Marinus Kunst

Bio: Marinus Kunst is an academic researcher. The author has contributed to research in topics: Pulse (physics) & Nanosecond. The author has an hindex of 6, co-authored 6 publications receiving 188 citations.

Proton mobility in ice

Abstract: Ice is frequently taken as a model when factors controlling proton transport in hydrogen-bonded molecular networks are discussed. Such discussions have increased with the acknowledgement that proton transfer across cell membranes may play a significant part in energy conversion and storage in biological systems1–4 and that this transfer may involve hydrogen-bonded chains spanning the membrane5,6. However, there is still much uncertainty about the basic mode of proton displacement and the external factors which effect bulk proton transport in ice itself. We present here results of a microwave conductivity pulse radiolysis technique which has been used to determine the mobility of bare protons in ice, and thus reduce some of the uncertainty over proton mobility.

Subnanosecond time-resolved optical absorption studies of electron solvation in ice

Abstract: Optical absorption studies of pulse-irradiated ice have been carried out with subnanosecond time resolution. A growth in the solvated electron absorption is observed with a risetime of 450 ps at -5/sup 0/C. The activation energy for relaxation of the matrix following localization is 0.3 eV. Below -35/sup 0/C localization at trapping sites becomes the rate-controlling step in solvation. Addition of 10/sup -2/M NH/sub 4/F considerably increases the overall solvation time by reducing the initial rate of dry electron localization at defects. A second absorbing species is produced immediately in the pulse with a yield Gepsilon/sub 660/ = 9 x 10 M/sup -3/ cm/sup -1/ (100 eV)/sup -1/, which is independent of temperature. This is tentatively ascribed to an exciton state. 4 figures, 1 table.

The Grotthuss mechanism

Abstract: Suggested mechanisms for proton mobility are confronted with experimental findings and quantum mechanical calculations, indicating that no model is consistent with the existing data. It is suggested that the molecular mechanism behind prototropic mobility involves a periodic series of isomerizations between H 9 O 4 + and H 5 O 2 + , the first trigerred by hyrdogen-bond cleavage of a second-shell water molecule and the second by the reverse, hydrogen-bond formation process.

Spectral signatures of hydrated proton vibrations in water clusters.

Abstract: The ease with which the pH of water is measured obscures the fact that there is presently no clear molecular description for the hydrated proton. The mid-infrared spectrum of bulk aqueous acid, for example, is too diffuse to establish the roles of the putative Eigen (H 3 O + ) and Zundel (H 5 O 2 + ) ion cores. To expose the local environment of the excess charge, we report how the vibrational spectrum of protonated water clusters evolves in the size range from 2 to 11 water molecules. Signature bands indicating embedded Eigen or Zundel limiting forms are observed in all of the spectra with the exception of the three- and five-membered clusters. These unique species display bands appearing at intermediate energies, reflecting asymmetric solvation of the core ion. Taken together, the data reveal the pronounced spectral impact of subtle changes in the hydration environment.

Voltage-Gated Proton Channels and Other Proton Transfer Pathways

Abstract: Proton channels exist in a wide variety of membrane proteins where they transport protons rapidly and efficiently. Usually the proton pathway is formed mainly by water molecules present in the protein, but its function is regulated by titratable groups on critical amino acid residues in the pathway. All proton channels conduct protons by a hydrogen-bonded chain mechanism in which the proton hops from one water or titratable group to the next. Voltage-gated proton channels represent a specific subset of proton channels that have voltage- and time-dependent gating like other ion channels. However, they differ from most ion channels in their extraordinarily high selectivity, tiny conductance, strong temperature and deuterium isotope effects on conductance and gating kinetics, and insensitivity to block by steric occlusion. Gating of H+ channels is regulated tightly by pH and voltage, ensuring that they open only when the electrochemical gradient is outward. Thus they function to extrude acid from cells. H+ch...

Elementary steps in excited-state proton transfer.

Abstract: The absorption of a photon by a hydroxy-aromatic photoacid triggers a cascade of events contributing to the overall phenomenon of intermolecular excited-state proton transfer. The fundamental steps involved were studied over the last 20 years using a combination of theoretical and experimental techniques. They are surveyed in this sequel in sequential order, from fast to slow. The excitation triggers an intramolecular charge transfer to the ring system, which is more prominent for the anionic base than the acid. The charge redistribution, in turn, triggers changes in hydrogen-bond strengths that set the stage for the proton-transfer step itself. This step is strongly influenced by the solvent, resulting in unusual dependence of the dissociation rate coefficient on water content, temperature, and isotopic substitution. The photolyzed proton can diffuse in the aqueous solution in a mechanism that involves collective changes in hydrogen-bonding. On longer times, it may recombine adiabatically with the excited base or quench it. The theory for these diffusion-influenced geminate reactions has been developed, showing nice agreement with experiment. Finally, the effect of inert salts, bases, and acids on these reactions is analyzed.

The study of charge carrier kinetics in semiconductors by microwave conductivity measurements. II.

Abstract: The study of the excess conductivity induced in a material by pulsed optical excitation yields information on the optoelectronic properties of the material and is receiving increasing attention. As conventional conductivity techniques are hampered by the need to apply electrical contacts, we have investigated the reliability and the possibilities of microwave conductivity measurements. This paper first presents the general background for excess conductivity measurements in the microwave range, and then derives the quantitative relationship between the reflected microwave signal and the change in conductivity for a wafer of single‐crystalline Si. For this sample, the theory of excess charge carrier kinetics is also developed. After a short description of our apparatus, kinetic measurements on a nano‐ and microsecond timescale are compared to theory.