Doubled frequency

Significant progress in signal enhancement methods for the central transition has been achieved by the implementation of double frequency sweeps (DFS) [62]. The basic idea of DFS, applicable for both static and MAS experiments, is to invert simultaneously the STs so that the populations of the outer spin levels are transferred to the CT energy levels before they are selectively excited (Fig. 4). 

A. P. M. Kentgens and R. Verhagen, Advantages of double frequency sweeps in static, MAS and MQMAS NMR of spin I = 3/2 nuclei. Chem. Phys. Lett., 1999,300, 435-443. 

D. luga, H. Schafer, R. Verhagen and A. P. M. Kentgens, Population and coherence transfer induced by double frequency sweeps in half-integer quadrupolar spin systems. /. Magn. Reson., 2000,147,192-209. 

A second method used to examine the same equilibria is the laser photoperturbation technique. Irradiation with a Q-switched laser pulse at 1060 nm depletes the tetrahedral isomer irradiation at the doubled frequency of 530 nm depletes the planar isomer. In both cases the same relaxation time of 0.93(4) jisec is observed for reestablishment of the equilibrium. 

RO, Fig. 3d) (2) higher-frequency, smaller amplitude, quasi-harmonic oscillations (QHO, Fig. 3a) and (3) double-frequency oscillations containing variable numbers of each of the two previous types. By far the most familiar feature of the BZ reaction, the relaxation oscillations of type 1 were explained by Field, Koros, and Noyes in their pioneering study of the detailed BZ reaction mechanism.15 Much less well known experimentally are the quasiharmonic oscillations of type 2,4,6 although they are more easily analyzed mathematically. The double frequency mode, first reported by Vavilin et al., 4 has been studied also by the present author and co-workers,6 who explained the phenomenon qualitatively on the basis of the Field-Noyes models of the BZ reaction. 

It has been mentioned in Section 7.3, and it was implicit all over Chapter 7, that a finite time is required to achieve selective saturation or inversion of a signal by a soft pulse, during which time polarization starts to be exchanged, causing non-linearity of the response (see also Section 9.3). It should be stressed that this is not the case in all common 2D experiments based on non-selective pulses, which have durations of the order of microseconds instead of milliseconds, as required for selectivity. Selectivity in 2D experiments is intrinsic because of the double frequency labeling along f and /2. 

The lack of accurate and stable frequency standards in the near-infrared spectral range, and in particular at 1083 nm, is a serious inconvenient to improve the present frequency stability of the He-locked master laser. On the other hand, hyperfine transitions of the iodine molecule has been defined as secondary frequency standard at different visible wavelengths, and in particular at 532 nm, the doubled frequency of the 1064 nm Nd YAG laser. Likewise, our idea has been to lock the master laser frequency to I2 hyperfine transitions at its doubled frequency, 541 nm. 

Using these numbers we find that the value M = 1.485 cm-1 reproduces their observed /I-doubling frequencies up to J = 25/2 extremely well. They point out, however, that the molecular parameters are strongly correlated we shall return to discuss their values later in this section. We will also discuss the relationship of this relatively simple example of /I-doubling (because mixing with only one excited electronic state is considered) to the more complicated cases encountered for other molecules. 

In this section we have described in considerable detail just one aspect of the spectroscopy of OH, namely, the measurement of zl-doubling frequencies and their nuclear hyperfine structure. This has led us to develop the theory of the fine and hyperfine levels in zero field as well as a brief discussion of the Stark effect. We should note at this point, however, that OH was the first transient gas phase free radical to be studied by pure microwave spectroscopy [121], We will describe these experiments in chapter 10. We note also that magnetic resonance investigations using microwave or far-infrared laser frequencies have also provided much of the most important and accurate information these studies are described in chapter 9, where we are also able to compare OH with the equally important radical, CH, a species which, until very recently, had not been detected and studied by either electric resonance techniques or pure microwave spectroscopy. 

Table 10.17. Experimental and calculated values (in MHz) for the A-doubling frequencies in 12 CH...

C. Metallo-1,2-enedithiolates as Double-Frequency Modulation-Based Probes / 383... 

---