Spontaneous Raman spectroscopy

The unique feature in spontaneous Raman spectroscopy (SR) is that field 2 is not an incident field but (at room temperature and at optical frequencies) it is resonantly drawn into action from the zero-point field of the ubiquitous blackbody (bb) radiation. Its active frequency is spontaneously selected (from the infinite colours available in the blackbody) by the resonance with the Raman transition at co - 0I2 r material. The effective bb field mtensity may be obtained from its energy density per unit circular frequency, the... 

Coherent and spontaneous Raman spectroscopy in shocked and unshocked liquids NATO ASI Series C 184 425... 

Special Raman Spectroscopies. The weakness of Raman scattering results typically in the conversion of no more than 10 of the incident laser photons into a usable signal, limiting the sensitivity of conventional spontaneous Raman spectroscopy. This situation can be improved using alternative approaches (8,215,216). 

We will first discuss spontaneous Raman spectroscopy with lasers (linear Raman effect) and then briefly some investigations of the nonlinear Raman effect. 

There are several articles reviewing spontaneous Raman spectroscopy with lasers 191-194) extensive references to the literature up to 1968. This survey therefore will be restricted to a selection of recent work in this field in order to demonstrate the progress achieved. 

Several types of time-resolved Raman spectroscopies have been reported and reviewed by Hamaguchi and co-workers and Hamaguchi and Gustafson. These include pump-probe spontaneous and time-resolved coherent Raman spectroscopy of the anti-Stokes and Stokes varieties [coherent anti-Stokes Raman spectroscopy (CARS) and coherent Stokes Raman spectroscopy (CSRS)], respectively). Here we will focus on pump-probe time-resolved spontaneous Raman spectroscopy. 

The pump pulse energy is controlled to minimize two-photon phenomena and to maximize the concentration of the desired excited-state or other reactive intermediate. The optimal average power of the probe pulse changes with a specific experiment but is often maintained at 10 mW peak powers in the range of 0.1-10 MW with repetition rates of 1 kHz-1 MHZ are best for picosecond spontaneous Raman spectroscopy. 

Spontaneous Raman spectroscopy has the ability to provide clinically relevant chemical concentration measurements of multiple analytes in biofluids. Blood serum, whole blood, and urine have all been studied. The detection limit (assuming a few hundred seconds of spectral acquisition) appears, based upon fundamental noise considerations, to be around 0.1 mM for most biochemicals this places several important analytes within reach but certainly precludes... 

E>vib and Our also show up in the theory of spontaneous Raman spectroscopy describing fluctuations of the molecular system. The functions enter the CARS interaction involving vibrational excitation with subsequent dissipation as a consequence of the dissipation-fluctuation theorem and further approximations (21). Equations (2)-(5) refer to a simplified picture a collective, delocalized character of the vibrational mode is not included in the theoretical treatment. It is also assumed that vibrational and reori-entational relaxation are statistically independent. On the other hand, any specific assumption as to the time evolution of vib (or or), e.g., if exponential or nonexponential, is made unnecessary by the present approach. Homogeneous or inhomogeneous dephasing are included as special cases. It is the primary goal of time-domain CARS to determine the autocorrelation functions directly from experimental data. 

Momentum also plays a role in ordinary spontaneous Raman spectroscopy. When the pump radiation at 532 nm is passed through a sample, the aE term of Eq. (2) produces scattering and, for the first Stokes case shown in Fig. la, the frequency is i si = where is the Raman-active vibration excited in the sample. It should be noted that there is an exchange between the radiation field and molecule not only of energy but also of momentum, represented by the vector ky. The direction and magnitude of k, are determined by the photon-scattering direction, which is random for this spontaneous event. The result is scattering in all directions so that there is no coherent addition of photon amplitudes, as expressed in the summation /(i si) = C8q The net intensity from this inco-... 

Similar difference measurements can be made using resonance-enhanced or spontaneous Raman spectroscopy. In Raman spectroscopy, in general, the signal from an aqueous solvent is not large (water is a very weak Raman scatterer). 

Inverse Raman scattering Inverse Raman scattering (IRS) is a coherent process involving stimulated loss at an anti-Stokes-shifted frequency. The term inverse Raman refers to the fact that, at resonance, the probe radiation is attenuated. In spontaneous Raman spectroscopy, on the other hand radiation at Raman-active frequencies would he generated in the course of the experiment. Inverse Raman scattering (IRS) and stimulated Raman gain (SRG) are closely related. While one involves stimulated gain at an anti-Stokes-shifted frequency, the other involves stimulated loss at a Stokes-shifted frequency. 

Schmidts C, Moore D S, Schiferl D, Chatelet M, Turner P, Shaner JW, Shampine D Land HoltWT 1986 Coherent and spontaneous Raman spectroscopy in shocked and unshocked liquids NATO ASI Series C 184 425... 

The experimental methods are extensively described in earlier papers (13,14, ). Temperature and major species profiles were determined by spontaneous Raman spectroscopy in free flames stabilized on a slot burner. Flame speeds were determined by particle tracks on the same burner and by the Guoy method on conical flames. Total NOx measurements were performed in the postflame region of a flat, water cooled, Meker burner. On the flat burner gas flows were adjusted to give a slightly wrinkled flame corresponding to free flame conditions. Gas samples are extracted with a water cooled, quartz microprobe analyzed with a chemiluminescent... 

So the great advantage of conventional spontaneous Raman spectroscopy is its simplicity, with complete rotational and vibrational spectra obtainable in a single run on a simple instrument using visible optics. The disadvantages include the inherent weakness of the... 

One definite advantage of CARS is the fact that the detector can be far away from the sample (remote Raman-spectroscopy) because the intensity of the signal does not decrease as 1/r as for spontaneous Raman spectroscopy or fluorescence... 

W. Zinth, M.C. Nuss, W. Kaiser, A picosecond Raman technique with resolution four times better than obtained by spontaneous Raman spectroscopy, in Picosecond Phenomena III,... 

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