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Vibrational Cooling in a Cold Ion Trap: Low-Temperature Electrospray Ionization Photoelectron Spectroscopy

presented by

Hin-Koon Woo
Washington State University

February 15, 2007

W.M Keck Foundation Ampitheater


Background:

     Hin-Koon Woo received his MPhil. in Materials Science from the City University of Hong Kong and his Ph.D. in Physical Chemistry from the University of California, Davis. His Ph.D. dissertation is two-color photoionization photoelectron spectroscopy using tunable vacuum ultraviolet and infrared lasers. After getting his Ph.D. in 2004, he moved to Washington State University and Pacific Northwest National Laboratory for his postdoctoral research, where he was involved in the construction of a low-temperature electrospray ionization photoelectron apparatus. His current research is focused on photodetachment photoelectron spectroscopy of anions.

Abstract:

     The construction of a newly developed electrospray ionization (ESI) photoelectron spectrometer and some results are presented. The machine consists of an ESI source, a quadrupole ion trap, a linear time-of-flight mass spectrometer, and a magnetic-bottle photoelectron spectrometer. The key feature of this apparatus is that the ion trap where ions are accumulated and cooled, is attached to a closed-cycle helium refrigerator cold head. The temperature of the cold head can be controlled from 350 to 20 K. Two particular experiments will be discussed: (1) Linear carboxylates, CH3(CH2)nCO2-, assume folded structures at low temperatures due to the formation of weak C-H?O hydrogen bonding between the terminal CH3 and ?CO2- groups for n ≥ 5. The strength of the C-H?O hydrogen-bonds were experimentally measured, ranging from 1.2 kcal/mol for n = 5 to 4.4 kcal/mol for n = 8. (2) Cysteine anion prefers to be in the thiolate form [-SCH2CH(NH2)CO2H], rather than the expected carboxylate form [HSCH2CH(NH2)CO2-] in the gas phase. An intramolecular hydrogen bond was found to exist between the carboxylic acid ?CO2H and the thiolate ?S-. The intramolecular O-H?S- hydrogen bond strength in cysteine thiolate anion was estimated.

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