Time-Resolved Infrared Fluorescence Detection

The energy transfer described by (13.16) can be monitored if M is excited by a short infrared laser pulse and the fluorescence of AB is detected by a fast cooled infrared detector (Sect. 4.5) with sufficient time resolution. Such measurements have been performed in many laboratories [13.6]. For illustration, an experiment carried out by Green and Hancock [13.77] is explained by Fig. 13.16a a pulsed HF laser excites hydrogen fluoride molecules into the vibrational level u = 1. Collisions with other molecules AB (AB = CO, N2) transfer the energy to excited vibrational levels of AB. The infrared fluorescence emitted by AB and HF has to be separated by spectral filters. If two detectors are used, the decrease of the density A(HF ) of vibrationally excited HF molecules and the build-up and decay of A(AB ) can be monitored simultaneously. 

For larger molecules M two different collisional relaxation processes have to be distinguished collisions M 4- AB may transfer the internal energy of M to AB (mr rmolecular energy transfer), or may redistribute the energy among the different vibrational modes of M (m ramolecular transfer) 

The energy transfer described by (8.16) can be monitored if M is excited by a short infrared laser pulse and the fluorescence of AB is detected by a fast cooled infrared detector (Vol. 1, Sect. 4.5) with sufficient time resolution. Such measurements have been performed in many laboratories [963]. For illustration, an experiment carried 

2 Time-Resolved Absorption and Double-Resonance Methods 

While collision-induced transitions in excited electronic states can be monitored through the satellite lines in the fluorescence spectrum (Sect. 8.2.2), inelastic collisional transfer in electronic ground states of molecules can be studied by changes in the absorption spectrum. This technique is particularly advantageous if the radiative lifetimes of the investigated rotational-vibrational levels are so long that fluorescence detection fails because of intensity problems. 

A successful technique for studying collision-induced transitions in electronic ground states is based on time-resolved double resonance [1040]. The method is explained by Fig. 8.17. A pulsed laser Li tuned to an infrared or optical transition 

The time-resolved measurement of /(Az, t) yields the time dependence of the population density Using a cw probe laser the absorption can be measured by 

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Sakai, M., Kawashima, Y., Takeda, A., Ohmori, T. and Fujii, M. (2007) Far-field infrared super-resolution microscopy using picosecond time-resolved transient fluorescence detected IR spectroscopy. Chem. Phys. Lett., 439, 171—176. 

FIGURE 1. Experimental set-up for detection of time and spectral resolved infrared fluorescence signals from water molecules fomied in Reaction (1). 

Zhu L, Stryjweski WJ, Soper SA (2004) Multiplexed fluorescence detection with micro-fabricated devices with both time-resolved and spectral-discrimination capabilities using near-infrared fluorescence. Anal Biochem 330 206-218... 

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

Rate constants for reaction of the CH radical with a number of atomic and molecular collision partners have been reported, with multiple-photon dissociation of suitable precursor molecules using either infrared or ultraviolet " laser radiation used as the pulsed photolysis source, and laser-induced fluorescence near 431 nm employed as a sensitive time-resolved detection method. A similar technique has been used to measure removal rates of CH2 and CDj with... 

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