Skip to main content

High Resolution Laser Spectroscopy of Radical Containing Complexes and Radical-Radical Reaction Products in Helium Nanodroplets

Date:
-
Location:
CP-137
Speaker(s) / Presenter(s):
Dr. Gary Douberly

 

High Resolution Laser Spectroscopy of Radical Containing Complexes and Radical-Radical Reaction Products in Helium Nanodroplets

Dr. Gary Douberly
Department of Chemistry, University of Georgia, Athens, Georgia 30602 USA

Helium nanodroplet isolation (HENDI) is a versatile technique for many forms of molecular spectroscopy.  Helium nanodroplets provide a dissipative medium for studying at 0.4 Kelvin, the structure and dynamics of novel systems such as free-radicals, metal clusters, and molecular clusters.  In this talk, I will discuss our recent use of HENDI and infrared laser spectroscopy to investigate the CH3 + O2 and C3H3 + O2 reactions.  HENDI is also used to investigate the effect of the superfluid helium environment on the spectroscopy and dynamics of the OH radical and its complexes with O2 and C2H2

Hydrocarbon radicals generated in an effusive pyrolysis source are picked-up by helium droplets and cooled to 0.4 K prior to the addition of single O2 molecules.  In this experimental configuration, a reaction may occur between sequentially picked-up and cold reactants.  The resulting products of this low temperature reaction are probed spectroscopically downstream from the pick-up zones.  The CH3 + O2 reaction leads barrierlessly to the methyl-peroxy radical, and although the droplets must dissipate an energy of ~30 kcal/mol, the infrared spectra reveal a large abundance of droplets containing the cold CH3O2 radical.  Theoretical studies have predicted an approximately 2-4 kcal/mol barrier in the entrance channel of the C3H3 + O2 reaction.  Because of this prediction, a weakly bound “entrance channel” C3H3--O2 van-der-Waals complex was expected, given the rapid cooling provided by the dissipative helium environment.  In addition to the trans-acetylenic isomer of the propargyl-peroxy (C3H3O2) radical, we now have experimental evidence for the metastable C3H3--O2 complex.

The fundamental vibrational band of the helium solvated hydroxyl radical (OH) has two sharp Q(3/2) lines.  The splitting is consistent with a fivefold increase in the parity (lambda type) doubling of the 2P3/2 state in helium droplets relative to the gas phase.  This parity splitting increase is rationalized in terms of the differences in the potential energy surfaces for He-OH(A') and He-OH(A").  Rotationally resolved OH and CH stretching bands of the T-shaped OH-C2H2 complex reveal that the electronic angular momentum of OH in the complex is only partially quenched, and to a similar degree as observed in the gas phase.1,2  This indicates that the helium droplet environment does not significantly affect the electronic intermolecular interactions in OH-C2H2.

HO3 and HOOO-(O2)n clusters have also been assembled in helium nanodroplets.  The trans-HOOO isomer is observed and has vibrational band origins within 1 cm-1 of the gas phase values.3,4  This supports recent theoretical calculations, which show that the HO + O2 reaction is barrierless.  Neither of the two other predicted stable isomers, namely cis-HOOO and the hydrogen-bound OH-O2 species,5 were found within a broad survey scan.  HOOO-(O2)n clusters grow in to the red of the n1 band of trans-HOOO as the O2 pick-up pressure is increased.  HOOO-(O2)n cluster bands are resolved up to n=4.  High level ab initio calculations are being carried out to help understand the geometries of these multiple O2 clusters.

(1)        J. B. Davey, M. E. Greenslade, M. D. Marshall, M. I. Lester, and M. D. Wheeler, J. Chem. Phys. 121, 3009 (2004)
(2)        M. D. Marshall, J. B. Davey, M. E. Greenslade, and M. I. Lester, J. Chem. Phys. 121, 5845 (2004)
(3)        Derro, E. L.; Murray, C.; Sechler, T. D.; Lester, M. I. J. Phys. Chem. A 2007, 111, 11592
(4)        Derro, E. L.; Sechler, T. D.; Murray, C.; Lester, M. I. J. Chem. Phys. 2008, 128.
(5)        Braams, B. J.; Yu, H. G. PCCP 2008, 10, 3150.

For more information on Dr. Douberly and his research, click here.

Faculty Host: Dr. Dong-Sheng Yang