Chemical reactions between ions and neutral gas are very important in the interstellar medium, since they usually have much lower barriers than neutral-neutral reactions. Thus, they are more likely to take place at low temperatures. In this work, the endothermic reactions of CH2D+, H2D+, and N+ with H2 have been investigated in a temperature variable 22-pole ion trap. Under the chosen conditions inside the trap the chemical processes are comparable to those in typical astrophysical environments. At low temperatures, the reactions mentioned above depend very sensitively on the translational and internal energies of the reaction partners. This dependence can be utilized to gain information on the energy levels or the population of the internal states. Based on this sensitivity, two different types of investigations were performed in this work. High resolution spectra of CH2D+ were obtained, resulting in improved predictions for pure rotational transitions. First steps were made towards a detailed understanding of the processes in laser induced reactions, often leading to non-Gaussian line profiles. In addition, the influence of internal states on ion-molecule reactions allowed to determine the purity of para hydrogen samples and yielded insight into the reaction kinetics of the system N+ + H2.

The mid-infrared (mid-IR) spectral region coincides with fundamental rotational-vibrational transitions of a large number of molecules in the gas phase. The strong interaction between mid-IR light sources and these gaseous molecules has enabled experimental investigations of energy-level structures of various molecules and also led to a series of spectroscopic applications in, for example, biomedicine, environmental monitoring, and combustion diagnostics. Nanosecond pulsed optical parametric sources tunable in the mid-IR are versatile tools offering the potential to access different molecular transitions. Their compact size and relatively high pulse energy allow convenient and sensitive detection in various types of spectroscopy. Achievable narrow linewidths close to Fourier transform limit also make it possible to resolve Doppler-limited fine absorption lines and distinguish different species with high selectivity. As such, this thesis aims at the further development of pulsed optical parametric sources in the mid-IR region, employing different nonlinear materials and down-conversion configurations, as well as their application towards high-resolution molecular spectroscopy. We have demonstrated optical parametric generation (OPG) in the newly invented nonlinear crystal, orientation-patterned gallium phosphide (OP-GaP). Pumped by a Q-switched Nd:YAG laser, the OPG source at 25 kHz repetition rate generates tunable radiation across 1721-1850 nm and 2504-2787 nm. Detailed characterization including temperature tuning, pump transmission, and OPG threshold validated the performance of OP-GaP as the next-generation quasi-phase-matched nonlinear material. Following the development of the OPG, we further successfully demonstrated and characterized a singly-resonant optical parametric oscillator (OPO) based on OP-GaP. Driven by the same Nd:YAG laser at 50 kHz repetition rate, the OPO delivers mid-IR idler in the spectral range of 2.8-3.1 μm. Absorption-induced thermal effects in the OP-GaP sample were revealed and studied using different pump power levels. We also demonstrated a stable, pulsed degenerate OPO at 2.1 μm based on MgO:PPLN and pumped by the same Nd:YAG laser with variable repetition rates from 65 kHz to 90 kHz. The OPO, in a Littrow-grating-cavity configuration, can provide up to 2.7 W of degenerate output with good pulse-to-pulse and long-term power stability. The spectral narrowing effect of the grating-cavity was also examined and compared to a plane-mirror linear cavity. Finally, we developed a high-resolution difference-frequency spectrometer in the spectral range of 3308.5-3317.3 nm. The mid-IR source for the spectrometer was based on single-frequency, mode-hop-free tunable, pulsed difference-frequency-generation (DFG) in MgO:PPLN. A part of the methane spectrum in the Q-branch of v3-band was resolved and compared to HITRAN simulation in both atmospheric pressure and reduced pressure. This conceptual technique can be extended to broader mid-IR regions for detecting various other molecules or to higher energy level for nonlinear spectroscopy with high resolution.

Trace gas sensing technologies are widely used in many applications, such as environmental monitoring, life science, medical diagnostics, and planetary exploration. On the one hand, laser sources have developed greatly due to the rapid development of laser media and laser techniques in recent years. Some novel lasers such as solid-state, diode, and quantum cascade lasers have experienced significant progress. At present, laser wavelengths can cover the range from ultraviolet to terahertz, which could promote the development of laser gas sensing technologies significantly. On the other hand, some new gas sensing methods have appeared, such as photothermal spectroscopy and photoacoustic spectroscopy. Laser spectroscopy-based gas sensing techniques have the advantages of high sensitivity, non-invasiveness, and allowing in situ, real-time observation. Due to the rapid and recent developments in laser source as well as the great merits of laser spectroscopy-based gas sensing techniques, this book aims to provide an updated overview of the state-of-the-art laser gas sensing technologies.

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