X-rays only when you want them: optimized pump-probe experiments using pseudo-single-bunch operation.

Laser pump-X-ray probe experiments require control over the X-ray pulse pattern and timing. Here, the first use of pseudo-single-bunch mode at the Advanced Light Source in picosecond time-resolved X-ray absorption experiments on solutions and solids is reported. In this mode the X-ray repetition rate is fully adjustable from single shot to 500 kHz, allowing it to be matched to typical laser excitation pulse rates. Suppressing undesired X-ray pulses considerably reduces detector noise and improves signal to noise in time-resolved experiments. In addition, dose-induced sample damage is considerably reduced, easing experimental setup and allowing the investigation of less robust samples. Single-shot X-ray exposures of a streak camera detector using a conventional non-gated charge-coupled device (CCD) camera are also demonstrated.

Pseudo-single-bunch with adjustable frequency: a new operation mode for synchrotron light sources.

We present the concept and results of pseudo-single-bunch (PSB) operation--a new operational mode at the advanced light source--that can greatly expand the capabilities of synchrotron light sources to carry out dynamics and time-of-flight experiments. In PSB operation, a single electron bunch is displaced transversely from the other electron bunches using a short-pulse, high-repetition-rate kicker magnet. Experiments that require light emitted only from a single bunch can stop the light emitted from the other bunches using a collimator. Other beam lines will only see a small reduction in flux due to the displaced bunch. As a result, PSB eliminates the need to schedule multibunch and timing experiments during different running periods. Furthermore, the time spacing of PSB pulses can be adjusted from milliseconds to microseconds with a novel "kick-and-cancel" scheme, which can significantly alleviate complications of using high-power choppers and substantially reduce the rate of sample damage.

Oxidation kinetics of an antiasthmatic, 2-[(4-hydroxyphenyl)amino]-5-methoxybenzenemethanol, and stabilization with ascorbic acid.

A novel antiasthmatic, 2-[(4-hydroxyphenyl)amino]-5-methoxybenzenemethanol (1), oxidizes to the corresponding iminoquinone in aqueous solutions. The reaction was monitored by a paired-ion reversed-phase HPLC method. The oxidation rate was highly dependent on the solution pH, with a large rate increase occurring above pH 6.1. In nonaqueous solution, the oxidation reaction was significantly slower. Ascorbic acid protects 1 from oxidation. The aqueous solution of the decomposed product is reduced to 1 in the presence of ascorbic acid.