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1997-RI-01.jpg 160.4Kb JPEG image Thumbnail of LASER EXCITED METASTABLE SPECTROSCOPY OF $C_{2}H_{2}$

Creators: Cunningham, Kevin L.; Drucker, Stephen; Field, R. W.; Humphrey, Susan J.; Morgan, Christopher G.; Wodtka, A. M.
Issue Date: 1997
Publisher: Ohio State University
Abstract: Triplet states have been detected near the 3$\nu_{3}$ vibrational level of the $S_{1}$ electronic surface of $C_{2}H_{2}$ using the highly selective technique of Laser Excited metastable (LEM) spectroscopy. LEM detects metastable molecular through the detection of an Auger electron emitted when the molecule impacts a metal surface. A supersonic molecules beam of $C_{2}H_{2}$ was crossed with a laser beam which excites $\pi^{*}-\pi$ transitions to metastable states. The molecules then travel $\sim$100 $\mu s$ before impacting a Au surface. During the collision, an electron from the conduction band of the metal may fall into the open $\pi$ bonding orbital of $C_{2}H_{2}$. The electron in the $\pi^{*}$ orbital is then ejected and detected. To be detected, the excited state must have a minimum lifetime on the order of 100 $\mu s$ and must have electronic excitation energy that exceeds the work function of the Au surface ($\phi=5.1 eV.$) The LEM and LIF spectra were recorded simultancously for a number of bands in the $\tilde{A}-\tilde{X}$ band system, including $3\nu_{3}$. LEM and LIF offer complementary information. LIF detects short-lived states which are of predominant singlet character; LEM detects long-lived states which are predominantly triplet. The spectra show a number of interesting features necessary to understanding the mechanism of singlet-triplet coupling in $C_{2}H_{2}$. Despite the low vibrational density of $T_{3}$ states near $3\nu_{3}$, the results show that a single vibrational level of the $T_{3}$ surface is strongly coupled to $3\nu_{3}$ and to the much denser manifold of $T_{1}$ and $T_{2}$ vibrational states.
Description: Author Institution: Department of Chemistry, Massachusetts Institute of Technology; Department of Chemistry, University of California at Santa Barbara
Other Identifiers: 1997-RI-01
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