Optical techniques spectral bandwidth

Fig. 86. Principle of background correction with the D2-lamp technique (A), Al spectral bandwidth of monochromator, BG non-element-specific background absorption, As element-specific absorption signal. (B), Optical...

This new device has led to a rapid growth in the nonlinear spectroscopy of atoms and molecules. In addition the narrow spectral bandwidth and great intensity per unit spectral range of these dye lasers have made it possible to extend the range of classic spectroscopic techniques such as absorption and fluorescence spectroscopy. Even the more precise spectroscopic methods such as the Hanle effect, the optical double resonance, and the optical pumping techniques have all benefitted from the increasing availability of tunable dye lasers. Selective step-wise excitation using dye... 

Two main techniques are available to perform 2D-IRS, namely a pulsed-Jrequency-domain technique [88] and a time-domain-pulsed-Fourier transform technique [89]. A hybrid method using acousto-optic modulation has also been proposed recently [90]. In the pulsed-frequency-domain experiments, an intense broadband femtosecond pulse is split into a pump pulse, which passes a filter to reduce the band width to sections of typically 10 cm, and an unfiltered probe pulse. Both pulses are focused into the sample within an adjustable time delay. The probe pulse (full bandwidth) measures the spectral changes of the sample after the arrival of the pump pulse. For this purpose, the intensity of the probe light beam is recorded using a spectrometer equipped with a broadband array detector. 

While for most experiments in Doppler-limited spectroscopy-discussed in Chaps.8 and 9-wultimode lasers can be used (e.g., for optical pumping experiments, laser-induced fluorescence of atoms and simple molecules, or for Doppler-limited absorption spectroscopy) only some of the sub-Doppler methods, treated in this chapter, may be performed with pulsed or cw multimode lasers. Most of these techniques demand narrow-band tunable single ode lasers with a bandwidth which should be smaller than the desired spectral resolution. If the natural linewidth 6v has to be resolved, the laser frequency jitter should be smaller than 6v. This demands frequency stabilization techniques (see Sect.6.5) and there are many examples in this branch of high-resolution laser spectroscopy where the achieved resolution is- indeed limited by the stability of the laser. 

---