Femtosecond Transient Grating Spectroscopy Research Articles

A tilted pulse-front setup for femtosecond transient grating spectroscopy in highly non-collinear geometries

We demonstrate a tilted pulse-front transient grating (TG) technique that allows to optimally utilize time resolution as well as TG line density while probing under grazing incidence as typically done in extreme ultraviolet (EUV) or soft x-ray (SXR) experiments. Our optical setup adapts the pulse front tilt of the two pulses that create the TG to the grazing incident pulse. We demonstrate the technique using all 800 nm femtosecond laser pulses for TG generation on a vanadium dioxide film. We probe that grating via diffraction of a third 800 nm pulse. The time resolution of 90 fs is an improvement by a factor of 30 compared to our previous experiments on the same system. The scheme paves the way for EUV and SXR probing of optically induced TGs on any material.

Photodissociation dynamics of organic molecules in condensed phase by femtosecond transient grating spectroscopy

In condensed phase, the dissociation mechanism of molecule is different from that of isolated molecule due to the effect of interaction between molecules. How to effectively trace the reaction process and products in condensed phase is a technical problem which needs to be solved urgently. In this paper, femtosecond transient grating spectroscopy is used to investigate dissociation dynamics in condensed phase. Transient grating spectroscopy, as a coherent spectral technique, has some advantages such as high signal-noise ratio and free background, thus it can identify trace numbers of reaction products in dissociation. The investigation about model molecules such as iodomethane and nitromethane demonstrates that the transient grating technique can observe relaxation in electronic excited state and also has ability to track reactants, products, and vibration of molecule or perssad. The dissociation dynamics in condensed phase material is significant for understanding the reaction mechanism in the fields of biochemistry and detonation. Thus the femtosecond transient grating has a wide application prospect in these fields. In addition, the transient grating technique, as a non-contact diagnostic approach, can be easily adapted to high temperature and high pressure conditions, etc. Thus, the transient grating technique also has a potential value in the fields of phase transform dynamics and high pressure synthesis, etc.

Torsional Dynamics and Intramolecular Charge Transfer in the S2(1(1)Bu(+)) Excited State of Peridinin: A Mechanism for Enhanced Mid-Visible Light Harvesting.

Of the carotenoids known in photosynthetic organisms, peridinin exhibits one of the highest quantum efficiencies for excitation energy transfer to chlorophyll (Chl) a acceptors. The mechanism for this enhanced performance involves an order-of-magnitude slowing of the S2 (1(1)Bu(+)) → S1 (2(1)Ag(-)) nonradiative decay pathway compared to carotenoids lacking carbonyl substitution. Using femtosecond transient grating spectroscopy with optical heterodyne detection, we have obtained the first evidence that the nonradiative decay of the S2 state of peridinin is promoted by large-amplitude torsional motions. The decay of an intermediate state termed Sx, which we assign to a twisted form of the S2 state, is substantially slowed by solvent friction in peridinin due to its intramolecular charge transfer (ICT) character.

Femtosecond Heterodyne Transient-Grating Studies of Nonradiative Decay of the S2 (1(1)Bu(+)) State of β-Carotene: Contributions from Dark Intermediates and Double-Quantum Coherences.

Femtosecond transient-grating spectroscopy with heterodyne detection was employed to characterize the nonradiative decay pathway in β-carotene from the S2 (1(1)Bu(+)) state to the S1 (2(1)Ag(-)) state in benzonitrile solution. The results indicate definitively that the S2 state populates an intermediate state, Sx, on an ultrafast time scale prior to nonradiative decay to the S1 state. Numerical simulations using the response function formalism and the multimode Brownian oscillator model were used to fit the absorption and dispersion components of the transient-grating signal with a common set of parameters for all of the relevant Feynman pathways, including double-quantum coherences. The requirement for inclusion of the Sx state in the nonradiative decay pathway is the observed fast rise time of the dispersion component, which is predominantly controlled by the decay of the stimulated emission signal from the optically prepared S2 state. The finding that the excited-state absorption spectrum from the Sx state is significantly red-shifted from that of S2 and S1 leads to a new assignment for the spectroscopic origin of the Sx state. Rather than assigning Sx to a discrete electronic state, such as the (1)Bu(-) state suggested in previous work, it is proposed that the Sx state corresponds to a transition-state-like structure on the S2 potential surface. In this hypothesis, the 12 fs time constant for the decay of the S2 state corresponds to a vibrational displacement of the C-C and C═C bond-length alternation coordinates of the conjugated polyene backbone from the optically prepared Franck-Condon structure to a potential energy barrier on the S2 surface that divides planar and torsionally displaced structures. The lifetime of the Sx state would be associated with a subsequent relaxation along torsional coordinates over a steep potential energy gradient toward a conical intersection with the S1 state. This hypothesis leads to the idea that twisted structures with intramolecular charge-transfer character along the S2 torsional gradient are active in excitation energy-transfer mechanisms to (bacterio)chlorophyll acceptors.

Ultrafast Transient Grating Spectroscopy in Silicon Quantum Dots

Transient grating spectroscopy detects directly the relaxation of the excited carriers rather than time-resolved photoluminescence and thus it is particularly desired for the indirect semiconductors such as silicon quantum dots. We investigate ultrafast carrier dynamics in silicon quantum dots embedded in silicon oxide matrix using femtosecond transient grating spectroscopy. Two ultrafast decay components are observed with decay time of 800 fs and 4 ps at various detection wavelengths, which are attributed to the transverse optical and transverse acoustic phonon assisted relaxation. Photoexcited electrons and holes are effectively trapped into the localized states on the surface of the silicon quantum dots where electrons and holes have a slow recombination in the time scale of microseconds.

Relaxation and diffusion of photoexcited carriers in ZnO epitaxial thin films

Relaxation and diffusion of carriers in an electron-hole plasma (EHP) in ZnO epitaxial thin films were studied by femtosecond transient grating spectroscopy. By performing several experiments of different fringe spacing, the lifetime and diffusion coefficient of the EHP state are determined to be 23.8 ps and 10.8 cm2/s, respectively. The observed lifetime is in good agreement with the previous values obtained by pump-probe and time-resolved optical Kerr gate luminescence measurements. Based on a simple diffusion equation, we conclude that the carrier diffusion in the EHP state does not contribute to reduction of the carrier population within such a short carrier lifetime of a few tens picosecond. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast Laser Induced Molecular Alignment and Deformation: Experimental Evidence from Neutral Molecules and from Fragment Ions

The alignment and molecular structure deformations induced by intense off-resonance excitation with ultrafast laser pulses are examined using femtosecond transient grating spectroscopy and by angle resolved multiphoton ionization in a molecular beam. The goal of this study is to correlate evidence obtained from the angular dependence of multiphoton ionization and from rotational recurrences observed in neutral molecules regarding alignment and molecular structure deformation. Structural parameters are determined by analysis of the field-free rotational recurrences, obtained by transient grating measurements, or by analysis of the anisotropy in the detected fragment ions, in the molecular beam experiments. Experimental data were obtained for CS2, CO2, acetylene, and benzene, for pulse intensities ranging from 1011 to 1014 W/cm2. The experimental results are consistent with molecular alignment resulting from a “kick” induced by the ultrafast off-resonance field. The results also provide evidence of molecular structure deformations. Results from transient grating experiments indicate that the electric field can induce alignment and bending in polyatomic molecules and that these effects can take place in the absence of ionization.

Rotational wavepacket revivals for phase modulation of ultrafast pulses

Phase modulation of ultrafast laser pulses has been demonstrated recently by taking advantage of the time-dependent refractive index that takes place during rotational wave packet revivals in a gaseous sample [Phys. Rev. Lett. 88 (2002) 013903]. Here we explore the effect of laser intensity, physical properties of molecules, and temperature on rotational wavepacket revivals by femtosecond transient grating spectroscopy. Theoretical analysis of experimental data is used to predict the optimum conditions for the phase modulation and compression of ultrafast laser pulses.

Nonlinear excited-state dynamics of a thin copper phthalocyanine film by femtosecond transient grating spectroscopy

Transient diffraction spectra and their rise and decay dynamics of a thin copper phthalocyanine film (≈ 50 nm thickness) were investigated by femtosecond transient grating spectroscopy using a white-light continuum as a probe pulse. Temporal profiles of first- and second-order diffractions were strongly dependent on the excitation intensity, which was interpreted in terms of exciton-exciton annihilation with a time-dependent rate constant.

Intersystem crossing of benzophenone by femtosecond transient grating spectroscopy

Transient grating spectroscopy using a femtosecond white-light continuum was developed and applied to the spectral analysis of intersystem crossing of benzophenone in solution. The diffraction spectra corresponding to S n ←S 1 and T n ←T 1 transient absorption were observed for the first time. The rate constants of intersystem crossing in acetonitrile and CCl 4 were estimated to be (9.6 ± 0.9 ps) −1 and (11.6 ± 0.9 ps) −1, respectively.