Ionic and Neutral Species in Pulse Radiolysis of Supercritical CO₂. 1. Transient Absorption Spectroscopy, Electric Field Effect, and Charge Dynamics

Abstract

Transient absorption spectroscopy was used to study the mechanism for radiolysis of dense, liquidlike supercritical (sc) CO2 (T = 41 °C, ρ = 0.84 g/cm3). The 350−1500 nm spectra are decomposed into the contributions from the solvent radical cation, solvent radical anion, and a long-lived neutral product that we associate with singlet carbon trioxide, CO3(1A1). These three species are characterized by their optical spectra, chemical behavior, kinetics, and the response of these kinetics to external electric field. The following mechanism for radiolysis of sc CO2 is suggested: Ionization of the solvent yields ≈5 pairs per 100 eV. Most of these pairs are comprised of the solvent hole and a thermalized quasifree electron; the prompt yield of CO3 - is <3% of the total ion yield. The electrons are trapped by the solvent in <200 ps. Because of high electron mobility, most of this trapping occurs after the charges escape each other's Coulomb field. Because of cross recombination of the electrons with nongeminate solvent holes, the lifetime of the quasifree electrons is further reduced. A theoretical model that accounts for these dynamics is suggested. It is shown that di- and triatomic molecules donate an electron to the solvent hole, and the resulting solute cations polymerize. Exotic ion species, such as (N2O)2 + and (CO) n +, can be produced this way. Using sc CO2 provides an opportunity to study such multimer cations in liquid solution.

Multimer Radical Ions and Electron/Hole Localization in Polyatomic Molecular Liquids: A Critical Review

“…Under the same physical conditions, the photodetachment and absorption spectra of the solvent anion are identical (Fig. 2b) [20], suggesting a bound-to-CB transition; the quantum yield of the photodetachment is close to unity. The photodetachment spectrum is similar to the photoelectron spectra of (CO 2 ) 6-9 -clusters observed by Tsukuda et al in the gas phase [24]; it is distinctly different from the electron photodetachment spectra of CO 2 -in hydrocarbon liquids [27].…”

“…If the lifetime were this short, the electrons would contribute negligibly either to the conductivity or the product formation. However, this extrapolation is not supported by experiment [18,20]. In low-temperature solid matrices (e.g., Ne and Ar at 5-11 K), the CO 2 -monomer, (CO 2 -)(CO 2 ) n multimer anion (n = 1, 2), and the C 2 O 4 -dimer were observed using IR spectroscopy; the latter species was the only one stable at 25-31 K [26].…”

“…Although in radiolysis, a large fraction of electrons have initial excess energies > 10 eV, the yield of CO 3 -in 20 MeV electron radiolysis of dense sc-CO 2 does not exceed 5% of the total ionization yield [20].…”

“…1/12/04 conduction band (CB) electron and a rapidly hopping multimer anion (a self-trapped electron) [18,20]. Both of these species exhibit unusual properties that account for many oddities observed in radiolysis of sc CO 2 .…”

“…This can be rationalized assuming that the scavenging reaction occurs systematically increase with the ionization potential of the solute [15,16]. These solvent holes are C 2h symmetrical O-O bound (CO 2 ) 2 + cations that exhibit a prominent charge resonance band in the visible, which suggests strong coupling in the dimer [15,16,19,20].…”