Wybren Jan Buma

Red spectral shift and enhanced quantum efficiency in phonon-free photoluminescence from silicon nanocrystals

Crystalline silicon is the most important semiconductor material in the electronics industry. However, silicon has poor optical properties because of its indirect bandgap, which prevents the efficient emission and absorption of light. The energy structure of silicon can be manipulated through quantum confinement effects, and the excitonic emission from silicon nanocrystals increases in intensity and shifts to shorter wavelengths (a blueshift) as the size of the nanocrystals is reduced. Here we report experimental evidence for a short-lived visible band in the photoluminescence spectrum of silicon nanocrystals that increases in intensity and shifts to longer wavelengths (a redshift) with smaller nanocrystal sizes. This higher intensity indicates an increased quantum efficiency, which for 2.5-nm-diameter nanocrystals is enhanced by three orders of magnitude compared to bulk silicon. We assign this band to the radiative recombination of non-equilibrium electron-hole pairs in a process that does not involve phonons.

Operation mechanism of a molecular machine revealed using time-resolved vibrational spectroscopy

Not So Random Walk In rotaxanes, a molecular ring can shuttle back and forth between docking sites along an axle. Panman et al. (p. 1255) traced the intricacies of this shuttling motion using vibrational spectroscopy. The kinetics were dominated by the slow scission of hydrogen bonds tying the ring to its starting site. Varying the length of the axle allowed the extraction of relative rates for forward and backward motion once the ring was free: Somewhat surprisingly, forward motion toward the destination site was slightly hindered relative to regression toward the starting place. Measuring the travel of a molecular ring along an axle explains its shuttling motion. Rotaxanes comprise macrocycles that can shuttle between docking stations along an axle. We explored the nanosecond shuttling mechanism by reversing the relative binding affinities of two stations through ultraviolet-induced transient reduction. We monitored the ensuing changes in the CO-stretching bands of the two stations and the shuttling macrocycle by means of an infrared probing pulse. Because hydrogen-bond scission and formation at the initial and final stations led to well-resolved changes in the respective CO-stretch frequencies, the departure and arrival of the macrocycle could be observed separately. We found that the shuttling involves two steps: thermally driven escape from the initial station, followed by rapid motion along the track ending either at the initial or final station. By varying the track’s length, we found that the rapid motion approximates a biased one-dimensional random walk. However, surprisingly, the direction of the overall motion is opposite that of the bias.

Lowest energy excited singlet state of isolated cis-hexatriene

In a previous letter [J. Chem. Phys. 92, 4622 (1990)] we reported the first observation of the 2 1Ag state of cis‐hexatriene in a supersonic jet expansion by using resonance enhanced multiphoton ionization spectroscopy. Here, the vibrational analysis of the 1 1Ag→2 1Ag excitation spectrum of cis‐hexatriene is presented. The excitation spectrum shows that cis‐hexatriene in the 2 1Ag state deviates slightly from planarity; a conclusion which is corroborated by ab initio calculations indicating that the nonplanarity primarily involves the terminal hydrogen atoms. Except for observable intensity in the low frequency modes associated with the small out of plane distortion, the vibronic development of the 1 1Ag→2 1Ag transition in cis‐hexatriene is similar to that observed for other polyenes: the 0–0 transition is the most intense feature and the next most intense band is the C■C stretching fundamental. Thus the general features of the electronic structure of the cis‐hexatriene 2 1Ag state are analogous to thos...

Separately doped upconversion-C60 nanoplatform for NIR imaging-guided photodynamic therapy of cancer cells

A highly efficient upconversion-C60 nanoplatform was demonstrated for NIR imaging-guided photodynamic therapy of cancer.

Resonance enhanced multiphoton ionization photoelectron spectroscopy on nanosecond and picosecond time scales of Rydberg states of methyl iodide

Rydberg states of methyl iodide have been investigated using resonance enhanced multiphoton ionization in combination with photoelectron spectroscopy with nanosecond and picosecond laser pulses. The study of the ns (6≤n≤10) Rydberg states in two‐, three‐, and four‐photon excitations has resulted in an unambiguous identification of state [1] in the 7s and 8s Rydberg states. As a consequence, it is concluded that the transition to 6s[1] in two‐ and three‐photon excitations is anomalously weak. The application of photoelectron spectroscopy to identify the electronic and vibrational nature of a resonance has led to a major reinterpretation of the excitation spectrum of the 6p Rydberg state in two‐photon excitation. In many of the recorded photoelectron spectra anomalous electrons are observed, which derive from a one‐photon ionization process. This process is suggested to find its origin in the mixing of 6p and 7s character into higher‐lying Rydberg states. The major difference between resonance enhanced mult...

Resonance enhanced multiphoton ionization photoelectron spectroscopy and pulsed field ionization via the F1Δ2(v'=0) and f3Δ2(v'=0) Rydberg states of HCl

In this paper, we report the first rotationally resolved one‐ and two‐color resonance enhanced multiphoton ionization photoelectron spectroscopy (REMPI‐PES) study of the HCl molecule. The agreement between our experimental branching ratios and theoretical investigations is excellent. We also report the first zero kinetic energy pulsed field ionization (ZEKE‐PFI) experiments carried out in a ‘‘magnetic bottle’’ electron spectrometer. A direct comparison is made between ZEKE‐PFI and REMPI‐PES spectra for ionization via several rotational levels of the F 1Δ2(v’=0) and f 3Δ2(v’=0) Rydberg states of HCl. Large differences in both the spin–orbit and rotational branching ratios are found between the ZEKE‐PFI and REMPI‐PES spectra. These differences can be understood qualitatively on the basis of rotational and spin–orbit autoionization mechanisms.

Proton transfer with a twist? Femtosecond Dynamics of 7‐(2‐pyridyl)indole in Condensed Phase and in Supersonic Jets

The results of spectral and photophysical studies strongly suggest that photoinduced proton transfer in 7-(2-pyridyl)indole (1) is accompanied by mutual twisting of the pyridyl and indole moieties. This conclusion is supported by the unusual finding that the photoreaction is faster in a cold, supersonicjet-isolated molecule than in solutions at room temperature, and by the ultrafast repopulation of the ground state substrate. The twisting and the presence of S1–S0 conical intersection (CI) are also predicted by calculations. The phenomenon may be quite general for several classes of organic molecules with intramolecular hydrogen bonds. Recent experimental and theoretical developments demonstrate that proton transfer—a fundamental chemical reaction—is by no means a one-dimensional process. [1] In particular, for tautomerization occurring along a hydrogen bond, coupling with modes that modulate the hydrogen bond strength may be crucial. [2] Much less explored is the possibility of large conformational changes accompanying proton/hydrogen transfer, although the role of torsional motion in the deactivation of phototautomerization products has been discussed in some depth. [3–9] Compound 1 is a member of a series of three isomeric 7pyridylindoles (Scheme 1), which were studied previously in solution. [10] Compounds 1 and 2 can exist in syn and anti

Resonance enhanced multiphoton ionization spectroscopy of carbon disulphide

Rydberg excited states of the CS2 molecule in the energy range 56 000–81 000 cm−1 have been further investigated via the two and three photon resonance enhancements they provide in the mass resolved multiphoton ionization (MPI) spectrum of a jet‐cooled sample of the parent molecule. Spectral interpretation has been aided by parallel measurements of the kinetic energies of the photoelectrons that accompany the various MPI resonances. Thus we have been able to extend, and clarify, previous analyses of the tangled spin–orbit split vibronic structure associated with the 3Πu and 1Πu states derived from the configuration [2Πg]4pσu and the 3Δu, 1Δu, and 1Σ+u states resulting from the configuration [2Πg]4pπu, and to deduce an approximate wave number for the origin of the hitherto unidentified 3Σ+u state derived from this same configuration. Moving to higher energies we are able to locate, unambiguously, the origins of the next (n=5) members of four of these [2Πg]np Rydberg series, and to identify extensive series...

Direct bandgap optical transitions in Si nanocrystals

The effect of quantum confinement on the direct bandgap of spherical Si nanocrystals has been modelled theoretically. We conclude that the energy of the direct bandgap at the Γ-point decreases with size reduction: quantum confinement enhances radiative recombination across the direct bandgap and introduces its “red“ shift for smaller grains. We postulate to identify the frequently reported efficient blue emission (F-band) from Si nanocrystals with this zero-phonon recombination. In a dedicated experiment, we confirm the “red“ shift of the F-band, supporting the proposed identification.

IR Spectroscopy of Molecular Ions by Nonthermal Ion Ejection from Helium Nanodroplets

Infrared spectroscopy provides a means to determine the intrinsic geometrical structures of molecules. Here we present a novel spectroscopic method that uses superfluid helium nanodroplets to record IR spectra of cold molecular ions, in this particular case aniline cations. The method is based on the detection of ions that are ejected from the helium droplets following vibrational excitation of these ions. We find that spectra can be recorded with a high sensitivity and that they exhibit only a small matrix shift. The widths of the individual transitions depend on the excited vibrational level and are thought to be related to the interaction of the ion with the surrounding helium solvent shells.