Molecular laser spectroscopy

This chapter is concerned with the following techniques in molecular laser spectroscopy (i) laser-Stark spectroscopy and electric field spectroscopy (ii) laser-Zeeman, or laser-magnetic-resonance spectroscopy (LMR) (iii) dispersed laser-induced fluorescence and (iv) double resonance spectroscopy. 

TECHNIQUES IN MOLECULAR LASER SPECTROSCOPY, R.N. DIXON Introduction... 

Doppler-Broadened Transition. Narrow Resonances of Two-Photon Transitions Willioul Doppler Broadening.—Nonlinear Resonances on Coupled Doppler-Broadened Transitions. Narrow Nonlinear Resonances in Spectroscopy. Nonlinear Atomic Laser Spectroscopy. Nonlinear Molecular Laser Spectroscopy. - Nonlinear Narrow Resonances in Quantum Electronics. Narrow Nonlinear Resonances in Experimental Physics. 

Gruebele M H W 1988 Infrared Laser Spectroscopy of Molecular Ions and Clusters (Berkeley University of California)... 

As described above, classical infrared spectroscopy using grating spectrometers and gas cells provided some valuable infonnation in the early days of cluster spectroscopy, but is of limited scope. However, tire advent of tunable infrared lasers in tire 1980s opened up tire field and made rotationally resolved infrared spectra accessible for a wide range of species. As for microwave spectroscopy, tunable infrared laser spectroscopy has been applied botli in gas cells and in molecular beams. In a gas cell, tire increased sensitivity of laser spectroscopy makes it possible to work at much lower pressures, so tliat strong monomer absorjDtions are less troublesome. 

DIott D D 1988 Dynamics of molecular crystal vibrations Laser Spectroscopy of Solids 7/ed W Yen (Berlin Springer) pp 167-200... 

Both molecular dynamics studies and femtosecond laser spectroscopy results show that molecules with a sufficient amount of energy to react often vibrate until the nuclei follow a path that leads to the reaction coordinate. Dynamical calculations, called trajectory calculations, are an application of the molecular dynamics method that can be performed at semiempirical or ah initio levels of theory. See Chapter 19 for further details. 

Laser spectroscopy is such a wide subject, with many ingenious experiments using one or two CW or pulsed lasers to study atomic or molecular stmcture or dynamics, that it is difficult to do justice to it at the level at which Modern Spectroscopy is aimed. In this edition 1 have expanded the section on supersonic jet spectroscopy, which is an extremely important and wide-ranging field. 

In dimers composed of equal molecules the dimer components can replace each other through tunneling. This effect has been discovered by Dyke et al. [1972] as interconversion splitting of rotational levels of (HF)2 in molecular beam electric resonance spectra. This dimer has been studied in many papers by microwave and far infrared tunable difference-frequency laser spectroscopy (see review papers by Truhlar [1990] and by Quack and Suhm [1991]). The dimer consists of two inequivalent HE molecules, the H atom of one of them participating in the hydrogen bond between the fluorine atoms (fig. 60). PES is a function of six variables indicated in this figure. 

Kinetics on the level of individual molecules is often referred to as reaction dynamics. Subtle details are taken into account, such as the effect of the orientation of molecules in a collision that may result in a reaction, and the distribution of energy over a molecule s various degrees of freedom. This is the fundamental level of study needed if we want to link reactivity to quantum mechanics, which is really what rules the game at this fundamental level. This is the domain of molecular beam experiments, laser spectroscopy, ah initio theoretical chemistry and transition state theory. It is at this level that we can learn what determines whether a chemical reaction is feasible. 

A technique which is not a laser method but which is most useful when combined with laser spectroscopy (LA/LIF) is that of supersonic molecular beams (27). If a molecule can be coaxed into the gas phase, it can be expanded through a supersonic nozzle at fairly high flux into a supersonic beam. The apparatus for this is fairly simple, in molecular beam terms. The result of the supersonic expansion is to cool the molecules rotationally to a few degrees Kelvin and vibrationally to a few tens of degrees, eliminating almost all thermal population of vibrational and rotational states and enormously simplifying the LA/LIF spectra that are observed. It is then possible, even for complex molecules, to make reliable vibronic assignments and infer structural parameters of the unperturbed molecule therefrom. Molecules as complex as metal phthalocyanines have been examined by this technique. 

Nonlinear Laser Spectroscopy and Dephasing of Molecules An Experimental and Theoretical Overview, M. J. Bums, W. K. Liu, and A. H. Zewail, in Spectroscopy and Excitation Dynamics of Condensed Molecular Systems, Series in Modem Problems in Condensed Matter Sciences, Vol. 4, V. M. Agranovich and R. M. Hochstrasser, Eds., North-Holland Publishing, Amsterdam, New York, Oxford, 1983, Chapter 7, p. 301. 

Optical Molecular Dephasing Principles of and Probings by Coherent Laser Spectroscopy, A. H. Zewail, Acc. Chem. Res. 13, 360 (1980). 

H. R. Kim, W. J. Marinelli, and N. Sivakumar, Laser Studies of Molecular Photodissociation Dynamics, to appear in Proceedings Fourth Symposium Recent Advances in Laser Spectroscopy, May 20, 1983, Brooklyn Polytechnic Institute of New York, Wiley, New York. 

Ionic crystals also support SEW, but again no data exists where they have been used as substrates for attached molecule studies. That such studies may be feasible is illustrated in Fig. 21, which shows measured and calculated propagation distances for SrTi03 in the far infrared.— Again, these measurements were made with a molecular laser as a source. Unfortunately, for many crystals the frequency region over which SEW exist is very narrow (between the transverse and longitudinal optic phonon frequencies), and propagation distances are very short. However, ferroelectrics (and near-ferroelectrics like SrTiC ) may prove useful substrates for SEW spectroscopy. 

Recent developments in the field of infrared laser molecular beam spectroscopy have lead to a wealth of information on both the structure (Jucks et al. 1988 Klemperer 1978, 1987 Lovejoy and Nesbitt 1987) and potential energy surfaces (Cohen and Saykally 1991a,b, 1992 Hutson 1988, 1990) associated with these weakly bound molecular complexes in the ground electronic state (Fraser and Pine 1989b Gough et al. 1977 Jucks et al., 1988 Kleiner et al. 1991 Mcllroy et... 

Intracavity dye laser spectroscopy (IDLS) can be a powerful technique for detecting trace species important in combustion. The technique is based on the phenomenal sensitivity of a laser to small optical losses within the laser cavity. Since molecular absorptions represent wavelength-dependent optical losses, the technique allows detection of minute quantities of free radicals by placing them inside the laser cavity and monitoring their effect on the spectral output of the laser. 

Dlott DD. Dynamics of molecular crystal vibrations. In Yen W, ed. Laser Spectroscopy of Solids II. Berlin Springer-Verlag, 1988 167-200. 

Landolt-Bornstein Numerical Data and Functional Relationships in Science and Technology, New Series Madelung, O., Ed.-in-Chief Group II Atomic and Molecular Physics Vol. 19 Molecular Constants Mostly from Microwave, Molecular Beam, and Sub-Doppler Laser Spectroscopy Subvol. a, b, and c Hiittner, W Ed. Springer Berlin, 1992. 

Dr. Rohlfing s research interests include the experimental characterization of transient molecules relevant to combustion processes, linear and nonlinear laser spectroscopies, trace detection of pollutants, molecular beam and mass spectrometric studies of carbon and metal clusters, and vibrational relaxation dynamics. He is the author of approximately 50 peer-reviewed articles, holds membership in the American Chemical Society and the American Physical Society, and is a fellow of the American Association for the Advancement of Science. 

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