Laser Spectroscopy of Highly Vibrationally Excited Molecules

Vladilen Stepanovich Letokhov

• Editorial: CRC Press
• Año: 2021
• ISBN:
  9781000447859
• Edición: 1
• Categoría: Sciences physiques
• Categoría: Physique

Laser spectroscopy has been perfected over the last fifteen years to become a precise tool for the investigation of highly vibrationally excited molecules. Intense infrared laser radiation permits both the multiple-photon resonant excitation and the dissociation of polyatomic molecules. In this book, the latest results of some of the foremost Soviet researchers are published for the first time in the West. Laser Spectroscopy of Highly Vibrationally Excited Molecules contains a comprehensive study of both the experimental and theoretical aspects of the basic photophysical interactions that occur in these processes. The book first focuses on the nonlinear interaction between the resonant vibrational mode and the intense infrared field and then examines the nonlinear interaction between the vibrational modes themselves due to anharmonicity. These interrelated processes determine all the characteristics of polyatomic molecules in an infrared field. The book also discusses related phenomena such as spectra broadening, optical resonance, photon echoes, and dynamical chaos. It includes examples of multiple-photon resonant excitation such as the excitation of OsO4 by CO^O2 laser radiation, which is detected by the visible luminescence that results. This book will be of great interest to researchers and postgraduate students in infrared laser spectroscopy and the laser chemistry of molecules and applications of isotope separation.

1 Introduction to the Physics of Multiple-Photon Excitation of Vibrations in Polyatomic Molecules

V S Letokhov

1.1 Introduction

The infrared (IR) spectroscopy of vibrational–rotational transitions of molecules is a well-developed field of molecular and optical physics and has numerous applications. Based on the quantum mechanical laws, a sufficiently deep insight has been gained into the rules governing the molecular vibrational–rotational absorption spectra due to quantum transitions from the ground or low-lying vibrational states. Also, vast information has been amassed on the IR absorption spectra of a large number of molecules. The results of these studies have been summarized in a series of excellent monographs [1.1-1.6].

All these results refer to quantum transitions between vibrational-rotational levels at the very minimum of the internuclear distance dependence of the molecular potential energy in the electronic ground state (figure 1.1(a)), where the molecular vibrations are almost harmonic. The anharmonicity of the vibrations of a polyatomic molecule and their interactions with the molecular rotations lead to a very complex structure of vibrational–rotational spectra which are, however, quite amenable to interpretation (see, for example, the reviews [1.7, 1.8]). For simple molecules, especially diatomic ones, substantial information has also been obtained on the excited vibrational states into which a molecule can transit, for example, from excited electronic states, giving rise to a rich spectrum of electronic–vibrational lines (figure 1.1(b)). Increasing the vibrational energy of a diatomic molecule causes no qualitatively new effects to occur in its vibrational–rotational transition spectrum, and the existing theories describe the experimental data quite well [1.1, 1.9].

Figure 1.1 Methods of studying vibrational levels and spectra of molecules in the electronic ground state: (a) traditional ir absorption spectroscopy of transitions from the ground state; (b) fluorescence spectroscopy of quantum transitions of a simple molecule from an excited electronic state into high-lying vibrational states of the electronic ground state; (c) IR multiple-photon spectroscopy of high-lying vibrational states of a polyatomic molecule.

The highly excited vibrational states of polyatomic molecules remained beyond the scope of systematic spectroscopic investigations because of the lack of adequate experimental techniques. The situation has changed fundamentally in the past decade when practical applications to experiment have found multiple-photon methods for exciting vibrations in polyatomic molecules by means of powerful IR laser pulses (figure 1.1(c)). The physics of highly vibrationally excited polyatomic molecules turned out to be much richer than that of simple (two- or three-atom) molecules. As the vibrational energy of a molecular vibrational system with many degrees of freedom grows higher, there occurs in the system a qualitatively new effect—intramolecular vibrational energy redistribution (IVR)—which usually cannot be observed on quantum transitions in the vicinity of the molecular potential energy minimum. It actually proved necessary to create a new trend in molecular spectroscopy, namely, the IR spectroscopy of polyatomic molecules in highly excited vibrational states.

At present, a qualitative understanding has been gained of the new effects arising here, but considerable theoretical and experimental efforts are still to be made to create a consummate picture. However, the results already obtained are of indubitable interest, for they allow one to understand the physics of the multiple-photon (MP) vibrational excitation of molecules by intense IR radiation, which is at the root of multiple-photon ir photochemistry and its practical applications. It should be emphasized that data on the spectral characteristics of highly excited polyatomic molecules exceed the limits of the ir multiple-photon excitation and dissociation of molecules, for these data are of independent interest, no matter what the method of exciting molecular vibrations. For this reason, we decided to sum up in this collective monograph the results obtained to date, so as to make them accessible to a wider circle of readers.

1.1.1 Methods of exciting high-lying vibrational levels

Let us first discuss the various laser excitation techniques of raising polyato...