Molecular and Laser Spectroscopy

Advances and Applications

V.P. Gupta

• Publisher: Elsevier
• Year: 2017
• ISBN:
  9780128498828
• Topic: Physical Sciences
• Subtopic: Spectroscopy & Spectrum Analysis

Molecular and Laser Spectroscopy: Advances and Applications provides students and researchers with an up-to-date understanding of the fast-developing area of molecular and laser spectroscopy. Editor V.P. Gupta has brought together the eminent scientists on a selection of topics to develop a systematic approach, first covering basic principles needed to understand each cutting-edge technique and application. This book acts as a standard reference for advanced students of molecular and laser spectroscopy and as a graduate text for new entrants in the field.

The book covers a wide range of applications of molecular and laser spectroscopy in diverse areas ranging from materials to medicine and defence, biomedical research, environmental monitoring, forensic investigations, food and agriculture, and chemical, pharmaceutical and petrochemical processes. Researchers and scientific personnel in these fields will learn the latest techniques in order to put them to practical use in their work.

• Covers several areas of spectroscopy research in a single volume, saving researchers time
• Includes exhaustive lists of research articles, reviews and books at the end of each chapter to point readers in the right direction for further learning
• Features illustrative examples of the varied applications
• Serves as a practical guide to those interested in using molecular and laser spectroscopy tools in their research and field applications

Chapter 1

Introduction and Overview

V.P. Gupta Editor

Abstract

Several developments have taken place in the recent past in the field of molecular and laser spectroscopy, which have found newer applications in diverse areas ranging from materials to medicines and defence—touching lives in all its hues. Lasers have greatly enriched the science of atoms and molecules, both by adding novel spectroscopic techniques and by reviving some old ones. These involve both linear and nonlinear processes reflected in the infrared and Raman spectra and the time-resolved spectroscopy. The integration of a microscope to a spectrometer and the development of fast spectroscopic imaging by infrared and Raman imaging techniques have allowed spatially resolved spectroscopy of large or multiple samples, enhancement of spatial resolution, and the investigation of molecular dynamics and biological mechanisms. Some of these advancements have been discussed in the present book. This chapter provides a concise overview of the book and the contents of its various chapters, which include recent advances in theory, instrumentation/experimental techniques, applications, limitations, and the future outlook of the spectroscopic techniques.

Keywords

2D correlation spectroscopy; Femtosecond spectroscopy; Fiber-optic probes; Infrared; Laser spectroscopy; Laser-induced breakdown spectroscopy; Linear and nonlinear spectroscopy; Matrix isolation spectroscopy; Microscopy; Molecular spectroscopy; Nanomaterials and random lasers; Photoacoustic spectroscopy; Raman, Resonance spectroscopy; Two-photon absorption; Waveguides

Chapter Outline

1. Introduction

2. Overview

2.1 Near-Infrared Spectroscopy

2.2 Mid-Infrared Spectroscopy

2.3 Terahertz Spectroscopy (0.2 to 30THz)

2.4 Linear and Nonlinear Raman and Infrared Spectroscopy and Microscopy

2.5 Resonance Raman Spectroscopy

2.6 Two-Photon Absorption Spectroscopy

2.7 Femtosecond Raman Spectroscopy

2.8 Two-Dimensional Correlation Spectroscopy

2.9 Laser Spectroscopy of Nanomaterials

2.10 Laser-Induced Breakdown Spectroscopy

2.11 Photoacoustic Spectroscopy

2.12 Matrix Isolation Spectroscopy

Reference

1. Introduction

One of the most exciting challenges in modern science has been to unravel the detailed code of molecular spectra because this code speaks directly of a molecule's energy levels. Chemical systems undergoing a change display spectral signatures in various regions of the electromagnetic spectrum. The details of these signature codes recorded in terms of the molecular spectra can be deciphered to get information about the related molecular structures and dynamic processes in the electronic ground and excited states. The study of fundamental regularities in the electronic (absorption, fluorescence, and phosphorescence), vibrational (both infrared and Raman), rotational, and hyperfine spectra has established their relation with molecular structure. They reveal not only the structural characteristics of molecules but also the strength of the bonding potential between atoms, not only for the ground state but also for the electronically excited states, and for unstable short-lived molecules. In particular, techniques based on vibrational spectroscopy have been widely used in all areas of science to provide a close understanding of the composition of materials at the molecular level. The field of molecular spectroscopy has continued to advance rapidly with measurements in a variety of molecules with increasing resolution, which lead to the observation of new effects and continued improvements in the theoretical description of the spectra. This expansion resulted from both improvements in existing instrumentation and the development of new techniques in the fields of Raman and infrared (IR) spectroscopy. Matrix isolation IR and Raman spectroscopy has been one such effort at developing a technique that, as a means of trapping reactive species, such as radicals, and unstable and transient species, allows for an extended lifetime of these species. This experimental technique offers advantages of sharp spectral linewidths and removal of spectral congestion, thereby leading to simple, highly resolved spectral features. Efforts have continued toward increasing spectral resolution and improved detection techniques. The replacement of dispersion technique used in grating or prism spectrographs by interferometry and Fourier transform (FT) analysis, as used in FT infrared (FTIR) spectrometers, has enhanced spectral resolution from 1 to 0.01 cm⁻¹. As an essential tool in establishing the nature of substances, molecular spectroscopy has expanded into new and exciting areas such as material science, biology, medical diagnostics, environmental science, industrial process control, homeland security, and space exploration.

Since the development of lasers, the conduct of the entire field of molecular spectroscopy has undergone major and far-reaching qualitative changes. Laser radiation is finding many applications for analysis and diagnostics based on the wavelength-dependent interaction between electromagnetic radiation and matter. Laser techniques have greatly enriched the science of atoms and molecules, both by adding novel spectroscopic techniques and by reviving some old ones. These involve both linear and nonlinear processes reflected in the IR and Raman spectra. The impact of lasers on spectroscopy can hardly be overestimated. With the use of even low-powered laser sources, such as semiconductor diode laser, which results in linear processes, it is now possible to have the advantages of high spectral resolution, supersensitive and superfast detection, remote sensing, etc. Many experiments, which could not be done before the application of lasers, because of lack of intensity or insufficient resolution, are now readily performed with lasers. While the narrow bandwidth of modern tunable lasers makes its interaction with free atoms and molecules having sharp spectral features extremely selective, the high spectral intensity available with pulsed as well as continuous-wave lasers increases the sensitivity and makes the detection of single atom or molecule possible. This represents the ultimate sensitivity in analytical chemistry.

From its beginning, laser spectroscopy has far surpassed conventional spectroscopic techniques in sensitivity, resolution, and measurement accuracy, and we are witnessing unabated progress in all the three directions. Laser spectroscopy provides the practical means of studying the spectra of short-lived molecules, such as transient molecules, free radicals, and molecular ions, which were impossible to be studied by conventional spectroscopy because of low sensitivity and slow scanning speed. Because of their extremely small bandwidth, single mode lasers allow a spectral resolution, which far exceeds that of conventional spectrometers. In recent years, the development of precisely controlled tunable narrow linewidth laser sources with appreciable power has provided powerful tools for the measurement and analysis of molecular spectra. In many cases the tunable lasers replace wavelength-selecting elements such as spectrometers and interferometers. With the advent of very short laser pulses, it has now been possible to conduct observations in timescales that are not only shorter than individual vibrational lifetimes or dephasing times but also shorter than vibrational oscillation periods. This has led to the development of the area of time-resolved spectroscopy. Using very short laser pulses, it is now possible to conduct observations in nano-, pico-, and femtosecond timescales. With such short pulses, it has been possible to track a chemical reaction and record each bond-forming and/or bond-breaking event. Femtosecond time-resolved spectroscopy therefore recreates the whole chemical event as a series of snapshots as if the dynamic process was frozen in time. Femtosecond Raman spectroscopy has proved to be a useful tool to probe evolution of molecular systems in the electronic excited state. Such vibrational spectroscopy is well suited for the study of excited electronic states, radical reactions, electron transfer, and vibrational dynamics. Molecular structures corresponding to unstable intermediates between the reactant and the product, crystal structure during phase transition, etc. have now been observed using femtosecond laser pulses. Fortunately, the relationship between laser spectroscopy and conventional molecular spectroscopy is not one of replacement but of complementarity, which makes laser spectroscopy an inseparable part of modern molecular spectroscopy.

Far-reaching changes have also come about in the instrumentation techniques in the near-IR (NIR) and mid-IR (MIR) regions with the development of intense laser sources, such as semiconductor diode lasers, room temperature–operated quantum cascade lasers (QCLs), and optical parametric oscillators, and detectors. Sophisticated instruments that provide spatial resolution beyond diffraction limit, fiber-optic probes, waveguides, miniature and highly integrated spectrometers, and low-cost FTIR minispectrometers with a size similar to a small sheet of paper are now on the way. Different accessories can be attached to these spectrometers for transmission or attenuated total reflection (ATR) measurements, making them attractive for various purposes. By combining with fiber-optic probes, the reach of the spectrometer can be tremendously extended. The flexibility of optical fibers used as waveguides permits probes to be applied within inaccessible sites and very small spaces. It also allows centralization of a spectrometer and analysis devices at a distance from the actual measurement and provides the option of multiplexing several probes. A wide variety of fiber-optic probes and configurations have now been developed, which allow IR radiation to be delivered to diverse environments outside the laboratory. In particular, flexible fiber probes based on silver halide material have several advantages, because they allow remote sensing with a fast and easy sample measurement within the NIR and MIR wavelength regions from 3 to 20 μm. A special class of systems, micro-opto-electro-mechanical systems (MOEMS) combining micro-electro-mechanical, optical, and electrical systems, present a significant step forward in the development of smart spectroscopic sensors, microsystem technology, and vibrational spectroscopy instrumentation.

QCLs from MIR to the terahertz spectral range represent a relatively recent development in the area of semiconductor lasers and have concrete impact in many technological applications such as trace gas analysis, optical communications, and real-time imaging. QCLs can generate short pulses with pico- and femto-second durations and have found application in time-resolved spectroscopy. Earlier MIR semiconductor lasers such as diode lasers were based on interband transitions, whereas QCLs utilize intersubband transitions. The photon energy, and also the wavelength of transitions, can be varied in a wide range by engineering the details of the semiconductor layer structure. Unlike diode lasers, QCLs can easily be designed to emit at multiple and widely differing wavelengths, and broadband tuning has been realized by using, for example, an external cavity (EC) grating as a wavelength-defining element. A spectral range of about 1000 cm⁻¹, including the most informative spectral fingerprint region, can thus be covered by multimodule EC-QCLs. Spectrometers with EC-QCLs have been suggested for clinical chemistry applications in quantitative blood substrate assays. Spectacular developments have taken place in imaging of cells and tissues using FTIR spectrometers with focal plane arrays and nanoscopy-based on NIR spectroscopy with spatial resolution similar to tip-enhanced Raman spectroscopy (TERS). Microscopes with QCLs have been developed and are now commercially available for spectral histopathology.

The spectroscopic technique that arises due to the inelastic scattering of light discovered by the Indian scientist Sir C.V. Raman in 1928 and named after him as Raman Spectroscopy has transcended all its barriers and has emerged as a technology that has wide applications in all the areas ranging from materials to medicines. It can be reckoned as a serious contender among all the other analytical tools because of its noninvasive and nondestructive nature. The Raman and IR transitions are like the two sides of a coin that provide complementary information. Development of lasers has resulted in the growth of areas of spontaneous Raman scattering and nonlinear Raman spectroscopies such as coherent anti-Stokes Raman scattering (CARS), resonance Raman scattering (RRS), surface-enhanced Raman scattering (SERS), stimulated Raman spectroscopy (SRS), and two-photon excitation fluorescence spectroscopy. Major advantage of these methods over the conventional Raman spectroscopy is the ability to obtain Raman spectra with very high resolution, which is solely determined by the linewidth of the input sources. High-resolution measurements utilizing these techniques preclude the necessity of using monochromators, interferometers, or other apparatuses usually used in conventional Raman spectroscopy. Applications of Raman spectroscopy saw new heights after the discovery of SERS, which enabled analyses of analytes at ultratrace levels pushing the limit of detection to single-molecule level. This advantage along with high signal-to-noise ratio offered new possibilities for Raman spectroscopy to be used as a sensitive analytical technique. Furthermore, following the discovery of lasers, significant advancements have taken place in an area such as photoacoustic spectroscopy (PAS), which although discovered about a century back could not find much practical application for want of a strong light source.

During the past decade, the integration of a microscope to a spectrometer has pushed the limits of the spectroscopic techniques to collect specific information from diffraction limited spots. A one-to-one mapping of the investigated volume of the sample constitutes a microscopic image. Much improved signal efficiency obtained by using lasers and availability of high-performance multichannel detectors, optimization of data acquisition, and the development of efficient evaluation and representation software have enabled several such techniques for high-speed microscopy, few of them up to video-rate imaging, and two-dimensional (2D) and three-dimensional (3D) mapping. The fast spectroscopic imaging by IR and Raman imaging techniques allows spatially resolved spectroscopy of large or multiple samples, enhancement of spatial resolution due to retention of radiation throughput, and the investigation of molecular dynamics and biological mechanisms. On the basis of their unique characteristics and capabilities, optical spectroscopy/microscopy techniques have got a variety of important applications in material science, biology, pharmaceutical, and medical fields. Raman imaging aims at separating molecular species present in the sample on the basis of their spectral differences, which is reflected in the 2D maps. Nonlinear techniques have been used to provide inherent 3D sectioning.

Although initially the terahertz region (frequency range 0.2 to 30 THz; 1 THz = 10¹² Hz) lying between the IR and microwave regions of the electromagnetic spectrum remained to be one of the least tapped regions of the electromagnetic spectrum, most probably due to the difficulty in generating and detecting the THz radiation, it has now become one of the extremely important regions. It has found several potential applications in condensed matter systems, material science, chemicals and pharmaceuticals, medical diagnostics (including imaging in biomaterial identification), agriculture, high-bandwidth short-distance secure communication/data transfer, and defense, mainly due to its nondestructive, nonionizing material evaluation properties. Intense research activities are taking place in the development of strong and b...