15min:

MANORI PERER, Department of Chemistry, University of Massachusetts Amherst^b, Amherst, MA.

TiO⁺(CO₂) is produced by reaction of laser-ablated titianium atoms with CO₂ and subsequent clustering, supersonically cooled and its electronic spectroscopy characterized by photofragment spectroscopy, monitoring loss of CO₂. The photodissociation spectrum consists of a vibrationally-resolved band in the visible, with extensive progressions in the covalent Ti-O stretch (952 cm^-1 vibrational frequency and 5 cm^-1 anharmonicity), and in the TiO⁺-(CO₂) stretch (186 cm^-1) and rock (45 cm^-1). The band origin is at 13918 cm^-1, assigned using titanium isotope shifts, and the spectrum extends to 17350 cm^-1. The excited state lifetime decreases dramatically with increasing internal energy, from 1100 ns for the lowest energy band (v_TiO=0), to <50 ns for v_TiO=3. The long photodissociation lifetime substantially reduces the photodissociation quantum yield at low energy, likely due to competition with fluorescence. Electronic structure calculations help to assign the spectrum of TiO⁺(CO₂) and predict allowed electronic transitions of TiO⁺ in the visible, which have not been previously measured. Time-dependent density functional calculations predict that the observed transition is due to B, ² X, ² in the TiO⁺ chromophore, and that binding to CO₂ red shifts the TiO⁺ transition by 1508 cm^-1, and lowers the Ti-O stretch frequency by 16 cm^-1. Combining the computational and experimental results, the ² state of TiO⁺ is predicted to lie at T₀=15426 cm^-1, with frequency e = 968 cm^-1 and anharmonicity exe = 5 cm^-1. The calculations also predict that there is only one low-lying ² state of TiO⁺, contrary to conclusions derived from photoelectron spectroscopy of TiO. Prospects for astronomical observation of TiO⁺ via the ² -² transition are also discussed.