Page 132 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

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Encyclopedia of Physical Science and Technology EN005E-212 June 15, 2001 20:32

Electron Spin Resonance 343

T is swept to produce the echo modulation pattern. This
can simplify analysis of the pattern. Also, the echo decay
becomes slower since it is usually dominated by the spin–
lattice relaxation time T 1 . This results in more detectable
FIGURE 9 Illustration of a two-pulse electron spin-echo signal.
modulation periods and hence more accurate analysis. A
Microwave pulses 1 and 2 separated by time τ produce the echo
four-pulse sequence in which an additional 180 pulse
◦
signal V at a second time τ after pulse 2. As τ is increased the
is introduced in the middle of the T period for a three-
echo amplitude generally decreases, and in solids the amplitude
may be modulated as shown here. The decrease in echo ampli- pulse sequence is also useful for measuring combination
tude is related to transverse magnetic relaxation times, and the frequencies.
modulation is related to weak anisotropic hyperfine coupling to Pulsed ENDOR has also become a more common tech-
nearby magnetic nuclei.
nique. This is achieved by adding a radio frequency pulse
within a spin-echo pulse sequence. Then, by detecting the
along the z axis to the x–y plane. During time τ between echo intensity while the radio frequency is swept, one can
the pulses the spins precess in the x–y plane. After time obtain a pulsed ENDOR spectrum which directly reveals
τ a second pulse (2) is applied, which is a 180 pulse and electron–nuclear hyperfine frequencies.
◦
which flips the spins into the other direction in the x–y
plane. The spins then precess back together, and at a sec-
ond time τ after the second pulse they coalesce and form X. APPLICATIONS
a burst of microwave energy called an echo (V). As the
time between the pulses is increased, the echo intensity Electron spin resonance is widely applicable to organic,
decreases; in liquids this decrease is exponential with a inorganic, and biological systems. The most common ap-
time constant that gives the spin–spin relaxation time T 2 . plication is probably the identification of paramagnetic
In solids the decay behavior is usually more complex and reaction intermediates in chemical reactions or in mate-
is only indirectly related to T 2 . rials after various physical or chemical treatments. This
In solids the decrease in echo intensity is often modu- identification is generally possible by determination of the
lated with increasing τ, as shown in Fig. 9; this modulation geometric structure of the paramagnetic species by virtue
is related to weak anisotropic hyperfine interactions with of hyperfine interaction with magnetic nuclei in the para-
surrounding nuclear spins. It is of particular interest that magnetic species. It is also often desired to determine the
this modulation is retained in a disordered sample such location of a paramagnetic species in a solid material. This
as a powder or a frozen solution, so that this technique can be accomplished, in principle, by detecting very weak
provides an interesting new approach to obtain structural hyperfine interactions with nuclei in the material and may
information about paramagnetic species in disordered sys- require double-resonance or time-domain electron mag-
tems. The interpretation of electron spin-echo modulation netic resonance techniques. The location of paramagnetic
patterns has been used to determine detailed geometric species in solid systems is particularly important for cata-
information about the solvation structure of paramagnetic lysts, polymers, and frozen systems of biological interest.
species such as metal cations, molecular anions, and even Another important application of electron spin reso-
solvated electrons. This electron spin-echo modulation nance is to directly determine the electronic structure
technique has also been used to study the coordination of of free radicals by measuring spin densities at various
paramagnetic species on catalytic oxide surfaces, as well locations within the radical species. Experimental spin
as in a variety of other systems of practical interest. The densities are also used to directly test the validity of ap-
information from electron spin-echo modulation analysis proximate molecular wave functions. Electron spin reso-
is essentially the same as what one would obtain from re- nance has been one of the major ways to evaluate various
solved ENDOR spectra in disordered systems. However, quantum-mechanical approximations for the determina-
in most actual disordered systems, resolved ENDOR is tion of molecular wave functions.
usually not seen, which demonstrates the advantage of the Since electron spin resonance is an excellent analyt-
electron spin-echo modulation method. ical method for paramagnetic species and free radicals,
In addition to the two-pulse spin echo illustrated in it can be used to obtain a variety of kinetic and ther-
Fig. 9, more-complicated pulse sequences are now being modynamic data. In this respect it is used in the same
routinely used. In a three-pulse sequence, the second 180 ◦ way as any other spectroscopic technique. Kinetic data
pulse in a two-pulse sequence is split into two 90 pulses can be obtained by studying radical intensity versus time.
◦
separated by time T . Then the first experimentally control- By using time-domain electron magnetic resonance tech-
lable interpulse time τ can be adjusted so as to eliminate niques such as electron spin-echo spectroscopy, one can
one nuclear modulation while the second interpulse time detect transient species with lifetimes as short as 100 nsec.