Absence of a Signature of Aqueous I(²P_1/2) after 200-nm Photodetachment of I^-(aq)

Abstract

Ultrafast pump-broadband probe spectroscopy was used to study the transient photoproducts following 200-nm photodetachment of I(-)(aq). Resonant detachment at 200 nm in the second charge-transfer-to-solvent (CTTS) band of I(-)(aq) is expected to produce an electron and iodine in its spin-orbit excited state, I*((2)P(1/2)). The transients in solution following photodetachment were probed from 200 to 620 nm. Along with strong absorption in the visible region due to solvated electrons and a strong bleach of the I(-)(aq) ground-state absorption, a weaker transient absorption near 260 nm was observed that is consistent with a previously assigned ground-state I((2)P(3/2)) charge-transfer band. However, no evidence was found for an equivalent I*(aq) charge-transfer absorption, and I((2)P(3/2)) was produced within the instrument response. This suggests either that I* is electronically relaxed in less than 300 fs or that excitation in the second CTTS band does not in fact lead to I*. The consequences for previous experimental work where I*(aq) production has been postulated, as well as for halogen electron ejection mechanisms, are discussed. In addition, the broad spectral coverage of this study reveals in the bleach recovery the rapid cooling of the solvent surrounding the re-formed iodide after geminate recombination of the iodine with the solvated electron.

Ultrafast Charge-Transfer-to-Solvent Dynamics of Iodide in Tetrahydrofuran. 2. Photoinduced Electron Transfer to Counterions in Solution

“…Thus, any spectroscopic dynamics associated with CTTS electron ejection from atomic ions must directly reflect the motions of solvent molecules. To this end, the CTTS behavior of solvated I -, including its steady-state spectroscopy in various solution environments, [37][38][39][40][41][42][43][44][45][46] photoinduced electron yields, 3,[47][48][49][50][51][52] and ultrafast electron-transfer dynamics, [1][2][3][4][5]8,9,28 has been characterized extensively over the last 75 years. Figure 1a shows the CTTS absorption spectrum of aqueous I -(blue solid curve), which consists of two broad and featureless bands that peak at 225 and 193 nm; these bands are associated with the two spinorbit states ( 2 P 3/2 and 2 P 1/2 ) of the neutral I atom photoproduct.…”

“…Figure 1a shows the CTTS absorption spectrum of aqueous I -(blue solid curve), which consists of two broad and featureless bands that peak at 225 and 193 nm; these bands are associated with the two spinorbit states ( 2 P 3/2 and 2 P 1/2 ) of the neutral I atom photoproduct. 41 The dynamics following one-photon CTTS excitation of I -in polar liquids has been investigated extensively by Bradforth and co-workers [1][2][3][4][5]7,8 (as well as by others 9,28 ). A small subset of Bradforth and co-workers' ultrafast spectral measurements associated with the CTTS excitation of aqueous I -are highlighted in Figure 1b; 53 excitation of the I -CTTS band leads to the rapid (<100 fs) appearance of hydrated electrons with a near-unit quantum yield.…”

“…8,102 We note, however, that the population kinetics of the two must be identical according to the stoichiometry for geminate recombination: I + e THF -f I -. Because we would expect there to be a significant amount of dynamic solvation of the newly formed I atom (as there is for the neutral sodium atom left behind following the CTTS excitation of Na -), 21,22 the fact that we observe identical kinetics at 650 nm and in the IR suggests that absorption due to the CTFS band of I is negligible at this probe wavelength.…”

Real-time observation of the charge transfer to solvent dynamics

“…The valence electrons of ground-state anions are bound by the nucleus, but the excited states are bound by solvent polarization 1 . The CTTS states of halides rapidly decay by ejection of an electron, which is then stabilized by solvation, leaving a neutral halogen atom behind [2][3][4][5][6][7][8][9][10][11][12][13][14] . Atomic anions are ideal for studies of the electron-transfer dynamics to the solvent, because the solute lacks internal (nuclear) degrees of freedom, such that the process of CTTSmediated electron ejection is entirely governed by the structure and motions of the solvation shell.…”

“…From the experimental point of view, there has been a huge effort aimed at observing the early-time dynamics of aqueous CTTS states using ultrafast transient absorption (TA) spectroscopy [1][2][3][4]15,22 and, more recently, ultrafast photoemission from liquid microjets 5,6 . However, these methods turned out to be exclusively sensitive to the electron signal, that is, the end product of the photodetachment process, because of its very large oscillator strength over a broad spectral range and the fact that the excited CTTS absorption is close to that of the solvated electron 5,6 .…”