to determine the individual rate coefficients, which are the crucial inputs to kinetic models. It can be particularly tricky to interpret the final product data, since there is typically time for multiple secondary reactions to occur. One can try to beat this problem by probing the immediate products spectroscopically, but some products are invisible to most probes.  To beat these problems, we have constructed an apparatus which simultaneously measures the overall reaction rate (by measuring depletion of a reactant by UV/vis or near-IR absorption spectroscopy) and the immediate products (by gentle single photon photoionization mass spectrometry). This allows us to determine all the rate coefficients for many gas phase reactions of the type A+B à multiple products, where A is a reactive intermediate that we generate by laser flash photolysis, and B is a stable molecule. The new apparatus incorporates several methods for increasing the signal-to-noise.  The capabilities of the new apparatus are demonstrated for reactions where A=C2H3 and B=an alkene, where the competition between two product channels depends strongly on how efficiently the bath gas can absorb the exothermicity of the reaction. Some of the advantages and disadvantages of this new experimental approach are discussed.

Grier Room, MIT Bldg 34-401

Refreshments served after the lecture
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