In the Raman effect, incident light is inelastically scattered from a sample and shifted in frequency by the energy of its characteristic molecular vibrations. Since its discovery in 1927, the effect has attracted attention from a basic research point of view as well as a powerful spectroscopic technique with many practical applications. The advent of laser light sources with monochromatic photons at high flux densities was a milestone in the history of Raman spectroscopy and resulted in dramatically improved scattering signals (for a general overview of modern Raman spectroscopy, see refs 1-5). In addition to this so-called spontaneous or incoherent Raman scattering, the development of lasers also opened the field of stimulated or coherent Raman spectroscopies, in which molecular vibrations are coherently excited. Whereas the intensity of spontaneous Raman scattering depends linearly on the number of probed molecules, the coherent Raman signal is proportional to the square of this number (for an overview, see refs 6 and 7). Coherent Raman techniques can provide interesting new opportunities such as vibrational imaging of biological samples,8 but they have not yet advanced the field of ultrasensitive trace detection. Therefore, in the following article, we shall focus on the spontaneous Raman effect, in the following simply called Raman scattering. Today, laser photons over a wide range of frequencies from the near-ultraviolet to the near-infrared region are used in Raman scattering studies, allowing selection of optimum excitation conditions for each sample. By choosing wavelengths which excite appropriate electronic transitions, resonance Raman studies of selected components of a sample or parts of a molecule can be performed.9 In the past few years, the range of excitation wavelengths has been extended to the near-infrared (NIR) region, in which background fluorescence is reduced and photoinduced degradation from the sample is diminished. High-intensity NIR diode lasers are easily available, making this region attractive for compact, low cost Raman instrumentation. Further, the development of low noise, high quantum efficiency multichannel detectors (chargecoupled device (CCD) arrays), combined with highthroughput single-stage spectrographs used in combination with holographic laser rejection filters, has led to high-sensitivity Raman spectrometers (for an overview on state-of-the-art NIR Raman systems, see ref 10). As we shall show in section 2, the nearinfrared region also has special importance for ultrasensitive Raman spectroscopy at the singlemolecule level. As with optical spectroscopy, the Raman effect can be applied noninvasively under ambient conditions in almost every environment. Measuring a Raman spectrum does not require special sample preparation techniques, in contrast with infrared absorption spectroscopy. Optical fiber probes for bringing excitation laser light to the sample and transporting scattered light to the spectrograph enable remote detection of Raman signals. Furthermore, the spatial and temporal resolution of Raman scattering are determined by the spot size and pulse length, respectively, of the excitation laser. By using a confocal microscope, Raman signals from femtoliter volumes (∼1 μm3) can by observed, enabling spatially resolved measurements in chromosomes and cells.11 Techniques such as multichannel Hadamard transform Raman microscopy12,13 or confocal scanning Fourier transform Raman microscopy14 allow generation of high-resolution Raman images of a sample. Recently, Raman spectroscopy was performed using near-field optical microscopy.15-17 Such techniques overcome the diffraction limit and allow volumes significantly smaller than the cube of the wavelength to be investigated. In the time domain, Raman spectra can be measured on the picosecond time scale, providing information on short-lived species such as excited 2957 Chem. Rev. 1999, 99, 2957−2975

Ultrasensitive Chemical Analysis by Raman Spectroscopy

The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

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Present and Future of Surface-Enhanced Raman Scattering

I. Introduction. Background. Focus of the review. Review procedure and results. The NEA-TDB data base system. Presentation of the selected data. II. Standards and Conventions. Symbols, terminology and nomenclature. Units and conversion factors. Standard and reference conditions. Fundamental physical constants. III. Selected Uranium Data. IV. Selected Auxiliary Data. V. Discussion of Data Selection. Elemental uranium. Aqua ions. Oxide, hydride and hydroxide species. VI. Discussion of Auxiliary Data Selection. Reference list. Authors list. Formula list. Discussion of selected references. Ionic strength corrections. Assigned uncertainties. The estimation of entropies.

Chemical thermodynamics of uranium

Cellular Ti-6Al-4V structures with interconnected macro porosity for bone implants fabricated by selective electron beam melting.

Theory of Raman scattering evolution and revolution of Raman instrumentation - application of available technologies to spectroscopy and microscopy Raman spectroscopy and its adaptation to the industrial environment Raman microscopy - confocal and scanning near-field Raman imaging the quest for accuracy in Raman spectra chemometrics for Raman spectroscopy Raman spectra of gases Raman spectroscopy applied to crystals - phenomena and principles, concepts and conventions Raman scattering of glass Raman spectroscopic applications to gemmology Raman spectroscopy on II-IV-semiconductor nanostructures medical applications of Raman spectroscopy - in vivo Raman spectroscopy some pharmaceutical applications of Raman spectroscopy low-frequency Raman spectroscopy and biomolecular dynamics - a comparison between different low-frequency experimental techniques collectivity of vibrational modes Raman spectroscopic studies of ion-ion interactions in aqueous and non-aqueous electrolyte solutions environmental applications of Raman spectroscopy to aqueous solutions Raman and surface enhanced resonance Raman scattering - applications in forensic science application of Raman spectroscopy to organic fibres and films applications of IR and Raman spectra of quasi-elemental carbon process Raman spectroscopy the use of Raman spectroscopy to monitor the quality of carbon overcoats in the disk drive industry Raman spectroscopy in the undergraduate teaching laboratory Raman spectroscopy in the characterization of archaeological materials.

Handbook of Raman Spectroscopy: From the Research Laboratory to the Process Line

The analytical potential of a new spectrochemical method based on surface-enhanced Raman spectrometry (SERS) on silver-spheres-coated substrates is presented for the first time in this work. A new method for preparing SERS-active substrate using submicron size silver-coated spheres deposited on filter paper substrates is described in detail. The analytical figures of merit and limitations of the technique are discussed. The detection limits for several organic compounds such as carbazole, 1-aminopyrene, and benzoic acid are 0.2, 1.4, and 0.3 ng, respectively. The results indicate that SERS shows great promise as a useful analytical tool for trace organic analysis.

Surface-enhanced Raman spectrometry for trace organic analysis

Raman spectra of anhydrous crystalline samples of all of the lanthanide orthophosphates (with the exception of PmP04) have been obtained. These data were interpreted in a systematic manner based on the known structures of these compounds. Assignments and correlations have been made for many of the observed bands. Accordingly, a series of investigations of the chemical and physical properties of the lanthanide (and related) orthophosphates is currently in progress. Single crystals of all of the lanthanide orthophosphates (except PmP0,) doped with small amounts of Gd3+ were prepared for the initial purpose of performing a series of electron paramagnetic resonance experiments. The availability of these samples made it possible to carry out a study of the Raman spectra for essentially the entire series of rare-earth orthophosphates. This study was directed toward the following objectives. First, baseline spectra were to be established that could be used as a basis of comparison with subsequent lanthanide orthophosphate crystals that would be doped with @-active actinides and, consequently, would be subjected to heavy-particle radiation damage. Second, a classification and correlation of the Raman spectra of the entire group of rare earth orthophosphates was desired. Yttrium orthophosphate was included in the present investigation because its properties are known to correlate well with those of the orthophosphates of the second half of the rare-earth series (i.e. TbP04 through LuP04) and also because YP04 is the analog of the natural mineral xenotime. In addition, a consider- able literature exists on the properties of YP04 that can be used for the purposes of verification and comparison with the lanthanide compounds. The lanthanide orthophosphates can be conveniently divided into two groups on the basis of the prevalent crystal structural form. The first group has the mono- clinic 'monazite' structure and consists of LaP04 through GdP04. The second group has the zircon struc- ture and consists of TbP04 through LuP04. Both YP04 and ScP04 also crystallize with the zircon structure and can be grouped with the orthophosphates formed by EXPERIMENTAL ~~~~ ~ ~

Raman spectra of the rare earth orthophosphates

The sodium titanates Na4TiO4, α, β-, and γ-Na2TiO3, Na8Ti5O14, Na2Ti3O7, and Na2Ti6O13, all of which can be synthesized in the temperature range 800° to 1200°C, have been characterized by X-ray diffraction (XRD) and Raman spectroscopy. The existing XRD data have been critically reviewed and several inconsistencies and errors have been identified.

Sodium Titanates: Stoichiometry and Raman Spectra

A Raman spectroscopy study of hydroxo and carbonato species of the uranyl (VI) ion

The majority of the potassium titanates described in the literature were synthesized, and their Raman spectra recorded. The identity of the compounds K2TiO3, K2Ti2O5, K2Ti4O9, K2Ti6O13, and K2Ti8O17 was confirmed by x-ray diffraction. Raman spectroscopy was then used to study the hydrolysis, under different conditions, of K2Ti2O5 and of K2Ti4O9. On drying of the hydrolysis products, the following species were found to from: K2(H2O)0.66Ti8O16(OH)2, K1.33(H2O)0.33Ti4O8.33(OH)0.67, and H2Ti8O17. On ignition at temperatures of 500-600°C these species converted, respectively, to K2Ti8O17, K2Ti6O13, and TiO2(B). Raman spectroscopy was used to establish that (1) K6Ti4O11 consists of a mixture of K2TiO3 and a new compound K4Ti3O8; (2) K2Ti3O7 consists of a mixture of K2Ti2O5 and K2Ti4O9, and (3) K2Ti5O11 consists of a mixture of K2Ti4O9 and K2Ti6O13. The temperature of decomposition and the identity of the products of the thermal decomposition of K2Ti8O17, K2Ti4O9, K2Ti2O5, and K4Ti3O8 were determined by Raman spectroscopy. The XRD data of the newly identified compounds are reported.

George M. Begun

Papers

Surface-enhanced Raman spectrometry for trace organic analysis

Raman spectra of the rare earth orthophosphates

Sodium Titanates: Stoichiometry and Raman Spectra

A Raman spectroscopy study of hydroxo and carbonato species of the uranyl (VI) ion

Raman Spectroscopy of Potassium Titanates: Their Synthesis, Hydrolytic Reactions, and Thermal Stability