Photodissociation Spectroscopy and Dissociation Dynamics of TiO+(CO2)


The copyright of this article (doi: 10.1021/jp809648c) is held by the American Chemical Society.


TiO+(CO2) is produced by reaction of laser-ablated titanium atoms with CO2 and subsequent clustering, supersonically cooled, and its electronic spectroscopy was 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 13 918 cm-1, assigned using titanium isotope shifts, and the spectrum extends to 17 350 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. The fluorescence rate is calculated to be kfl = 7.5 × 105 s-1, based on the measured excited-state lifetimes and relative band intensities. This corresponds to an integrated oscillator strength of f = 0.0056. 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 = 15 426 ± 200 cm-1, with frequency ωe = 968 ± 5 cm-1 and anharmonicity ωexe = 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.


Physical Chemistry