Comparing the electronic relaxation dynamics of aniline and d₇-aniline following excitation at 272–238 nm

Abstract

Femtosecond time-resolved photoelectron spectroscopy experiments have been used to compare the electronic relaxation dynamics of aniline and d7-aniline following photoexcitation in the range 272-238 nm. Together with the results of recent theoretical investigations of the potential energy landscape [M. Sala, O. M. Kirkby, S. Guérin and H. H. Fielding, Phys. Chem. Chem. Phys., 2014, 16, 3122], these experiments allow us to resolve a number of unanswered questions surrounding the nonradiative relaxation mechanism. We find that tunnelling does not play a role in the electronic relaxation dynamics, which is surprising given that tunnelling plays an important role in the electronic relaxation of isoelectronic phenol and in pyrrole. We confirm the existence of two time constants associated with dynamics on the 1(1)πσ* surface that we attribute to relaxation through a conical intersection between the 1(1)πσ* and 1(1)ππ* states and motion on the 1(1)πσ* surface. We also present what we believe is the first report of an experimental signature of a 3-state conical intersection involving the 2(1)ππ*, 1(1)πσ* and 1(1)ππ* states.

New insights into the dissociation dynamics of methylated anilines

“…The first excited state of aniline is a bound, valence 1 pp* state 8,9 (labelled 1 1 pp*, see Fig. 1), photoexcitation to which results in an extended excited state lifetime [9][10][11][12][13][14] and high fluorescence quantum yield; 15 photoexcitation to this state does not result in N-H bond dissociation. 16,17 In the Franck-Condon (FC) region, the 1 1 ps* state is largely Rydberg in character (N 3s) and weakly bound 18 while, at elongated N-H bond lengths, the state takes on increasing s* character.…”

“…17 Alternatively, IC to the 1 1 pp* state results in 'trapped' population which again persists for an extended excited state lifetime. 10,12,14,19 While several computational investigations have found several other low-lying Rydberg excited states in aniline, 8,9,19 these states have received little experimental attention. Indeed, most of the recent experimental work on aniline photodynamics has focused on excitation to a second, higher lying 1 pp* state at ca.…”

“…240 nm. Relaxation from this state, henceforth referred to as 2 1 pp*, has been observed to occur in r50 fs 14 and may decay through the aforementioned Rydberg states. 9,20 Though a consensus on the exact decay pathways remains elusive (namely the degree to which 2 1 pp* state population decays to the 1 1 ps* and 1 1 pp* states, cf.…”

“…17 Theoretical explorations have found several conical intersections (CIs) between the 2 1 pp*, 1 1 ps*, 1 1 pp*, and S 0 states, 17,21 including an apparent 2 1 pp*/1 1 ps*/1 1 pp* threestate CI which lies along the minimum energy 2 1 pp* -S 0 relaxation pathway. 21 Recent time-resolved photoelectron spectroscopy (TR-PES) by Fielding and co-workers 14 has yielded considerable insight into the 2 1 pp* decay mechanisms, concluding that photoexcitation at 238 nm populates the 2 1 pp* which decays almost exclusively to S 0 with some IC to both the 1 1 ps* and 1 pp* states via the aforementioned multi-state CI. 21 The latter process is clearly revealed through a delayed rise in photoelectron signal due to the sequential population of the 1 1 pp* state.…”

Multidimensional Analysis of Time-Resolved Charged Particle Imaging Experiments

“…It is important to note that the multidimensional nature of the fit allows the discrimination of the different contributions to the images, in a manner that a reduced-dimensionality analysis cannot achieve. This method has revealed its value in a number of contributions where the need for discrimination of overlapping contributions in charged particle images was crucial [13][14][15][16][17][18]. The method is analogous to the global 2D fit approach used in Stolow's group [19][20][21][22] but can easily be extended to extra dimensions such as the anisotropy, the temperature, the intensity dependence, measurements in coincidence, etc.…”