Ph.D., 1978, Cornell
I. Chemistry in cold molecular beams
Molecular beam methods have had an enormous range of application in chemical physics, chemistry, and analytical chemistry. The powerful and increasingly common use of electrospray and MALDI mass spectrometry were developed from basic molecular beam work.
A. novel organosilicon species
One especially fruitful area of current molecular beam based research, and in our work, involves the production, stabilization and study of reactive species (e.g. radical fragments). Small molecule Si analogs of carbon compounds such as acetone and triply bonded Si species are highly reactive; one cannot simply take some out of a bottle for study. However, using the properties of supersonic gas expansions that underlie the production of cold molecular beams, one can potentially produce and stabilize the target species from precursors at the start of the gas expansions. Generally a laser pulse is used to initiate photochemistry in a precursor. One particularly promising approach developed in earlier beam work has been the laser ablation of condensed phase strained ring precursor molecules (1-3). As these experiments proceed, we sometimes have found the need to do additional method/apparatus developments (4-5).
B. probing complex product chemistry at an early reactive product stage
This approach, which is central in much of our current work, enables potentially powerful applications in a number of areas of chemistry, environmental science, and materials science. Although the use of molecular beam methods for characterization of complex product chemistry is not yet a widely employed method, it appears to have large general potential.
1. nonthermal plasma discharge of volatile organic compounds
The remediation of volatile organic compounds (VOCs) vented into the atmosphere is a significant environmental problem. Probably the most effective and economical approach to remediation is through the use of nonthermal discharges on the exhaust flues; much process engineering work using this approach has been reported in the last twenty years. Through measurement of final stable remediated products, the effectiveness of the remediation can be related to characteristics of the discharges. A key aspect of the foregoing work has involved efforts to infer, from final stable products detected, the important chemistry that happened at an earlier stage—just post discharge, involving intermediates that generally would not be detectable as stable final products. Molecular beam methods have the potential of directly studying the product chemistry at earlier post discharge stages (6-8). We have carried out mass spectrometric identification of post discharge neutral products using vuv (118 nm) single photon ionization via frequency tripling of 355 nm ns duration pulses as well as 800 nm fs pulse ionization. Pre-existing ionic products can also be mass detected in the molecular beam.
2. product chemistry following immediately pyrolysis of plant oils
With the high price of petroleum, there has been growing interest in developing economically viable methods of obtaining desired organic products from other renewable sources. Recently there have been indications of practical pyrolysis methods for renewable feedstocks; actual practical application will require further tweaking of conditions and understanding of relevant mechanisms. The necessary progress will require detailed product characterizations. Application of molecular beam methods could play a pivotal role in these developments. We are collaborating with a consortium of researchers in this area to develop molecular beam based product analysis experiments.
II. State of the art time correlated single photon counting (tcspc)
Time correlated single photon counting is a digital, not analog, method of recording luminescence decays (9). Using excitation pulses that are ps or less in duration with the best available microchannel plate photomultiplier tubes, it is possible to attain a tcspc luminescence decay instrumental resolution of a few tens of ps. We employ a ps pumped dye laser affording tunable excitation available from ~220 nm to 800 nm. Quite intentionally, we use a ps, not a fs, system, because the attainable linewidth of the pulses is then ~ 1 cm-1 (much broader linewidths with fs pulses). A 1 cm-1 excitation pulse linewidth affords the possibility of exciting selected narrow absorbance features in isolated molecule samples that have been jet cooled; we have successfully used this method in the past. It is also possible to excite conventional samples, normally solutions in cuvettes. A current collaborative project with Prof. Janan Jayawickramarajah in the department involves the measurement of luminescence lifetimes in porphyrin aggregates.
1. M. Sulkes, M.J. Fink, S. E. Gottschling and K. M. Baines, "Laser Ablation of Hexamesitylcyclotrigermane and Hexamesitylcyclosiladigermane in a Molecular Beam", Organometallics 21, 2138 (2002).
2. M. Sulkes and M. J. Fink, "Laser Ablation of Ring Strained Precursor Molecules: Selective Product Fragment Generation", Chem. Phys. Lett. 379, 60 (2003).
3. M. Sulkes, "Selective Bond Fragmentation in Pulsed Laser Ablation of Ring Strained Compounds: Evidence for a Thermal Mechanism", Chem. Phys. Lett. 402, 165 (2005).
4. R. Subramanian, M. Sulkes, "Production of a slit skimmer for use in cold supersonic molecular beams", Review of Scientific Instruments, 79 (1), 016101 (2008).
5. Wei Shen and Mark Sulkes, "High Temperature Pulsed Solenoid Valve, Rev. Sci. Instr.", Rev. Sci. Instrum. 81, 016101 (2010).
6. Luning He, Wei Shen and Mark Sulkes, "Early Chemical Intermediates Following Corona Discharge on Benzene Derivatives: CH Addition Favored Products", Chem. Phys. Lett., 471, 210 (2009).
7. Luning He and Mark Sulkes, "Chemical intermediate detection following corona discharge on volatile organic compounds: general method using molecular beam techniques", J. Phys. D, 44, 274006 (2011).
8. Luning He and Mark Sulkes, "Post Discharge Chemistry of Aromatic Molecules in Rare Gas", Plasma Chemistry and Plasma Processing, DOI 10.1007/s11090-011-9324-1, 2011.
9. Mark Sulkes and Zoe Sulkes, "Measurement of luminescence decays: High performance at low cost", Am. J. Phys. 79, 1104 (2011).
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