| Autor: |
Thøgersen J; Department of Chemistry, Aarhus University. Langelandsgade 140, DK-8000 Aarhus C, Denmark. frj@chem.au.dk., Weidner T; Department of Chemistry, Aarhus University. Langelandsgade 140, DK-8000 Aarhus C, Denmark. frj@chem.au.dk., Jensen F; Department of Chemistry, Aarhus University. Langelandsgade 140, DK-8000 Aarhus C, Denmark. frj@chem.au.dk. |
| Jazyk: |
angličtina |
| Zdroj: |
Physical chemistry chemical physics : PCCP [Phys Chem Chem Phys] 2023 May 24; Vol. 25 (20), pp. 14104-14116. Date of Electronic Publication: 2023 May 24. |
| DOI: |
10.1039/d3cp00236e |
| Abstrakt: |
We study the primary photolysis dynamics of aqueous carbonate, CO 3 2- (aq), and hydrogen carbonate, HCO 3 - (aq), when they are excited at λ = 200 nm. The photolysis is recorded with sub-picosecond time resolution using UV pump-Vis probe and UV pump-IR probe transient absorption spectroscopy and interpreted with the aid of density functional theory calculations. When CO 3 2- is excited via single photon absorption at λ = 200 nm, Φ ( t = 20 ps) = 82 ± 5% of the excited di-anions either detach an electron or dissociate. The electron detachment takes place from the excited state in t < 1 ps and forms ground state CO 3 ˙ - and e aq - . Dissociation occurs from both the electronic ground and excited states of CO 3 2- . Dissociation from the CO 3 2- excited state is assisted by water molecules and forms CO 2 ˙ - , OH˙ and OH - . The dissociation occurs both directly from the Franck-Condon region in t < 1 ps and indirectly with a time constant of τ = 13.9 ± 0.5 ps as the excited state relaxes. Dissociation of vibrationally excited CO 3 2- molecules in the electronic ground state is also assisted by water molecules and forms CO 2 and two OH - anions. The dissociation and subsequent vibrational relaxation of CO 2 occur with a time constant of τ = 10.2 ± 0.5 ps. The residual 1 - Φ ( t = 20 ps) = 18 ± 5% of the excited CO 3 2- di-anions return by internal conversion to the equilibrated CO 3 2- ground state with a time constant of τ = 4.0 ± 0.4 ps. The extinction coefficient of aqueous hydrogen carbonate HCO 3 - (aq) at λ = 200 nm is an order of magnitude smaller than that of carbonate, so even though the hydrogen carbonate anions dominate the carbonate di-anions in the hydrogen carbonate solution, the primary photolysis of hydrogen carbonate is obscured by the photo-products of carbonate. Hence, we are unable to assess the primary photolysis of hydrogen carbonate. However, the weak one-photon absorption facilitates two-photon ionization of water, which forms hydronium, H 3 O + , cations. The sudden increase in the acidity induced by two-photon ionization protonates the ground state hydrogen carbonate molecules, thus offering a rare spectroscopic glimpse of aqueous carbonic acid. |
| Databáze: |
MEDLINE |
| Externí odkaz: |
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