Time resolved infrared spectra

Our efforts to collect, characterize, model, and derive classifiers from time-resolved infrared spectra of conventional explosives was summarized in this document. Detailed... 

Fig. 8.28 S eries of time-resolved infrared spectra of sulfate adsorbed on the Au(lll)...

Fig. 15 Time-resolved infrared spectra of the reduction of heptyl viologen, HV +, at a silver electrode during a potential step from —0.2 to —0.55 V versus Ag/AgCl. The spectra were taken with a 100-ps acquisition time, but only the spectra of every 1-ms interval are shown, for clarity, see text for details. (From the work of Osawa and...

Figure 6.15 Time-resolved infrared spectra of the compounds trans-M2(T PB)2(02CCgH4-p-CN)2 (M = and W (b)) showing the C=N stretching vibration in the photoexcited state. (Taken from Ref. [76].)...

Figure 6.20 Time-resolved infrared spectra of the compounds Mo2(02CCgH4-p-C=X)4, here X = CH (a) or N (b) recorded in THF at room temperature. (Reprinted with permission from Ref. [81]. Copyright 2013 American Chemical Society.)...

Figure 20.1 Millisecond time-resolved infrared spectra measured during a reaction of CpCo(CO)2 with CINO [3], (a) Spectra at room temperature and (b) spectra at 10°C. Cp, cyclopentadienyl. (Source Reproduced with permission from [3], Copyright 2010, SAS.)...

In the middle of the 1990s, a new type of an ultrafast time-resolved measurement system was developed by Hamm et al. [24] femtosecond infrared pulses over a broadband were dispersed by a polychromator and detected by a multichannel infrared detector. They obtained broadband infrared pulses tunable over a wide frequency range (pulse width 400 fs, spectral bandwidth (FWHM) 65cm ), and divided these pulses into two one was used for probing the sample and the other for reference. The infrared pulses passing through the sample and the reference pulses were detected separately by two MCT array detectors each with 10 pixels. By this system, time-resolved infrared spectra of photoexcited molecules were measured [24]. 

The time-resolved infrared spectrum of [CpFe(CO)2]2 in cyclohexane solution 5 jlls after a UV flash shows peaks at 1938cm-1 and 1823cm-1, corresponding to (b) and (c), respectively. 

Fig. 16. Time-resolved infrared spectrum obtained after UV flash photolysis of f(r)r>-C5H5)Fe(CO)2]2, VIII (A), and MeCN in cyclohexane solution at 25°C. The bands are labeled thus B, (r)5-C6H5)Fe(CO)2, X C, (p -Cr,Hs)2Fe2(p-CO).i, IX and D (7f-C5Hs),Fe,-(CO)s(MeCN). The first three spectra correspond to the duration of the firing of the UV flash lamp, and subsequent spectra are shown at intervals of 10 /us. The negative peaks in the first spectrum are due to material destroyed by the flash these have been omitted from the subsequent traces to avoid undue confusion [reproduced with permission from Dixon, A. J. Healy, M. A. Poliakoff, M. Turner, J. J. J. Chem. Soc., Chern. Com-mun. 1986, 994],...

Figure 20.6 (a) Picosecond time-resolved infrared spectrum of RR-P3HT (regioregular poly (3-hexylthiophene)) at about lOOps delay time after photoexcitation [22] and (b) infrared difference spectrum of RR-P3HT doped with FeClj [23J. AAbsorbance, absorbance difference. (Source Reproduced with permission from [22] and [23], Copyright (2009) and (2003), Elsevier.)... 

A kinetic trace for a particular metal carbonyl intermediate is recorded at a specific wavelength obtained from the time-resolved infrared absorption spectrum. Suitable data analysis allows determination of the kinetics of the decay of the intermediate. 

Laser flash photolysis methods have also been applied to the study of nitrenium ion trapping rates and hfetimes. This method relies on short laser pulses to create a high transient concentration of the nitrenium ion, and fast detection technology to characterize its spectrum and lifetime The most frequently used detection method is fast UV-vis spectroscopy. This method has the advantage of high sensitivity, but provides very little specific information about the structure of the species being detected. More recently, time-resolved infrared (TRIR) and Raman spectroscopies have been used in conjunction with flash photolysis methods. These provide very detailed structural information, but suffer from lower detection sensitivity. 

A variety of spectroscopic methods has been used to determine the nature of the MLCT excited state in the /ac-XRe(CO)3L system. Time-resolved resonance Raman measurements of /ac-XRe(CO)3(bpy) (X = Cl or Br) have provided clear support for the Re -a- n (bpy) assignment of the lowest energy excited state [44], Intense excited-state Raman lines have been observed that are associated with the radical anion of bpy, and the amount of charge transferred from Re to bpy in the lowest energy excited state has been estimated to be 0.84 [45], Fast time-resolved infrared spectroscopy has been used to obtain the vibrational spectrum of the electronically excited states of/ac-ClRe(CO)3(bpy) and the closely related/ac-XRe(CO)3 (4,4 -bpy)2 (X = Cl or Br) complexes. In each... 

These conclusions were supported by transient absorption spectroscopy, which revealed signals corresponding to the formation of the diimine radical anion, with lifetimes in close agreement with the luminescence lifetimes. Time-resolved infrared spectroscopy of the acetylide C = C bonds provides further conclusive evidence for the MLCT assignment. Thus, in the ground state IR spectrum of 4, there are two v(C=C) bands at 2115 and 2124 cm-1, whilst the step-scan FTIR difference spectrum obtained 50 ns after irradiation at 355 nm reveals bleaching of the parent bands, and the formation... 

Fig. 15. Schematic representation of the time-resolved infrared (TRIR) flash photolysis apparatus used at Nottingham. The UV pulse laser generates transient species the continuous IR laser monitors the change in transmission at a particular IR frequency, producing a trace showing the IR absorbance as a function of time. The experiment is repeated at different IR frequencies so that a complete IR spectrum of the transient can be built up [reproduced with permission from (97), p. 103],...

Flash-photolysis studies of the hexacarbonyls Cr(CO)g, Mo(CO)g and W(CO)g are, like previous years, still abundant Nayak and Burkey have found that there are low quantum yields for Cr(CO)g substitution in fluorocarbon solvents - which provides yet more evidence that metal-fluorocarbon interactions are very weak There have been estimates made of solvent-metal bond strengths in (Solvent)M(CO)5 complexes (Solvent = Benzene [M = Mo, W] and Tetrachloromethane [M = Cr]) and the photolysis of W(CO)g in the presence of hex-l-ene has been reported Flash-photolysis studies of the photochemical reactions of silanes with Cr(CO)g have been published Using time-resolved infrared spectroscopy RIR), Turner and co-workers have captured the IR spectrum of the MLCT excited state of W(CO)5(4-cyanopyridine) which rapidly decays to W(CO)5. The excited state of W(CO)5(4-cyanopyridine) is relatively long lived, which makes the experiment possible. This reporter will be interested to see how this technique develops. 

Other integral methods include in situ spectroscopic techniques, such as time-resolved infrared spectroscopy, which measures concentration or conversion as well as provides information on chemical identities of components in the reactor. In contrast to the chemical sampling method, the infi ared (IR) spectrum of the reacting system may be collected at a much faster pace than that normally possible for the chemical sampling method. Consequently, rate data may be derived with good accuracy by differentiating the IR intensity data with respect to time. 

In this chapter, millisecond time-resolved infrared measurements are first described in Section 20.2 for this time scale, time resolution is set by the time needed to measure (scan) a spectrum. Then, microsecond to nanosecond time-resolved measurements, which are limited by the detector response time are described in Section 20.3, and finally, picosecond to femtosecond time-resolved measurements, the time resolution for which is determined by the width of the laser pulse used for the measurement, are described in Section 20.4. 

In ultrafast, time-resolved infrared absorption measurements by the pump-probe method, the sample is first excited by an ultrashort pump pulse, and then irradiated by an ultrashort infrared pulse (probe pulse) after a certain delay time from the excitation by the pump pulse. The delay time of the probe pulse from the pump pulse is usually changed by the difference in the optical path lengths of the pump and probe pulses (a delay time of 1 ps arises from a path difference of about 0.3 mm). When the infrared spectrum of a molecule in an excited electronic state is measured, pulses in the ultraviolet to visible region are used for the pump purpose, and pulses in the infrared region are used for the probe purpose. When a vibrationally excited molecule is the target of such a measurement, pulses in the infrared region are used for both the pump and probe purposes. The transient (or time-resolved) infrared absorption spectra by this method are usually measured as the difference in absorption intensities for the probe pulses between the measurements with the pump pulses and those without the pump pulses. 

Material response in THz frequency region, which corresponds to far- and mid-infrared electromagnetic spectrum, carries important information for the understanding of both electronic and phononic properties of condensed matter. Time-resolved THz spectroscopy has been applied extensively to investigate the sub-picosecond electron-hole dynamics and the coherent lattice dynamics simultaneously. In a typical experimental setup shown in Fig. 3.5, an... 

Terahertz spectroscopy uses continuous wave (CW) and short pulsed laser excitation in the spectrum region between infrared and microwave frequencies. Pulsed laser excitation using pulse widths in the range of 10-100 femtoseconds has enabled the use of time-resolved terahertz spectroscopy, which is capable of capturing dynamic information at subpicosecond time scales. 

Figure 1.12. Comparison of the transient infrared spectra on the microsecond time scale of hypericin, O-hexamethoxy hypericin, and a hypericin analog that lacks carbonyl groups (the hexaacetoxy analog). The salient feature of the data is that the latter two compounds, which cannot execute excited state H-atom transfer owing to the absence of either labile protons or appropriate carbonyl groups, lack the feature at 1450 cm-1. Ab initio calculations at the Hartree-Fock 3-21G level for the normal and two monotautomeric forms of the hypericin triplets indicate normal modes with substantial character in the region 1400-1460 cm-1 [77]. While these preliminary results do not demonstrate a time-resolved H-atom transfer, they do clearly point to a region of the spectrum that must be investigated in further studies. Hypericin and hexamethoxy hypericin, solid line, 0-1 p,s and dashed line, 14-18 ps reduced analog, solid line, 0-0.5 ps, and dashed line, 7-9 ps.

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