Field modulation

Choice of magnetic field modulation frequency is based on the following considerations (1) minimization of noise by converting resonance information to a relatively noise-free frequency, (2) experiments where the field modulation is itself in the spin Hamiltonian and is a crucial physical parameter such as saturation transfer spec- 

There is an intrinsic experimental problem when using magnetic field modulation in ESR spectroscopy that is associated with spurious resonance-like modulations transferred to the microwave carrier and arising from modulations of the resonator microwave characteristics. This phenomenon, perhaps apocryphally, was called potato by F. Bloch because of the resemblance when the effect is displayed as a lissajous figure. 

Potato of the first kind involves acoustic coupling to the resonator of vibrations in the field modulation coils that arise from forces determined by the cross product of current and magnetic field. It is controlled by mechanical damping and acoustic isolation. 

Potato of the second kind arises from forces determined by the cross product of the magnetic field with eddy currents induced in metallic walls of the resonator by the modulation field. It is controlled by interrupting eddy current paths and using resonators with wall thicknesses less than one skin depth thickness at the field modulation frequency. 

Mechanical displacements associated with potato are in the micron range. Quantitation of potato effects is difficult, and no perfect solution to potato problems exists. Every ESR spectroscopist is aware of the problem, and partial solutions can often be found based on the conceptual models outlined in this section. 

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Figure Bl.15.5. Effect of small-amplitude 100 kHz field modulation on the detector output current. The static magnetic field is modulated between the limits and The corresponding detector current varies between the limits 1 and/. The upper diagram shows the recorded 100 kHz signal as a fiinction of B. After [3].

ColvinVL, Cunningham K L and Alivisatos A P 1994 Electric field modulation studies of optical absorption in CdSe nanocrystals dipolar character of the excited state J. Chem. Phys. 101 7122... 

For each EA spectrum, the transmission T was measured with the mechanical chopper in place and the electric field off. The differential transmission AT was subsequently measured without the chopper, with the electric field on, and with the lock-in amplifier set to detect signals at twice the electric-field modulation frequency. The 2/ dependency of the EA signal is due to the quadratic nature of EA in materials with definite parity. AT was then normalized to AT/T, which was free of the spectral response function. To a good approximation [18], the EA signal is related to the imaginary part of the optical third-order susceptibility ... 

Meyers F, Marder SR, Pierce BM, Bredas JL (1994) Electric field modulated nonlinear optical properties of donor-acceptor polyenes sum-over-states investigation of the relationship between molecular polarizabilities (a, p, and y ) and bond length alternation. J Am Chem Soc 116 10703-10714... 

In this experimental system, it was necessary to trigger the laser pulse, the microwave pulse, and field modulation at the right timing, then to... 

Figure 1.5 Small-amplitude field modulation converts the absorption curve into a first-derivative.

Microwave frequency Center field Modulation frequency Modulation phase... 

With most spectrometers, you have a choice of either 100 kHz or a lower frequency of field modulation. The higher frequency generally gives better S/N, but if the lines are unusually sharp (<0.08 Gauss), 100 kHz modulation leads to side bands , lumps in the line shape that confuse the interpretation of the spectrum. This effect is illustrated in Figure 1.12. Under such circumstances, use the lower frequency for which the sidebands are closer together and thus less likely to be a problem. 

When the absorption is detected via small amplitude field modulation, the signal is proportional to the first derivative of absorption ... 

Fig. 10. The ESR signal produced at various points on the resonant line in a magnetic field modulated spectrometer. The vertical magnetic field modulation interacts with the bell-shaped adsorption curve [F(H)1 to produce the horizontal ESR signal. Here AH is the half amplitude line width and Hu is the center of resonance (S3).

The measurement of the Stark effect were carried out with the electric-field modulation technique at room temp, in vacuo (about 10 3 torr). A sinusoidal ac voltage (500 Hz) was applied between the A1 electrodes. Then, the change in transmittance induced by the applied electric field were measured with a phase-sensitive detector (NF Electronic Instruments LI-575A) at the fundamental frequency. 

Fio. 3. Imaginary (x O and real parts of rf susceptibility as a function of magnetic field strength H. Field modulation and resulting NMR signal are schematically illustrated for the absorption curve (x ). 

EPR spectra were recorded with a Varian E9 X-band spectrometer using field (100 kHz) and light (13 or 83 Hz) modulation with phase-sensitive detection at the modulation frequencies (19). Typically, the field modulation amplitude employed ranged from 20 to 40 gauss, the microwave power from 0.1 to 0.5 mW. Measurements were performed on frozen solutions of the porphyrins at about 100 K using the standard Varian variable temperature accessory or at about 10 R with an Oxford Instruments helium gas cryostat. Light sources used for photoexcitation were a 1000 W Xe arc source powered by a Photochemical Research Associates Supply with electronic modulation... 

Figure 3, Triplet EPR spectra of TCP 10 M) in CH Cl-CH OH recorded at about 10 K. Microwave power 0,5 mW field modulation 20 G (100 kHz) excitation with square wave modulated (83 Hz) light of an argon laser (514.5 am 0.5 W). (a) No addition (b) with NaCl (- 5 x 10 M) and (c) with KCl ( 5 X 10 N). Absorption and emission peaks have been labeled A and E. respectively.

Figure 4, Triplet EPR spectra of ZnTCP ( -10 ) in Cn Cl-CII OH recorded at 10 K before (solid line) and after (dotted line) addition of KCl ( S x 10 ). Microwave power 0.5 mW, field modulation 20 gauss (100 kHz), excitation with square wave modulated (13 Hz) light from an Xe high pressure arc (1000 W) passed through a CuSO heat filter.

Figure 4.5 Schematic drawing of system and bath, (a) Amplitude noise (AN) (red) combatted by AC-Stark shift modulation (green), (b) Phase noise (PN) (red) combatted by resonant-field modulation (green). (See color plate section for the color representation of this figure.)...

A thorough discussion Is given of the field modulation technique, a new stationary relaxation method based on electric field perturbation of Ionic equilibria. Concomitantly the theory of electric field effect In Ionic systems Is reviewed especially stressing their Importance for conductance phenomena In low polar solutions. 

Conductance relaxation Is also shown to be critically dependent upon aggregation equilibria affecting non-conducting (ion-pairs) as well as Ionic species. The relaxation behavior In the presence of quadrupoles (ion-pair dimers) and triple Ions Is thoroughly analyzed. The experimental results show the potential of the field modulation techniques as a method for the Investigation of ionization processes, independent of conductance measurements. 

To investigate the dynamic properties of ionic species present at very low concentration in apolar solvents the field modulation method is particularly suited In this... 

In this paper we review the field modulation method especially to indicate the applicability for the investigation of organic reaction mechanisms where ionic species may intervene. 

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