Detectors Stokes shift

Fig.l. Normalized steady-state fluorescence spectra of the PSBR in methanol, ethanol and octanol. The fluorescence spectra are corrected for detector response and converted onto an energy scale. Note that absorption and emission are lacking mirror symmetry. The total Stokes shift is 6.870 cm 1 octanol and 7.900 cm 1 in methanol. 

An important aspect of FT-Raman instrumentation is the necessity for optical filtering. The first task is to eliminate the stray light caused by the laser excitation because it will saturate the detector and electronics. The filtering must be capable of reducing the Rayleigh line, which is 106 stronger than the Stokes-shifted lines in the Raman spectrum. In order to be sufficiently... 

Block the laser beam or spectrometer entrance slit and adjust the spectrometer to an anti-Stokes shift of 1000 cm Caution Exposure of the sensitive phototube to the intense Rayleigh scattering line can seriously damage the detector. Scan the anti-Stokes spectrum from 1000 to 150 cm in the parallel polarization configuration and, using appropriate sensitivity expansion, j measure the ratio of anti-Stokes to Stokes peak heights for each band. 

Secondly, the Stokes shift is very large, almost 300 nm. This difference between excitation and emission peaks means that the fluorescence measurement is made at a wavelength where the influence of non-specific signals is minimal. In addition, the emission peak is very sharp which means that the detector can be set to very fine limits and that the emission signals from different lanthanide chelates can be easily distinguished from each other (Figure 1). 

Upconversion materials, which emit high-energy photons under excitation by the NIR light (anti-Stokes shift) were first discovered in the 1960s, but have primarily been exploited for the development of some remarkably effective optical devices such as infrared quantum counter detectors, ... 

The recent availability of inexpensive Q"Switched frequency-doubled diode-pumped Nd YAG lasers may encourage more study of coherent Raman effects. Stimulated Raman spectroscopy (SRS) is especially easy to observe [13]. Only the laser, a short-focal-length monochromator and a spectrograph are needed. Because the stimulated effect is strong, an interline transfer CCD video camera and frame grabber can serve as the detector system. With this simple apparatus (Fig. 6), Grant and Hartwick observed the stimulated Raman spectra of common organic solvents, such as benzene and acetonitrile. The second Stokes shift for benzene is shown in Fig. 7. Stokes shifts up to the fourth are observable. 

Germanium detectors are almost always cooled to 77 K with LN2. The first versions of these detectors were operated as low-bias diodes, and their noise performance was significantly worse than that of InGaAs detectors. The newer Ge detectors are fabricated from high-purity germanium elements and are biased near 100 V. These detectors cut on at about 6000 cm (3400-cm Stokes shift) and have excellent sensitivity, with an NEP that can be less than 10 W Like the InGaAs detector, the frequency response of Ge detectors... 

Fig. 6.8. A Principle of frequency-multiplexed CARS microspectroscopy A narrow-bandwidth pump pulse determines the inherent spectral resolution, while a broad-bandwidth Stokes pulse allows simultaneous detection over a wide range of Raman shifts. The multiplex CARS spectra shown originate from a 70 mM solution of cholesterol in CCI4 (solid line) and the nonresonant background of coverglass (dashed line) at a Raman shift centered at 2900 cm-1. B Energy level diagram for a multiplex CARS process. C Schematic of the multiplex CARS microscope (P polarizer HWP/QWP half/quarter-wave plate BC dichroic beam combiner Obj objective lens F filter A analyzer FM flip mirror L lens D detector S sample). D Measured normalized CARS spectrum of the cholesterol solution. E Maximum entropy method (MEM) phase spectrum (solid line) retrieved from (D) and the error background phase (dashed line) determined by a polynomial fit to those spectral regions without vibrational resonances. F Retrieved Raman response (solid line) calculated from the spectra shown in (E), directly reproducing the independently measured spontaneous Raman response (dashed line) of the same cholesterol sample...

Fig. 2.3 Raman spectrum of a polydimethylsiloxane (PDMS) sample showing the peaks due to anti-Stokes (negative Raman shifts) and Stokes (positive Raman shifts) scattering. The anti-Stokes side of the spectrum has been multiplied by a factor of 1,000 times so it can be seen clearly. The section shadowed is the region of the spectrum which was heavily attenuated by filters, in order to reduce the Rayleigh scattering into the detector...

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