Laser-microwave double resonance spectroscopy

H. Jones, Laser microwave-double-resonance and two-photon spectroscopy. Comments At. Mol. Phys. 8, 51 (1978)... 

Other experiments, carried out recently and based on nonlinear effects, involve laser-microwave double resonance measurements with intense Stark fields on H2CO, spectroscopy on NH3 and NH3 by intracavity... 

Spectroscopy conducted on ions which have been trapped by an appropriate configuration of fields leads to results void of inhomogeneous broadening sources and allows measurements with high spectral resolution. In a recent laser-microwave double resonance experiment conducted on Yb trapped ions, Blatt and co-workers (1983) resolved the ground state hyperfine splitting of the 171 isotope with a Q-factor, i.e., Avlv, of 10 . 

Optical-microwave double resonance (OMDR) can considerably improve the situation and extends the advantages of microwave spectroscopy to excited vibrational or electronic states, because selected levels in these states can be populated by optical pumping. Generally dye lasers or tunable diode lasers are used for optical pumping. However, even fixed frequency lasers can often be used. Many lines of intense infrared lasers (for example, CO2, N2O, CO, HF, and DF lasers) coincide with rotational-vibrational transitions of polyatomic molecules. Even for lines that are only close to molecular transitions the molecular lines may be tuned into resonance by external magnetic or electric fields (Sect. 1.6). The advantages of this OMDR may be summarized as follows ... 

Major advantages of microwave-optical polarization spectroscopy are narrower linewidths as compared to conventional laser-rf double resonance and smaller intensities required for the laser light field and the micro-waves, so that strongly saturating conditions can be avoided. Therefore, the sensitivity as well as the resolution can be greatly enhanced. 

Clarke and Hofeldt determined the depopulation rates for the individual triplet state spin sublevels of chlorophyll a and chlorophyll b by microwave-modulated fluorescence intensity measurements. The species was dissolved in n-octane at a temperature of 2 K. The solvent n-octane is a low-temperature host matrix which allows high-resolution spectroscopy in the chlorophyll triplet state. Triplet absorption detection of magnetic resonance as well as fluorescence-microwave double resonance techniques were applied. The experimental arrangement was described in Ref. 167. In the case of fluorescence detection, chlorophyll b was irradiated with the 457.9-nm single-mode line of an Ar" laser. Microwave transitions were... 

Meth. MW MBER Last IRIRDR IRMWDR method of measurement for ft microwave spectroscopy molecular beam electric resonance Laser Stark spectroscopy infrared-infrared double resonance infrared-microwave double resonance... 

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... 

Millimeter wave spectroscopy with a free space cell such as a Broida oven is more sensitive than lower frequency microwave spectroscopy. However, the higher J transitions monitored by millimeter wave spectroscopy often do not show the effects of hyperfine structure. In the case of CaOH and SrOH, the proton hyperfine structure was measured in beautiful pump-probe microwave optical double resonance experiments in the Steimle group [24,68], They adapted the classic atomic beam magnetic resonance experiments to work with a pulsed laser vaporization source and replaced the microwave fields in the A and C regions by optical fields (Fig. 15). These sensitive, high-precision measurements yielded a very small value for the proton Fermi contact parameter (bF), consistent with ionic bonding and a... 

Volume 11/19 brings the spectroscopic data on diamagnetic and paramagnetic molecules as well as on molecular ions up to date considering the publications up to and partly including 1990. The spectroscopic information collected in this volume has been obtained principally from gas phase microwave measurements. In addition, gas phase data have been included derived from methods related to microwave spectroscopy by employing a coherent radiation source. These are molecular beam techniques, radio frequency spectroscopy, electron resonance spectroscopy, laser spectroscopy, and double resonance techniques. Some other methods are considered if the accuracy of the derived molecular parameters is comparable to that of micro-wave spectroscopy and no microwave data are available. Examples would be Fourier infirared spectroscopy or electric deflection method. 

Mode 2 is a particularly sensitive method to detect mw transitions as will be shown in chapter III and is called microwave-optical polarization spectroscopy (MOPS). Polarization spectroscopy techniques require less intensity of laser and mw radiation than the corresponding nonlinear methods based on fluorescence detection. Power broadening is avoided which is the reason for the largely improved resolution of MOPS compared to conventional microwave optical double resonance (MOOR) spec-... 

Laser-microwave spectroscopy based on nonlinear phenomena developed from the type of experiments on molecules already discussed in Section 3.2 which make use of optical pumping or double resonance. Occasionally, the laser and the rf power were high enough to create the nonlinear phenomena mentioned above, i.e., to saturate the transitions involved and/or to induce multiphoton transitions. The intermediate level in, e.g., two-photon transitions did not have to be a real state but could be virtual as well. Therefore, a drawback often encountered in earlier infared laser-microwave experiments could be avoided if the laser transition frequency did not exactly coincide with the molecular absorption line the Stark or Zeeman effect had to be used for tuning. This results in an undesired line splitting. With laser-microwave multiphoton processes, however, the laser can be operated at its inherent transition frequency. Exact resonance with molecular lines is then achieved by using a nonlinear effect, i.e., a radiofrequency quantum is added to or subtracted from the laser frequency (see Figure 28). 

Spectroscopy utilizing tunable laser and microwave sources has been applied widely in exploring atoms, molecules, and condensed matter. Besides the classical areas of optical double resonance and optical pumping the extension of these or related methods to difference frequency measurements in the optical range seems to be of increasing importance. This includes heterodyne techniques. Laser microwave schemes can also play an essential role for the generation of modem frequen( standards. Last but not least, there will be many technical applications like infrared detectors, wavemeters, magnetometers, etc. 

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... 

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