Microsecond time-resolved infrared

Towrie, M., Gabrielsson, A., Matousek, R, Parker, A.W., Rodriguez, A.M.B. and Vlcek, Jr., A. (2005) A high-sensitivity femtosecond to microsecond time-resolved infrared vibrational spectrometer. Appl. Spectrosc., 59,467-473. 

Despite the considerable amount of information that has been garnered from more traditional methods of study it is clearly desirable to be able to generate, spectroscopically characterize and follow the reaction kinetics of coordinatively unsaturated species in real time. Since desired timescales for reaction will typically be in the microsecond to sub-microsecond range, a system with a rapid time response will be required. Transient absorption systems employing a visible or UV probe which meet this criterion have been developed and have provided valuable information for metal carbonyl systems [14,15,27]. However, since metal carbonyls are extremely photolabile and their UV-visible absorption spectra are not very structure sensitive, the preferred choice for a spectroscopic probe is time resolved infrared spectroscopy. Unfortunately, infrared detectors are enormously less sensitive and significantly slower... 

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. 

Microsecond to Nanosecond Time-Resolved Infrared Absorption Measurements... 

For microsecond to nanosecond time-resolved infrared absorption measurements, three types of spectroscopic methods have been developed (i) a method using an infrared laser, (ii) a method using a dispersive spectrometer, and (iii) a method using an FT-IR spectrometer. The time resolution of each of these is limited to the fastest time-response capability of the detector used. 

In 2005, Towrie et al. [30] developed another time-resolved infrared spectrometer capable of performing femtosecond to microsecond time-resolution measurements, by adding to their spectrometer described in Ref [29] a sub-nanosecond Q-switch Nd YV04 laser (wavelength 1064 nm, pulse width 0.6 ns). The pulses generated by this laser were electronically synchronized with the probe pulses with about 0.3 ns jitter, and the harmonics of pulses from this laser were used as the pump pulses. 

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.

Abstract This chapter describes recent breakthroughs in the instrumentation for far-ultraviolet (FUV) spectroscopy. The key technique is attenuated total reflection (ATR) that is frequently used in the infrared region. ATR technique decreases the absorbance of samples with strong absorptivity because of the penetration depth of the evanescent wave less than 100 nm. Therefore, ATR-FUV spectroscopy realizes the measurement of FUV spectra of samples in liquid and solid states. Some applications (in-line monitoring, characterization of polymers and time-resolved spectroscopy in sub-microsecond) are introduced in terms of instrumentation. This chapter explains not only the detail of the instruments but also the mathematical correction for ATR spectra to separate the absorption and refraction indices. 

Slow dissociation rates (10 -10 s ) have been measured in Dunbar s laboratory by time-resolved photodissociation, which consists of trapping ions in an ICR cell during a variable delay time after a phot-odissociating photon pulse. The technique called time-resolved photoionization mass spectrometry , developed by Lifshitz, consists of trapping photoions in a cylindrical trap at very low pressure to avoid bimolecular collisions, and then ejecting them into a mass filter after a variable delay covering the microsecond to millisecond range. When the dissociation rate constant becomes lower than ca. 10 s competition with infrared fluorescence takes place and limits the lifetime of the decomposition process. This has to be taken into account to extract the dissociation rate constant from the experimental data. 

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