Ignacio B. Martini

ULTRAFAST STUDY OF ELECTRON-PHONON COUPLING IN COLLOIDAL GOLD PARTICLES

Abstract The hot electron dynamics in 11±2 nm diameter Au particles has been examined by transient absorption and bleach recovery measurements. These experiments show that the timescale for energy relaxation by electron–phonon coupling decreases as the pump laser power decreases, and that the relaxation time is independent of the pump or probe wavelength at low pump power. The electron–phonon relaxation time measured in our experiments (∼650 fs) is very similar to the time constant obtained from experiments with bulk gold samples. These observations are explained using the two-temperature model for electron–phonon coupling.

Near-field two-photon nanolithography using an apertureless optical probe

Near-field two-photon optical lithography is demonstrated by using ∼120 fs laser pulses at 790 nm in an apertureless near-field optical microscope, which produces lithographic features with ∼70 nm resolution. The technique takes advantage of the field enhancement at the extremity of a metallic probe to induce nanoscale two-photon absorption and polymerization in a commercial photoresist, SU-8. Even without optimization of the resist or laser pulses, the spatial resolution of this technique is as high as λ/10, nearly a factor of 2 better than techniques based on far field two-photon lithography.

Controlling optical gain in semiconducting polymers with nanoscale chain positioning and alignment.

We control the chain conformation of a semiconducting polymer by encapsulating it within the aligned nanopores of a silica host. The confinement leads to polarized, low-threshold amplified spontaneous emission from the polymer chains. The polymer enters the porous silica film from only one face and the filling of the pores is therefore graded. As a result, the profile of the index of refraction in the film is also graded, in the direction normal to the pores, so that the composite film forms a low-loss, graded-index waveguide. The aligned polymer chains plus naturally formed waveguide are ideally configured for optical gain, with a threshold for amplified spontaneous emission that is twenty times lower than in comparable unoriented polymer films. Moreover, the optimal conditions for ASE are met in only one spatial orientation and with one polarization. The results show that nanometre-scale control of semiconducting polymer chain orientation and position leads to novel and desirable optical properties.

Ultrafast competition between energy and charge transfer in a functionalized electron donor/fullerene derivative

Abstract The fact that fullerenes are good electron acceptors has generated interest in covalently linked complexes between electron donors and fullerenes; photoinduced charge transfer in these dyads has great potential for use in photovoltaic devices. In this Letter, we investigate the excited-state properties of a perylene–fulleropyrrolidine dyad using steady-state and femtosecond time-resolved spectroscopies. Following photoexcitation, charge separation and energy transfer occur in nearly equal proportion; both processes take place on a sub-picosecond timescale. This suggests that competition between energy and charge transfer is common in these molecular systems, so that the best molecules for device applications are not necessarily those with the fastest electron transfer rates.

OBSERVATION OF ACOUSTIC QUANTUM BEATS IN NANOMETER SIZED AU PARTICLES

Quantum beats due to coherently prepared acoustic vibrational modes have been observed for 14 to 17 nm diameter Au particles. The beat frequency is inversely proportional to the particle size. The dephasing time is ca. 15 ps, which is limited by the broad size distribution (±2 nm) of the samples.

Ultrafast study of interfacial electron transfer between 9-anthracene-carboxylate and TiO2 semiconductor particles

The excited state dynamics of 9-anthracene-carboxylic acid adsorbed onto the surface of TiO2 semiconductor particles were examined with ca. 250 fs time resolution. A combination of transient absorption and time-resolved anisotropy measurements show that approximately 76% of the photo-excited dye molecules transfer an electron to the TiO2 particles. The time scale for the forward electron transfer reaction was determined to be ⩽1 ps. The 9-anthracene-carboxylate radical cations produced by this reaction undergo back electron transfer on a 54 ps time scale. A more accurate estimate of the forward electron transfer reaction time is not possible, due to the contribution to the transient absorption signal from adsorbed dye molecules that do not transfer electrons to TiO2. These nonreactive species are deactivated by either nonradiative decay or fluorescence emission. The fluorescence spectrum from the dye molecules bound to the TiO2 particles is very different to that of the free dye in solution. The free dye ...

Mechanisms of the ultrafast production and recombination of solvated electrons in weakly polar fluids: Comparison of multiphoton ionization and detachment via the charge-transfer-to-solvent transition of Na− in THF

The processes by which solvated electrons are generated and undergo recombination are of great interest in condensed phase physical chemistry because of their relevance to both electron transfer reactions and radiation chemistry. Although most of the work in this area has focused on aqueous systems, many outstanding questions remain, especially concerning the nature of these processes in low polarity solvents where the solvated electron has a fundamentally different structure. In this paper, we use femtosecond spectroscopic techniques to explore the dynamics of solvated electrons in tetrahydrofuran (THF) that are produced in two different ways: ejection by multiphoton ionization of the neat solvent, and detachment via the charge-transfer-to-solvent (CTTS) transition of sodide (Na−). Following multiphoton ionization of the solvent, the recombination of solvated electrons can be well described by a simple model that assumes electrons are first ejected to a given thermalization distance and then move diffusi...

Direct observation of charge-transfer-to-solvent (CTTS) reactions: Ultrafast dynamics of the photoexcited alkali metal anion sodide (Na−)

Charge-transfer-to-solvent (CTTS) transitions have been the subject of a great deal of interest recently because they represent the simplest possible charge transfer reaction: The CTTS electron transfer from an atomic ion to a cavity in the surrounding solvent involves only electronic degrees of freedom. Most of the work in this area, both experimental and theoretical, has focused on aqueous halides. Experimentally, however, halides make a challenging choice for studying the CTTS phenomenon because the relevant spectroscopic transitions are deep in the UV and because the charge-transfer dynamics can be monitored only indirectly through the appearance of the solvated electron. In this paper, we show that these difficulties can be overcome by taking advantage of the CTTS transitions in solutions of alkali metal anions, in particular, the near-IR CTTS band of sodide (Na−) in tetrahydrofuran (THF). Using femtosecond pump–probe techniques, we have been able to spectroscopically separate and identify transient ...

Relaxation dynamics in the first excited singlet state of a cyanine dye: HITC

The S1 state relaxation dynamics of HITC were examined with 200 fs time resolution. The sample was excited at 780, 700, 650 or 600 nm, and the transmission at 780 nm was recorded. In the nondegenerate experiments there is an instantaneous ground state depletion and a stimulated emission signal with a bi-exponential rise with time constants of 300 ± 100 fs and 2.9 ± 0.3 ps. The degenerate experiments show the ground state depletion component and a small stimulated emission signal with a 1.1 ± 0.2 ps decay. The dynamics are assigned to vibrational relaxation and/or solvation effects.

Solvent effects on the ultrafast dynamics and spectroscopy of the charge-transfer-to-solvent reaction of sodide

In “outer sphere” electron transfer reactions, motions of the solvent molecules surrounding the donor and acceptor govern the dynamics of charge flow. Are the relevant solvent motions determined simply by bulk solvent properties such as dielectric constant or viscosity? Or are molecular details, such as the local solvent structure around the donor and acceptor, necessary to understand how solvent motions control charge transfer? In this paper, we address these questions by using ultrafast spectroscopy to study a photoinduced electron transfer reaction with only electronic degrees of freedom: the charge-transfer-to-solvent (CTTS) reaction of Na− (sodide). Photoexcitation of Na− places the excited CTTS electron into a solvent-bound excited state; motions of the surrounding solvent molecules in response to this excitation ultimately lead to detachment of the electron. The detached electron can then localize either in an “immediate” contact pair (in the same cavity as the Na atom), which undergoes back electr...