Photofragmentation spectroscopy of Al+(C2H4)

We have studied the structure and dissociation dynamics of the weakly bound bimolecular complex Al+(C2H4) by photodissociation spectroscopy in the 216-320 nm spectral region. Experimental studies are supported by ab initio electronic structure calculations of the ground and low-lying excited states of the complex. Al+ is the dominant photofragment observed throughout the absorption profile. C2H4+ charge transfer product is also observed for shorter photolysis wavelengths, lambda < 252 nm. The Al+-C2H4 bond dissociation energy is measured as D-0=0.37 +/- 0.15 eV. Three molecular absorption bands are observed and assigned to the transitions (2 (1)A(1),1 B-1(1),1 B-1(2) <-- 1 (1)A(1)) in C-2v equilibrium complex geometry. The excited states are of predominantly charge-transfer character correlating with the product channel Al(3s(2)3p)+(C2H4)(+). The 2 (1)A(1) and 1 B-1(2) <-- 1 (1)A(1) absorption bands appear broad and structureless. This observation is consistent with ab initio results that suggest a pathway for rapid nonadiabatic dissociation through a 1 B-1(2)-1 (1)A(1) surface crossing facilitated by a stretch in the C-C bond of ethylene. In contrast the 1 B-1(1) <-- 1 (1)A(1) molecular band shows significant vibrational structure. Spectroscopic analysis yields a band origin (0(0)(0)=40 042 cm(-1)) and corresponding vibrational mode frequencies for the 1 B-1(1) excited state. The observed modes have been assigned to the intermolecular Al-C2H4 stretch of a(1) symmetry (nu(2)=230 cm(-1)), the Al-C2H4 out-of-plane wag with b(1)-symmetry (nu(3)=328 cm(-1)), and two intramolecular ethylene modes of a(1) symmetry at 1264 and 1521 cm(-1). The assignment for these higher frequency ethylene modes is not conclusive. (C) 1999 American Institute of Physics. [S0021-9606(99)02124-8].