Raman spectroscopy intracavity

Fig. 3. Diagram of continuous wave (cw) laser sources suitable for metalloprotein resonance Raman spectroscopy. The best quality spectra are provided by Ar, Kr, He-Ne, and He-Cd lasers operating at fixed frequencies (the lengths of the lines indicate the relative output for a given laser) throughout the visible and near-UV region. An intracavity frequency-doubled (ICFD) Ar laser has been developed with five useful cw excitation wavelengths in the far-UV region (257, 248, 244, 238, and 228.9 nm). The high-powered Ar and Kr lasers can also be used to pump dye lasers which are tunable between the near-UV and near-IR region. The cw Nd YAG laser with a fundamental at 1064 nm is the primary excitation source in FT Raman spectrometers.

Fig. 3.5 Experimental arrangement for intracavity Raman spectroscopy with an argon laser CM, multiple reflection four-mirror system for efficient collection of scattered light LM, laser-resonator mirror DP, Dove prism, which turns the image of the horizontal interaction plane by 90° in order to match it to the vertical entrance slit S of the spectrograph FPE, Fabry-Perot etalon to enforce single-mode operation of the argon laser LP, Littrow prism for line selection [315]...

Fig. 3.12 Intracavity Raman spectroscopy of molecules in a cold jet with spatial resolution...

One example is intracavity Raman spectroscopy of molecules in a supersonic jet, demonstrated by van Helvoort et al. [327]. If the intracavity beam waist of an argon-ion laser is shifted to different locations of the molecular jet (Fig. 3.12), the vibrational and rotational temperatures of the molecules (Sect. 4.2) and their local variations can be derived from the Raman spectra. 

The 632.8 nm helium-neon (HeNe) laser is also very popular for Raman spectroscopy because it is small, portable, mature, and very inexpensive. The resulting Raman wavelengths are well matched to the optimum sensitivity of charge-coupled device (CCD) detectors. HeNe lasers used for Raman spectroscopy typically deliver 5-30 mW of optical power. Sometimes they also sporadically deliver a weaker laser-like beam at 650 nm that can cause a huge line in the Raman spectrum at 418cm. The 650nm radiation is due to an intracavity pumped laser Raman transition. 

Ethylene has a center of symmetry and no dipole moment and, therefore, the Raman spectrum is an important source of structural information. After the early work on the rotational [269] and rovibrational Raman spectrum [270], these spectra were thoroughly studied in a series of publications [271-273]. Overtones and combination bands were measured in an intracavity Raman experiment by Knippers et al. [274]. The Q branch of the V2 band was resolved by pulsed CARS spectroscopy in a molecular beam experiment [275] and the band by inverse Raman spectroscopy [90]. 

Fig. 9.6. Intracavity Raman source unit for Raman spectroscopy of gases. CM = multiple reflection four-mirror system for efficient collection of scattered radiation LM = highly reflecting laser-resonator mirror [9.20]...

The sensitivity of this intracavity technique (Sect. 1.2.3) can even be enhanced by modulating the magnetic field, which yields the first derivative of the spectrum (Sect. 1.2.2). When a tunable laser is used it can be tuned to the center vo of a molecular line at zero field = 0. If the magnetic field is now modulated around zero, the phase of the zero-field LMR resonances for AM = +1 transitions is opposite to that for AM = — 1 transitions. The advantages of this zero-field LMR spectroscopy have been proved for the NO molecule by Urban et al. [143] using a spin-flip Raman laser. 

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