Intersystem crossing in acetylene : a mechanistic study

Abstract

The triplet electronic states of acetylene and the intersystem crossing process that populates these states were studied using a second-generation, supersonic jet molecular beam machine with a multispectral capability. Surface Electron Ejection (SEELEM), Ultraviolet Laser Induced Fluorescence (UV-LIF), and Infrared Laser Induced Fluorescence (IR-LIF) spectra were acquired simultaneously following excitation in the vicinity of the A1Au [upsilon]3 = 3 [left arrow]X1[sum of]+g [upsilon] = 0 transition of acetylene. The A1Au [upsilon]3 = 3 level of acetylene is known to be coupled strongly to a background of near-degenerate triplet states. Simultaneous analysis of the complementary SEELEM and UV-LIF data using a set of recently developed analytical tools, yielded mechanistic insights into the non-radiative relaxation (i.e. intersystem crossing) process that distributes the optically accessible bright state over the background of dark states. The potential of the IR-LIF signal as an indirect probe of the triplet states of acetylene has also been demonstrated. Mechanistic insights into the SEELEM detection process itself were gained through a comparison of SEELEM data obtained separately in the first-generation and second-generation molecular beam machines. The two versions of the apparatus differed in their respective operating pressures. The operating background pressure has been found to be an important external factor in the SEELEM process. The much higher sensitivity (a factor of 103) of low-work function SEELEM surfaces such as Cs ([Phi]=2.1 eV) compared to that of other metals such as Au ([Phi]=5.1 eV) is observed only under low operating background pressures (4x 10-7 Torr).