Photons, UV spectroscopy

As an example of the use of two-photon spectroscopy in assigning excited states that are not observed in one-photon UV spectroscopy, the two-photon absorption spectrum of naphthalene is shown in Figure 1.17. Since for the point group all Bg states have a theoretical polarization degree of H = 3/2, the polarization measurement reveals immediately a Bg state near 42,000 cm . In the two-photon absorption spectrum this shows up only as a shoulder, whereas the maximum at 44,500 cm" can be assigned to an Ag state. Neither state is prominent in the one-photon spectrum. 

This chapter deals mainly with (multi)hyphenated techniques comprising wet sample preparation steps (e.g. SFE, SPE) and/or separation techniques (GC, SFC, HPLC, SEC, TLC, CE). Other hyphenated techniques involve thermal-spectroscopic and gas or heat extraction methods (TG, TD, HS, Py, LD, etc.). Also, spectroscopic couplings (e.g. LIBS-LIF) are of interest. Hyphenation of UV spectroscopy and mass spectrometry forms the family of laser mass-spectrometric (LAMS) methods, such as REMPI-ToFMS and MALDI-ToFMS. In REMPI-ToFMS the connecting element between UV spectroscopy and mass spectrometry is laser-induced REMPI ionisation. An intermediate state of the molecule of interest is selectively excited by absorption of a laser photon (the wavelength of a tuneable laser is set in resonance with the transition). The excited molecules are subsequently ionised by absorption of an additional laser photon. Therefore the ionisation selectivity is introduced by the resonance absorption of the first photon, i.e. by UV spectroscopy. However, conventional UV spectra of polyatomic molecules exhibit relatively broad and continuous spectral features, allowing only a medium selectivity. Supersonic jet cooling of the sample molecules (to 5-50 K) reduces the line width of their... 

Mons M, Piuzzi F, Dimicoli I, Gorb L, Lesczynski J (2006) Near-UV resonant two-photon ionization spectroscopy of gas phase guanine evidence for the observation of three rare tautomers. J Phys Chem A 110 10921-10924... 

Most of these types of spectroscopy are of use in chemistry. The levels probed by low-energy photons are sensitive to the detailed structure of molecules, and can be used to help identify compounds that have been newly synthesized. Visible and UV spectroscopy is important in the study of chemical bonding. X-ray spectra are characteristic of particular atoms, and are important in some methods of chemical analysis. The application of the quantum theory to the appropriate types of energy levels is essential in all these applications. A few relevant examples will be given in later chapters. 

Characterisation of vesicles was achieved using a combination of methods, including photon-correlation spectroscopy, video-enhanced and cryo-electron microscopy. Measurements of the cmc of the surfactants (in the absence of salt) were made using uv-visible spectrophotometry and electrical conductivity (k). For cmc measurements, there is a convenient change in the extinction coefficient of the benzene chromophore at 262 nm. The onset of vesicle formation, and hence the determination of the esc, can be measured by 90° scattering and 180° optical turbidity measurements at 300 nm. 

The measurement and control of UV/VIS radiation and/or the quantification of the photon flow (Dp of an industrial lamp is of vital importance to AOP developments. It can also be aciiieved by using radiometers or UV sensors (McCluney, 1994). Moreover, it is essential to determine the UV/VIS absorbance spectrum of the water or air to be treated by UV spectroscopy prior to further research and development efforts. 

The samples were studied by room temperature UV-spectroscopy and PL. UV-VIS absorption spectra of films transferred onto quartz substrates were measured using a Shimatzu spectrophotometer. PL was excited using a pulsed N2 laser (337 nm). The PL spectra of LB films on silicon substrates were registered using a monochromator equipped with photomultiplier operated in a photon counting mode. 

Abstract The linear and nonlinear optical properties of polysilanes, and in particular poly(di-n-hexylsilane), have been investigate by vacuum UV spectroscopy and by two-photon induced processes. The electronic structure of the polymer inferred from these measurements is consistent with models which view the polymer as a one-dimensional semiconductor quantum wire. The exciton related optical nonlinearities are also large enough to allow polysUanes to be considered for various nonlinear switching applications. 

Fig. 2 Schematic representation of (a) a one-colour two-photon REMPI schane (b) the modus operand of IR-UV ion-dip spectroscopy (c) IR probing scheme for the excited state using UV— IR-UV spectroscopy (d) a general scheme for stimulated emission pumping for conformer A and B...

As it is common in the Raman scattering process to observe Raman band intensities of ca. 10 of the incident photons (UV, VIS, NIR) provided by a monochromatic laser source, Raman spectroscopy is an inherently insensitive analytical method that usually requires molecular concentrations of >0.01 M. Raman spectroscopy probably represents the single largest application of laser spectroscopy in industrial analysis and is being used in industry only as from the 1980s for the analysis of a wide range of materials, mainly solids. Raman spectroscopy is... 

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