Ground-state Vibrational Wavepacket Research Articles

Electronic State-Resolved Multimode-Coupled Vibrational Wavepackets in Oxazine 720 by Two-Dimensional Electronic Spectroscopy.

The difference between the excited- and ground-state vibrational wavepackets remains to be fully explored when multiple vibrational modes are coherently excited simultaneously by femtosecond pulses. In this work, we present a series of one- and two-dimensional electronic spectroscopy for studying multimode wavepackets of oxazine 720 in solution. Fourier transform (FT) maps combined with time-frequency transform (TFT) are employed to unambiguously distinguish the origin of low-frequency vibrational wavepackets, that is, an excited-state vibrational wavepacket of 586 cm-1 with a dephasing time of 0.7 ps and a ground-state vibrational wavepacket of 595 cm-1 with a dephasing time of 1.3-1.7 ps. We also found the additional low-frequency vibrational wavepackets resulting from the coupling of the 595 cm-1 mode to a series of high-frequency modes centered at 1150 cm-1 via electronic transitions. The combined use of FT maps and TFT analysis allows us to reveal the potential vibrational coupling of wavepackets and offers the possibility of disentangling the coupling between the electronic and vibrational degrees of freedom in condensed-phase systems.

Coherent wavepackets in the Fenna-Matthews-Olson complex are robust to excitonic-structure perturbations caused by mutagenesis.

Femtosecond pulsed excitation of light-harvesting complexes creates oscillatory features in their response. This phenomenon has inspired a large body of work aimed at uncovering the origin of the coherent beatings and possible implications for function. Here we exploit site-directed mutagenesis to change the excitonic level structure in Fenna-Matthews-Olson (FMO) complexes and compare the coherences using broadband pump-probe spectroscopy. Our experiments detect two oscillation frequencies with dephasing on a picosecond timescale-both at 77 K and at room temperature. By studying these coherences with selective excitation pump-probe experiments, where pump excitation is in resonance only with the lowest excitonic state, we show that the key contributions to these oscillations stem from ground-state vibrational wavepackets. These experiments explicitly show that the coherences-although in the ground electronic state-can be probed at the absorption resonances of other bacteriochlorophyll molecules because of delocalization of the electronic excitation over several chromophores.

Excited state wavepacket dynamics in NO2 probed by strong-field ionization.

We present an experimental femtosecond time-resolved study of the 399 nm excited state dynamics of nitrogen dioxide using channel-resolved above threshold ionization (CRATI) as the probe process. This method relies on photoelectron-photoion coincidence and covariance to correlate the strong-field photoelectron spectrum with ionic fragments, which label the channel. In all ionization channels observed, we report apparent oscillations in the ion and photoelectron yields as a function of pump-probe delay. Further, we observe the presence of a persistent, time-invariant above threshold ionization comb in the photoelectron spectra associated with most ionization channels at long time delays. These observations are interpreted in terms of single-pump-photon excitation to the first excited electronic X̃ 2A1 state and multi-pump-photon excitations to higher-lying states. The short time delay (<100 fs) dynamics in the fragment channels show multi-photon pump signatures of higher-lying neutral state dynamics, in data sets recorded with higher pump intensities. As expected for pumping NO2 at 399 nm, non-adiabatic coupling was seen to rapidly re-populate the ground state following excitation to the first excited electronic state, within 200 fs. Subsequent intramolecular vibrational energy redistribution results in the spreading of the ground state vibrational wavepacket into the asymmetric stretch coordinate, allowing the wavepacket to explore nuclear geometries in the asymptotic region of the ground state potential energy surface. Signatures of the vibrationally "hot" ground state wavepacket were observed in the CRATI spectra at longer time delays. This study highlights the complex and sometimes competing phenomena that can arise in strong-field ionization probing of excited state molecular dynamics.

Control of Phonons in Semiconductor Nanocrystals via Femtosecond Pulse Chirp-Influenced Wavepacket Dynamics and Polarization

The realistic electronic structure of semiconductor nanocrystals is characterized by excitonic fine structure and atomistic symmetry breakings that are challenging to resolve experimentally. Exciton-phonon coupling is one of the most sensitive measures of the excitonic wave functions of the nanocrystals. Here, we exploit this sensitivity via chirped pulse and polarization resolved femtosecond pump/probe spectroscopy of colloidal CdSe nanocrystals. Pulse chirp measurements and simulations are used to explore the contributions of excited- and ground-state vibrational wavepackets to the observed coherent phonons in the pump/probe signals. Polarization resolved pump/probe spectroscopy is used to explore electronic and vibrational polarization anisotropies. We find no electronic polarization anisotropy, whereas vibrational anisotropy is preserved.

Characterization of vibrational wave packets by core-level high-harmonic transient absorption spectroscopy

Author(s): Hosler, ER; Leone, SR | Abstract: The ground-state vibrational wave packet produced by strong-field ionization of Br2 is characterized by femtosecond high-harmonic transient absorption spectroscopy. Vibrational motion is observed in time by a change in the 3d core-level transition energy with bond length 0.14 eV/pm to higher energies at shorter bond lengths. The wave packet has the expected period of 104 fs for the predominant v0v1 vibrational quantum beat, which is prepared near the outer turning point with a phase of 0.21π±0.05π due to the preferential ionization at short bond lengths. Simultaneous observation of a wave packet on the Br2+2Π g,3/2 ground state, prepared at the equilibrium bond distance of the ion, confirms preferential ionization near the inner turning point of the neutral. The Br2 ground-state wave packet has a degree of coherence ranging between 0.19 and 0.24, where unity is perfect coherence. The results utilize the sensitivity of core-level transient absorption spectroscopy to bond length and charge state, providing a means to analyze the formation and evolution of vibrational wave packets. © 2013 American Physical Society.

Origin of the Bathochromic Shift of Astaxanthin in Lobster Protein: 2D Electronic Spectroscopy Investigation of β-Crustacyanin

We report on ultrafast spectroscopy study of β-crustacyanin, the carotenoprotein responsible for the coloration of the lobster shell. β-Crustacyanin is formed by two closely positioned astaxanthin molecules encapsulated in protein. The 2D electronic spectroscopy together with two-color pump-probe was applied to investigate the electronic structure, the excited-state dynamics, and the influence of the excitonic interaction between the two carotenoids in β-crustacyanin. By using the ∼20 fs laser pulses tuned to absorption bands of the S0-S2 and S1-Sn transitions of carotenoids, we were able to trace full excitation relaxation dynamics, starting with S2-S1 relaxation on the ∼30 fs time scale and finishing with the ground-state recovery of 3.2 ps. Superimposed on the relaxation dynamics in the 2D spectra, we observed long-lived beating signals at the characteristic frequencies of astaxanthin vibrational modes. We assign these oscillations to the ground-state vibrational wavepacket dynamics. All major features of the 2D spectra, including amplitude and phase maps of the long-lived oscillations, were reproduced by employing the exciton-vibronic model. Consistent modeling of all optical properties of β-crustacyanin (including absorption and circular dichroism spectra) points to the relatively weak coupling between the two astaxanthin molecules (∼250 cm(-1)). This implies that the excitonic coupling provides insignificant contribution to the bathochromic shift in β-crustacyanin. We discuss the origin of the shift and propose that it is caused by two major effects: conformational changes of astaxanthin molecules (increase in effective conjugation length) together with increased charge-transfer character of the S2 state. We put the bathochromic shift in the broad perspective of other "blue" carotenoids properties.

Mechanistic Origin of the Vibrational Coherence Accompanying the Photoreaction of Biomimetic Molecular Switches

The coherent photoisomerization of a chromophore in condensed phase is a rare process in which light energy is funneled into specific molecular vibrations during electronic relaxation from the excited to the ground state. In this work, we employed ultrafast spectroscopy and computational methods to investigate the molecular origin of the coherent motion accompanying the photoisomerization of indanylidene-pyrroline (IP) molecular switches. UV/Vis femtosecond transient absorption gave evidence for an excited- and ground-state vibrational wave packet, which appears as a general feature of the IP compounds investigated. In close resemblance to the coherent photoisomerization of rhodopsin, the sudden onset of a far-red-detuned and rapidly blue-shifting photoproduct signature indicated that the population arriving on the electronic ground state after nonadiabatic decay through the conical intersection (CI) is still very focused in the form of a vibrational wave packet. Semiclassical trajectories were employed to investigate the reaction mechanism. Their analysis showed that coupled double-bond twisting and ring inversions, already populated during the excited-state reactive motion, induced periodic changes in π-conjugation that modulate the ground-state absorption after the non-adiabatic decay. This prediction further supports that the observed ground-state oscillation results from the reactive motion, which is in line with a biomimetic, coherent photoisomerization scenario. The IP compounds thus appear as a model system to investigate the mechanism of mode-selective photomechanical energy transduction. The presented mechanism opens new perspectives for energy transduction at the molecular level, with applications to the design of efficient molecular devices.

Iodine−Benzene Complex as a Candidate for a Real-Time Control of a Bimolecular Reaction. Spectroscopic Studies of the Properties of the 1:1 Complex Isolated in Solid Krypton

The properties of the 1:1 iodine-benzene complex isolated in a solid Kr matrix at low temperatures have been studied using UV-vis absorption, FTIR, resonance Raman, and femtosecond coherent anti-Stokes Raman spectroscopy (fs-CARS). The use of all these techniques on similar samples provides a wide view on the spectroscopic properties of the complex and allows comparison and combination of the results from different methods. The results for the complex cover its structure, the changes in the iodine molecule's vibrational frequencies and electronic absorption spectrum upon complexation, and the dynamics of the complexed I(2) molecule on both ground and excited electronic states. In addition, polarization beats between uncomplexed benzene and iodine molecules are detected in the fs-CARS spectra, showing an amplification of an electronically nonresonant CARS signal by the resonant iodine signal. The possibility of controlling the charge-transfer reaction of the I(2)-Bz complex using the excitation of a well-defined ground-state vibrational wavepacket, according to the Tannor-Rice-Kosloff scheme, is discussed on the basis of the experimental findings.

Formation of Ground-State Vibrational Wave Packets in Intense Ultrashort Laser Pulses

The formation of coherent vibrational wave packets in the electronic ground state of neutral molecules in intense ultrashort laser pulses and their subsequent detection by means of recently developed pump-probe experiments are discussed. The wave packet formation is due to the pronounced dependence of the strong-field ionization rate on the internuclear distance. This leads to a deformation of the initial wave function due to an internuclear-distance dependent depletion. The phenomenon is demonstrated with a time-dependent wave packet study for molecular hydrogen.

Quantum-Phase Resolved Mapping of Ground-State VibrationalD2Wave Packets via Selective Depletion in Intense Laser Pulses

Applying 7 fs pump-probe pulses (780 nm, 4 x 10(14) W/cm2) we observe electronic ground-state vibrational wave packets in neutral D2 with a period of T=11.101(70) fs by following the internuclear separation (R-)dependent ionization with a sensitivity of Delta<R><or=0.02 A. The absolute phase of the wave packet's motion provides evidence for R-dependent depletion of the ground state by nonlinear ionization, to be the dominant preparation mechanism. A phase shift of about pi found between pure ionization (D2+) and dissociation (D+ + D) channels opens a pathway of quantum control.

Preparation and monitoring of high-ground-state vibrational wavepackets by femtosecond coherent anti-Stokes Raman scattering

Femtosecond time-delayed coherent anti-Stokes Raman scattering is presented not only as a tool for monitoring but also as a viable method for the preparation of vibrational wavepackets with very high quantum numbers in the ground electronic state of molecules. We experimentally demonstrate a particularly useful approach of using two separate time delays between the pulses for preparing vibrational wavepackets as high as v″=38 [ΔEv=7000 cm−1] in bulk gas-phase molecular iodine. By means of an ultrashort laser pulse, we prepare a wavepacket in an electronic excited state, optimize the frequency and timing of a second pulse to efficiently generate the targeted ground-state vibrational wavepacket, and monitor the wavepacket by coherent scattering from a third pulse. The method is further used to probe interference effects in femtosecond four-wave-mixing signals generated by molecular wavepackets.

Ultrafast Coherent Control in the Electronic Ground State of a Molecule

AbstractWe report on a coherent control experiment to selectively excite a well localized nuclear probability density in the electronic ground state of isolated, jet cooled potassium dimer molecules. The basic idea of this control scheme is to take advantage of the motion of a vibrational wave packet created in an intermediate excited electronic state, from which a Franck‐Condon window to the desired final state in the ground state emerges. To realize this concept we apply a resonant stimulated Raman process, in which the two involved laser pulses are separated by a time delay τ1 and their wavelengths are independently tunable. The method enables us to excite ground state vibrational wave packets, whereby the vibrational energy of the wave packet and the position of its generation along the internuclear separation coordinate of the ground state potential energy surface can in principle be freely chosen by the experimentalist. The results we present for the chosen model system K2 strongly support the idea that this control concept might be one possible approach to selectively populate a desired chemical reaction channel in the electronic ground state of a molecule.

Vibrational wave packet generation and time evolution in diatomic ions

A theoretical calculation of the generation and the time evolution of vibrational wave packets in diatomic ions is described. Under the irradiation of a short laser pulse, the alkali-rare gas van der Waals molecule may undergo photoionization and the ground state vibrational wavepacket is set up at the resulting alkali-rare gas ion potential. The temporal behavior and the spontaneous emission of the wave packets are described. The effect of the vibrationally and rotationally excited states of the initial electronic ground state on the dispersion of the generated total vibrational wave packet is also discussed.