PHOTODISSOCIATION SPECTROSCOPY AND DISSOCIATION DYNAMICS OF TiO$^+$(CO$_2$)
Date
2009
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Publisher
Ohio State University
Abstract
TiO$^+$(CO$_2$) is produced by reaction of laser-ablated titianium atoms with CO$_2$ and subsequent clustering, supersonically cooled and its electronic spectroscopy characterized by photofragment spectroscopy, monitoring loss of CO$_2$. 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$^+$-(CO$_2$) 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}$=3. The long photodissociation lifetime substantially reduces the photodissociation quantum yield at low energy, likely due to competition with fluorescence. Electronic structure calculations help to assign the spectrum of TiO$^+$(CO$_2$) and predict allowed electronic transitions of TiO$^+$ in the visible, which have not been previously measured. Time-dependent density functional calculations predict that the observed transition is due to B, $^2\Pi\leftarrow$ X, $^2\Delta$ in the TiO$^+$ chromophore, and that binding to CO$_2$ 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 $^2\Pi$ state of TiO$^+$ is predicted to lie at T$_0=15426$ cm$^{-1}$, with frequency $\omega$e = 968 cm$^{-1}$ and anharmonicity $\omega$exe = 5 cm$^{-1}$. The calculations also predict that there is only one low-lying $^2\Sigma$ state of TiO$^+$, contrary to conclusions derived from photoelectron spectroscopy of TiO. Prospects for astronomical observation of TiO$^+$ via the $^2\Pi$-$^2\Delta$ transition are also discussed.
Description
Author Institution: Department of Chemistry, University of Massachusetts Amherst$^b$, Amherst, MA