DIRECT IDENTIFICATION OF PHOTOFRAGMENT STRUCTURES FORMED IN THE 193-NM PHOTODISSOCIATION OF THIOPHENE

An experimental scheme which involves vacuum ultraviolet photoionizati on mass spectrometric sampling of laser photodissociation products in a pulsed supersonic molecular beam has been used to identify the isome ric structures of the 193 nm photofragments from thiophene. The primar y products observed are vinylacetylene (H2C=CH-C=CH), acetylene (C2H2) , and thioketene (H2C=C=S). Using the 2 + 1 resonance-enhanced multiph oton (REMPI) scheme, we find that S atoms are produced predominantly ( greater than or equal to 96%) in P-3(J) states with a fine structure d istribution of P-3(2):P-3(1):P-3(0) = 0.82 +/- 0.03:0.15 +/- 0.03:0.03 +/- 0.03. These experimental results indicate that the major dissocia tion channels are CH2=CH-C=CH + S(P-3) and CH=CH + CH2=C=S. Ab initio multiconfiguration self-consistent-field calculations have been made t o rationalize these experimental observations.