Toward two-dimensional photon echo spectroscopy with 200 nm laser pulses.

Knowledge of elementary relaxation processes in small molecules and proteins motivates the extension of two-dimensional photon echo (2DPE) spectroscopy further into the UV wavelength range. Here, we describe our development of a four-wave mixing spectrometer employing 200 nm laser pulses. Filamentation of laser beams in both air and argon yields 200 nm pulses with 60 fs durations. These 200 nm pulses are used to probe dynamics initiated at 267 nm in transient grating and 2DPE experiments conducted on adenosine. This study demonstrates that these femtosecond spectroscopies may indeed be carried out at the shortest wavelengths feasible in aqueous solutions.

Interplay between vibrational energy transfer and excited state deactivation in DNA components.

Femtosecond laser spectroscopies are used to examine a thymine family of systems chosen to expose the interplay between excited state deactivation and two distinct vibrational energy transfer (VET) pathways: (i) VET from the base to the deoxyribose ring; (ii) VET between neighboring units in a dinucleotide. We find that relaxation in the ground electronic state accelerates markedly as the molecular sizes increase from the nucleobase to the dinucleotide. This behavior directly reflects growth in the density of vibrational quantum states on the substituent of the base. Excited state lifetimes are studied at temperatures ranging from 100 to 300 K to characterize the thermal fluctuations that connect the Franck-Condon geometries and the conical intersections leading back to the ground state. An Arrhenius analysis yields an approximate excited state energy barrier of 13 meV in the thymine dinucleotide. In addition, we find that the transfer of vibrational energy from the base to the substituent suppresses thermal fluctuations across this energy barrier. The possibility that the solvent viscosity imposes friction on the reaction coordinate is examined by comparing thymine and adenine systems. Experiments suggest that the solvent viscosity has little effect on barrier crossing dynamics in thymine because the conical intersection is accessed through relatively small out-of-plane atomic displacements. Overall, we conclude that the transfer of vibrational quanta from thymine to the deoxyribose ring couples significantly to the internal conversion rate, whereas the neighboring unit in the dinucleotide serves as a secondary heat bath. In natural DNA, it follows that (local) thermal fluctuations in the geometries of subunits involving the base and deoxyribose ring are most important to this subpicosecond relaxation process.

Two-Dimensional Electronic Spectroscopy in the Ultraviolet Wavelength Range.

Coherent two-dimensional (2D) spectroscopies conducted at visible and infrared wavelengths are having a transformative impact on the understanding of numerous processes in condensed phases. The extension of 2D spectroscopy to the ultraviolet spectral range (2DUV) must contend with several challenges, including the attainment of adequate laser bandwidth, interferometric phase stability, and the suppression of undesired nonlinearities in the sample medium. Solutions to these problems are motivated by the study of a wide range of biological systems whose lowest-frequency electronic resonances are found in the UV. The development of 2DUV spectroscopy also makes possible the attainment of new insights into elementary chemical reaction dynamics (e.g., electrocyclic ring opening in cycloalkenes). Substantial progress has been made in both the implementation and application of 2DUV spectroscopy in the past several years. In this Perspective, we discuss 2DUV methodology, review recent applications, and speculate on what the future will hold.

Communication: uncovering molecule-TiO2 interactions with nonlinear spectroscopy.

Femtosecond transient grating experiments are used to investigate electronic structures and transport mechanisms in dye-sensitized nanocrystalline TiO(2) films. This study examines two molecular sensitizers spanning the weak (a phosphonated Ruthenium complex) and strong (catechol) molecule-TiO(2) coupling regimes. It is shown that strong molecule-TiO(2) interactions give rise to photoinduced vibrational coherences at the interface between species. We suggest that the amplitudes of these coherences reflect the molecule-TiO(2) coupling strength and signify the delocalization of excited state wavefunctions.

Influence of temperature on thymine-to-solvent vibrational energy transfer.

At the instant following the non-radiative deactivation of its ππ* electronic state, the vibrational modes of thymine possess a highly non-equilibrium distribution of excitation quanta (i.e., >4 eV in excess energy). Equilibrium is re-established through rapid (5 ps) vibrational energy transfer to the surrounding solvent. The mechanisms behind such vibrational cooling (VC) processes are examined here using femtosecond transient grating and two-dimensional photon echo spectroscopies conducted at 100 K and 300 K in a mixture of methanol and water. Remarkably, we find that this variation in temperature has essentially no impact on the VC kinetics. Together the experiments and a theoretical model suggest three possible mechanisms consistent with this behavior: (i) vibrational energy transfer from the solute to solvent initiates (directly) in intramolecular modes of the solute with frequencies >300 cm(-1); (ii) the relaxation induced increase in the temperature of the environment reduces the sensitivity of VC to the temperature of the equilibrium system; (iii) the time scale of solvent motion approaches 0.1 ps even at 100 K. Mechanism (i) deserves strong consideration because it is consistent with the conclusions drawn in earlier studies of isotope effects on VC in hydrogen bonding solvents. Our model calculations suggest that mechanism (ii) also plays a significant role under the present experimental conditions. Mechanism (iii) is ruled out on the basis of long-lived correlations evident in the photon echo line shapes at 100 K. These insights into photoinduced relaxation processes in thymine are made possible by our recent extension of interferometric transient grating and photon echo spectroscopies to the mid UV spectral region.

Probing ultrafast dynamics in adenine with mid-UV four-wave mixing spectroscopies.

Heterodyne-detected transient grating (TG) and two-dimensional photon echo (2DPE) spectroscopies are extended to the mid-UV spectral range in this investigation of photoinduced relaxation processes of adenine in aqueous solution. These experiments are the first to combine a new method for generating 25 fs laser pulses (at 263 nm) with the passive phase stability afforded by diffractive optics-based interferometry. We establish a set of conditions (e.g., laser power density, solute concentration) appropriate for the study of dynamics involving the neutral solute. Undesired solute photoionization is shown to take hold at higher peak powers of the laser pulses. Signatures of internal conversion and vibrational cooling dynamics are examined using TG measurements with signal-to-noise ratios as high as 350 at short delay times. In addition, 2DPE line shapes reveal correlations between excitation and emission frequencies in adenine, which reflect electronic and nuclear relaxation processes associated with particular tautomers. Overall, this study demonstrates the feasibility of techniques that will hold many advantages for the study of biomolecules whose lowest-energy electronic resonances are found in the mid-UV (e.g., DNA bases, amino acids).

Influence of vibronic coupling on band structure and exciton self-trapping in α-perylene.

Exciton sizes influence transport processes and spectroscopic phenomena in molecular aggregates and crystals. Thermally driven nuclear motion generally localizes electronic states in equilibrium systems. Exciton sizes also undergo dynamic changes caused by nonequilibrium relaxation in the lattice structure local to the photoexcitations (i.e., self-trapping). The α-phase of crystalline perylene is particularly well-suited for fundamental studies of exciton self-trapping mechanisms. It is generally agreed that a subpicosecond self-trapping process in α-perylene localizes photoexcited excitons onto pairs of closely spaced molecules (i.e., dimers), which then relax through excimer emission. Here, electronic relaxation dynamics in α-perylene single crystals are investigated using a variety of nonlinear optical spectroscopies in conjunction with a Frenkel exciton model. Linear absorption and photon echo spectroscopies suggest that excitons are delocalized over less than four unit cells (16 molecules) at 78 K prior to self-trapping. Stimulated Raman spectroscopies conducted on and off electronic resonance reveal significant vibronic coupling in a mode at 104 cm(-1), which corresponds to the displacement between perylene molecules comprising a dimer. Strong vibronic coupling in this mode suggests that motion along the interdimer axis is instrumental in driving the self-trapping process. The results are discussed in the context of our recent study of tetracene and rubrene single crystals in which similar experiments and models were employed.