Dynamics of excited tetrakis(dimethylamino)ethylene solvated by argon atoms

Abstract

The supersonic expansion of a mixture of the title molecule (TDMAE) with argon generates a beam carrying a log-normal distribution of TDAME(Ar) n clusters, broadly centered atn ≈ 60. The femtosecond pump-probe technique is used to investigate the excited state dynamics of these clusters up to a 220 ps delay between the pump and the probe. This documents the effect of the argon environment on the TDMAE dynamics. The TDMAE molecule is excited in the valence state V within the cluster by the pump laser at 266 nm. It undergoes deformation in the excited potential energy surface that brings the initial wavepacket to a conical intersection (CI) where the electronic configuration of the molecule switches to a Zwitterionic configuration Z. Compared to the behaviour of free TDMAE, the effect of the argon environment is a slow down of the wavepacket movement and an increase of the time scale of the V-Z energy transfer from 300 fs to 400 ± 50 fs. This slow down effect, that we call a chistera effect, differs from a standard cage effect. Here, the deforming molecule does not experience a hard sphere collision with the argon cage, rather it pushes it away. Furthermore, umbrella oscillations of the dimethylamino groups are excited when the initial wavepacket passes the CI region. Because of the argon environment, the sharp 250 fs oscillation period of the free molecules is transformed into a broad structure of 40 fs width (FWHM) centred at about 240 fs. In addition, breathing oscillations of the argon environment with respect to the TDMAE molecule are observed with a period of 410 ± 40 fs. Finally, long delays between the pump and the probe lasers allow us to investigate the non radiative energy transfer from the Z electronic configuration of TDMAE(Ar) n to a charge transfer state. The effect of the evaporation of argon atoms in the neutral and the ionised clusters has been taken into account, as its time scale accompanies that of the observed phenomena.

Solvation shift of a conical intersection in clusters of excited tetrakis(dimethyl amino)ethylene with ammonia and acetonitrile molecules

“…Importantly, as in our former experiment on TDMAE(Ar) n , evaporation phenomena are active and signals measured at a given ion mass do not reflect the behaviour of a neutral parent of the same mass, but the behaviour of parents carrying a larger number of solvent molecules [4]. As in reference [4] also, the signals shown in Figure (2) can be understood as resulting from a sequential 3 level scheme and can be fitted by the following expression after convoluting S(t) by the cross-correlation function of the lasers:…”

“…As in reference [4] also, the signals shown in Figure (2) can be understood as resulting from a sequential 3 level scheme and can be fitted by the following expression after convoluting S(t) by the cross-correlation function of the lasers:…”

“…The crosscorrelation width of the lasers is 110 fs. This excitation scheme is common with former experiments performed in this group [1,4].…”

“…Tetrakis(dimethyl amino)ethylene is an alkene-like molecule of structural formula (1) Its real time dynamics has been investigated in the present laboratory following excitation to the S 1 energy surface, either when the TDMAE molecule is free [1,2] or solvated in an argon cluster [3,4] or within a TDMAE cluster [5].…”

“…The experimental apparatus is extensively described elsewhere [1,4]. It associates a pulsed supersonic beam with a Time-Of-Flight Mass-Spectrometer (TOF-MS).…”