Photofragmentation spectroscopy of Al~+(C_2H_4)

摘要

We have studied the structure and dissociation dynamics of the weakly bound bimolecular complex Al~(C_2H_4) 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. C_2H_4~+ charge transfer product is also observed for shorter photolysis wavelengths, #lambda# < 252 nm. The Al~+-C_2H_4 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 ~1A_1, 1 ~1B_1, 1 ~1B_2 <- 1 ~1A_1) in C_(2v) equilibrium complex geometry. The excited state are of predominantly charge-transfer character correlating with the product channel Al(3s~23p) + (C_2H_4)~+. The 2 ~1A_1 and 1 ~1B_2 <- 1 ~1A_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 ~1B_2 <- 1 ~1A_1 surface crossing facilitated by a stretch in the C-C bond of ethylene. In contrast the 1 ~1B_1 <- 1~1A_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 ~1B_1 excited state. The observed modes have been assigned to the intermolecular Al-C_2H_4 stretch of a_1 symmetry (v_2 = 230 cm~(-1)), the Al-C_2H_4 out-of-plane wag with b_1-symmetry (v_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.