Spectroscopy excitation

In the following, we describe such techniques, which are, besides time-resolved spectroscopy, modulation excitation spectroscopy (MES) (14,48,91) and singlebeam signal reference (SBSR) spectroscopy (14). 

Data acquisition is synchronized with the stimulation. The response (absorbance) can be retarded with respect to the stimulation (characterized by a phase lag 0) (AS). 

Mathematically, a phase-sensitive detection (PSD) is carried out by multiplying A(v, f) by a sine function of the same frequency as the stimulation or harmonics thereof, sin(/rfflt + followed by a normalized integration of the product over 

Concentration modulation experiments have been reported for applications to heterogeneous catalysis (48). The experimental implementation was accomplished by periodically flowing solutions with different (reactant) concentrations over the catalyst immobilized on the IRE. Fast concentration modulation in the liquid phase is limited by mass transport (diffusion and convection), and an appropriately designed cell is essential. The cell depicted in Fig. 12 has two tubes ending at the same inlet (65). This has the advantage that backmixing in the tubing upstream of the cell can be avoided. With this cell, concentration modulation periods of about 10 s were achieved (45,65). 

The high sensitivity of lock-in amplification can be applied to detect the small periodic changes in transmittance caused by modulated e.xcitation with UV light. Measurements of the amplitude and phase shift of the response signal allow us to determine the spectra and lifetimes of the transient species. 

One of the first applications of this chopped-beam irradiation techni iue to the measurement of triplet spectra was reported by Labhart From a knowledge of the intensity of the irradiation light, he determined the quantum yield of triplet generation to be 0.55 0.11 for outgassed solutions of 1,2-benzanthrazene in hexane at room temperature. Hunziker 32) has applied this method to the study of the gas-phase absorption spectrum of triplet naphthalene. A gas mixture of 500 torr Na, 0.3 mtorr Hg, and about 10 mtorr naphthalene was irradiated by a modulated low-pressure mercury lamp. The mercury vapor in the cell efficiently absorbed the line spectrum of the lamp and acted as a photosensitizer. The triplet state of naphthalene was formed directly through collisional deactivation of the excited mercury atoms. 

A very high sensitivity in the visible and ultraviolet regions can be achieved if the absorption of laser photons is monitored through the laser-excited fluorescence. When the laser wavelength is tuned to an absorbing molecular transition E. - the number of photons absorbed per s and pathlength Ax is 

The number of fluorescence photons emitted per second from the excited lev- 

7) to all levels with E Ej. The quantum efficiency nj = gives the ratio of the spontaneous transition rate to the total deactivation rate which may also include a radiationless transition rate (e.g., collision-induced transitions). For n = 1 the number n of fluorescence photons 

Unfortunately only a fraction 6 of the fluorescence photons, emitted into all directions, can be collected on the photomultiplier cathode where again only the fraction = p /np of these photons produces on the average np photoelectrons. The quantity npj is called the quantum effioienoy of the photocathode (see Sect.4.5.2). The number np of photoelectrons counted per second is then 

Modern photomultipliers reach quantum efficiencies of np = 0.2. With carefully designed optics it is possible to achieve a collection factor 6 = 0.1. Using photon counting techniques and cooled multipliers (dark pulse rate 4IO counts/s), counting rates of np = 100 counts/s are already sufficient to obtain a signal-to-noise ratio S/R 8 at integration times of Is. 

---

A dispersive element for spectral analysis of PL. This may be as simple as a filter, but it is usually a scanning grating monochromator. For excitation spectroscopy or in the presence of much scattered light, a double or triple monochromator (as used in Raman scattering) may be required. 

Band gaps in semiconductors can be investigated by other optical methods, such as photoluminescence, cathodoluminescence, photoluminescence excitation spectroscopy, absorption, spectral ellipsometry, photocurrent spectroscopy, and resonant Raman spectroscopy. Photoluminescence and cathodoluminescence involve an emission process and hence can be used to evaluate only features near the fundamental band gap. The other methods are related to the absorption process or its derivative (resonant Raman scattering). Most of these methods require cryogenic temperatures. 

Several characteristics of the metal beam have been studied in detail. It is well known that metal clusters and metal oxides are formed as a result of the ablation process. However, these potentially interfering species have been studied in detail130 and it has been concluded that they do not introduce any doubt as to the validity of the experimental results. Much more important than cluster or oxide formation are the atomic electronic state populations of the metal beams. For each metal reactant, these have been characterized using laser-induced fluorescence (LIF) excitation spectroscopy. For Y, only the two spin-orbit states of the ground electronic state (a Dz/2 and a D-3,/2) were observed.123... 

A number of experimental techniques are available for the determination of triplet energy levels. Those most commonly employed are phosphorescence spectroscopy, phosphorescence excitation spectroscopy, singlet to triplet... 

Fluorescence energy transfer experiments, in which the energy transfer from the excited DNA bases to a fluorescent ligand is monitored by fluorescence excitation spectroscopy, has been used to analyze the binding of the bisquinolizinium species 35 to DNA <2004ARK219>. 

NHE OCP ONO OPS PCD PDS PL PLE PMMA PP PP PS PSG PSL PTFE PVC PVDF normal hydrogen electrode (= SHE) open circuit potential oxide-nitride-oxide dielectric oxidized porous silicon photoconductive decay photothermal displacement spectroscopy photoluminescence photoluminescence excitation spectroscopy polymethyl methacrylate passivation potential polypropylene porous silicon phosphosilicate glass porous silicon layer polytetrafluoroethylene polyvinyl chloride polyvinylidene fluoride... 

A. P. Demchenko, Red-edge-excitation spectroscopy of single-tryptophan proteins, Eur. Biophys. J. 16, 121-129 (1988). 

In the preceding, we have assumed that the molecules are all oriented at a fixed angle 0 relative to the surface normal. Thompson efa/.(10) have utilized a distribution function in angle in place of our more restricted assumption. Inasmuch as our major interest is in showing the manner in which the particle resonances affect the excitation spectroscopy, we will continue to use the more restrictive assumption. 

Fluorescence excitation spectroscopy is thus a powerful technique for obtaining molecular information about systems of cellular size. At present, the technique is restricted to single small objects because of the requirement of angular integration of the emitted fluorescence. As work progresses, similiar information will be obtainable from spectra taken at a particular angle with respect to the exciting beam. This will allow extension of the photoselection concept to suspensions of particles and perhaps to individual cells. 

The very weak Tm - So transitions are hard to observe directly by absorption spectroscopy. Even with long cells, the high concentrations required present solubility — and what is more important — purity problems. An impurity of 1 10 may give rise to absorption bands which have the same intensity as the expected Ti So absorption. The experimental conditions, therefore, have to be chosen to allow an increase of the Ti- - So oscillator strength to be achieved through perturbation by paramagnetic species (O2 or NO) or heavy atoms. Alternatively, an indirect method, phosphorescence excitation spectroscopy, which has high sensitivity and selectivity, may be applied. 

Two orientations have been chosen (Fig. 17) for polarized excitation spectroscopy. The first is used to distinguish between in-plane and out-of-plane effects, while the second one allows a comparison of the two in-plane directions. 

Phosphorescence excitation spectroscopy also allows us to observe the transitions starting at 389 nm to the second triplet state, which is of (n,n ) nature. Direct spin-orbit coupling (mechanism I) to a Sn n,n ) state introduces strong in-plane, long-axis polarization. Indeed, in-plane polarization is preferred over out-of-plane polarization by 3 1, and long-axis polarization is about four times stronger than the short-axis contribution. 

Hirota used doped crystals to observe weak Ti-<- So absorption spectra by phosphorescence excitation spectroscopy. Triplet excitons of the host are formed by direct light absorption. The guest molecules, chosen to have lower triplet energy, act as traps and emit guest phosphorescence. 

Jones, C. R., Kearns, D. R., Wing, R. M. Investigation of singlet-triplet transitions by phosphorescence excitation spectroscopy. X. A simple i, i -unsaturated ketone. J. Chem. Phys. 58, 1370 (1973). 

Physico-chemical properties. In the fifties and sixties, several studies on the conformation of ACTH in solution were carried out. Among the used techniques were ORD, CD, fluorescence depolarization studies and kinetics of deuterium hydrogen exchange (for a review see ref. 2). The results pointed to a highly flexible random coil in solution however, Eisinger (40) found that the distance between Tyr and Trp [in ACTH-(1-24)] as measured by excitation spectroscopy, was in better agreement with some form of loop or helical structure. In addition. Squire and Bewley noted 11-15% helical content, located in the N-terminal 1-11 part of the molecule, when measuring the ORD of ACTH at pH 8.1 (41) (a random coil was found at neutral and acidic pH values, 2). 

---