15min:

KEVIN L. CUNNINGHAM, STEPHEN DRUCKER AND ROBERT W. FIELD, Department of Chemistry, Massachusetts Institute of Technology, Cambridge MA 02139-4307; SUSAN J. HUMPHREY, CHRISTOPHER G. MORGAN AND ALEC M. WODTKE, Department of Chemistry, University of California at Santa Barbara, Santa Barbara CA 93106.

Triplet states have been detected near the 3₃ vibrational level of the S₁ electronic surface of C₂H₂ using the highly selective technique of Laser Excited Metastable (LEM) spectroscopy. LEM detects metastable molecules through the detection of an Auger electron emitted when the molecule impacts a metal surface. A supersonic molecular beam of C₂H₂ was crossed with a laser beam which excites ^*- transitions to metastable states. The molecules then travel ~100 µs before impacting a Au surface. During the collision, an electron from the conduction band of the metal may fall into the open bonding orbital of C₂H₂. The electron in the ^* orbital is then ejected and detected. To be detected, the excited state must have a minimum lifetime on the order of 100 µs and must have electronic excitation energy that exceeds the work function of the Au surface ( = 5.1 eV). The LEM and LIF spectra were recorded simultaneously for a number of bands in the A-X band system, including 3₃. 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 C₂H₂. Despite the low vibrational density of T₃ states near 3₃, the results show that a single vibrational level of the T₃ surface is strongly coupled to 3₃ and to the much denser manifold of T₁ and T₂ vibrational states.