LASER EXCITED METASTABLE SPECTROSCOPY OF $C_{2}H_{2}$

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.