The ion yields for direct (no matrix) desorption-ionization with a nitrogen laser (0.5 ns pulse width) were studied for 3 organic molecules deposited as thin layers on different metallic substrates. These molecules have a low absorption coefficient at the laser wavelength (337 nm). One molecule (aminotriazole) forms positive (MH)(+) ions, the two other molecules (dinoterb and ioxynil) form negative (M-H)(-) ions. The yields were measured at 240 MW/cm(2) (similar to 120 mJ/cm(2)) for dinoterb on Al, Au, Co, Cu, CuZn, Fe, Mo, Pd, stainless steel, Ti and Zr. The yield variation was also studied for the three molecules, on brass (CuZn) and on stainless steel, between 50 and 250 MW/cm(2). It was observed that the yields are strongly substrate-dependent, with very low yields for the noble metals Au and Pd. For the three molecules, the yield is higher for CuZn than for stainless steel, with the difference becoming smaller as the irradiance (MW/cm(2)) increases. The observed phenomenon cannot be explained in terms of heating due to absorption of light by the metal alone; it is proposed that the superficial oxide layer plays a major role in the observed phenomenon. (Int J Mass Spectrom 177 (1998) 217-223) (C) 1998 Elsevier Science B.V.
