Raman intracavity

In Raman measurements [57], the 514-nm line of an Ar+ laser, the 325-nm line of a He-Cd laser, and the 244-nm line of an intracavity frequency-doubled Ar+ laser were employed. The incident laser beam was directed onto the sample surface under the back-scattering geometry, and the samples were kept at room temperature. In the 514-nm excitation, the scattered light was collected and dispersed in a SPEX 1403 double monochromator and detected with a photomultiplier. The laser output power was 300 mW. In the 325- and 244-nm excitations, the scattered light was collected with fused silica optics and was analyzed with a UV-enhanced CCD camera, using a Renishaw micro-Raman system 1000 spectrometer modified for use at 325 and 244 nm, respectively. A laser output of 10 mW was used, which resulted in an incident power at the sample of approximately 1.5 mW. The spectral resolution was approximately 2 cm k That no photoalteration of the samples occurred during the UV laser irradiation was ensured by confirming that the visible Raman spectra were unaltered after the UV Raman measurements. 

IRS and OHD-RIKES are less effected by the background interference, but the signal levels tend to be low, and they are subject to noise in the broad-band dye laser probe. Attempts to eliminate the remaining noise by time dispersal of the Raman signal and local oscillator by means of a streak camera (9) or a smoothing of the dye probe mode structure by intracavity phase shifting of the dye radiation are under consideration. 

Figure 6 Block diagram of the two-color optical parametric amplifier (OPA) and IR-Raman apparatus. CPA = Chirped pulse amplification system Fs OSC = femtosecond Ti sapphire oscillator Stretch = pulse stretcher Regen = regenerative pulse amplifier SHGYAG = intracavity frequency-doubled Q-switched Nd YAG laser YAG = diode-pumped, single longitudinal mode, Q-switched Nd YAG laser KTA = potassium titanyl arsenate crystals BBO = /J-barium borate crystal PMT = photomultiplier tube HNF = holographic notch filter IF = narrow-band interference filter CCD = charge-coupled device optical array detector. (From Ref. 96.)...

Ethylene has no dipole moment and a center of symmetry and therefore the Raman spectrum is an important source of structural information. After the early work on the rotational (Dowling and Stoicheff, 1959) and rovibrational Raman spectrum (Feldman et ah, 1956) these spectra were thoroughly studied in a series of publications (Hills and Jones, 1975 Hills et ah, 1977 Foster et ah, 1977). Overtones and combination bands were measured in an intracavity Raman experiment by Knippers et ah (1985). The Q-branch of the U2 band was resolved by pulsed CARS spectroscopy in a molecular beam experiment (Byer et ah, 1981). 

Since the He-Ne is based on emission of gas-phase atoms, the frequency accuracy is excellent, and the output linewidth is sufficiently narrow for most Raman applications without special accessories such as an intracavity etalon. Like ion lasers, He-Ne lasers exhibit a variety of atomic emission lines from the DC discharge that must be filtered out before reaching the sample. 

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.

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. 

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. 

Nevertheless, it is possible to give a formal description of a statistical-limit molecule in the same terms as previously used in the strong-coupling case. It is well known that the emission spectrum of large molecules (studied up to now only in condensed phases) is composed of narrow bands (considered as the resonance Raman scattering) and broad-band fluorescence. The relative intensity of the first component is enhanced in presence of fluorescence quenchers (Friedman and Hochstrasser, 1975), or in laser intracavity experiments (Bobovich and Bortkevich, 1977). The first component may be related to the emission from nonstationary s> states with redistribution time shorter than the exciting-pulse duration. The second component would be due to the rapid vibrational redistribution. In the limiting case of nonfluorescent molecules only the resonance Raman spectrum persists. The nonradiative deactivation of the excited state would be more rapid here than the vibrational redistribution. 

Because of the increased sensitivity of an intracavity arrangement, even weak vibrational overtone bands can be recorded with rotational resolution. For illustration. Fig. 8.5 shows the rotationally resolved Q-branch of the D2 molecule for the transitions (v = 2 v" = 0) [8.28]. The photon counting rate for the overtone transitions was about 5000 times smaller than those for the fundamental (v = v = 0) band. This overtone Raman spec-... 

In Munich, we have developed our spectrometer around a Jarrell-Ash double monochromator equipped with holographic gratings, using photomultiplier detection [48-51]. An extracavity multiple-reflection cell [49] as well as intracavity excitation with transfer of the Raman-scattered light with optical fibres [50,51] were applied for signal enhancement. The multiple reflection cell has been transferred to Florence and installed at a Jobin-Yvon UlOOO spectrometer equipped with a charge-coupled device (CCD) camera [52]. 

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]...

---