eV Pump Research Articles

Ultrafast internal conversion dynamics of highly excited pyrrole studied with VUV/UV pump probe spectroscopy.

We study the relaxation dynamics of pyrrole after excitation with an 8 eV pump pulse to a state just 0.2 eV below the ionization potential using vacuum ultraviolet/ultraviolet pump probe spectroscopy. Our measurements in conjunction with electronic structure calculations indicate that pyrrole undergoes rapid internal conversion to the ground state in less than 300 fs. We find that internal conversion to the ground state dominates over dissociation.

From the (1B) Spectroscopic State to the Photochemical Product of the Ultrafast Ring-Opening of 1,3-Cyclohexadiene: A Spectral Observation of the Complete Reaction Path.

All stages of the electrocyclic ring-opening of 1,3-cyclohexadiene (CHD) were observed by time-resolved photoionization-photoelectron spectroscopy. Spectra of the 1B state, previously unobserved using time-resolved methods, were obtained upon optical excitation using ultrashort laser pulses at 4.60 or 4.65 eV, followed by ionization with pulses at 3.81, 3.85, and 4.10 eV, revealing a 1B lifetime of 30 fs. In an experiment using 3.07 eV probe photons and a 4.69 eV pump, we observed a time-sequenced progression of Rydberg states that includes s, p, and d states of the series n = 3 to 6. The sequentiality of the Rydberg signals points to an ionization mechanism that captures the molecule on different points along the reaction path in 2A. A dynamic fit of the Rydberg signals, coupled with MS-CASPT2 calculations, reveals that as the wavepacket moves down the potential energy surface it acquires kinetic energy at a rate of 28 eV/ps before reaching the conical intersection to the 1A ground state. During the reaction, the terminal carbon atoms separate at a speed of 16 Å/ps. A deconvolution of the Rydberg signals from a broad feature assigned to structurally disperse 1,3,5-hexatriene (HT) shows the formation of the open-chain hexatriene structure with an onset 142 fs after the initial absorption of a pump photon. The experimental observations are discussed in the context of recent ultrafast X-ray scattering experiments and theoretical quantum dynamics simulations.

Distance Dependence of Exciton Dissociation at a Phthalocyanine–C60 Interface

Exciton dissociation at donor–acceptor (DA) interfaces is critical for the operation of organic photovoltaic (OPV) devices, yet a detailed physical understanding of this process is lacking. This work examines an important aspect of this process, namely the dependence of the exciton dissociation rate on distance from the DA interface. Time-resolved two-photon photoemission (TR-2PPE) measurements were performed on bilayer H2Pc\C60 heterojunctions fabricated using organic molecular beam epitaxy (MBE) with varying H2Pc thickness. In the measurements, the dynamics of the H2Pc S1 exciton population created with a 1.55 eV pump pulse were monitored via photoemission with a delayed UV probe pulse. The depth sensitivity of TR-2PPE, due to the short electron attenuation length, provides the means to follow excited state dynamics as a function of H2Pc thickness. Analysis of the S1 population decay as a function of H2Pc thickness revealed that the electron transfer rate for the first H2Pc layer, adjacent to C60, is kCT = (2.3 ± 0.4) × 1012 s–1. Exciton dissociation is reduced by a factor of at least 10 for the second H2Pc layer and beyond.

Possible Bose-condensate behavior in a quantum phase originating in a collective excitation in the chemically and optically doped Mott-Hubbard system UO2+x

X-ray pair distribution function (pdf) and U L${}_{3}$ extended x-ray absorption fine structure (EXAFS) and neutron pdf measurements that give identical results for UO${}_{2}$ show U(VI)-oxo moieties with x rays for mixed valence U${}_{4}$O${}_{9}$ and U${}_{3}$O${}_{7}$, in contrast to the neutron data that indicate only U(V) sites with no short U-O bonds as well as other differences. In addition, although the EXAFS spectra of UO${}_{2}$ are essentially identical at 30, 100, and 200 K, those of the UO${}_{2+x}$ compounds exhibit different nearest-neighbor U-O distributions at each temperature. Further tunneling polaron-type behavior is found in the broadening of the features of the O K-edge x-ray absorption spectra (XAS) of the UO${}_{2+x}$ compounds. Raman spectra of powders also show a large increase in scattering cross section with increasing O content that would originate in a change in the electronic structure that increases the overall polarizability. The XAS and Raman also show that U${}_{4}$O${}_{9}$ does not behave as a linear combination of the UO${}_{2}$ and U${}_{3}$O${}_{7}$ fluorite endpoints. The properties induced by mobile rather than static charged quasiparticles were explored by optical pumping of the metal-to-metal charge-transfer transition. The temperature dependence of 4.71 eV pump--1.57 eV probe reflectivity on UO${}_{2}$ that initially populates the U 6$d$-dominated portion of the upper Hubbard band (UHB) shows a sharp 28-\ensuremath{\mu}sec lifetime peak at 25 K that may be associated with the fluctuations of its antiferromagnetic transition. Pumping at 3.14 eV into the 5$f$-dominated portion of the UHB shows an analogous 2.8-\ensuremath{\mu}sec peak but also a plateau bracketing this peak that ends in a cusp at 50--60 K and an abrupt change in the hardening rate of a novel 12--15 GHz phonon that is the signature for the quasiparticle quantum phase. The different results for the different excitation channels indicate a highly specific nonthermal relaxation mechanism. These results constitute the first observation of a distinct phase of photoinduced quasiparticles that is sufficiently coupled to the lattice to undergo a gap-opening transition. When the intragap state is probed with a terahertz time domain spectroscopy (TTDS) measurement 33 psec after a 3.14 excitation pulse, it shows increased absorption in the 0.5--1.1 THz range with a decrease in temperature from \ensuremath{\sim}30 to 10 K instead of the expected decrease, a result consistent with the presence of a condensate. These results are too extreme to originate in the dynamical, nonadiabatic, coupled charge-transfer--phonon/tunneling polaron scenario previously used for doped Mott-Hubbard insulators with intermediate electron-phonon coupling and therefore indicate novel physics. One possibility that could cause all of these behaviors is that a collective, dynamical, charge transfer-coupled Peierls distortion involving the 2 U(V) \ensuremath{\leftrightarrow} U(IV)+U(VI)-oxo excitation occurs coherently over an entire domain to cause the atoms in this domain to condense into a system with Bose-Einstein or Bose-Einstein-Hubbard properties.

Direct Observation of Sub-100 fs Mobile Charge Generation in a Polymer-Fullerene Film

The formation of mobile charges in a roll-to-roll processed poly-3-hexylthiophene-fullerene bulk heterojunction film is observed directly by using transient terahertz spectroscopy with sub-100fs temporal resolution. The transient terahertz ac conductivity reveals that 20% of the incident pump photons are converted into highly delocalized charges within the 40fs, 3.1eV pump pulse duration, which then rapidly becomes localized within 120fs. Approximately 2/3 of these carriers subsequently decay, possibly into an exciton, on a 1ps time scale.

Solvent-dependent activation of intermediate excited states in the energy relaxation pathways of spheroidene

In carotenoids internal conversion between the allowed (S(2)) and forbidden (S(1)) excited states occurs on a sub-picosecond timescale; the involvement of an intermediate excited state(s) (S(x)) mediating the process is controversial. Here we use high time resolution (sub-20 fs) broadband (1.2-2.5 eV) pump-probe spectroscopy to study the solvent dependence of excited state dynamics of spheroidene, a naturally-occurring carotenoid with ten conjugated double bonds. In the high polarizability solvent, CS(2), we find no evidence of an intermediate state, and the traditional three-level (S(0), S(1), S(2)) model fully accounts for the S(2)→ S(1) process. On the other hand, in the low polarizability solvent, cyclohexane, we find that rapid (~30 fs) relaxation to an intermediate state, S(x), lying between S(1) and S(2) is required to account for the data. We interpret these results as due to a shift of the S(2) energy, which positions the state above or below the energy of S(x) in response to changes in solvent polarizability.

Ultrafast Dynamics of BiFeO3 Thin Films Studied by Dual-Color Femtosecond Spectroscopy

The ultrafast dynamics in (100)-oriented BiFeO3 thin films were studied from 20 K to 300 K by using the dual-color femtosecond pump-probe spectroscopy, which were performed by the 400 nm (3.1 eV) pump pulses above the energy gap of 2.67 eV and the 800 nm (1.55 eV) probe pulses below the energy gap of 2.67 eV. From the temperature-dependent transient reflectivity changes (ΔR/R), the anomalous changes were clearly observed below 200 K caused by magnons. Thus, the femtosecond pump-probe spectroscopy could unambiguously reveal the magnetization dynamics in BiFeO3 with strong magnetoelectric coupling.

Photodesorption of hydrogen molecules physisorbed on Ag: Isotope dependence of translational-energy distribution

We report findings of our investigation on the non-thermal photodesorption dynamics of H 2(D 2) initially physisorbed on Ag. We photodesorbed the H 2(D 2) using a 6.4 eV pump photon, and then state-selectively probed the desorbing H 2(D 2) using the resonance-enhanced multi-photon ionization technique. The corresponding time-of-flight (TOF) spectra measured for the initial rotational states J = 1 and J = 0 indicate no particular J dependence. The mean translational temperature was ∼8 K for both H 2 and D 2. Using a coherent DIET (desorption induced by electronic transitions) model, we were able to reproduce the TOF spectra, including the observed isotope dependence.

Time-resolved relaxation dynamics of Hgn− (11⩽n⩽16,n=18) clusters following intraband excitation at 1.5 eV

Electron-nuclear relaxation dynamics are studied in Hg(n) (-) (11 <or= n <or= 16,n = 18) using time-resolved photoelectron imaging. The excess electron in the anion uniquely occupies the p band and is excited intraband by 1.53 eV pump photons; the subsequent dynamics are monitored by photodetachment at 3.06 eV and measurement of the photoelectron images as a function of pump-probe delay. The initially excited state decays on a time scale of approximately 10 ps, and subsequent relaxation dynamics reveal a smooth evolution of the photoelectron spectra towards lower electron kinetic energy over 50-100 ps. Qualitatively, the relaxation process is captured by a simple kinetic model assuming a series of radiationless transitions within a dense manifold of electronic states. All the clusters studied show similar dynamics with the exception of Hg(11) (-) in which the initially prepared state does not decay as quickly as the others.

Femtosecond studies of mixed valence state formation in strongly correlated electron materials

We present the results of our ultrafast optical experiments on mixed-valence manganites La0.7X0.3MnO3(X=Ca,Sr) and the heavy-fermion compounds YbXCu4(X=Ag,In). For the manganites, 1.5eV pump and either optical or terahertz probe reveals a dynamic spectral weight transfer that, as a function of temperature, allows us to determine the spin–lattice relaxation time. For YbXCu4, pump–probe measurements at 1.5eV are shown to be sensitive to the onset of coherence. These results are discussed in terms of a simple hybridization gap model for the f and s electrons.

Oscillation of the optical second-harmonic generation intensity during Ag thin film growth on aSi(111)7×7surface

The optical second harmonic generation (SHG) intensity demonstrated clear oscillations during Ag thin film growth on an $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ surface at room temperature. The first intensity peak appeared at two monolayer (ML) for 1.20, 1.30, and 1.40 eV pump photon energies, and the second peak shifted to a lower coverage side with increases in the excitation photon energy. The SHG spectra and atomic force microscope images indicated that the first peak was due to local field enhancement by the plasmon confined in isolated Ag islands. The second peak was caused by resonance with the quantized electronic states confined in the flat Ag thin film.

Ultrafast optical switching of second-harmonic generation at the C60 single-crystal surface

The switching of second-harmonic generation (SHG) at a C60 single-crystal surface has been observed in a pump-and-probe experiment. The SHG signal from a picosecond 1.17 eV laser pulse is suppressed by one order of magnitude upon illuminating the crystal surface with a 3.49 eV pump pulse. The nonlinear optical response is faster than 45 ps and persists for longer than 20 ns. SHG suppression to 1/e occurs at pump densities as low as 2.8 μJ/cm2. We suggest that nonlocalized excited electronic states determine the change in the nonlinear optical behavior.

Time-resolved measurements of substrate to adsorbate energy transfer

The use of ultrafast infrared pulses to characterize the transient response of visible and ultraviolet pulse-excited CO/Pt(111) is described. A 15 200 cm–1(1.9 eV) pump pulse of 1.0 ps duration and incident energy density of ca. 16 mJ cm–2 excites electron–hole pairs in the platinum. The accompanying transient response leads to an overall heating of the system, as revealed by a decrease in infrared reflectivity of the platinum and a shift to lower frequency of the 2106 cm–1 CO-stretching mode. Unlike the transient response observed for resonant infrared pumping of the CO stretch, the shift associated with visible pumping persists for over 50 ps and is believed to reflect activation of the 60 cm–1 frustrated CO-translation mode which is anharmonically coupled to the CO stretch. By using a density matrix formalism for the IR response to the pump-induced CO(v= 0 → 1) frequency shift, and incorporating a three-temperature model for the CO/Pt(111) surface (Te, Tlat, and Tads), fitting of the measured spectral response as a function of time-delay places limits on the timescale for energy transfer from the substrate to the adsorbed CO. Similar results were obtained with 34 612 cm–1(4.3 eV) pump pulses of <1.2 ps duration.

Photoelectron Spectroscopy of Conduction-Electron Energy Distributions in Laser-Excited Silicon

ABSTRACTElectron energy distributions, extending from equilibrium surface states through transiently populated conduction band states, have been observed in laser excited silicon by means of transient photoelectron spectroscopy. Atomically clean (111) wafers in UHV were excited by 90 nsec, 2.33 eV or 65 nsec, 4.66 eV pulses. Photoelectron spectra were obtained with the 4.66 eV pulses. Photovoltage shifts have been measured and found to saturate. We have identified a feature in the excited state portion of the spectra due to electrons which have equilibrated in the conduction band minima (CBM). Electron temperatures as a function of 2.33 eV pump fluence have been extracted from the spectra and are shown to be in equilibrium with the lattice above 1100 K. Very hot non-equilibrium electrons are also observed to ~ 4 eV above the CBM. A method of determining the diffusion depth of CBM electrons has been developed and applied to our data.