Xenon lamp lifetime

Fig. 6.13. Data obtained by the phase-modulation technique with a Fluorolog tau-3 instrument (Jobin Yvon-Spex) operating with a xenon lamp and a Pockel s cell. Note that because the fluorescence decay is a single exponential, a single appropriate modulation frequency suffices for the lifetime determination. The broad set of frequencies permits control of the proper tuning of the...

Lifetimes were measured by flashing the crystals with a 1-joule discharge from a xenon lamp. Data were collected at room temperature. The decays were detected with a photomultiplier tube and displayed on an oscilloscope. The traces were then photographed. 

By far the most common lamps used in AAS emit narrow-line spectra of the element of interest. They are the hollow-cathode lamp (HCL) and the electrodeless discharge lamp (EDL). The HCL is a bright and stable line emission source commercially available for most elements. However, for some volatile elements such as As, Hg and Se, where low emission intensity and short lamp lifetimes are commonplace, EDLs are used. Boosted HCLs aimed at increasing the output from the HCL are also commercially available. Emerging alternative sources, such as diode lasers [1] or the combination of a high-intensity source emitting a continuum (a xenon short-arc lamp) and a high-resolution spectrometer with a multichannel detector [2], are also of interest. 

H NMR spectra were recorded on a Bruker AC 400 spectrometer with tetramethylsilane (TMS) as internal reference. Infrared spectra were obtained as KBr pellets on a Perkin-Elmer 580 B FT-IR spectrometer. Electrospray (ES) mass spectra were recorded on an LCQ Finnigan Mat spectrometer. The excitation and emission spectra were obtained on a SPEX FL-2T2 spectrofluorimeter with slit at 0.8 mm and equipped with a 450 W lamp as the excitation source. Luminescence lifetimes were measured with a SPEX 1934D phosphorimeter using a 7 W xenon lamp as the excitation source with the pulse width at 3 ps. Powder X-ray diffraction patterns were recorded on Rigaku D/Max-IIB diffractometer using Cu-Ka radiation. 

Light sources pose a difficulty on an industrial scale. Lamps used in photochemistry include medium- and high-pressure mercury lamps, xenon lamps and halogen lamps, all of which are costly to run. These have a limited lifetime and additionally tend to generate a large amount of heat and therefore require additional cooling systems. 

The unit cell of a crystal of VO(acac)2 (acac = acetylacetonato, 511702 ) (Figure 5-26) expanded significantly when the single crystal at low temperature was irradiated with the xenon lamp. The VO(acac)2 complex was proposed to have a short-lived excited species due to a d-d transition under irradiation with visible light, [43] but the lifetime is too short to observe the emission. Because the anisotropy in expansion of the unit cell on photoirradiation clearly differed from the thermal one, it is possible to observe the excited structure of the VO(acac)2 complex in the equilibrium state even if the lifetime of the excited state is small. 

Trivalent neodymium absorbs strongly in broad bands in the green, red and near infra-red with reasonable coincidence in the output of a xenon flash lamp. Nearly all the electrons excited into the pump bands revert to the metastable upper level 4F3/2 through non-radiative decay which has a lifetime of 230 ps. Radiative transition then occurs to a set of lower energy levels 4I)5/2,4Ii3/2,4I 1/2,4l9/2- All these levels are multiplets and the lowest of 4l9/2 is... 

Europium cryptates are excited in the UV wavelength range either by a xenon flash lamp or by a nitrogen laser. Their fluorescence occurs in a wavelength range between 550 and 710 nm with typical narrow emission lines. Since the electronic transitions of the europium ion are forbidden by quantum mechanical rules, the cryptate fluorescence lifetime is exceptionally long, in the range of 100-1000 J,s. 

The basic principle is to observe the change in absorbance after an intense radiation pulse has created a significant population of short-lived reactive intermediates. Early experiments used xenon flash lamps as excitation sources and were able to detect intermediates with lifetimes >10 second. Modern experiments use pulsed lasers as excitation sources. The monochromatic output of a laser allows selective excitation the narrow pulse width allows detection of species with lifetimes as low as 10 second. A complementary experiment uses a pulse of electrons from a linear accelerator to generate the reactive species. More experimental detail is available in many reviews [139]. 

The use of a xenon flash lamp as the source enables the study of fluorescence after the source has been switched off. This method, used in immunology, in which fluorescing lanthanide complexes are obtained having a fluorescencing lifetime of around 1 ms, is sufficient for very sensitive measurements (in the picomolar concentration range). 

The principle of laser flash photolysis and its oxygen-sensing application is shown in Fig. 10-7 [69]. In laser flash photolysis techniques, a strong xenon or laser lamp is used to generate Ti states via So-to-Si absorption, and this is followed by intersystem crossing from Si to T], The concentrations of species in the triplet state are determined via the transient T]-to-T2 absorption, which is monitored by means of a second light source. This method is particularly useful when the lifetimes of the triplet state are short (in the range of nanoseconds). 

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