Quantum effects in the diffusion of hydrogen on Ru(0001)

An understanding of hydrogen diffusion on metal surfaces is important, not just for its role in heterogeneous catalysis and hydrogen fuel cell technology, but also because it provides model systems where tunneling can be studied under well-defined conditions. Here we report helium spin-echo measurements of the atomic-scale motion of hydrogen on the Ru(0001) surface between 75 and 250 K. Quantum effects are evident at temperatures as high as 200 K, while below 120 K we observe a tunneling-dominated temperature independent jump rate of 1.9$\times$10$^9$ s$^{-1}$, many orders of magnitude faster than previously seen. Quantum transition state theory calculations based on ab initio path-integral simulations reproduce the temperature dependence of the rate at higher temperatures and predict a crossover to tunneling-dominated diffusion at low temperatures, although the tunneling rate is under-estimated, highlighting the need for future experimental and theoretical studies of hydrogen diffusion on well-defined surfaces.Comment: 15 pages, 3 figure

Theoretical and Practical Arguments for Modeling Labor Supply as a Choice among Latent Jobs

Models of labor supply derived from stochastic utility representations and discretized sets of feasible hours of work have gained popularity because they are more practical than the standard approaches based on marginal calculus. In this paper we argue that practicality is not the only feature that can be addressed by means of stochastic choice theory. This theory also offers a powerful framework for developing a more realistic model for labor supply choices, founded on individuals having preferences over jobs and facing restrictions on the choice of jobs and hours of work. We discuss and clarify how this modeling framework deviates from both the conventional discrete approach (Van Soest, 1995), as well as the standard textbook approach based on marginal calculus (Hausman, 1985). It is argued that a model based on job choice opens up for a more realistic representation of the choice environment, and consequently offers the possibility of conducting a richer set of simulations of alternative policies.labor supply, random utility models, tax reform

The Boson peak in supercooled water

We perform extensive molecular dynamics simulations of the TIP4P/2005 model of water to investigate the origin of the Boson peak reported in experiments on supercooled water in nanoconfined pores and in hydration water around proteins. We find that the onset of the Boson peak in supercooled bulk water coincides with the crossover to a predominantly low-density-like liquid below the Widom line TW. The frequency and onset temperature of the Boson peak in our simulations of bulk water agree well with the results from experiments on nanoconfined water. Our results suggest that the Boson peak in water is not an exclusive effect of confinement. We further find that, similar to other glass-forming liquids, the vibrational modes corresponding to the Boson peak are spatially extended and are related to transverse phonons found in the parent crystal, here ice Ih.We thank S. V. Buldyrev and S. Sastry for helpful discussions. The simulations were in part performed using resources provided by the Swedish National Infrastructure for Computing (SNIC) at the NSC and HPC2N centers. LGMP, KTW and DS were supported by the Swedish Research Council. KTW is also supported by the Icelandic Research Fund through the START programme. PK acknowledges the support of National Academies Keck Future Initiatives award. HES thanks NSF Grants No. CHE0911389, No. CHE0908218, and No. CHE-1213217. (Swedish Research Council; Icelandic Research Fund through the START programme; National Academies Keck Future Initiatives award; CHE0911389 - NSF; CHE0908218 - NSF; CHE-1213217 - NSF)Published versio

The Boson peak in supercooled water

We perform extensive molecular dynamics simulations of the TIP4P/2005 model of water to investigate the origin of the Boson peak reported in experiments on supercooled water in nanoconfined pores, and in hydration water around proteins. We find that the onset of the Boson peak in supercooled bulk water coincides with the crossover to a predominantly low-density-like liquid below the Widom line $T_W$. The frequency and onset temperature of the Boson peak in our simulations of bulk water agree well with the results from experiments on nanoconfined water. Our results suggest that the Boson peak in water is not an exclusive effect of confinement. We further find that, similar to other glass-forming liquids, the vibrational modes corresponding to the Boson peak are spatially extended and are related to transverse phonons found in the parent crystal, here ice Ih.Comment: 25 pages, 9 figure

Nanosecond laser photolysis studies of chlorosomes and artificial aggregates containing bacteriochlorophyll e: Evidence for the proximity of carotenoids and bacteriochlorophyll a in chlorosomes from Chlorobium phaeobacteroides strain CL1401

Time-resolved, laser-induced changes in absorbance, ΔA(λ; t), have been recorded with a view to probing pigment–pigment interactions in chlorosomes (control as well as carotenoid-depleted) and artificial aggregates of bacteriochlorophyll e (BChle). Control chlorosomes were isolated from Chlorobium phaeobacteroides strain CL1401, whose chromophores comprise BChle, bacteriochlorophyll a (BChla) and several carotenoid (Car) pigments; Car-depleted chlorosomes, from cells grown in cultures containing 2-hydroxybiphenyl. Artificial aggregates were prepared by dispersing BChle in aqueous phase in the presence of monogalactosyl diglyceride. In chlorosomes ΔA(λ; t) shows, besides a signal attributable to triplet Car (with a half-life of about 4 μs), signals in the Qy regions of both BChl. The BChla signal decays at the same rate as the Car signal, which is explained by postulating that some Car are in intimate contact with some baseplate BChla pigments, and that when a ground-state Car changes into a triplet Car, the absorption spectrum of its BChla neighbors undergoes a concomitant change (termed transient environment-induced perturbation). The signal in the Qy-region of BChle behaves differently: its amplitude falls, under reducing conditions, by more than a factor of two during the first 0.5 μs (a period during which the Car signal suffers negligible diminution), and is much smaller under nonreducing conditions. The BChle signal is also attributed to transient environment-induced perturbation, but in this case the perturber is a BChle photoproduct (probably a triplet or a radical ion). The absence of long-lived BChle triplets in all three systems, and of long-lived BChla triplets in chlorosomes, indicates that BChle in densely packed assemblies is less vulnerable to photodamage than monomeric BChle and that, in chlorosome, BChla rather than BChle needs, and receives, photoprotection from an adjacent Car.Research Council of Norway. EU (contract FMRX-CT96-0081)Peer reviewe

Regularized Field Map Estimation in MRI

In fast magnetic resonance (MR) imaging with long readout times, such as echo-planar imaging (EPI) and spiral scans, it is important to correct for the effects of field inhomogeneity to reduce image distortion and blurring. Such corrections require an accurate field map, a map of the off-resonance frequency at each voxel. Standard field map estimation methods yield noisy field maps, particularly in image regions with low spin density. This paper describes regularized methods for field map estimation from two or more MR scans having different echo times. These methods exploit the fact that field maps are generally smooth functions. The methods use algorithms that decrease monotonically a regularized least-squares cost function, even though the problem is highly nonlinear. Results show that the proposed regularized methods significantly improve the quality of field map estimates relative to conventional unregularized methods.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85871/1/Fessler22.pd