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

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

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

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

A kinetic study of the CH2OO Criegee intermediate reaction with SO2, (H2O)(2), CH2I2 and I atoms using OH laser induced fluorescence

The OH laser induced fluorescence method was used to study the kinetics of CH2OO reacting with SO2, (H2O)(2), CH2I2 and I atoms. Decay of CH2OO is not strictly first-order since its self-reaction is rapid. With this consideration, we derived the rate coefficient of CH2OO + SO2/(H2O)(2)/CH2I2/I taking into account the contribution of the CH2OO self-reaction. For the CH2OO + SO2 reaction, the rate coefficient is measured to be (3.88 +/- 0.13) x 10(11) cm(3) molecule (1) s (1) at 10 Torr, which agrees very well with a previously reported value obtained by directly monitoring CH2OO using the UV absorption method with the CH2OO self-reaction considered. We did not observe obvious evidence for SO2 catalysed CH2OO isomerization or the intersystem crossing effect in this reaction. CH2OO + (H2O)(2) is supposed to account for the major sink of CH2OO in the atmosphere, but previous rate coefficient measurements were not in good agreement. We have revisited this reaction including the self-reaction of CH2OO and obtained the rate coefficient to be (7.53 +/- 0.38) x 10(12) cm(3) molecule(-1) s(-1) at 60 Torr and 300 K. The rate coefficients of CH2OO + CH2I2 and CH2OO + I were measured to be (5.2 +/- 2.6) x 10(-14) and (2.2 +/- 1.1) x 10 (-12) cm(3) molecule(-1) s(-1) respectively