Tunable radiation sources

Most fundamental rotation-vibration bands are located in the mid-infrared region from 4000 - 400 cm". A few vibrational bands appear in the far infrared where purely rotational spectra of light molecules with two or three atoms are also observed. This is in contrast to heavier polyatomic molecules the study of their rotational spectra is the domain of the microwave spectroscopist who employs different equipment, particularly, monochromatic tunable radiation sources. Rotational constants determined from IR-work are therefore usually less accurate than those obtained by microwave spectroscopy. 

Work on EXAFS then progressed very little until the advent of the synchrotron radiation source (storage ring), described in Section 8.1.1.1. This type of source produces X-ray radiation of the order of 10 to 10 times as intense as that of a conventional source and is continuously tunable. These properties led to the establishment of EXAFS as an important structural tool for solid materials. 

The discrete line sources described above for XPS are perfectly adequate for most applications, but some types of analysis require that the source be tunable (i.e. that the exciting energy be variable). The reason is to enable the photoionization cross-section of the core levels of a particular element or group of elements to be varied, which is particularly useful when dealing with multielement semiconductors. Tunable radiation can be obtained from a synchrotron. 

One of the most exciting developments in modem X-ray spectroscopy is the now widespread availability of synchrotron radiation sources. By virtue of its much higher intensity and the tunability of its wavelength over a broad range, synchrotron radiation permits more sophisticated experiments to be performed [43]. 

A useful source of continuously tunable radiation from the near UV to the near-IR with unexplored potential in fluorescence studies is the optical parametric oscillator (OPO). These devices have been around since the 1960s(73) and have received a lot of coverage recently in laser and optoelectronic journals/74 This resurgence of interest in OPOs has been brought about by recent improvements in nonlinear crystals and the development of all-solid-state pump-laser sources with the required levels of coherence and intensity. 

Instrumental methods have become more sophisticated to face these challenges. In particular, Westmoreland and Cool have developed a flame-sampling mass spectrometer that has provided several revelations in terms of relevant molecular intermediates in combustion. " Their setup couples a laminar flat-flame burner to a mass spectrometer. This burner can be moved along the axis of the molecular beam to obtain spatial and temporal profiles of common flame intermediates. By using a highly tunable synchrotron radiation source, isomeric information on selected mass peaks can be obtained. This experiment represents a huge step forward in the utility of MS in combustion studies lack of isomer characterization had previously prevented a full accounting of the reaction species and pathways. 

Tunable lasers allow selection of the emission wavelength within a definite wavelength range. They function both as radiation sources and as monochromators. 

The sensitivity 7 is the derivative of the signal by the concentration (Eqs. 3.3-38, 45, 48). It should be as high as possible in order to reduce the detection limit and to enhance the precision. Ideally, a sample should give rise to a narrow line with a high peak intensity, and the spectrometer should only .see the. spectral band at which 7 is at its maximum. This is possible with spectrometers which use tunable lasers as radiation sources for absorption spectroscopy or by using spectrometers with high resolution, e.g., for trace analyses of atmo.spheric gases. 

A suprananosecond kinetic spectrometer is schematically shown in Figure 2. The excitation source is a Q-switched Nd YAG laser (Continuum Surelite 1) which is capable of a iO-Hz repetition rate but is typically used in the replicate-one-shot mode. The nominal 6-ns pulse can contain up to 450 mJ at 1064 nm, which decreases after harmonic conversions to 532 nm, 355 nm, or 266 nm. In addition to these harmonic lines, the 355-nm line can be used to pump an OPO (Opotek Magieprism), which provides tunable radiation in the range 420 nm to ca. 900 nm. The selected photolysis beam is incident on one face of a 10 mm x 10 mm quartz cuvette containing the sample. 

Accordingly, it was very soon found that using sources for which the physical widths of the emitted analyte lines are low is more attractive. This is necessary so as to obtain high absorbances, as can be understood from Fig. 76. Indeed, when the bandwidth of the primary radiation is low with respect to the absorption profile of the line, a higher absorption results from a specific amount of analyte as compared with that for a broad primary signal. Primary radiation where narrow atomic lines are emitted is obtained with low-pressure discharges as realized in hollow cathode lamps or low-pressure rf discharges. Recently, however, the availability of narrow-band and tunable laser sources, such as the diode lasers, has opened up new per-... 

The MAD method is performed on a single crystal, but it requires access to tunable radiation (synchrotron source). Moreover, selenomethionine-labeled protein must be produced, purified and crystallized. This is more easily done for proteins which can be expressed in E. coli. 

Refinements in vuv spectroscopy W, aided by the development of synchrotron radiation (7 ) and equivalent-photon electron-impact ( ) tunable light sources, and closely related advances in photoelectron, fluorescence-yield, and electron-ion coincidence spectroscopy measurements of partial cross sections (9), have provided the complete spectral distributions of dipole intensities in many stable diatomic and polyatomic compounds. Of particular importance is the experimental separation of total absorption and ionization cross sections into underlying individual channel contributions over very broad ranges of incident photon energies. 

Fig. 1 shows a schematic of the time resolved resonance Raman apparatus for radiation chemical studies. The essential components of the experimental set up are (a) a pulsed electron radiation source for inducing the reactions, (b) a tunable pulsed laser to probe the Raman scattering, (c) a Spex double monochromator for analyzing the scattered light, and (d) a gated detector for recording the spectra. 

For the initiation of photochemistry two sources are popular. The most widely used is the arc lamp. As well as being cheap and easy to operate arc lamps provide continuously tunable radiation from the near ir to the UV. Thus it is possible to obtain the action spectrum, which yields the most direct information on the mechanism of adsorbate photochemistry. Another advantage is that the incident power is relatively low, so surface heating is negligible and there is no competition from thermal reactions. A diagram of a typical surface photochemical experiment is shown in figure 1. The importance of the control of polarisation and angle of incidence will be described below. 

The measurement of very small absorption coefficients (down to lO-5 cm-1) of optical materials has been carried out by laser calorimetry. In this method, the temperature difference between a sample illuminated with a laser beam and a reference sample is measured and converted into an absorption coefficient at the laser energy by calibration [13]. Photoacoustic spectroscopy, where the thermal elastic waves generated in a gas-filled cell by the radiation absorbed by the sample are detected by a microphone, has also been performed at LHeT [34]. Photoacoustic detection using a laser source allows the detection of very small absorption coefficients [14]. Photoacoustic spectroscopy is also used at smaller absorption sensitivity with commercial FTSs for the study of powdered or opaque samples. Calorimetric absorption spectroscopy (CAS) has also been used at LHeT and at mK temperatures in measurement using a tunable monochromatic source. In this method, the temperature rise of the sample due to the non-radiative relaxation of the excited state after photon absorption by a specific transition is measured by a thermometer in good thermal contact with the sample [34,36]. 

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