Overview & General Strategy

FREQUENCY

3. PERFORMANCE COMPARISON OF ENDOR COILS

3.1 Overview & General Strategy

2.4.4 ENDOR Spectra Above 50 MHz

One does not often see reports of ENDOR spectra above 50 MHz, and the reason for this may be inferred from Figure 5 or in practice by watching the amplifier power meter as one sweeps the rf carrier frequency upwards beyond 30 MHz; the power is dramatically attenuated beyond 50 MHz with the standard probehead consisting of a 20 tum wire coil and a cylindrical cavity because the impedance has simply increased to the point where the coil behaves as a cut-off filter. The remedy for this is to either use a low-inductance coil or incorporate an impedance matching network into the circuit so that the transmitter's impedance is close to the optimum 50

n

(Section 4).

4. ENDORCOILS AND RELATED RAoIOFREQUENCY CIRCUITS 169

which is detected as a power fluctuation at the detector. The power fluctuation is proportional to the loaded resonator's quality factor, that is,

dP

= t

77X" PQ, ⁽⁵⁾

and therefore the sensitivity of the experiment will be deleteriously affected if the Q-factor is lowered (cf. Poole, 1983; Talpe, 1971).

ENDOR coils were initially wound externally to high Qcavities (Feher&

Gere, 1959; Hyde, 1965; Christidis^& Heineken, 1973; Gruberetal., 1974).

This approach on the one hand has the advantage that the cavity Qis not spoiled, but, on the other hand, has the disadvantage that the volume enclosed by the coil is large. The generation of a radiofrequency field that is suitable for ENDOR spectroscopy therefore requires operation with very high powers because the field is inversely proportional to the coil diameter (H

=

No!Ir, Section 2.1). External coils are used to best advantage with small resonators, such as high frequency cavities (Yagi et al., 1970; Wang ^&

Chasteen, 1995) and loop-gap resonators (Newton& Hyde, 1991).

The most efficient means of applying the radiofrequency field to the sample is by placing the coil inside the microwave cavity resonator and compensating for Q-factor spoilage by carefully selecting the cavity mode, sample orientation, and coil type. For example, solution ENDOR requires both high microwave and radiofrequency power in order to overcome the rapid relaxation rates of free radicals, and because many liquids also tend to be lossy, cavityQis also lowered. The cylindrical TM_lIocavity is well suited to these experiments because a capillary tube containing the sample material can be inserted along the cavity's cylindrical axis with minimum perturbation to the electric field lines of TMlIomode. The TMlIocavity is further suited to solution EMR by the fact that the mode's Q-factor increases with the cavity's length . At room temperature, where there is little concern for temperature gradients along the sample length, an elongated cavity will accommodate more sample material while at the same time feature a higher Q.Modification of the TMI IO cavity for ENDOR (Biehl et al., 1977;

Zweygart et al., 1994) can be achieved by inserting a wire helix into the cavity with the coil's cylindrical axis colinear with the cavity's. The wire turns of the coil run very nearly perpendicular to the electric field lines of the TM110 mode and the resultant probehead geometry is therefore suited for optimum signal-to-noise. Silver wire helices wound onto quartz tubing of

1-2mm thickness and outer diameter between 8 and II mm' have been used in ENDOR studies of free radicals in solution when used with the

5 Poole (1983) and Alger (1968) list cavity factors that affectSIN. Likewise, the Varian operation manual features a section by Hyde on radiofrequency field concentration by quartz tubing inserts; for example, an optimum signal intensity was achieved in a rectangular TE cavity with a quartz tube~o=10 mm and~i=6 mm

commercial TMllO cavity (Bruker Instruments; part number ENB-250).

Prototype TMll o cavities with slightly longer axial lengths (l

=

5 em ^VS. 4 em; Bender, unpublished) yielded higher Qand signal-to-noise for identical sample concentrations. Leniart (1979) describes the coil arrangement of the now obsolete Bruker ER-420 ENDOR probe, which was the forerunner of the present version.

The elongated TMl lo cavity can be used for ENDOR spectroscopy of solid samples and frozen solutions, but one must be concerned about the sample length in flow dewars ." A better cavity design tactic for ENDOR studies of frozen solutions with the TMll o cavity entails decreasing the cavity's axial length, which increases the filling factor II in Equation (5).

There is some sacrifice in cavity Q, but the sample now occupies a greater portion of the cavity's axial length and thereby optimizes n . The combined decrease inQand increase of II is a favorable design feature for electron spin echo detected ENDOR. TM_llOcavity prototypes with axial lengths as short as 0.5" have been successfully tested by the author with stable radical-containing powders (y-irradiated sucrose and citric acid); details of their construction are given in Appendix I.

The cylindrical TEo I I and TEI 12 cavities are also excellent sample resonators for ENDOR on solid samples that require temperature control.

Both cavities have an inherently high Q-factor and can be used with two or four parallel posts that are arranged inside the cavity so that they are parallel to the cavity's central axis. The posts are externally joined and configured as a loop in order to generate the radiofrequency field, although the magnitude of the field produced by the loop is less than that of the helix for the same current . This method may also be applied to the rectangular TElo2cavity, and one variant (Castner & Doyle, 1968) orients the posts horizontally in a rectangular TE201cavity.

The following section describes several coil types and their network characteristics that are derived from S-parameter measurements made by using a Hewlett-Packard 8753A Network Analyzer and S-Parameter Test Set. Graphical and numerical data are presented for such parameters as ^Sl), SWR, reactance, and the Smith Chart. For each case study the test coils were constructed so as to be compatible with the Bruker ENB-250 cylindrical TM_llO(helix), the Varian or Bruker rectangular TE102 (posts, stripline), or Varian V4533 cylindrical TEoll (post, stripline) cavities. Wire loops and coils were wound onto quartz tubes of specified diameter and had a length of 1 inch. Parallel posts were made compatible with the cylindrical TEo1Icavity

6Liquid immersion dewars (nitrogen or helium) can be used with the TM₁₁₀cavity, but one must use care to prevent bubbling of the cryogen. See Alger, 1968.

4. ENDOR COILS AND RELAT ED RADIOFREQUENCY CIRCUITS 171 by using special modified end plates.' All coils were soldered directly to a50

o

Type-N bulkhead connector that mated a pair of 1 m cables that connected the device to the S-Parameter Test set and network analyzer. The test coils were constructed of commercial magnet wire (Belden Wire and Cable, Richmond, IN; MWS Wire Industries, Westlake Village, CA) that had an enamel coat; in spectroscopic experiments the coils are fabricated from silver wire (helices) or German silver (posts). The network behavior of the standard magnet wire was identical to silver wire of the same gauge.

Table J.Network parameters ofa 20-tum wire coil (24 ga.) wound ontoalOmm o.d.

quartz tube (1 mm wall).

f(MHz) 0.15 0.25 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50 10.00

R(O) 49.8 50.2 50 .5 51.6 53.3 55.7 59.0 63.2 68.9 75 .8 85.0 96 .9 112 132 156 187 219 245 254 237 201

160

X(O) 5.7 5.2 6.9 12.1 17.8 23 .6 29 .6 35 .8 42.4 48.9 55.6 62.0 67.5 71.0 70.0 61.0 38 .7 0.0 -53.0

-110 -144 -160

LorC 6.0 J.1H 3.3 J.1H 2.2 J.1H 1.9 J.1H 1.9 J.1H 1.9 J.1H 1.9 J.1H 1.9 J.1H 1.9 J.1H 1.9 J.1H 2.0 J.1H 2.0 J.1H 1.9 J.1H 1.9 J.1H 1.7 J.1H 1.9 J.1H 0.8 J.1H 25 nF 350 pF 170 pF 120 pF 100 pF

SWR(1: ) 1.12 1.10 1.15 1.27 1.42 1.58 1.76 1.94 2.16 2.38 2.63 2.90 3.19 3.50 3.80 4.16 4.53 4.19 5.30 5.74 6.18 6.60

, The walls of the V4533 cons ist of a winding of silver wire that can be detached from the end plates, which are aluminum and were duplicated in the shop with fittings for mounting the ENDORcoil(s) .

8 An alloy of copper, nickel and zinc. Also known as nickel silver (Goodfellow Corp., Cambridge, UK and Berwyn, PA).