Ultrafast Dynamics of Excited Electronic States in Nitrobenzene Measured by Ultrafast Transient Polarization Spectroscopy

Abstract

We investigate ultrafast dynamics of the lowest singlet excited electronic state in liquid nitrobenzene using Ultrafast Transient Polarization Spectroscopy (UTPS), extending the well known technique of Optical Kerr-Effect (OKE) spectroscopy to excited electronic states. The third-order non-linear response of the excited molecular ensemble is highly sensitive to details of excited state character and geometries and is measured using two femtosecond pulses following a third femtosecond pulse that populates the S 1 excited state. By measuring this response as a function of time delays between the three pulses involved, we extract the dephasing time of the wave-packet on the excited state. The dephasing time measured as a function of time-delay after pump excitation shows oscillations indicating oscillatory wave-packet dynamics on the excited state.From the experimental measurements and supporting theoretical calculations, we deduce that the wave-packet completely leaves the S 1 state surface after three traversals of the inter-system crossing between the singlet S 1 and triplet T 2 states. Graphical TOC Entry

Ultraviolet Excitation Dynamics of Nitrobenzenes

“…We note, however, that S 1 ( n 1 π 1 *) lifetimes of 5–6 ps reported in NB following direct excitation at wavelengths > 320 nm are far shorter than the τ 3 values we observe here when using a much higher energy 267 nm pump. , Given this observation, we therefore instead assign the τ 1 ≤ 30 fs DAS to a cascaded decay of the initially exited S 3 (π 1 π 1 *) and S 4 (π 2 π 1 *) states all of the way down to S 1 ( n 1 π 1 *), which then exhibits a lifetime described by τ 2 =160–190 fs. This extremely rapid sequential cascade through the singlet manifold (potentially including passage via the S 2 ( n 2 π 1 *) state) is consistent with the interpretation of earlier time-resolved photoelectron spectroscopy measurements made in NB using a 200 nm pump, as well as theoretical predictions. , An S 1 ( n 1 π 1 *) lifetime of 160–190 fs is also broadly consistent with recent transient polarization spectroscopy data obtained from pure samples of liquid NB using a multiphoton 780 nm pump …”

“…Earlier work from the same group also noted some dependence on triplet state lifetime with both solvent and temperature, and supporting theoretical work indicated that bending of the nitro group out of the phenyl plane is a key reaction coordinate in driving efficient ISC . Much more recent measurements conducted on pure samples of liquid NB have reported S 1 ( n 1 π 1 *) lifetimes of just a few hundred femtoseconds . In the gas phase, Zewail and co-workers employed ultrafast electron diffraction to investigate the photochemical dynamics operating in NB following 267 nm excitation .…”

“…11,14 An S 1 (n 1 π 1 *) lifetime of 160−190 fs is also broadly consistent with recent transient polarization spectroscopy data obtained from pure samples of liquid NB using a multiphoton 780 nm pump. 27 Once population has transferred to the S 1 (n 1 π 1 *) state, potential nonradiative decay pathways are either further IC back to the S 0 ground state or ISC to the triplet manifold. Radiative decay via fluorescence is not believed to be a significant factor.…”

“…16 Much more recent measurements conducted on pure samples of liquid NB have reported S 1 (n 1 π 1 *) lifetimes of just a few hundred femtoseconds. 27 In the gas phase, Zewail and co-workers employed ultrafast electron diffraction to investigate the photochemical dynamics operating in NB following 267 nm excitation. 28 This work suggests that ISC and structural rearrangement yields NO and phenyl radical products from the T 1 state in 8.8 ps.…”

Electric Field Measurement of Femtosecond Time-Resolved Four-Wave Mixing Signals in Molecules

“…Our work demonstrates that ultrafast electric field-resolved nonlinear spectroscopy can be a sensitive measure of electron dynamics. When applied to nonlinear spectroscopy of excited states [26,27], such as molecules and materials excited by attosecond pulses, electric field measurements can track the induced transient nonlinear polarization, which could offer new insight into ultrafast coherent electron dynamics.…”