This article aims to overview infrared (IR) spectroscopy. Simultaneously, it outlines mid-infrared (MIR), near-infrared (NIR), and far-infrared (FIR) or terahertz (THz) spectroscopy separately, and compares them in terms of principles, characteristics, advantages, and applications. MIR spectroscopy is the central spectroscopic technique in the IR region, and is mainly concerned with the fundamentals of molecular vibrations. NIR spectroscopy incorporates both electronic and vibrational spectroscopy; however, in this review, I have chiefly discussed vibrational NIR spectroscopy, where bands due to overtones and combination modes appear. FIR or THz spectroscopy contains both vibrational and rotational spectroscopy. However, only vibrational FIR or THz spectroscopy has been discussed in this review. These three spectroscopy cover wide areas in their applications, making it rather difficult to describe these various topics simultaneously. Hence, I have selected three key topics: hydrogen bond studies, applications of quantum chemical calculations, and imaging. The perspective of the three spectroscopy has been discussed in the last section.

本文旨在概述红外 (IR) 光谱。同时，分别概述了中红外（MIR）、近红外（NIR）和远红外（FIR）或太赫兹（THz）光谱，并从原理、特点、优势和应用等方面进行了比较。中红外光谱是红外区域的核心光谱技术，主要涉及分子振动的基本原理。近红外光谱结合了电子和振动光谱；然而，在这篇评论中，我主要讨论了振动 NIR 光谱，其中出现了由于泛音和组合模式而产生的谱带。FIR 或 THz 光谱包含振动光谱和旋转光谱。然而，本综述仅讨论了振动 FIR 或 THz 光谱。这三种光谱学涵盖了广泛的应用领域，因此很难同时描述这些不同的主题。因此，我选择了三个关键主题：氢键研究、量子化学计算的应用和成像。三种光谱的观点在上一节已经讨论过。