Femtosecond Spectrometer Research Articles

Transient Absorption Spectra of Photopolymeric Films with a Periodic Silver-Nanoparticle Substructure

Spectral-kinetic characteristics of planar plasmonic nanostructures based on polymer films with ordered silver nanoparticles (Ag-NPs) were studied in the spectral range near the surface-plasmon resonance absorption (SPRA) using a femtosecond spectrometer. The Ag-NPs were synthesized in the polymer matrix from a precursor that was added beforehand to the starting photopolymerizable composition and was ordered using several laser beams to photopolymerize the composition. The influence of the degree of Ag-NP ordering on the transient SPRA spectra and relaxation dynamics of induced changes near the SPRA was discussed.

Tracking energy transfer between light harvesting complex 2 and 1 in photosynthetic membranes grown under high and low illumination

Energy transfer (ET) between B850 and B875 molecules in light harvesting complexes LH2 and LH1/RC (reaction center) complexes has been investigated in membranes of Rhodopseudomonas palustris grown under high- and low-light conditions. In these bacteria, illumination intensity during growth strongly affects the type of LH2 complexes synthesized, their optical spectra, and their amount of energetic disorder. We used a specially built femtosecond spectrometer, combining tunable narrowband pump with broadband white-light probe pulses, together with an analytical method based on derivative spectroscopy for disentangling the congested transient absorption spectra of LH1 and LH2 complexes. This procedure allows real-time tracking of the forward (LH2→LH1) and backward (LH2←LH1) ET processes and unambiguous determination of the corresponding rate constants. In low-light grown samples, we measured lower ET rates in both directions with respect to high-light ones, which is explained by reduced spectral overlap between B850 and B875 due to partial redistribution of oscillator strength into a higher energetic exciton transition. We find that the low-light adaptation in R. palustris leads to a reduced elementary backward ET rate, in accordance with the low probability of two simultaneous excitations reaching the same LH1/RC complex under weak illumination. Our study suggests that backward ET is not just an inevitable consequence of vectorial ET with small energetic offsets, but is in fact actively managed by photosynthetic bacteria.

A novel ultra-broadband transient spectrometer with microsecond measurement range based on a supercontinuum fiber laser

We show that the combination of a free-running 24 kHz supercontinuum fiber laser with a kHz femtosecond laser renders a sensitive and easy to implement transient spectrometer. It can directly extend the observation range of a standard broadband femtosecond spectrometer from the usual limit of a few nanoseconds to 1 millisecond and with the additional use of a chopper to seconds. The transmission signal from all supercontinuum pulses is recorded and assigned to the proper delay time by data post-processing. To demonstrate the performance of the device, the relaxation dynamics of Nd:YAG is measured.

Dynamics of collisional relaxation of the optically induced anisotropy of para-quaterphenyl in the gas phase

The decay kinetics of the optically induced anisotropy of para-quaterphenyl molecules is theoretically and experimentally studied in a wide range of pressures of buffer gases, pentane and argon. The anisotropy of absorption is measured in real time using a femtosecond spectrometer. Comparison with the theoretical models of orientational relaxation shows that the collisional interruption of the angular momentum corresponds to a situation that is intermediate between the situations described by the J-diffusion model and by the model of the Fokker-Planck equation and is closer to the latter situation. In terms of the Keilson-Storer model, such a regime corresponds to the collision efficiency parameter γ ≈ 0.78 for argon and 0.66 for pentane; i.e., 4.5 collisions with argon or 2.9 collisions with pentane are necessary to completely randomize the angular momentum of para-quaterphenyl molecules.

Photoinduced Electron Transfer Processes in Dye-sensitized Solar Cells

Electron injection processes in dye-sensitized solar cells have been studied. Transient absorption spectra were measured in a wide wavelength range (400—3000 nm) to probe photoproducts, oxidized dyes and conducting electrons. We estimated the efficiency of electron injection as a function of free energy change (ΔG)and found that surface of nanocrystalline films is energetically heterogeneous. Using a femtosecond spectrometer, we observed that electron injection occurs through several pathways, i.e., direct injection into the conduction band and injection via exciplex-like intermediates.