Decay dynamics of nascent acetonitrile and nitromethane dipole-bound anions produced by intracluster charge-transfer

Abstract

Decay dynamics of nascent dipole bound states of acetonitrile and nitromethane are examined using time-resolved photoelectron imaging of iodide-acetonitrile (I(-)·CH3CN) and iodide-nitromethane (I(-)·CH3NO2) complexes. Dipole-bound anions are created by UV-initiated electron transfer to the molecule of interest from the associated iodide ion at energies just below the vertical detachment energy of the halide-molecule complex. The acetonitrile anion is observed to decay biexponentially with time constants in the range of 4-900 ps. In contrast, the dipole bound state of nitromethane decays rapidly over 400 fs to form the valence bound anion. The nitromethane valence anion species then decays biexponentially with time constants of 2 ps and 1200 ps. The biexponential decay dynamics in acetonitrile are interpreted as iodine atom loss and autodetachment from the excited dipole-bound anion, followed by slower autodetachment of the relaxed metastable ion, while the dynamics of the nitromethane system suggest that a dipole-bound anion to valence anion transition proceeds via intramolecular vibrational energy redistribution to nitro group modes in the vicinity of the iodine atom.

Time-resolved radiation chemistry: Dynamics of electron attachment to uracil following UV excitation of iodide-uracil complexes

“…It should be noted that at 30 meV and 100 meV, there is some electron signal near t = 0 between −0.2 eV and 0 eV eBE, in the vicinity of feature I. This electron signal is attributed to two-color twophoton detachment of the iodide-uracil complex, observed previously with Initromethane and Iacetonitrile, 35 and is not included in the analysis of feature I.…”

“…Features I and II are transient features due to probe detachment from the photo-excited complex, while feature III is primarily due to direct detachment of the ground state iodideuracil cluster induced by the excitation pulse. 15,16,35 Feature III is very high in intensity compared to features I and II, and in Figure 2(d) it is largely off scale and colored grey. Upon negative-time background subtraction, feature III is only seen as residual noise in Figure 2(e).…”

“…Based on previous experiments, features I and II can be identified as photo-detachment from the dipole bound (DB) and valence bound (VB) anions of uracil, respectively. 16,35 Table I reports the intensity ratio of feature I to feature II, which was measured at a set time delay where both features are close to their maxima, approximately 400-500 fs. This ratio decreases with increasing excitation energy.…”

“…35 The acetonitrile anion is a DB state, with no low-lying VB state. 36,37 Nitromethane, however, supports both low-lying DB and VB anions with a VB anionic ground state.…”

“…Experiments on I -CH 3 CN complexes showed evidence only for the DB anion of acetonitrile, while experiments on I -CH 3 NO 2 clusters showed initial formation of the DB CH 3 NO 2 anion decaying to the VB anion on a timescale of 400-600 fs. 35 The work on I -CH 3 CN and I -CH 3 NO 2 suggested that the early-time electron attachment dynamics in I -U might be substantially different at excitation energies closer to the VDE of the complex. In this paper, we carry out such experiments, using excitation energies ranging from 4.00 to 4.21 eV, below and above the VDE of I -U, as well as at 4.66 eV, in the range of our previous experiments.…”