Raman Photons Research Articles

Improved Fluorescence Rejection in Measurements of Raman Spectra of Fluorescent Compounds

Practical ways to achieve fluorescence rejection were developed by using the technique of Raman difference spectroscopy and a gated diode-array detector. In order to improve the signal-to-noise ratio, we had to circumvent the problem presented by the fact that the sensitivities of each element of the diode-array detector are not always uniform and linear—a problem which results in an apparent noise in the observed spectra. Therefore, in this study we tried to separately observe a Raman-plus-fluorescence spectrum ( ST) and a pure fluorescence spectrum ( SF), with a high S/N ratio, and to obtain a Raman spectrum ( SR) as their difference. With a cw Ar+ laser, ST and SF were excited at different wavelengths but recorded in the same wavelength region. This procedure was easy and quite successful for compounds, such as methyl orange and methylene blue, that have a Raman-to-fluorescence intensity ratio ( IR/ IF) of greater than 0.01. With a mode-locked and Q-switched Nd:YAG laser, ST and SF were obtained from different timings of the detector gate (6 ns), i.e., −6 to 0 ns for ST and 3 to 9 ns for SF. The temporal discrimination between Raman and fluorescence photons combined with the Raman difference technique significantly improved the 532-nm excited Raman spectrum of the tetraphenylporphyrin solution. This method is applicable to compounds with an IK/ IF of smaller than 0.01. The time-correlated single photon counting method with a time-to-amplitude converter was improved, to enable it to work with a high repetition rate, and was applied to a Rhodamine 6G solution, which was also examined by the gated Raman difference technique. Advantages and problems associated with each technique and appropriate methods for a given sample will be discussed.

H2 Raman‐shifted YAG laser ultraviolet Raman spectrometer operating at wavelengths down to 184 nm

AbstractApparatus is described for the acquisition of Raman spectra with excitation at wavelengths as short as 184 nm. The H2 Raman‐shifted output of an Nd:YAG laser is directed on to a free‐flowing sample, and the Raman photons are collected and dispersed by a scanning 1.26 m single monochromator equipped with a 2400 groove holographic grating and a solar blind phototube with integrating electronics, or by a 0.5 m spectrograph with an 1800 groove holographic grating and a reticon multi‐channel detector. Data for air‐sensitive samples are obtained by enclosing the sample jet in an inert gas purged shroud equipped with a UV‐transparent optical window. Raman spectra are reported for phenylalanine at a series of excitation wavelengths down to 184 nm. They show a sharp transition between 200 and 192 nm from vibronic to Franck–Condon scattering. At 200 nm, the intensities of the vibronically induced ν8a and ν8b modes, at 1606 and 1586 cm−1, are nearly the same as that of the 1000 cm−1 ν1 ring breathing mode, whereas at 192 nm the relative intensities of the former bands are dramatically dimished. In contrast, the ν8a and ν8b modes of tyrosine continue to show enhancement comparable to the ring breathing modes at 192 nm, the maximum of the strong 1Bα, b tyrosine absorption. The spectrum of insulin at 192 nm shows comparable contributions from tyrosine, phenylalanine and amide vibrations, whereas at 200 and 218 nm excitation the aromatic contributions are dominant.

Correlation between time- and depth-resolved simulated lidar signals

Abstract In this letter, the results from the analysis of the recent runs of SALMON code are presented. One important result is that the effective attenuation coefficient at excitation wavelength shows a maximum correlation between depth-and time-resolved signals for variable field of view and variable chlorophyll concentration. Under the situation considered here the effective attenuation coefficients are related as cz,l ≅ 2ct,l. It is also found that the effective velocity of Raman photons decreases as the chlorophyll concentration increases. The incremental Raman normalized fluorescence signal as a function of depth lags behind the one as a function of time.

Surface Plasmon Enhanced Vibrational Spectra of Monolayer Assemblies on Gratings and Rough Metal Surfaces

Abstract The vibrational spectra of monolayer assemblies of cadmium arachidate on smooth and rough silver substrates were obtained by surface infrared and surface plasmon enhanced Raman spectroscopy. The assemblies were laid down by the Langmuir-Blodgett deposition technique. For Raman scattering the intensity of the incident light was enhanced by grating coupling to surface plasmon optical modes of the metal or to localized plasmon modes in the case of rough surfaces. It was found that the different vibrational frequency regions corresponding, for example, to C-C and C-H stretching modes, were enhanced by selecting different scattering angles for collecting the inelastically scattered light. The Raman spectra of monolayer assemblies in contact with silver islands showed evidence of conformational disorder, i.e., the alkyl chains of some molecules were not in the all-trans configuration. In contrast, the infrared spectra did not show evidence of similar disorder. These observations were explained by assuming that the infrared photons sensed the majority undistorted molecular species, while the Raman photons came from a distorted minority species located in regions where optical electromagnetic fields were enhanced by shape plasmon resonances of the rough silver surface.

Surface infrared and surface enhanced Raman vibrational spectra of monolayer assemblies in contact with rough metal surfaces

Surface infrared and surface enhanced Raman spectroscopy have been used to obtain the vibrational spectra of monolayer assemblies of cadmium arachidate in contact with a variety of rough silver surfaces. The assemblies were laid down by the Langmuir–Blodgett monolayer deposition technique. The Raman spectra of these assemblies in contact with silver islands show evidence of conformational disorder, i.e., the alkyl chains are not in the all trans configuration. In contrast, the infrared spectra do not show evidence of similar disorder. These results and others, described in the text, are explained by assuming that the infrared photons sense the majority undistorted molecular species, while the Raman photons come from a distorted minority species located in regions where optical electromagnetic fields are enhanced by shape effects, plasmon resonances, etc., of the rough silver surface.

Two-Raman-photon emission in an intense electromagnetic field

Two-Raman-photon emission in hydrogen atoms in the presence of an intense electromagnetic field has been calculated utilizing the method of Dalgarno and Lewis. We give a general formulation for ($n\ensuremath{-}2$)-photon absorption and two-photon emission and also give numerical results for two-photon emission in the $1s\ensuremath{-}2p$ transition in hydrogen atoms. It is found that the emission of two spontaneous Raman photons could become important when the energy of the intense photon approaches the ionization energy. Finally, in the spectrum of the emitted Raman photon, faint lines are expected corresponding to the resonant peaks in the cross section.

Raman Processes in Intense Fields

Transition amplitudes corresponding to bound-bound multiphoton transitions with the emission or absorption of one and two Raman photons in the hydrogen atom have been evaluated following a recent method of Reiss. Results show that the transition probability varies smoothly for low values of $N$, where $N$ is the number of intense photons absorbed or emitted during the transition. For larger values of $N$ the amplitude, however, has oscillatory behavior. A particular feature of the present method of calculation is that we find the "reduced" transition amplitude for induced emission of two Raman photons is enhanced compared with single-Raman-photon emission. Another distinctive feature is the appearance of many more extrema in the transition amplitude for higher transitions for multiphoton processes at high intensity.

Polarization effects in stimulated Raman scattering and related phenomena

Polarization effects in stimulated Raman and stimulated Rayleigh scattering and self focusing are investigated from the viewpoint of the scattering of polarized photons by molecules. Both stimulated vibrational Raman scattering in liquids and stimulated rotational Raman scattering in gases are considered, and it is demonstrated that the polarization changes and relative power thresholds for different laser polarizations should be somewhat different to those predicted in other theories. For example, the ellipticity eta 0 of the stimulated rotational Raman photons in the forward direction from Delta J=+or-2 transitions in gaseous symmetric top molecules is given in terms of the initial ellipticity eta 1 by sin 2 eta 0= -5/7 sin 2 eta 1, whereas conventional theories predict that eta 0 and eta 1 are equal in magnitude and opposite in sign. Furthermore it is shown that deviations from this relation could provide a measure of the degree of optical orientation of the molecules.