Applications of Ultrafast Transient Infrared Spectroscopies

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Abstract
Broadband femtosecond and longer timescale mid-infrared and far-infrared (THz) probing techniques being explored at NIST using multi-element, mid-infrared focal plane arrays and solid-state THz generators/detectors are presented. Ultrafast infrared pulse generation, detection and pump-probe spectroscopy schemes are first reviewed in detail. These methods are used to examine transitory vibrational, chemical and electron transfer phenomena amenable to time-resolved infrared spectroscopy. Direct monitoring of electron transfer rates (> 5×1012 s) from dye sensitizers ([Ru(dceb)(bpy)2]+2 and Ru(dcb)2(NCS)2 derivatives) anchored to semiconductor solar-cell substrates (nanoparticle TiO2, ZrO2 and SnO2 films) is accomplished by measuring increased mid-IR absorption originating from C=O groups and injected electrons in the substrate. Coherent control of CO-stretching vibrational overtone population distributions in liquid-phase metal-hexacarbonyl systems using deliberately chirped mid-IR excitation pulses has also been explored. Identification of molecular steric and diffusional properties involved in internal ring-closing reaction dynamics of ultraviolet (UV) photo-dissociated (RCp)Mn(CO)3 systems in n-hexane will be presented. Terahertz spectroscopic methods and pulsed THz spectra of calf-thymus DNA, proteins and model dipeptides are also presented and their potential use for extracting bio-molecular functional dynamics discussed.