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High-intensity phasing with x-ray free-electron lasers
Sang-Kil SonCCP4 Study Weekend
(Nottingham, UK, January 7-9, 2015) [invited talk]
X-ray free-electron lasers (XFELs) show promise for revealing molecular structure using serial femtosecond crystallography (SFX), but the associated phase problem remains largely unsolved. Many of ab initio phasing methods with synchrotron radiation employ anomalous scattering from heavy atoms, for example, multiwavelength anomalous diffraction (MAD). Because of the extremely high intensity of XFELs, samples experience severe and unavoidable electronic radiation damage, especially to heavy atoms. The scattering factors of heavy atoms are dramatically changed due to ionization during an intense x-ray pulse, which hinders direct implementation of those phasing techniques with XFELs. In this talk, I will discuss how to overcome this challenge and propose new phasing methods with XFELs, taking advantage of electronic radiation damage at high x-ray intensity. I will start with a theoretical model of how heavy atoms are ionized during an XFEL pulse, comparing with a series of LCLS and SACLA experiments. It will be demonstrated that the scattering factors are reformulated including electronic damage dynamics and that a Karle-Hendrickson-type equation in the high-intensity regime exists in spite of the high degree of ionization. Then I will present new phasing methods such as high-intensity MAD and high-intensity RIP. New opportunities and challenges of high-intensity phasing methods with XFELs will be discussed.
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