12 research outputs found
Ultrafast Photo-Induced Charge Transfer Unveiled by Two-Dimensional Electronic Spectroscopy
The interaction of exciton and charge transfer (CT) states plays a central
role in photo-induced CT processes in chemistry, biology and physics. In this
work, we use a combination of two-dimensional electronic spectroscopy (2D-ES),
pump-probe measurements and quantum chemistry to investigate the ultrafast CT
dynamics in a lutetium bisphthalocyanine dimer in different oxidation states.
It is found that in the anionic form, the combination of strong CT-exciton
interaction and electronic asymmetry induced by a counter-ion enables CT
between the two macrocycles of the complex on a 30 fs timescale. Following
optical excitation, a chain of electron and hole transfer steps gives rise to
characteristic cross-peak dynamics in the electronic 2D spectra, and we monitor
how the excited state charge density ultimately localizes on the macrocycle
closest to the counter-ion within 100 fs. A comparison with the dynamics in the
radical species further elucidates how CT states modulate the electronic
structure and tune fs-reaction dynamics. Our experiments demonstrate the unique
capability of 2D-ES in combination with other methods to decipher ultrafast CT
dynamics.Comment: 14 pages, 11 figures, and Supporting informatio
Two-dimensional electronic spectroscopy of beta-carotene.
Two-dimensional electronic spectroscopy (2D) has been applied to beta-carotene in solution to shine new light on the ultrafast energy dissipation network in carotenoids. The ability of 2D to relieve spectral congestion provides new experimental grounds for resolving the rise of the excited state absorption signal between 18,000 and 19,000 cm(-1). In this spectral region, the pump-probe signals from ground state bleach and stimulated emission overlap strongly. Combined modeling of the time-evolution of 2D spectra as well as comparison to published pump-probe data allow us to draw conclusions on both the electronic structure of beta-carotene as well as the spectral densities giving rise to the observed optical lineshapes. To account for the experimental observations on all time scales, we need to include a transition in the visible spectral range from the first optically allowed excited state (S(2)-->S(n2)). We present data from frequency resolved transient grating and pump-probe experiments confirming the importance of this transition. Furthermore, we investigate the role and nature of the S* state, controversially debated in numerous previous studies. On the basis of the analysis of Feynman diagrams, we show that the properties of S*-related signals in chi(3) techniques like pump-probe and 2D can only be accounted for if S* is an excited electronic state. Against this background, we discuss a new interpretation of pump-deplete-probe and intensity-dependent pump-probe experiments
Double-quantum two-dimensional electronic spectroscopy of a three-level system: Experiments and simulations
Double-quantum coherence two-dimensional (2Q2D) electronic spectroscopy is utilized to probe the dynamic fluctuations of electronic states in a solvated molecule at approximately twice the energy of the ground state bleach transition. The 2Q2D spectrum gives insight into the energetic position and spectral fluctuations (system-bath interaction) of the probed excited states. Combining it with single-quantum two-dimensional (1Q2D) electronic spectroscopy enables one to determine the strength of the excited state absorption transition and the relative detuning of electronic states, as well as the dynamics of the single-quantum coherence. To investigate the correlation of spectral fluctuations in different electronically excited states, we have carried out experiments on a solvated dye (Rhodamine 6G) with 23 fs pulses centered at the maximum of the linear absorption spectrum. The 2Q2D spectrum reveals three peaks of alternating signs with the major negative peak located at higher frequencies along the emission axis compared to the single positive peak. The 1Q2D spectrum, on the other hand, shows a negative peak stemming from excited state absorption at lower frequencies along the emission axis. Analysis of the signal in the homogeneous limit fails to account for this observation as well as the number of peaks in the 2Q2D spectrum. Employing a three-level model in which all time correlations of the third-order response function are accounted for via second-order cumulant expansion gives good agreement with both the 1Q2D and 2Q2D data. Furthermore, the analysis shows that the fluctuations of the probed electronic states are highly correlated, reflecting the modulation by a common nuclear bath and similarities in the nature of the electronic transitions. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3474995
Electronic Double-Quantum Coherences and Their Impact on Ultrafast Spectroscopy: The Example of beta-Carotene
The energy level structure and dynamics of biomolecules are important for understanding their photoinduced function. In particular, the role of carotenoids in light-harvesting is heavily studied, yet not fully understood. The conventional approach to investigate these processes involves analysis of the third-order optical polarization in one spectral dimension. Here, we record two-dimensional correlation spectra for different time-orderings to characterize all components of the transient molecular polarization and the optical signal. Single- and double-quantum two-dimensional experiments provide insight into the energy level structure as well as the ultrafast dynamics of solvated beta-carotene. By analysis of the lineshapes, we obtain the transition energy and characterize the potential energy, surfaces of the involved states. We obtain direct experimental proof for an excited state absorption transition in the visible (S-2 -> S-n2). The signatures of this transition in pump-probe transients are shown to lead to strongly damped oscillations with characteristic pump and probe frequency dependence
System-Dependent Signatures of Electronic and Vibrational Coherences in Electronic Two-Dimensional Spectra
In this work, we examine vibrational coherence in a molecular monomer,
where time evolution of a nuclear wavepacket gives rise to oscillating
diagonal- and off-diagonal peaks in two-dimensional electronic spectra.
We find that the peaks oscillate out-of-phase, resulting in a cancellation
in the corresponding pump–probe spectra. Our results confirm
the unique disposition of two-dimensional electronic spectroscopy
(2D-ES) for the study of coherences. The oscillation pattern is in
excellent agreement with the diagrammatic analysis of the third-order
nonlinear response. We show how 2D-ES can be used to distinguish between
ground- and excited-state wavepackets. On the basis of our results,
we discuss coherences in coupled molecular aggregates involving both
electronic and nuclear degrees of freedom. We conclude that a general
distinguishing criterion based on the experimental data alone cannot
be devised
Vibronic and Vibrational Coherences in Two-Dimensional Electronic Spectra of Supramolecular J‑Aggregates
In J-aggregates of cyanine dyes,
closely packed molecules form
mesoscopic tubes with nanometer-diameter and micrometer-length. Their
efficient energy transfer pathways make them suitable candidates for
artificial light harvesting systems. This great potential calls for
an in-depth spectroscopic analysis of the underlying energy deactivation
network and coherence dynamics. We use two-dimensional electronic
spectroscopy with sub-10 fs laser pulses in combination with two-dimensional
decay-associated spectra analysis to describe the population flow
within the aggregate. Based on the analysis of Fourier-transform amplitude
maps, we distinguish between vibrational or vibronic coherence dynamics
as the origin of pronounced oscillations in our two-dimensional electronic
spectra