Pulsed ENDOR


Shortly after the advent of pulsed EPR, the first pulsed ENDOR experiments were performed but did not receive the interest they deserved. It took two decades until the great potential of this method was rediscovered. With the technical and methodological advances in pulsed EPR, pulsed ENDOR has also experienced a rapid development in the last few years.

Figure 6.1
Figure 6.1: Types of pulsed ENDOR

Types of Pulsed ENDOR

In contrast to CW-ENDOR, where microwave and radio frequency fields are applied continuously, both of these fields are only applied during short time intervals in a pulsed ENDOR experiment. The microwave pulses are used to generate an electron spin echo whose intensity is then recorded as a function of the pulsed radio frequency. If the radio frequency hits a nuclear spin transition, the electron spin echo intensity will change and the rf sweep over a certain frequency range results in the ENDOR spectrum.

Pulsed ENDOR spectroscopy has several advantages over the conventional CW-technique:

  • it requires no critical balance of rf power and relaxation times, a condition which has to be met in many cases of CW-ENDOR
  • it is less susceptible to artifacts as there is no rf and no microwave field applied during the detection period
  • it gives access to all relaxation times of a spin system (electron T1 & T2, nuclear T1 & T2, cross relaxation)
  • it complements the results obtained from Electron Spin Echo Envelope Modulation (ESEEM)
  • it allows the manipulation of the spin system to observe one particular spin property with high selectivity and sensitivity
  • the ENDOR effect can be as large as the electron spin echo intensity itself

A great number of special techniques have been developed to improve spectral clarity, selectivity and sensitivity. We mention only a few of those which are part of the E560D-P capability:

  • suppression of the matrix line
  • separation of large and small hyperfine couplings which arise from different nuclei
  • hyperfine selection via FT-EPR detection
  • counting the number of equivalent nuclei
  • correlation spectroscopy (TRIPLE, 2D-ENDOR, ENDOR induced EPR and HYEND)
  • measurement of nuclear relaxation times with nuclear spin echoes
  • measurement of the natural ENDOR linewidth

ESEEM and Pulsed ENDOR

Both ESEEM and Pulsed ENDOR are methods used in conjunction with a FT-EPR instrument to study the hyperfine interaction between electron spins and nuclear spins. Depending on the interaction parameters, the sensitivity of the two techniques can be drastically different. There are cases where only one of the techniques can be employed and there are situations where complementary results are obtained from ESEEM and Pulsed ENDOR. The sensitivity of ESEEM is highest at low hyperfine transition frequencies and decreases with increasing frequency due to the limited microwave field strength. The sensitivity of Pulsed ENDOR (and also CW ENDOR) is approximately proportional to the hyperfine transition frequency. At zero frequency, the ENDOR sensitivity is zero. ENDOR is therefore extremely difficult at low transition frequencies whereas it has a high sensitivity for high frequencies. ESEEM can only be used for solid samples. In liquid solution, the ESEEM sensitivity is zero. This restriction does not apply to Pulsed ENDOR.

The ESEEM line shapes of powder samples are predominantly due the ESEEM transition probabilities, while Pulsed ENDOR powder line shapes reflect the statistical powder average.

Figure 6.2
Figure 6.2: The line pair at approximately 10 and 20 MHz has a smaller separation in the ESEEM spectrum (top) than in the pulsed ENDOR spectrum (bottom).

Time Domain Pulsed ENDOR

A number of time-domain pulsed ENDOR sequences have been developed to measure nuclear transient nutations, FIDs and spin echoes. These techniques open the way to the nuclear relaxation times and provide the ultimate resolution in ENDOR spectroscopy. The technical demands of short rf pulses and rf phase cycling capabilities have been fulfilled and the incorporation of these advanced pulse schemes into PulseSPEL eases the use considerably.

Nulcear Spin Echo

In general, inhomogeneously broadened lines can be studied by spin echo techniques. This now also applies to ENDOR spectra. Rf pulses of 2µs were used to generate the nuclear coherences in the center of an ENDOR line. The nuclear FID and echo is then indirectly detected as a change in the ESE amplitude.

Figure 6.3
Figure 6.3: Nuclear spin echo

Transient Nutation

The number of equivalent nuclei belonging to a certain ENDOR line can be determined by a transient nutation experiment. Two equivalent nuclei produce a notation pattern like the one shown in the top trace, while only one nuclear spin results in a nutation signal as shown in the lower trace.

Figure 6.4
Figure 6.4: Transient nutation

Nuclear Spin FID

Figure 6.5 shows the FID of the PNT ENDOR line at 6.4 MHz and its Fourier transformed spectrum. The FID was excited by an rf pulse of 1.5 μs at a carrier frequency of 6.6 MHz. As there is no driving field applied during detection, the ultimate resolution is obtained. This is demonstrated by the line splitting of 14 kHz. A 4-step rf phase cycle was employed for elimination of the electron spin relaxation decay and for quadrature detection. Note that the frequency scale of the FT spectrum is relative to the rf carrier.

Figure 6.5
Figure 6.5: nuclear spin FID