85 Lifetime Spectroscopy

Spectroscopy, 490. See also 13C NMR spectroscopy FT Raman spectroscopy Fourier transform infrared (FTIR) spectrometry H NMR spectroscopy Infrared (IR) spectroscopy Nuclear magnetic resonance (NMR) spectroscopy Positron annihilation lifetime spectroscopy (PALS) Positron annihilation spectroscopy (PAS) Raman spectroscopy Small-angle x-ray spectroscopy (SAXS) Ultraviolet spectroscopy Wide-angle x-ray spectroscopy (WAXS)... 

Luminescence lifetime spectroscopy. In addition to the nanosecond lifetime measurements that are now rather routine, lifetime measurements on a femtosecond time scale are being attained with the intensity correlation method (124), which is an indirect technique for investigating the dynamics of excited states in the time frame of the laser pulse itself. The sample is excited with two laser pulse trains of equal amplitude and frequencies nl and n2 and the time-integrated luminescence at the difference frequency (nl - n2 ) is measured as a function of the relative pulse delay. Hochstrasser (125) has measured inertial motions of rotating molecules in condensed phases on time scales shorter than the collision time, allowing insight into relaxation processes following molecular collisions. 

Because of the underlying photophysics, fluorescence lifetimes are intrinsically short, usually on the order of a few nanoseconds. Detection systems with a high timing resolution are thus required to resolve and quantify the fluorescence decays. Developments in electronics and detector technology have resulted in sophisticated and easy to use equipment with a high time resolution. Fluorescence lifetime spectroscopy has become a popular tool in the past decades, and reliable commercial instrumentation is readily available. 

The output from the TAC is an analog signal that is proportional to the time difference between the start and stop pulses. The next step consists of digitizing the TAC output and storing the event in a multichannel analyzer (MCA). After repeating this process many times, a histogram of the arrival times of photons is accumulated in the memory of the MCA. In fluorescence lifetime spectroscopy the histogram usually contains 512-2048 channels... 

The influence of alkali cations on the structure of zeolite precursor gels investigated by positron lifetime spectroscopy... 

In this work positron annihilation lifetime spectroscopy (PALS) was used to investigate structural diversity inside zeolite precursor matrix caused by the presence of alkali cations Na, K, Rb and Cs. PALS is an established and well-proven method for structural investigations of various materials, extensively used for metals and alloys, semiconductors and porous materials [3, 4]. In the investigations of zeolites PALS has been mostly used for their void structure and size study [5, 6, 7, 8], also in correlation to... 

The operation and application of streak cameras in fluorescence lifetime spectroscopy has been reviewed previously (see, e.g., Refs. 91 and 92). Streak cameras are useful in 2-D time-resolved imaging applications such as microscopy or multiwavelength array fluorometry. The operating principle is based on converting an optical pulse into a photoelectron pulse and spatially dispersing the electron image on a phosphor by means of a synchronized deflection voltage across two plates. 

G. Hungerford, The application of spark source fluorescence lifetime spectroscopy to the study of infrared fluorescence, transient species and DCM, Ph.D. thesis. University of Strathclyde, Glasgow, Scotland (1991). 

The applicability of positron lifetime spectroscopy for the characterization of the partly charged nickel hydroxide was investigated [90]. The positron lifetime spectra of 8-Ni(OH)2/j8-NiOOH systems were presented. Three different parts of the annihilation curves were observed and identified. 

Positron annihilation lifetime spectroscopy (PALS) provides a method for studying changes in free volume and defect concentration in polymers and other materials [1,2]. A positron can either annihilate as a free positron with an electron in the material or capture an electron from the material and form a bound state, called a positronium atom. Pnra-positroniums (p-Ps), in which the spins of the positron and the electron are anti-parallel, have a mean lifetime of 0.125 ns. Ortho-positroniums (o-Ps), in which the spins of the two particles are parallel, have a mean lifteime of 142 ns in vacuum. In polymers find other condensed matter, the lifetime of o-Ps is shortened to 1-5 ns because of pick-off of the positron by electrons of antiparallel spin in the surrounding medium. 

In this chapter we consider the physics of the positronium atom and what is known, both theoretically and experimentally, of its interactions with other atomic and molecular species. The basic properties of positronium have been briefly mentioned in subsection 1.2.2 and will not be repeated here. Similarly, positronium production in the collisions of positrons with gases, and within and at the surface of solids, has been reviewed in section 1.5 and in Chapter 4. Some of the experimental methods, e.g. lifetime spectroscopy and angular correlation studies of the annihilation radiation, which are used to derive information on positronium interactions, have also been described previously. These will be of most relevance to the discussion in sections 7.3-7.5 on annihilation, slowing down and bound states. Techniques for the production of beams of positronium atoms were introduced in section 1.5. We describe here in more detail the method which has allowed measurements of positronium scattering cross sections to be made over a range of kinetic energies, typically from a few eV up to 100-200 eV, and the first such studies are summarized in section 7.6. 

Lozano-Castello D, Cazorla-Amoros D, Linares-Solano A, Hall PJ, and Fernandez JJ. Characterization of activated carbon fibers by positron annihilation lifetime spectroscopy (PALS). In Unger KK, et al., eds. Studies in Surface Science and Catalysis, Characterisation of Porous Solids V, vol. 128, the Netherlands Elsevier Science. 2000 523-532. 

Positron lifetime spectroscopy radiochemical (nuclear) methods in electrochemistry... 

The most important techniques are -> tracer methods (i), Mossbauer spectroscopy (ii), neutron activation (iii), thin layer activation (TLA) (iii), ultrathin layer activation (UTLA) (iii), and positron lifetime spectroscopy (iv). 

Independent of whether or not a well-defined crossover temperature can be observed in NS data above Tg, it has been well known for a considerable time that on heating a glass from low temperatures a strong decrease of the Debye-Waller factor, respectively Mossbauer-Lamb factor, is observed close to Tg [360,361], and more recent studies have confirmed this observation [147,148,233]. Thus, in addition to contributions from harmonic dynamics, an anomalously strong delocalization of the molecules sets in around Tg due to some very fast precursor of the a-process and increases the mean square displacement. Regarding the free volume as probed by positron annihilation lifetime spectroscopy (PALS), for example, qualitatively similar results were reported [362-364]. 

The main techniques used are positron annihilation lifetime spectroscopy (PALS) and the Doppler broadening (DB) or angular correlation (AC) techniques. The PALS parameters are the relative intensities (I j) and the... 

Shaefer, H.-E., Wurschum, R. Schwarz, R., Slobodin, D. et al. (1986) Amorphous hydrogenated silicon studied by positron lifetime spectroscopy , Appl. Phys. A 40, 145. 

Suzuki, R., Kobayashi, Y., Mikado, T., Ohgaki, H. et al. (1992) Investigation of near surface defects by variable-energy positron lifetime spectroscopy", Mater. Sci. Forum 105-110,1459. 

Suzuki, R., Ohdaira, T., Shioya, Y. and Ishimaru, T. (2001) Pore characteristics of low-dielectric-constant films grown by plasma-enhanced chemical vapor deposition studied by positron annihilation lifetime spectroscopy , Jpn. J. Appl. Phys. 40, L414. 

Petkov, M. P., Weber, M. H., Lynn K.G. and Rodbell, K. P. (2000) Probing capped and uncapped mesoporous low-dielectric constant films using positron annihilation lifetime spectroscopy , Appl. Phys. Lett. 77, 2470. 

The aim of this chapter is to introduce the reader to the application of positron annihilation techniques to polymers. An extensive review of the large volume of publications related to positron studies in polymers will not be presented. Rather it is intented to introduce the reader to the theory and techniques used in polymer studies and indicate the types of information that can be obtained about different polymer systems. The main focus of this chapter will be on the use of positron annihilation lifetime spectroscopy (PAL) in polymer studies. Chapter 11 discusses the use of monoenergetic slow positron beams used to study polymers surfaces. One of the interesting new developments in the application of positron annihilation techniques in polymers is the positron age-momentum correlation technique (AMOC). This technique promises to shed new light on the mechanisms of positronium formation and annihilation in polymer systems. A more detailed discussion of this technique can be found elswhere in this text. 

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