Beam energy distribution influences on density modulation efficiency in seeded free-electron lasers

The beam energy spread at the entrance of undulator system is of paramount importance for efficient density modulation in high-gain seeded free-electron lasers (FELs). In this paper, the dependences of high harmonic micro-bunching in the high-gain harmonic generation (HGHG), echo-enabled harmonic generation (EEHG) and phase-merging enhanced harmonic generation (PEHG) schemes on the electron energy spread distribution are studied. Theoretical investigations and multi-dimensional numerical simulations are applied to the cases of uniform and saddle beam energy distributions and compared to a traditional Gaussian distribution. It shows that the uniform and saddle electron energy distributions significantly enhance the performance of HGHG-FELs, while they almost have no influence on EEHG and PEHG schemes. A numerical example demonstrates that, with about 84keV RMS uniform and/or saddle slice energy spread, the 30th harmonic radiation can be directly generated by a single-stage seeding scheme for a soft x-ray FEL facility

Linearized Relative Positional Encoding

Relative positional encoding is widely used in vanilla and linear transformers to represent positional information. However, existing encoding methods of a vanilla transformer are not always directly applicable to a linear transformer, because the latter requires a decomposition of the query and key representations into separate kernel functions. Nevertheless, principles for designing encoding methods suitable for linear transformers remain understudied. In this work, we put together a variety of existing linear relative positional encoding approaches under a canonical form and further propose a family of linear relative positional encoding algorithms via unitary transformation. Our formulation leads to a principled framework that can be used to develop new relative positional encoding methods that preserve linear space-time complexity. Equipped with different models, the proposed linearized relative positional encoding (LRPE) family derives effective encoding for various applications. Experiments show that compared with existing methods, LRPE achieves state-of-the-art performance in language modeling, text classification, and image classification. Meanwhile, it emphasizes a general paradigm for designing broadly more relative positional encoding methods that are applicable to linear transformers. The code is available at https://github.com/OpenNLPLab/Lrpe.Comment: Reviewed by TMLR, decision pending. Yiran Zhong is the corresponding author. Code is available at https://github.com/OpenNLPLab/Lrp

Sequential bonding of CO molecules to a titanium dimer: A photoelectron velocity-map imaging spectroscopic and theoretical study of Ti-2(CO)(n)(-) (n=1-9)

Binuclear titanium carbonyl cluster anions, Ti-2(CO)(n)(-) (n = 4-6), are produced via a laser vaporization supersonic cluster source and characterized by mass-selected photoelectron velocity-map imaging spectroscopy. Quantum chemical calculations are carried out for Ti-2(CO)(n)(-) (n = 1-9) to explore the trend of sequential bonding of CO molecules to a titanium dimer. It has been found that the CO molecules bind to Ti-2 in a side-on fashion and form a stable Ti-2[eta(2)(mu(2)-C,O)](3) structure at n = 3, the motif of which retains up to n = 5. Starting at n = 6, a new building block of two CO groups side-on-bonded to Ti-2 is favored, the structure of which persists up to n = 9. In the larger clusters (n = 6-9), the side-on-bonded CO molecule can be stabilized via the removal of two electrons from an anionic titanium carbonyl, which is different from the effect of charge on CO binding in rhodium carbonyls where bridge-bonded CO molecules are selectively destabilized by the removal of an electron from a neutral rhodium carbonyl. The present study provides a stepwise picture for molecular-level understanding of CO bonding on transition-metal clusters, which is directly relevant to the elementary processes of CO at metal catalysts. Published by AIP Publishing

Effect of Multilayer Methanol and Water in Methanol Photochemistry on TiO2

We have conducted a series of temperature programmed desorption (TPD) experiments to study the photoinduced reactions of methanol (CH3OH) on a rutile(R)TiO2(110) surface with 355 nm light. Products formaldehyde (CH2O) and water (H2O) have been detected. The results clearly show that the photochemistry of CH3OHTisc is hindered by the coadsorbed multilayer CH3OH or H2O. For multilayer CH3OH adsorption, CH3OH molecules hydrogen-bonded to the bridge bonded oxygen sites inhibit the CH3OHTisc photochemistry. Combined with previous theoretical studies (Sci. China Chem. 2015, 58, 614-619), CH3OH molecules hydrogen bonded to the BBO sites increase the energy barrier of C-H bond dissociation and decrease the energy barrier of the reverse reaction, resulting in lowering the efficiency of CH3OHTisc photolysis. As the coverage of CH3OH keeps increasing to multilayer, the efficiency of the CH3OHTisc photolysis does not decrease any more. However, with the coadsorbed H2O molecules, the efficiency of CH3OHTi5c photolysis decreases continually with increasing H2O coverage, which is likely due to the decrease of the, chemisorbed CH3OHTisc molecules via molecule exchange between the H2O films and the chemisorbed CH3OH layer that occurs at similar to 100 K and the complicated hydrogen bonds formed between H2O and CH3OHTisc molecules

Acetone Formation from Photolysis of 2-Propanol on Anatase-TiO_2(101)

Photocatalysis of 2-propanol on A-TiO2(101) has been investigated using a temperature programed desorption method with 266 nm laser light. A clear mechanism is proposed for photodissociation of 2-propanol on A-TiO2(101). Acetone product on five coordinate Ti4+ sites is formed in a stepwise manner in which the O-H dissociation proceeds first and then followed by secondary C-H dissociation of 2-propanol while H atoms are transferred to the adjacent bridge bond oxygen (BBO) sites. Low temperature water is formed in a thermally driven process via H-atom on BBO in exchange with isopropyl groups of molecule 2-propanol, while isopropyl radical desorbs at high temperature during the TPD process. The observation demonstrates the prospect of TiO2 as a photocatalyst for degradation of organics

Acetone Formation from Photolysis of 2-Propanol on Anatase-TiO_2(101)

Photocatalysis of 2-propanol on A-TiO2(101) has been investigated using a temperature programed desorption method with 266 nm laser light. A clear mechanism is proposed for photodissociation of 2-propanol on A-TiO2(101). Acetone product on five coordinate Ti4+ sites is formed in a stepwise manner in which the O-H dissociation proceeds first and then followed by secondary C-H dissociation of 2-propanol while H atoms are transferred to the adjacent bridge bond oxygen (BBO) sites. Low temperature water is formed in a thermally driven process via H-atom on BBO in exchange with isopropyl groups of molecule 2-propanol, while isopropyl radical desorbs at high temperature during the TPD process. The observation demonstrates the prospect of TiO2 as a photocatalyst for degradation of organics

State-to-state dynamics of the H*(n) plus HD -> D*(n ') + H-2 reactive scattering

The state-to-state dynamics of the H*(n) + HD -> D*(n') + H-2 reactive scattering at the collision energy of 0.5 eV have been carried out for the first time by using H-atom Rydberg tagging time-of-flight technique. Experimental results show that the angular distribution of the total H-2 products presents clearly forward-backward asymmetric, which considerably differs from that of the corresponding H+ + HD -> D+ + H-2 reaction predicted by previously theoretical calculations. Such disagreement between these two processes suggests that the Fermi independent-collider model is also not valid in describing the dynamics of isotopic variants of the H* + H-2 reaction. The rotational state distribution of the H-2 products demonstrates a saw-toothed distribution with odd-j' > even-j'. This interesting observation is strongly influenced by nuclear spin statistics. (C) 2014 AIP Publishing LLC

Observation of Extremely High Vibrational Excitation in O-2 from Inelastic Scattering of Rydberg H Atom with O-2

The state-resolved differential cross sections for the Rydberg-atom (RA) inelastic scattering process H*(n = 46) + O-2(v = 0, j = 1,3) -> H*(n') + O-2(v', j') have been measured by using the H-atom Rydberg tagging time-of-flight (HRTOF) technique. Extensive vibrational excitation of O-2 products has been observed at the two collision energies of 0.64 and 1.55 eV. Experimental results show that the O-2 products in the low vibrationally excited states are clearly forward-scattered, whereas those in the highly vibrationally excited states are mainly backward-scattered. Partially resolved rotational structures were also observed and assigned. The striking observation of extremely high energy transfer from translational to vibrational excitation at the backward direction could be explained involving charge transfer between proton and O-2 molecule and possibly complex formation during the scattering process