Transition States and Nonlinear Excitations in Chloroform Observed with a Sub-5 fs Pulse Laser

Photoluminescent ellipsometric circular dichroism

A novel spectroscopic technique, photoluminescent ellipsometric circular dichroism (PECD), which distinguishes all radiative electronic transitions related to molecular chiral centers. Additionally, it is proposed as complementary to the ellipsometric Raman spectroscopy (ERS) technique, thus establishing a relationship between vibrational modes and electronic transitions, associated with molecular chiral centers. In this way, PECD turns into a powerful technique for chiral material characterization. The PECD technique was performed on a chiral oligomer (1R,2R)-diiminocyclohexane, and its derivative polymer. A complete photophysical characterization in solution was performed to corroborate the new PECD technique.

Crystallization of highly soluble thioglucopyranoside ejected by coherent molecular vibrational excitation using a visible 10 fs pulsed laser

Under irradiation with a visible 10 fs pulsed laser, crystals can be formed by ejection from the solution phase to the gas phase due to coherent molecular vibrational excitation.

Formation of thioglucoside single crystals by coherent molecular vibrational excitation using a 10-fs laser pulse

Compound crystallization is typically achieved from supersaturated solutions over time, through melting, or via sublimation. Here a new method to generate a single crystal of thioglucoside using a sub-10-fs pulse laser is presented. By focusing the laser pulse on a solution in a glass cell, a single crystal is deposited at the edge of the ceiling of the glass cell. This finding contrasts other non-photochemical laser-induced nucleation studies, which report that the nucleation sites are in the solution or at the air-solution interface, implying the present crystallization mechanism is different. Irradiation with the sub-10-fs laser pulse does not heat the solution but excites coherent molecular vibrations that evaporate the solution. Then, the evaporated solution is thought to be deposited on the glass wall. This method can form crystals even from unsaturated solutions, and the formed crystal does not include any solvent, allowing the formation of a pure crystal suitable for structural analysis, even from a minute amount of sample solution.

Surface-Enhanced Impulsive Coherent Vibrational Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) has attracted a lot of attention in molecular sensing because of the remarkable ability of plasmonic metal nanostructures to enhance the weak Raman scattering process. On the other hand, coherent vibrational spectroscopy triggered by impulsive excitation using ultrafast laser pulses provides complete information about the temporal evolution of molecular vibrations, allowing dynamical processes in molecular systems to be followed in "real time". Here, we combine these two concepts and demonstrate surface-enhanced impulsive vibrational spectroscopy. The vibrational modes of the ground and excited states of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), spin-coated on a substrate covered with monodisperse silver nanoparticles, are impulsively excited with a sub-10 fs pump pulse and characterized with a delayed broad-band probe pulse. The maximum enhancement in the spectrally and temporally resolved vibrational signatures averaged over the whole sample is about 4.6, while the real-time information about the instantaneous vibrational amplitude together with the initial vibrational phase is preserved. The phase is essential to determine the vibrational contributions from the ground and excited states.

Ultrafast dynamics of uracil and thymine studied using a sub-10 fs deep ultraviolet laser

Single 9.6 fs deep ultraviolet pulses with a spectral range of 255–290 nm are generated by a chirped-pulse four-wave mixing technique for use as pump and probe pulses.

A new reaction mechanism of Claisen rearrangement induced by few-optical-cycle pulses: Demonstration of nonthermal chemistry by femtosecond vibrational spectroscopy

Time-resolved vibration spectroscopy is the only known way to directly observe reaction processes. In this work, we measure time-resolved vibration spectra of the Claisen rearrangement triggered and observed by few-optical-cycle pulses. Changes in molecular structure during the reaction, including its transition states (TSs), are elucidated by observing the transient changes of molecular vibration wavenumbers. We pump samples with visible ultrashort pulses of shorter duration than the molecular vibration period, and with photon energies much lower than the minimum excitation energy of the sample. The results indicate that the “nonthermal Claisen rearrangement” can be triggered by visible few-optical-cycle pulses exciting molecular vibrations in the electronic ground state of the sample, which replaces the typical thermal Claisen rearrangement.

Sub-10-fs deep-ultraviolet light source with stable power and spectrum

In some applications of ultrafast spectroscopy that employ sub-10-fs pulses, the pulse spectrum and power need to be stable for several tens of minutes. In this study, we generate sub-10-fs deep-ultraviolet (DUV) pulses with such stabilities by chirped-pulse four-wave mixing. A power fluctuation of less than 3% rms was realized by employing stabilization schemes that employ a power stabilizer. The pulses generated in this study have been applied to transient absorption spectroscopy in the DUV with a sub-10-fs time resolution [Phys. Chem. Chem. Phys.14, 6200 (2012).10.1039/c2cp23649d]. This sub-10-fs DUV source has a similar performance to widely used noncollinear optical parametric amplifiers.

Ultrafast spectroscopy with sub-10 fs deep-ultraviolet pulses

Time-resolved transient absorption spectroscopy with sub-9 fs ultrashort laser pulses in the deep-ultraviolet (DUV) region is reported for the first time. Single 8.7 fs DUV pulses with a spectral range of 255-290 nm are generated by a chirped-pulse four-wave mixing technique for use as pump and probe pulses. Electronic excited state and vibrational dynamics are simultaneously observed for an aqueous solution of thymine over the full spectral range using a 128-channel lock-in detector. Vibrational modes of the electronic ground state and excited states can be observed as well as the decay dynamics of the electronic excited state. Information on the initial phase of the vibrational modes is extracted from the measured difference absorbance trace, which contains oscillatory structures arising from the vibrational modes of the molecule. Along with other techniques such as time-resolved infrared spectroscopy, spectroscopy with sub-9 fs DUV pulses is expected to contribute to a detailed understanding of the photochemical dynamics of biologically significant molecules that absorb in the DUV region such as DNA and amino acids.

Generation of stable sub-10 fs pulses at 400 nm in a hollow fiber for UV pump-probe experiment

Stable sub-10-fs pulses useful for many pump-probe experiments with center wavelength at 400 nm were obtained using a hollow-fiber compression technique with a beam-pointing stabilizing system. The output power stability was improved by around 2-times with the beam-pointing stabilizer. A 1-mm-thick cell sample of perylene dissolved in cyclohexane was used to test the pulse using for the pump-probe experiment. Even the high C-H stretching of vibration mode at around 2860 cm(-1), 2916 cm(-1), and 2955 cm(-1) were real-time resolved with vibrational phase information.