Reaction mechanisms following gas-surface collisions may differ depending on the site of impact. On stepped Pt surfaces at least three dissociation mechanisms have been suggested to occur in... Show moreReaction mechanisms following gas-surface collisions may differ depending on the site of impact. On stepped Pt surfaces at least three dissociation mechanisms have been suggested to occur in parallel. Using supersonic molecular beams and a curved Pt single crystal, we unravel these site-dependent contributions and quantify absolute reaction cross sections using these mechanisms' different dependencies on kinetic energy in the range of 0.7 to 13.4 kJ/mol. A sum of three dissociation mechanism probabilities fits our data well and allows us to extract absolute cross sections for indirect and direct non-activated dissociation at steps. We find good agreement with theoretical predictions. Show less
Cao, K.; Lent, R. van; Kleyn, A.W.; Juurlink, L.B.F. 2018
Using semi-empirical density functional theory and the quasi-classical trajectory (QCT) method, a specific reaction parameter (SRP) density functional is developed for the dissociation of... Show moreUsing semi-empirical density functional theory and the quasi-classical trajectory (QCT) method, a specific reaction parameter (SRP) density functional is developed for the dissociation of dihydrogen on Pt(1 1 1). The validity of the QCT method was established by showing that QCT calculations on reaction of D2 with Pt(1 1 1) closely reproduce quantum dynamics results for reaction of D2 in its rovibrational ground state. With the SRP functional, QCT calculations reproduce experimental data on D2 sticking to Pt(1 1 1) at normal and off-normal incidence with chemical accuracy. The dissociation of dihydrogen on Pt(1 1 1) is non-activated, exhibiting a minimum barrier height of −8 meV. Show less
Alessandrini, A.; Gerunda, M.; Canters, G.W.; Verbeet, M.P.; Facci, P. 2003
Cu- and Zn-azurin chemisorbed on Au(111) have been comparatively investigated by electrochemical scanning tunnelling microscopy in buffer solution. Cu-azurin shows a marked tunnelling current... Show moreCu- and Zn-azurin chemisorbed on Au(111) have been comparatively investigated by electrochemical scanning tunnelling microscopy in buffer solution. Cu-azurin shows a marked tunnelling current resonance upon substrate potential at -0.21 V (vs SCE), whereas Zn counterparts do not. These data, discussed in terms of current theories on electron tunnelling through redox adsorbates, demonstrate the role of the electroactive metal ion present in the active site in assisting electron transfer via this metalloprotein, (C) 2003 Elsevier B.V. All rights reserved. Show less
An optical single-molecule study is reported of a quickly frozen solution of 2.3,8.9-dibenzanthanthrene (DBATT) in n-tetradecane at 1.4 K. The orientation has been measured of several hundreds of... Show moreAn optical single-molecule study is reported of a quickly frozen solution of 2.3,8.9-dibenzanthanthrene (DBATT) in n-tetradecane at 1.4 K. The orientation has been measured of several hundreds of DBATT molecules within a confocal detection volume of similar to 10 mum(3) as a function of their resonance frequency in the range of the two 0-0 bands in the fluorescence-excitation spectrum. Each band is found to correspond to a distinct distribution of orientations of DBATT molecules. A particular resonance frequency within a band is not correlated with a specific molecular orientation. (C) 2001 Elsevier Science B.V. All rights reserved. Show less
We report the spectroscopic observation of single pentacene molecules in the matrices n-tetradecane and n-hexadecane, using a confocal microscope operating at liquid-helium temperatures. A maximum... Show moreWe report the spectroscopic observation of single pentacene molecules in the matrices n-tetradecane and n-hexadecane, using a confocal microscope operating at liquid-helium temperatures. A maximum detected photon emission rate of only 30 counts per second (cps) is found for pentacene in n-hexadecane and 160 cps for pentacene in n-tetradecane. For the latter system, the low count rate is shown to be caused by a high S-1 --> T-1 intersystem crossing yield of about 40% in combination with a triplet lifetime of 33 +/- 3 mu s. Pentacene molecules in this polycrystalline host are found to show little spectral diffusion on a timescale of seconds. (C) 2000 Elsevier Science B.V. All rights reserved. Show less
La Penna, G.; Buda, F.; Bifone, A.; Groot, H.J.M. de 1998
The energy landscape of the reaction center protein of the photosynthetic bacterium Rhodobacter sphaeroides R-26 has been investigated with electron spin echo envelope modulation (ESEEM)... Show moreThe energy landscape of the reaction center protein of the photosynthetic bacterium Rhodobacter sphaeroides R-26 has been investigated with electron spin echo envelope modulation (ESEEM) spectroscopy of the modulations induced by spin-spin interactions (dipolar and exchange) between radicals in the light-induced spin-polarized radical pair D(+)Q(A)(-) (D, primary electron donor; Q(A), primary electron acceptor). At temperatures above similar to 100 K the values of the dipolar and spin-exchange couplings between D+ and Q(A)(-) were found to be -(115 +/- 5) mu T and 0.7 mu T (with uncertainty of + 1.5 and -0.3 mu T), respectively. Abrupt changes of the linewidth of the Fourier-transformed ESEEM spectrum were observed near 25, 40 and 80 K. The lineshapes could be simulated assuming that the distance between the two radicals is distributed within a range of about 4 Angstrom and that the distribution depends stepwise on the temperature. The similarity between our results and those obtained with optical spectroscopies on Zn-protoporphyrin substituted myoglobin suggests that the stepwise changes in distance distribution are related to relaxation along a hierarchical self-similar pattern of minima in the multidimensional potential surface of the protein, and that this energy landscape is a global property of the protein. Show less
The binding of the primary quinone anion radical Q(A)(-.) in reaction centers (RCs) of Rhodobacter (Rb.) sphaeroides species was investigated with electron spin echo envelope modulation (ESEEM).... Show moreThe binding of the primary quinone anion radical Q(A)(-.) in reaction centers (RCs) of Rhodobacter (Rb.) sphaeroides species was investigated with electron spin echo envelope modulation (ESEEM). The ESEEM spectra, at two microwave frequencies, of Zn-substituted RCs of Rb. sphaeroides R26 showed interactions of the unpaired electron of Q(A)(-.) with two nitrogen nuclei in the protein matrix. From an analysis of the experimental data the nitrogen nuclear quadrupole resonance parameters were determined: e(2)qQ/h = 1.52 MHz, eta = 0.82 and e(2)qQ/h = 3.04 MHz, eta = 0.66, which are assigned to the N-14(8(1))-H group of His M219 and the peptide N-14 of Ala M260, The ESEEM spectrum of Q(A)(-.) in reaction centers of the Rb, sphaeroides mutant W(M252)Y shows that the nitrogen of Trp M252 is not interacting with Q(A)(-.), that of the mutant H(M266)C shows that manipulating the Fe binding site strongly affects the Q(A)-binding site. Show less
We report Shpol'skii-type luminescence spectra of C-70 in n-pentane at 1.5 K with vibronic linewidths of 3 cm(-1). The vibronic structure of both fluorescence and phosphorescence has been... Show moreWe report Shpol'skii-type luminescence spectra of C-70 in n-pentane at 1.5 K with vibronic linewidths of 3 cm(-1). The vibronic structure of both fluorescence and phosphorescence has been unravelled. The fluorescence intensity largely shows up in vibronic bands corresponding to transitions to ground-state infrared-active vibrational modes and it is concluded that the first excited singlet state of C-70 in alkanes is of A'(2) character. Show less
Rohrer, M.; Gast, P.; Mobius, K.; Prisner, T.F. 1996
Pulsed high-field/high-frequency EPR (3.4 T magnetic field, 95 GHz microwave frequency) is performed on the primary electron acceptor ubisemiquinone-10 anion radical (UQ-10(-.)) in Zn-substituted... Show morePulsed high-field/high-frequency EPR (3.4 T magnetic field, 95 GHz microwave frequency) is performed on the primary electron acceptor ubisemiquinone-10 anion radical (UQ-10(-.)) in Zn-substituted photosynthetic reaction centers (RCs) of Rhodobacter sphaeroides R26. In addition, in vitro measurements of UQ-10(-.) in a frozen solution of deuterated isopropanol are presented. The method is two-dimensional EPR, with the swept magnetic field as the first and the pulse separation time as the second variable of the echo-detected spectra. The high Zeeman field leads to a spectral resolution of the anisotropic g-tensor. This allows a sensitive detection of the orientation-dependent echo-decay functions. The T-2 relaxation anisotropy at high field is dominated by librations. Comparison of in vivo and in vitro measurements shows different anisotropies of the echo-decay functions. The results are interpreted by calculations considering angular motions around the principal axes of the g-tensor. Hydrogen bonds between UQ-10(-.) and its protic environment are considered. Show less
Using continuous wave EPR spectroscopy with a high time resolution, a new short-lived signal at g = 2 is observed in bacterial photosynthetic reaction centers in which electron transfer past the... Show moreUsing continuous wave EPR spectroscopy with a high time resolution, a new short-lived signal at g = 2 is observed in bacterial photosynthetic reaction centers in which electron transfer past the bacteriopheophytin Phi(A) is blocked. This signal decays with a time constant equal to the rise time of the triplet state of the primary donor D-3, and is therefore attributed to the primary radical pair [D(.+)Phi(A)(.-)]. Applying the theory of spin-correlated radical pairs, the spectrum could be satisfactorily simulated, yielding the following magnetic interaction parameters between D-.t and Phi(A)(.-): exchange interaction J(D Phi) = -0.9 +/- 0.1 mT and J(D Phi) = -1.7 +/- 0.1 mT for reaction centers of Rhodobacter sphaeroides R26 and Rhodopseudomonas viridis, respectively, and an axial dipolar interaction D-D Phi = -3.0 +/- 0.5 mT for Rhodopseudomonas viridis, The implications of We magnitude and sign of the exchange parameters for the energetics of photosynthetic electron transfer are discussed. Show less
Radicals produced by illumination or ionizing radiation are often produced in pairs, which quickly decay by recombination or by diffusion and subsequent reactions. For maximizing the yield of... Show moreRadicals produced by illumination or ionizing radiation are often produced in pairs, which quickly decay by recombination or by diffusion and subsequent reactions. For maximizing the yield of products, and for facilitating the study of reaction pathways, it is desirable to minimize the probability of radical pair recombination. We present a way of controlling the radical pair lifetime through the application of a pulse of resonant microwaves in the presence of a magnetic field. Herewith, two radical pair triplet states are coherently populated, from which the pair cannot recombine directly to the singlet ground state because of spin conservation. We illustrate the method with a photosynthetic photochemical reaction, where we have achieved an increase in the radical pair lifetime of up to two orders of magnitude. Show less
Köhler, J.; Brouwer, A.C.J.; Groenen, E.J.J.; Schmidt, J. 1996
Applying methods developed for single molecule spectroscopy to small ensembles, we have recorded high-resolution fluorescence-excitation spectra for pentacene in all substitutional sites of a p... Show moreApplying methods developed for single molecule spectroscopy to small ensembles, we have recorded high-resolution fluorescence-excitation spectra for pentacene in all substitutional sites of a p-terphenyl single crystal. The difference in intersystem crossing efficiency for pentacene molecules in the various substitutional sites is discussed on the basis of these spectra and data from optically detected magnetic resonance experiments. Show less
A Car-Paninello ab initio molecular dynamics calculation is presented of all-trans and 11-cis retinals. The minimum energy configurations of the two isomers have been determined by a simulated... Show moreA Car-Paninello ab initio molecular dynamics calculation is presented of all-trans and 11-cis retinals. The minimum energy configurations of the two isomers have been determined by a simulated annealing procedure. The backbone conjugation is properly described within the local density approximation. The vibrational frequencies have been determined from the molecular dynamics trajectories. The theoretical results show an excellent agreement with experiment and provide grounds for an analysis of the retinal vibrations in terms of localized modes. Show less
An electron spin echo envelope modulation (ESEEM) study is performed on the reduced primary electron-accepting ubiquinone-10 (Q(A)(-.)) in Zn-substituted reaction centers of the photosynthetic... Show moreAn electron spin echo envelope modulation (ESEEM) study is performed on the reduced primary electron-accepting ubiquinone-10 (Q(A)(-.)) in Zn-substituted reaction centers of the photosynthetic bacterium Rhodobacter sphaeroides R26. The ESEEM spectra showed hyperfine and quadrupolar couplings of Q(A)(-.) to nitrogens in the protein matrix. Simulation of the spectra revealed the following N-14 coupling parameters: hyperfine interaction: A(iso)=1.85 MHz, T-11=0.32, alpha=0(0), beta=45(0); nuclear quadrupole interaction: e(2)qQ/h=1.52 MHz, eta=0.82. Comparison of the quadrupole values with data in the literature shows that Q(A)(-.) is coupled to the N-delta(1)-H group of the M219 heterocycle, most probably through a hydrogen bond with the 4-C carbonyl group of the quinone. Show less
Electron spin echo envelope modulation (ESEEM) of the transient spin-polarised P(+)Q(A)(-) pairs has been studied at 20 and 220 K. The observed strong out-of-phase modulation is interpreted as... Show moreElectron spin echo envelope modulation (ESEEM) of the transient spin-polarised P(+)Q(A)(-) pairs has been studied at 20 and 220 K. The observed strong out-of-phase modulation is interpreted as resulting from electron-electron dipolar and exchange interactions. Fourier-transformed echo envelopes are consistent with the theory developed recently by Tang, Thurnauer and Norris and allow us to obtain readily the values of dipolar and exchange couplings. The results are insensitive to N-15 enrichment. Show less
Coremans, J.W.A.; Gastel, M. van; Poluektov, O.G.; Groenen, E.J.J.; Blaauwen, T. den; Pouderoyen, G. van; ... ; Messerschmidt, A. 1995
We report electron-nuclear double-resonance experiments on a single crystal of azurin at 95 GHz and electron-spin-echo envelope-modulation experiments on frozen solutions of azurin and of the H117G... Show moreWe report electron-nuclear double-resonance experiments on a single crystal of azurin at 95 GHz and electron-spin-echo envelope-modulation experiments on frozen solutions of azurin and of the H117G mutant at 9 GHz. The hyperfine and quadrupole tensors of the two remote nitrogens of the histidine ligands of copper are assigned and discussed. A third nucleus is found to contribute to the echo-modulation spectrum and this probably concerns an amide nitrogen of the peptide backbone. Show less
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