Femtosecond time-resolved infrared

Diller R, Malt S, Walker G 0, Cowen B R, Pippenger R, Bogomoini R A and Hochstrasser R M 1995 Femtosecond time-resolved infrared laser study of the J-K transition of bacteriorhodopsin Chem. Rhys. Lett. 241 109-15... 

Deeper insight into this mechanism was afforded by femtosecond time-resolved infrared studies that enabled observation of intermediates and the calculation of relative energy barriers. Thus, upon UV irradiation 430 loses CO (<100fs) to afford a 16-electron monocarbonyl complex that is rapidly solvated (barrier-less process) to afford Tp Rh(CO)(RH), which vibrationally cools in 20 ps. Thermal mono-dechelation of the Tp ligand (zlG=4.2kcalmol ) proceeds... 

Picosecond to Femtosecond Time-Resolved Infrared Absorption Measurements... 

Many photochemical and photophysical phenomena occur on a time scale shorter than a nanosecond. In order to follow such fast phenomena by infrared spectroscopy, picosecond to femtosecond time-resolved infrared measurements are required. Since time resolving in this time range cannot be performed by utilizing the fast-response capability of a detector and the time-resolving power of an electronic circuit (gate circuit, etc.), the following optical methods are mainly used (i) a method based on the upconversion (optical gating) process, and (ii) a method which detects pulsed infrared radiation itself. At present, the latter method is commonly used for picosecond to femtosecond time-resolved measurements. 

Picosecond to femtosecond time-resolved infrared absorption measurements were initiated in the middle of the 1980s. In 1984, Heilweil et al. [17] studied the dynamics of vibrational relaxation by using picosecond infrared pulses obtained from an OPA (LiNb03) excited by a mode-locked Nd YAG laser. 

Figure 20.8 Explanatory schematic illustration of the femtosecond time-resolved infrared spectrometer reported in Ref [32].

The C=0 group of coumarin (98, R = CH3) is a potential hydrogen bond acceptor. By using subpicosecond time-resolved infrared absorption spectroscopy, following photoexcitation of the cumarin chromophore of 98, in its complex with aniline, the hydrogen bond dissociation rate is in the order of femtoseconds and the aniline reorients itself by reformation of the hydrogen bond with a new geometry166. 

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 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]. 

Since about the end of the 1990s, generation of ultrashort pulses has become easier due to the progress of laser technology, and, as a consequence, a measuring method based on femtosecond Ti sapphire regenerative amplifier with a kilohertz repetition rate has become the mainstream of fast time-resolved infrared absorption measurements. 

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. 

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. 

In practice, an electromagnetic pulse with an infinitely short width does not exist, but ultrashort laser pulses are now used for various spectroscopic measurements. Terahertz spectrometry described in Chapter 19 is based on femtosecond laser pulses. In Chapter 20, time-resolved infrared spectroscopic methods using picosecond to femtosecond laser pulses are described. Such ultrashort laser pulses have large spectral widths in the frequency domain. Let us discuss the relation between the pulse width in the time domain and its spectral width expressed in either frequency or wavenumber. 

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. 

While the first experiments of time-resolved IR spectroscopy were conducted with pulse durations exceeding 10 ps, the improved performance of laser systems now offers subpicosecond (12) to femtosecond (13-15) pulses in the infrared spectral region. In addition, the pump-probe techniques have been supplemented by applications of higher-order methods, e.g., IR photon echo observations (16). 

Figure 7. Spectral contributions of transient electronic configurations triggered by the femtosecond UV excitation of aqueous chloride ions. The relative spectral contributions are obtained from the computed analysis of time-resolved UV-IR femtosecond spectroscopic data. A first photophysical channel, including a non-adiabatic transition from a p-like excited hydrated electron state (e hydV to an s-like ground hydrated electron state. B spectral contributions of two well-defined transient fe Cl pairs. The presence of counterions (Na ) influences the dual behavior of these transient electronic configurations. C Direct identification of the spectral band assigned to near-infrared fe Cl pairs, made by using a cooled Optical Multichannel Analyzer (OMA 4) equipped with CCD detectors (1024 X...

The high costs associated with specialist ultrafast laser techniques can make their purchase prohibitive to many university research laboratories. However, centralised national and international research infrastructures hosting a variety of large scale sophisticated laser facilities are available to researchers. In Europe access to these facilities is currently obtained either via successful application to Laser Lab Europe (a European Union Research Initiative) [35] or directly to the research facility. Calls for proposals are launched at least annually and instrument time is allocated to the research on the basis of peer-reviewed evaluation of the proposal. Each facility hosts a variety of exotic techniques, enabling photoactive systems to be probed across a variety of timescales in different dimensions. For example, the STFC Central Laser Facility at the Rutherford Appleton Laboratory (UK) is home to optical tweezers, femtosecond pump-probe spectroscopy, time-resolved stimulated and resonance Raman spectroscopy, time-resolved linear and non-linear infrared transient spectroscopy, to name just a few techniques [36]. 

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