William Peters

Electronic resonance with anticorrelated pigment vibrations drives photosynthetic energy transfer outside the adiabatic framework

The delocalized, anticorrelated component of pigment vibrations can drive nonadiabatic electronic energy transfer in photosynthetic light-harvesting antennas. In femtosecond experiments, this energy transfer mechanism leads to excitation of delocalized, anticorrelated vibrational wavepackets on the ground electronic state that exhibit not only 2D spectroscopic signatures attributed to electronic coherence and oscillatory quantum energy transport but also a cross-peak asymmetry not previously explained by theory. A number of antennas have electronic energy gaps matching a pigment vibrational frequency with a small vibrational coordinate change on electronic excitation. Such photosynthetic energy transfer steps resemble molecular internal conversion through a nested intermolecular funnel.

Bulklike Hot Carrier Dynamics in Lead Sulfide Quantum Dots

Hot electronic dynamics in lead sulfide nanocrystals is interrogated by degenerate pump-probe spectroscopy with 20-25 fs pulses over a broad frequency range around three times the nanocrystal band gap. For each nanocrystal diameter, an initial reduction in absorption is seen only at the peak of the quantum confined E1 transition, while increased absorption is seen at all other wavelengths. The signals from the nanocrystals are approximately 300 times weaker than expected for a two-level system with the same absorbance and molar extinction coefficient and are weaker near time zero. These results appear to be inconsistent with quantum confinement of the initially excited high energy states. Arguments based on carrier scattering length, the wave packet size supported by the band structure, and effective mass are advanced to support the hypothesis that, for many direct-gap semiconductor quantum dots, the carrier dynamics at three times the band gap is localized on the 1-2 nm length scale and essentially bulklike except for frequent collisions with the surface.

Energy transfer: Vibronic coherence unveiled.

Pigment assemblies with high-efficiency electronic energy transfer have recently been observed to show unusual and persistent coherence, but its origin is not fully understood. Now, a combination of 2D electronic spectroscopy and theoretical modelling has allowed the excitonic coherence signal of a strongly coupled homodimer to be isolated.

Absolute measurement of femtosecond pump-probe signal strength.

The absolute femtosecond pump-probe signal strength of deprotonated fluorescein in basic methanol is measured. Calculations of the absolute pump-probe signal based on the steady-state absorption and emission spectrum that use only independently measured experimental parameters are carried out. The calculation of the pump-probe signal strength assumes the pump and probe fields are both weak and includes the following factors: the transverse spatial profile of the laser beams; the pulse spectra; attenuation of the propagating pulses with depth in the sample; the anisotropic transition probability for polarized light; and time-dependent electronic population relaxation. After vibrational and solvent relaxation are complete, the calculation matches the measurement to within 10% error without any adjustable parameters. This demonstrates quantitative measurement of absolute excited state population.

Ultrafast 25-fs relaxation in highly excited states of methyl azide mediated by strong nonadiabatic coupling

Significance Highly excited states of neutral molecules behave qualitatively differently than the lower excited states that are commonly studied in photochemistry. Such states are involved in ionospheric and astrochemical phenomena, as well as in detonation processes. However, highly excited states are poorly understood due to experimental and theoretical challenges in probing their complex dynamics. Here, we apply vacuum-UV femtosecond laser sources and an imaging photoelectron–photoion coincidence spectrometer to directly probe the surprisingly fast 25-fs reaction pathway of the energetic molecule methyl azide. Combined with advanced calculations, we conclude that the electronic relaxation is driven by strong nonadiabatic coupling and that population transfer occurs along a seam well above the minimum energy conical intersection. Highly excited electronic states are challenging to explore experimentally and theoretically—due to the large density of states and the fact that small structural changes lead to large changes in electronic character with associated strong nonadiabatic dynamics. They can play a key role in astrophysical and ionospheric chemistry, as well as the detonation chemistry of high-energy density materials. Here, we implement ultrafast vacuum-UV (VUV)-driven electron–ion coincidence imaging spectroscopy to directly probe the reaction pathways of highly excited states of energetic molecules—in this case, methyl azide. Our data, combined with advanced theoretical simulations, show that photoexcitation of methyl azide by a 10-fs UV pulse at 8 eV drives fast structural changes and strong nonadiabatic coupling that leads to relaxation to other excited states on a surprisingly fast timescale of 25 fs. This ultrafast relaxation differs from dynamics occurring on lower excited states, where the timescale required for the wavepacket to reach a region of strong nonadiabatic coupling is typically much longer. Moreover, our theoretical calculations show that ultrafast relaxation of the wavepacket to a lower excited state occurs along one of the conical intersection seams before reaching the minimum energy conical intersection. These findings are important for understanding the unique strongly coupled non-Born–Oppenheimer molecular dynamics of VUV-excited energetic molecules. Although such observations have been predicted for many years, this study represents one of the few where such strongly coupled non-Born–Oppenheimer molecular dynamics of VUV-excited energetic molecules have been conclusively observed directly, making it possible to identify the ultrafast reaction pathways.

Uncovering Highly-Excited State Mixing in Acetone Using Ultrafast VUV Pulses and Coincidence Imaging Techniques

Understanding the ultrafast dynamics of highly excited electronic states of small molecules is critical for a better understanding of atmospheric and astrophysical processes, as well as for designing coherent control strategies for manipulating chemical dynamics. In highly excited states, nonadiabatic coupling, electron-electron interactions, and the high density of states govern dynamics. However, these states are computationally and experimentally challenging to access. Fortunately, new sources of ultrafast vacuum ultraviolet pulses, in combination with electron-ion coincidence spectroscopies, provide new tools to unravel the complex electronic landscape. Here we report time-resolved photoelectron-photoion coincidence experiments using 8 eV pump photons to study the highly excited states of acetone. We uncover for the first time direct evidence that the resulting excited state consists of a mixture of both ny → 3p and π → π* character, which decays with a time constant of 330 fs. In the future, this approach can inform models of VUV photochemistry and aid in designing coherent control strategies for manipulating chemical reactions.

Nodeless vibrational amplitudes and quantum nonadiabatic dynamics in the nested funnel for a pseudo Jahn-Teller molecule or homodimer

The nonadiabatic states and dynamics are investigated for a linear vibronic coupling Hamiltonian with a static electronic splitting and weak off-diagonal Jahn-Teller coupling through a single vibration with a vibrational-electronic resonance. With a transformation of the electronic basis, this Hamiltonian is also applicable to the anti-correlated vibration in a symmetric homodimer with marginally strong constant off-diagonal coupling, where the non-adiabatic states and dynamics model electronic excitation energy transfer or self-exchange electron transfer. For parameters modeling a free-base naphthalocyanine, the nonadiabatic couplings are deeply quantum mechanical and depend on wavepacket width; scalar couplings are as important as the derivative couplings that are usually interpreted to depend on vibrational velocity in semiclassical curve crossing or surface hopping theories. A colored visualization scheme that fully characterizes the non-adiabatic states using the exact factorization is developed. The nonadiabatic states in this nested funnel have nodeless vibrational factors with strongly avoided zeroes in their vibrational probability densities. Vibronic dynamics are visualized through the vibrational coordinate dependent density of the time-dependent dipole moment in free induction decay. Vibrational motion is amplified by the nonadiabatic couplings, with asymmetric and anisotropic motions that depend upon the excitation polarization in the molecular frame and can be reversed by a change in polarization. This generates a vibrational quantum beat anisotropy in excess of 2/5. The amplitude of vibrational motion can be larger than that on the uncoupled potentials, and the electronic population transfer is maximized within one vibrational period. Most of these dynamics are missed by the adiabatic approximation, and some electronic and vibrational motions are completely suppressed by the Condon approximation of a coordinate-independent transition dipole between adiabatic states. For all initial conditions investigated, the initial nonadiabatic electronic motion is driven towards the lower adiabatic state, and criteria for this directed motion are discussed.

Electronic energy transfer through non-adiabatic vibrational-electronic resonance. I. Theory for a dimer

Non-adiabatic vibrational-electronic resonance in the excited electronic states of natural photosynthetic antennas drastically alters the adiabatic framework, in which electronic energy transfer has been conventionally studied, and suggests the possibility of exploiting non-adiabatic dynamics for directed energy transfer. Here, a generalized dimer model incorporates asymmetries between pigments, coupling to the environment, and the doubly excited state relevant for nonlinear spectroscopy. For this generalized dimer model, the vibrational tuning vector that drives energy transfer is derived and connected to decoherence between singly excited states. A correlation vector is connected to decoherence between the ground state and the doubly excited state. Optical decoherence between the ground and singly excited states involves linear combinations of the correlation and tuning vectors. Excitonic coupling modifies the tuning vector. The correlation and tuning vectors are not always orthogonal, and both can be asymmetric under pigment exchange, which affects energy transfer. For equal pigment vibrational frequencies, the nonadiabatic tuning vector becomes an anti-correlated delocalized linear combination of intramolecular vibrations of the two pigments, and the nonadiabatic energy transfer dynamics become separable. With exchange symmetry, the correlation and tuning vectors become delocalized intramolecular vibrations that are symmetric and antisymmetric under pigment exchange. Diabatic criteria for vibrational-excitonic resonance demonstrate that anti-correlated vibrations increase the range and speed of vibronically resonant energy transfer (the Golden Rule rate is a factor of 2 faster). A partial trace analysis shows that vibronic decoherence for a vibrational-excitonic resonance between two excitons is slower than their purely excitonic decoherence.

Femtosecond Electronic Dynamics via a Conical Funnel

Femtosecond polarization spectroscopy measures electronic wavepacket motion after vibrational wavepackets are excited near an energetically inaccessible conical intersection in a free-base naphthalocyanine. Partial equilibration via the conical funnel takes place within ~100 fs.

Sample exchange by beam scanning with applications to noncollinear pump–probe spectroscopy at kilohertz repetition rates

In laser spectroscopy, high photon flux can perturb the sample away from thermal equilibrium, altering its spectroscopic properties. Here, we describe an optical beam scanning apparatus that minimizes repetitive sample excitation while providing shot-to-shot sample exchange for samples such as cryostats, films, and air-tight cuvettes. In this apparatus, the beam crossing point is moved within the focal plane inside the sample by scanning both tilt angles of a flat mirror. A space-filling spiral scan pattern was designed that efficiently utilizes the sample area and mirror scanning bandwidth. Scanning beams along a spiral path is shown to increase the average number of laser shots that can be sampled before a spot on the sample cell is resampled by the laser to ∼1700 (out of the maximum possible 2500 for the sample area and laser spot size) while ensuring minimal shot-to-shot spatial overlap. Both an all-refractive version and an all-reflective version of the apparatus are demonstrated. The beam scanning apparatus does not measurably alter the time delay (less than the 0.4 fs measurement uncertainty), the laser focal spot size (less than the 2 μm measurement uncertainty), or the beam overlap (less than the 3.3% measurement uncertainty), leading to pump-probe and autocorrelation signal transients that accurately characterize the equilibrium sample.