Microwave rotational spectroscopy resonance

Electron spin resonance (e.s.r.) spectroscopy, applied to free radicals in condensed phases, is a long established technique with several commercially available spectrometers. The gas phase applications we will describe have little in common with condensed phase studies, and are much more a part of rotational spectroscopy. However, the experimental methods used for condensed phase studies can be applied to the study of gases with rather little change, so it is appropriate first to describe a typical microwave magnetic resonance spectrometer, as illustrated schematically in figure 9.1. 

The main conclusion from these results is that the observed hyperfine splitting is determined primarily by a linear combination of the hyperfine constants corresponding to the three separate interactions. The spectrum depends upon the axial component ofthe total magnetic hyperfine interaction, which we designate / 3/2 (= a + (1 /2)(b + 2c/3)), and in a good case (a) system it is not usually possible to separate the individual contributions from the microwave magnetic resonance spectrum alone. The solution to the problem lies in the combination of these studies with pure rotational spectroscopy, as we shall see later in this chapter. 

An important development in microwave/optical double resonance, called microwave/optical polarisation spectroscopy, was described by Ernst and Torring [42], The principles of this technique are illustrated in figure 11.22. A linearly polarised probe beam from a tunable laser is sent through the gas sample and a nearly crossed linear polariser, before its final detection. Polarised microwave radiation resonant with a rotational transition in the gas sample is introduced via a microwave horn as shown, and resonant absorption results in a partial change in polarisation of... 

Fourier transform microwave (FTMW) spectroscopy is very similar to nuclear magnetic resonance, except it probes rotational levels of molecules,... 

Double-resonance Spectroscopy.—A review has been given of double-resonance methods in spectroscopy.378 Attention will be focused here on optically (usually phosphorescence) detected magnetic resonance experiments (ODMR). Microwave-optical double-resonance experiments have been carried out on the spectrum of gaseous N02,379 permitting assignment of the rotational = 0—4 side-bands of the 493 nm band. 

There are many experimental techniques for the determination of the Spin-Hamiltonian parameters g, Ux, J. D, E. Often applied are Electron Paramagnetic or Spin Resonance (EPR, ESR), Electron Nuclear Double Resonance (ENDOR) or Triple Resonance, Electron-Electron Double Resonance (ELDOR), Nuclear Magnetic Resonance (NMR), occasionally utilizing effects of Chemically Induced Dynamic Nuclear Polarization (CIDNP), Optical Detections of Magnetic Resonance (ODMR) or Microwave Optical Double Resonance (MODR), Laser Magnetic Resonance (LMR), Atomic Beam Spectroscopy, and Muon Spin Rotation (/iSR). The extraction of data from the spectra varies with the methods, the system studied and the physical state of the sample (gas, liquid, unordered or ordered solid). For these procedures the reader is referred to the monographs (D). Further, effective magnetic moments of free radicals are often obtained from static... 

Over the years a number of physical methods have been applied to study the mechanism of dye sensitization of semiconductor electrodes. The techniques can be broadly divided into photoelectrochemical and spectroscopic ones. The following are some that fall under the spectroscopic methods flash photolysis (diffuse reflection or internal reflection), photoacoustic spectroscopy. Resonance Raman spectroscopy and microwave absorption. Amongst electrochemical methods, potential modulation and rotating ring disc techniques are the commonly used ones... 

A new very sensitive and accurate double-resonance technique is the Microwave-Optical double-resonance Polarization Spectroscopy (MOPS) developed by Ernst et. al [10.93]. This technique detects microwave transitions in a sample between crossed polarizers through the change in transmission of a polarized optical wave. The sensitivity of the method has been demonstrated by measurements of the hfs of rotational transitions in the electronic ground state of CaCl molecules which were produced by the reaction 2Ca+ CI2 - CaCl in an argon flow. In spite of the small concentrations of CaCl reaction products and the short absorption pathlength in the reaction zone a good signal-to-noise ratio could be achieved at linewidths of lf2 MHz [10.94]. 

See also Atmospheric Pressure Ionization in Mass Spectrometry Cosmochemical Applications Using Mass Spectrometry Environmental Applications of Electronic Spectroscopy Interstellar Molecules, Spectroscopy of Microwave Spectrometers Rotational Spectroscopy, Theory Solid State NMR, Rotational Resonance Vibrational, Rotational and Raman Spectroscopy, Historical Perspective. 

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