Photofragmentation spectroscopy of MgC2H+4

MgC2H+4 has been studied by photodissociation spectroscopy. We identify two distinct molecular absorption bands correlating with Mg+ (3pπ−3sσ) excitation in the spectral range 350–510 nm. The lowest energy band, assigned as 12B2 ← 12A1, is a structureless continuum indicative of a fast predissociation in the upper state. The higher energy band, assigned as 12B1 ← 12A1, exhibits vibrational structure, including a progression in the Mg+C2H4 intermolecular stretch (ν2), and combination bands built on the Mg+C2H4 intermolecular out-of-plane wag (ν3) and a CH2CH2 wag (ν7). The vibrational constants for these modes are ω2 = 329 cm−1, x22 = −2.3 cm−1, ω3 = 439 cm−1, and ω7 = 1024 cm−1.