Time-resolved detection spectroscopy

Scherer N F, Carlson R J, Matro A, Du M, Ruggiero A J, Romero-Rochin V, Cina J A, Fleming G R and Rice S A 1991 Fluorescence-detected wave packet interferometry time resolved molecular spectroscopy with sequences of femtosecond phase-locked pulses J. Chem. Rhys. 95 1487... 

Especially with LTG GaAs, materials became available that were nearly ideal for time-resolved THz spectroscopy. Due to the low growth temperature and the slight As excess incorporated, clusters are fonned which act as recombination sites for the excited carriers, leading to lifetimes of <250 fs [45], With such recombination lifetunes, THz radiators such as dipole anteimae or log-periodic spirals placed onto optoelectronic substrates and pumped with ultrafast lasers can be used to generate sub-picosecond pulses with optical bandwidths of 2-4 THz. Moreover, coherent sub-picosecond detection is possible, which enables both... 

Miller R J and Closs G L 1981 Application of Fourier transform-NMR spectroscopy to submicrosecond time-resolved detection in laser flash photolysis experiments Rev. Sc/. Instrum. 52 1876-85... 

Well before the advent of modern analytical instruments, it was demonstrated by chemical techniques that shear-induced polymer degradation occurred by homoly-tic bond scission. The presence of free radicals was detected photometrically after chemical reaction with a strong UV-absorbing radical scavenger like DPPH, or by analysis of the stable products formed from subsequent reactions of the generated radicals. The apparition of time-resolved ESR spectroscopy in the 1950s permitted identification of the structure of the macroradicals and elucidation of the kinetics and mechanisms of its formation and decay [15]. 

Fujii, R. et al., Cis-to-trans isomerization of spheroidene in the triplet state as detected by time-resolved absorption spectroscopy, J. Phys. Chem. A, 106, 2410, 2002. Montenegro, M.A. et al., Model studies on the photosensitized isomerization of bixin, J. Agric. Food Chem., 52, 367, 2004. 

Detection of Transient Organometallic Species by Fast Time-Resolved IR Spectroscopy... 

Until recently, fast time-resolved IR spectroscopy has been a technique fraught with difficulty. Generally it has been easier to use low temperature techniques, particularly matrix isolation (2,4), to prolong the lifetime of the fragments so that conventional spectrometers can be used. In the last 5 years, however, there have been major advances in fast IR spectroscopy. It is now posssible to detect metal carbonyl intermediates at room temperature in both solution and gas phase reactions. In Section II of this article, we explain the principles of these new IR techniques and describe the apparatus involved in some detail. In Section III we give a self-contained summary of the organometallic chemistry that has already been unravelled by time-resolved IR spectroscopy. 

Poliakoff, M. Weitz, E. Detection of Transient Organometallic Species by Fast Time-Resolved IR Spectroscopy, in Stone, F.G.A. R. West, Eds Advances in Organometallic Chemistry, 1986, 25, 277. 

The use of short (fs) laser pulses allows even highly transient ion-radical pairs with lifetimes of t 10 12 s to be detected, and their subsequent (dark) decay to products is temporally monitored through the sequential spectral changes. As such, time-resolved (ps) spectroscopy provides the technique of choice for establishing the viability of the electron-transfer paradigm. This photochemical (ET) mechanism has been demonstrated for a variety of donor-acceptor interactions, as presented in the foregoing section. 

Time-resolved luminescence spectroscopy may be extremely effective in minerals, many of which contain a large amount of emission centers simultaneously. With the steady state technique only the mostly intensive centers are detected, while the weaker ones remain unnoticed. Fluorescence in minerals is observed over time range of nanoseconds to milliseconds (Table 1.3) and this property was used in our research. Thus our main improvement is laser-induced time-resolved spectroscopy in the wide spectral range from 270 to 1,500 nm, which enables us to reveal new luminescence centers in minerals previously hidden by more intensive centers. 

After a delay of several ps, the luminescence of Eu " is already very weak, and narrow long-lived lines of trivalent RE dominate in the spectrum. The lines at 589, 617, 651, and 695 nm (Fig. 4.1c) have never been detected in natural apatite by steady-state spectroscopy. According to their spectral position they may be ascribed to Eu ", but they are different from known lines in synthetic apatites activated by Eu (Jagannathan and Kottaosamy 1995 Morozov et al. 1970 Piriou et al. 1987 Piriou et al. 2001 Voronko et al. 1991). In order to clarify this problem we studied artificially activated samples by laser-induced time-resolved luminescence spectroscopy. 

Time-resolved luminescence spectroscopy of zircon revealed luminescence lines, which maybe confidentially ascribed to a Eu center (Fig. 4.38d). Usually they are hidden by a broad band yellow emission of zircon and may be detected only with a long delay time using its much longer decay time compared to yellow luminescence. 

Time-Resolved IR Spectroscopy. More recently, time-resolved IR (TRIR) experiments have been used to characterize species with lifetimes of micro-and even nanoseconds. Since IR spectra provide structural information in more detail than UV, this technique will be more powerful than TRUV-vis if one can find a carbene that can be detected and studied by this technique. To date, however, only one carbene has been studied by using TRIR. The matrix IR study shows that the planar triplet and twisted singlet states of 2-naphthyl(methoxycartbonyl) carbenes (NMC, 17) show distinctly different IR bands (see Section 3.1.4). Both NMC and NMC are detected by TRIR in solution and their kinetics have been studied. Such experiments provide clear cut data for the reaction kinetics as well as energetics of both states (see Sections 4.2 and 4.3... 

Direct detection of DPC is made by time-resolved EPR spectroscopy. In this method, DPC is first generated by photolysis of 30 in a hydrocarbon matrix at 16 K and is excited by a 465-nm laser, which corresponds to a T-T absorption of the To state of DPC. The transient triplet spectrum of the species having a decay rate of 160 ns at 16 K is assigned to the EPR spectrum of DPC. The ZFS parameters are determined by computer simulation to be D = 0.201 m and E = 0.0085 cm The D values observed by different methods are essentially identical. 

The putative benzyloxynitrene can be intercepted with tetramethylethylene to form the expected aziridine. Time-resolved IR spectroscopy was unable to detect the 0-nitrene, but detected the presence of PhCH2N=0 formed with a time constant of 250 ns after the laser pulse. Thus, the lifetime of benzyloxynitrene is also 250 ns. The TRIR spectroscopic studies indicated that benzyloxynitrene reacts with oxygen a rate constant of 10 M s. This value strongly suggests that the 6>-nitrene, in contrast to the Al-nitrenes has a triplet ground state. 

A promising recent development in the study of nitrenium ions has been the introduction of time-resolved vibrational spectroscopy for their characterization. These methods are based on pulsed laser photolysis. However, they employ either time resolved IR (TRIR) or time-resolved resonance Raman (TRRR) spectroscopy as the mode of detection. While these detection techniques are inherently less sensitive than UV-vis absorption, they provide more detailed and readily interpretable spectral information. In fact, it is possible to directly calculate these spectra using relatively fast and inexpensive DFT and MP2 methods. Thus, spectra derived from experiment can be used to validate (or falsify) various computational treatments of nitrenium ion stmctures and reactivity. In contrast, UV-vis spectra do not lend themselves to detailed structural analysis and, moreover, calculating these spectra from first principles is still expensive and highly approximate. 

Time-resolved Photoacoustic Spectroscopy. In photoacoustic spectroscopy (PAS) the heat evolved by the absorption of light in the sample is transformed into sound waves which are detected by a microphone. In steady-state spectroscopy the light is continuous, but it is also possible to use a pulsed laser and to observe the change in the intensity of the sound signal with time. In this respect time-resolved PAS is somewhat similar to thermal lensing, but both techniques have different limitations and advantages. 

The first subsurface bone tissue Raman spectroscopic measurements were performed using picosecond time-resolved Raman spectroscopy on excised equine cortical bone [56, 57], In these experiments it was shown that a polystyrene backing could be detected through 0.3 mm of bone. The same picosecond technology was used to perform the first transcutaneous Raman spectroscopic measurements of bone tissue [58]. In this study, the cortical bone mineral/matrix ratios of excised limbs of wild type and transgenic (oim/oim) mice were compared and the differences demonstrated. 

Decomposition of 2,6-difluorophenyl azide by LFP (266 nm) generated a singlet nitrene which was detected by time-resolved IR spectroscopy (1404 cm-1).75 The nitrene could only be detected between 243 and 283 K. At 298 K, the nitrene decay products, a ketenimine (1576 cm-1) and a triplet nitrene (1444 cm-1), were observed. The IR assignments were consistent with DFT calculations and previous UV-visible detection results. 

Most of the time-resolved emission spectroscopy setups are home made in the sense that they are built from individual devices (laser, detection system,. ..) hence they are not of a plug and press type, so that their exact characteristics may vary from one installation to the other. Some of these differences have no impact on the overall capabilities of the system but some have a drastic influence on the way the collected data are processed and analysed. This aspect will be detailed in the next section, while this section deals with a general description of the apparatus. The most basic type of apparatus will be described, with no reference to sophisticated techniques such as Time Correlated Single Photon Counting or Circularly Polarized Luminescence devices. 

In this review we have shown that time-resolved FTIR spectroscopy (with temperature and pressure held constant during the experiment) and temperature-time resolved FTIR spectroscopy (with both temperature and time changing simultaneously in a controlled way during the experiment) allow the detection of surface transient species in surface-catalyzed reactions. Several examples are reported, which can be classified in the following main categories ... 

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