Laser in Raman

It is certainly no exaggeration to say that the application of lasers in Raman spectroscopy has revolutionized this field, giving new impetus to many investigations and opening up new areas of research. There are several reasons why lasers are in many cases superior to conventional lamps. 

If the incident radiation is plane-polarized, such as that produced by lasers in Raman spectroscopy, scattering theory predicts that totally symmetric modes... 

The absolute differential scattering cross section of the standard needs to be determined as precisely as possible. A number of measurements has been performed ovm-the past forty years [260,278-287]. The introduction of lasers in Raman spectroscopy and of computer processing of spectral data has improved highly the accuracy of Raman intensity measurements. As a result, the absolute differential Raman scattering cross section of nitrogen reported frrom different laboratories deviates within a few percent only [260,284,285,287]. 

Due to the very high intensity of the laser beams and their coherent nature they may be used in a variety of ways where controlled energy is required. Lasers are used commercially for excitation with a specific energy, e.g. in Raman spectroscopy or isotope separation. 

The 70 years since these first observations have witnessed dramatic developments in Raman spectroscopy, particularly with the advent of lasers. By now, a large variety of Raman spectroscopies have appeared, each with its own acronym. They all share the conunon trait of using high energy ( optical ) light to probe small energy level spacings in matter. 

Perhaps the best known and most used optical spectroscopy which relies on the use of lasers is Raman spectroscopy. Because Raman spectroscopy is based on the inelastic scattering of photons, the signals are usually weak, and are often masked by fluorescence and/or Rayleigh scattering processes. The interest in usmg Raman for the vibrational characterization of surfaces arises from the fact that the teclmique can be used in situ under non-vacuum enviromnents, and also because it follows selection rules that complement those of IR spectroscopy. 

Major breakthroughs in early ultrafast VER measurements were made in 1972 by Laubereau et al [22], who used picosecond lasers in an SRS pump-incoherent anti-Stokes Raman probe configuration, to study VER of C-H... 

Lasers having wavelengths ranging from the deep uv to the near infrared have been used in Raman spectroscopy. In industrial laboratories, the most common laser is the Nd YAG operating at 1.06 pm. Increasingly, diode lasers or other lasers operating in the 750—785-nm region are encountered. These... 

Raman spectroscopy, long used for quaHtative analysis, has been revitalized by the availabiHty of laser sources. Raman spectroscopy is based on scattering of light with an accompanying shift in frequency. The amount by which the frequency is shifted is characteristic of the molecules that cause the scattering. Hence, measurement of the frequency shift can lead to identification of the material. 

Laser stimulation of a silver surface results in a reflected signal over a million times stronger than that of other metals. Called laser-enhanced Raman spectroscopy, this procedure is useful in catalysis. The large neutron cross section of silver (see Fig. 2), makes this element useful as a thermal neutron flux monitor for reactor surveillance programs (see Nuclearreactors). 

Raman scattering is essentially undelayed with respect to the arrival of the incident light, in this technique the detector is activated only during each laser pulse and deactivated at all other times. This allows only Raman signals to be recorded but fluorescence signals and detector noise are gated out (Fig. 19). Improvement in Raman signal to fluorescence ratio has been achieved as illustrated in Fig. 20. The technique, however, at present seems to be restricted by several instrumental limitations [37). 

A large number of nonlasing plasma lines emitted from the discharge plasma tube often interfere in the recorded Raman spectra. Loader (40) listed tables of plasma lines when using the argon ion and argon/krypton ion lasers as Raman sources. 

UV-Vis spectroscopy may also provide valuable information if small molecules are studied. However, the photochemical sensitivity of many sulfur-containing molecules may trigger changes in the composition of the sample during irradiation. For instance, this phenomenon has been observed in Raman spectroscopy using the blue or green hnes of an argon ion laser which sometimes decompose sensitive sulfur samples with formation of Sg [2, 3]. Reliable spectra are obtained with the red hnes of a krypton ion or a He-Ne laser as well as with the infrared photons of a Nd YAG laser. 

The large energy differences between the global minimum structure of C2v symmetry and the other isomers indicate that equilibrium sulfur vapor will contain only minute amounts of the latter, even at very high temperatures. However, under non-equilibrium conditions as in electrical discharges or by illumination with a laser as in Raman spectroscopy unstable isomers may be formed and detected. 

Fig. 3 a UV-Vis DRS spectra of dehydrated TS-1 catalyst reporting the typical 208 nm (48000cm i) LMCT hand, see Fig. 2h also reported are the four excitation laser lines used in this Raman study near-lR (dotted), visible (full), near-UV (dashed) and far-UV (dot-dashed), b Raman spectra of dehydrated TS-1 obtained with four different lasers emitting at 7 = 1064,422,325, and 244 nm (dotted, full, dashed, and dot-dashed lines, respectively). Raman spectra have been vertically shifted for clarity. Although the intensity of each spectrum depends upon different factors, the evolution of the 7(1125)//(960) ratio by changing the laser source is remarkable. The inset reports the Raman spectrum collected with the 244 nm laser in its full scale, in order to appreciate the intensity of the 1125 cm enhanced mode. Adapted from [48] with permission. Copyright (2003) by The Owner Societies 2003... 

Demming et al. [17], and compared it with their experimental results. The observed increase in Raman intensity by a factor of 15 within the laser focus of about 2 pm, corresponds to a maximum local enhancement at the center of the tip apex by a factor of about 12 000 under the assumption that the tip radius is 100 nm. More precisely, about 20% of the Raman intensity is considered to originate from an area with a radius of 14 nm under the tip, which is roughly equivalent to 400 molecules out of 2 X 10 in the total focused area. 

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. 

Raman spectroscopy has enjoyed a dramatic improvement during the last few years the interference by fluorescence of impurities is virtually eliminated. Up-to-date near-infrared Raman spectrometers now meet most demands for a modern analytical instrument concerning applicability, analytical information and convenience. In spite of its potential abilities, Raman spectroscopy has until recently not been extensively used for real-life polymer/additive-related problem solving, but does hold promise. Resonance Raman spectroscopy exhibits very high selectivity. Further improvements in spectropho-tometric measurement detection limits are also closely related to advances in laser technology. Apart from Raman spectroscopy, areas in which the laser is proving indispensable include molecular and fluorescence spectroscopy. The major use of lasers in analytical atomic... 

The first laser Raman spectra were inherently time-resolved (although no dynamical processes were actually studied) by virtue of the pulsed excitation source (ruby laser) and the simultaneous detection of all Raman frequencies by photographic spectroscopy. The advent of the scanning double monochromator, while a great advance for c.w. spectroscopy, spelled the temporary end of time resolution in Raman spectroscopy. The time-resolved techniques began to be revitalized in 1968 when Bridoux and Delhaye (16) adapted television detectors (analogous to, but faster, more convenient, and more sensitive than, photographic film) to Raman spectroscopy. The advent of the resonance Raman effect provided the sensitivity required to detect the Raman spectra of intrinsically dilute, short-lived chemical species. The development of time-resolved resonance Raman (TR ) techniques (17) in our laboratories and by others (18) has led to the routine TR observation of nanosecond-lived transients (19) and isolated observations of picosecond-timescale events by TR (20-22). A specific example of a TR study will be discussed in a later section. 

An interesting and powerful new development in Raman spectroscopy of catalysts is the use of a UV laser to excite the sample. This has two major advantages. First, the scattering cross section, which varies with the fourth power of the frequency, is substantially increased. Second, the Raman peaks shift out of the visible region of the spectrum where fluorescence occurs. The reader is referred to Li and Stair for applications of UV Raman spectroscopy on catalysts [40]. 

Figure 9.27 In Raman spectroscopy, light from a laser is shone at a sample. It is monochromated at a frequency of v0. Most of the light is transmitted. Most of the scattered light is scattered elastically, so its frequency remains at v0 this is Rayleigh scattered light. Raman scattered light has a frequency V(SCattered) = v0 — vibration) The sample is generally in solution...

Laser flash photolysis, 13 429 Laser frequency, in Raman scattering, 21 324... 

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