Ultrafast Curve Crossing Dynamics through Conical Intersections in Methylated Cyclopentadienes

Abstract

We explored the curve crossing dynamics of 1,2,3,4-tetramethyl-cyclopentadiene (TMCPD) and 1,2,3,4,5-pentamethyl-cyclopentadiene (PMCPD) upon pi --> pi* excitation to the 1B(2) state using time-resolved, resonance-enhanced multiphoton ionization mass and photoelectron spectroscopy. Upon excitation with a femtosecond laser pulse at 267 nm, the energy relaxation pathway is observed by a time-delayed probe pulse at 400 nm, which ionizes the molecule through Rydberg states that reveal the momentary state of the molecule in the photoelectron spectra. We observe that the initially populated 1B(2) state decays to the 2A(1) surface in 135 fs in TMCPD and 183 fs in PMCPD, followed by a crossing to the ground state 1A(1) surface on 57 and 60 fs time scales for TMCPD and PMCPD, respectively. The spectroscopic signatures of the 2A(1) states are clearly revealed in the two-photon ionization photoelectron spectra. In both systems we observe that the ground states are recovered completely, indicating that no new molecular structures are created on the time scale of the experiment.

Hexamethylcyclopentadiene: time-resolved photoelectron spectroscopy and ab initio multiple spawning simulations

“…Although the TRPES data for CPD and its derivatives were earlier analyzed via a biexponential fit, 9,10 we here show on the example of CPDMe 4 PrH that the dynamics can be described equally well via a single exponential decay and a shift in time zero or a fit according to eqn (4). The most appropriate model can only be chosen with the help of the theoretical knowledge of the underlying dynamics and the direct simulations of observables.…”

“…CPD was previously investigated by TRPES and time-resolved mass spectroscopy. 5,[8][9][10] The photoinduced dynamics were observed to be complete within less than 200 fs after photoexcitation. No shift in time zero was observed and the data were analyzed assuming a relaxation scheme which included three diabatic electronic states: the photoexcited state, a dark state, and the ground state.…”

“…This is especially important because a profound understanding of photoinduced dynamics almost always necessitates a detailed comparison of experiment with theory. Many model systems have been investigated by time-resolved experiments [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] or dynamical simulations. [20][21][22][23][24][25][26] We believe that fundamental questions could be answered more easily by joint studies connecting experimental approaches with such simulations.…”

Between ethylene and polyenes - the non-adiabatic dynamics of cis-dienes

“…54,57 Despite the inability to directly locate this dark state in absorption spectroscopy, several time-resolved studies have invoked non-adiabatic transitions involving it to explain the sub 200 fs dynamics observed in ion-and photoelectron signals subsequent to excitation to the bright (p,p * )-state. 23,58,59 The short timescales involved can also be inferred from the lack of detectable fluorescence of CPD, 60 further hinting at the involvement of a doubly excited state. The timescales of the observed dynamics have furthermore been suggested to be dependent on specific nuclear motion as exemplified in the slow-down of the dynamics from CPD to the substituted 1,2,3,4-tetramethylcyclopentadiene (Me 4 -CPD).…”

Ultrafast structural and isomerization dynamics in the Rydberg-exited Quadricyclane: Norbornadiene system

“…The sensitivity toward molecular structure coupled with the insensitivity toward vibrational excitation makes photoionization out of a Rydberg state an ideal tool to study ultrafast structural changes in large molecules. In recent publications we utilized Rydberg electron binding energy spectra to explore ultrafast curve crossing dynamics in cyclic dienes, 16 the conformational dynamics in large aliphatic systems 17,18 and to identify the structure of a molecule after conversion to the ground electronic state potential energy surface. 19 In all these measurements, we observed the kinetics of the molecular reaction, i.e., the distillation of a molecular structure from one potential energy well to another.…”