The Mechanism Of Surface Electron Ejection By Laser Excited Metastable Molecules

Surface Electron Ejection by Laser Excited Metastables (SEELEM) is a useful but poorly understood form of laser excitation spectroscopy, whereby nominally forbidden transitions that result in the excitation of molecules into long-lived, electronically excited states are selectively (and sensitively) detected. When a molecule in a metastable (lifetime > 100 mus) electronically excited state impacts a metal surface, an electron is ejected and detected, provided that the vertical electronic excitation energy exceeds the work function of the metal. The interaction between the excited molecule and metal surface is sensitively dependent on extrinsic and intrinsic factors, such as, respectively, metal surface contamination and whether the electronic deexcitation is electron spin-allowed or forbidden. SEELEM spectra of acetylene in the region of the (A) over tilde (1)A(u) <-- (X) over tilde (1)Sigma(g)(+) (S-1 <-- S-0) (V0K01)-K-3 band illustrate the effects of detector surface contamination on the relative detectivities of S-1, T-3, and T-1 electronic states on Air (Phi = 5.1 eV), Y (Phi = 3.1 eV), and Cs (Phi = 2.1 eV) surfaces. Deexcitation via a spin-allowed transition is shown to be much more robust with respect to surface contamination than a spin-forbidden deexcitation. When the metal surface is contaminated by adsorbed acetylene, the efficiency of SEELEM detection is significantly reduced, and the surviving detectivity derives from the minuscule fractional S-1 character in predominantly T-1 eigenstates. When the SEELEM surface is less contaminated, the relative detectivities and spectral profiles on Au, Y, and Cs surfaces reflect the similar to1000 times larger density of T-1, T-2 than S-1, T-3 vibrational states.