- Email: [email protected]
Adiabatic rapid passage ESR of natural diamond
JOURNAL
OF MAGNETIC
RESONANCE
74, 155- 157 ( 1987)
Adiabatic Rapid Passage ESR of Natural Diamond I.D. CAMPBELL CSIRO, Division of Materials Science and Technology, Normanby Road, Clayton, Locked Bag 33, Clayton, Victoria 3168, Australia Received December 15, 1986; revised February 24, 1987
A comprehensive review of the ESR spectrum of natural and synthetic diamonds has been published by Loubser and van Wyk (1). Spectra listed include the well-known three line spectrum, PI, due to isolated nitrogen atoms substituting for carbon in the diamond lattice, the Nl and N4 spectra due to pairs of nitrogen atoms, the P2 spectrum due to three nitrogen atoms, and spectra due to more complex configurations. More recent work can be found in Refs. (2-7). Such ESR spectra were obtained by tuning the microwave spectrometer bridge to the low power, in-phase absorption mode. Alternatively, the bridge could have been set to detect the out-of-phase response to record the sample dispersion. Using conventional magnetic field modulation, both absorption and dispersion are recorded as derivatives with respect to the magnetic field sweep. However, at higher microwave power levels, when the in-phase response saturates, Hyde (8) has shown that the absorption can be recorded from the out-of-phase response directly, not as a derivative with respect to the magnetic field sweep, provided certain “adiabatic rapid passage” (ARP) conditions are satisfied. These conditions were used in the present investigation. The spectrometer was a conventional Varian 4502 apparatus equipped with a dual sample cavity. A “strong pitch” sample was used as the standard comparison sample. Since this sample cannot easily be power saturated, the bridge can be unambiguously tuned to either the in-phase or out-of-phase response by observing the signal of the strong pitch sample. All the present diamond spectra were recorded at 293 K. The present work arose from an ESR survey of 25 diamond samples which had also been characterized by a variety of other spectroscopic techniques. Only three typical diamond samples are described here. The incentive to use the ESR ARP mode arose from the fact that all three diamonds saturated in the conventional in-phase absorption mode unless 20 dB of attenuation was applied. Since the average sample weight was only 17 mg, computer signal averaging was necessary to detect the absorption mode with acceptable signal to noise. However the ARP signal was clearly recorded with a signal-to-noise ratio of 50: 1. The diamond samples were obtained from Drukker and Zn., Amsterdam, and classified as type la. ESR spectra due to nitrogen atoms in a variety of configurations were therefore to be expected. A typical ARP signal is shown in Fig. 1. The diamond was carefully aligned with a ( 100) axis parallel to the magnet field Ho. Centered on g = 2.003 is a broad, 15 G 155
0022-2364187 $3.00 Copyright 0 1987 by Academic Ress, Inc. All rights of reproduction in any form reserved.
156
NOTES
,
I -40
I
I 0
1
I
I
I 4oc
RG. 1. Adiabatic rapid passage spectrum of nitrogen in diamond showing the three sharp lines of the Pl spectrum of isolated nitrogen atoms and the central broad line of aggregated nitrogen. A computed Gaussian curve (dotted) is fitted to the broad line. Also shown is the derivative of an absorption spectrum taken from Ref. (7) see text.
line recognizable as the P2 spectrum attributed to three nitrogen atoms substituting for three next nearest carbon atoms in a { 1 1 1 } plane. A computed Gaussian curve (dotted line) gave a good fit to this central broad line. This confirms that the spectrum represents an inhomogeneously broadened line recorded under ARP conditions (8). Also shown is the absorption derivative spectrum obtained by Loubser and Wright (7). The comparison of line positions is good but not exact. Possibly different nitrogen aggregates are present in the two samples. The three sharp lines of the Pl spectrum (centered on g = 2.0024, each 0.5 G wide at half height) due to isolated substitutional nitrogen atoms are also clearly visible. An expanded trace of the low-field line is shown in Fig. 2a. The line profile cannot be fitted by a Gaussian curve but is described rather well by a Lorentzian curve (dotted line). The magnitude of the audio modulation (100 kHz) was estimated as 0.2 G peak-to-peak, well within the overall linewidth, but greater than the individual contributing spin packets, assuming inhomogeneous broadening. -1 I
0 I
1C I
-5 I
0 I
5c 1
FIG. 2. (A) An expanded trace of the low-field line of the Pl spectrum of Fig. I. The satisfactory fit of a superposed Lorentzian curve (dotted) suggests line narrowing due to strong dipole-dipole interactions. (B) An expanded trace of a similar line in another diamond sample, recorded with gain X8. (Note change of horizontal scale.) The fit of a superposed Gaussian curve indicates line broadening due to weak dipoledipole interactions.
The value of T, was estimated by using the relationship
derived by Portis (9)
tan($ - 90”) = w,T,, connecting the observed audio modulation phase lag of the recorded signal, 4, with the modulation frequency w, . As Mailer and Taylor (10) point out this method of determining Tl “does not depend upon the difficult and uncertain determination of the value of Hi .” Experimentally one maximizes the chart record of the diamond by carefully adjusting the lock-in reference phase, &, of the lock-in detector. The modulation cable is then transferred to the strong pitch reference sample and the optimum phase 4, determined. The relative phase lag,
is found to change from 180” to 90” as the modulation frequency is reduced in steps from 100 kHz to 35 Hz. Despite poor signal to noise at the lower frequencies, a value of T, = 1.7 f 0.7 ms was derived. This value is not significantly different from the value 2.7 + 1.2 ms measured by Barklie and Guven (3) for a similar diamond. It is of interest that this value of T, applies to the whole ESR spectrum observed in the present experiment, despite the presence of contributions from a variety of paramagnetic centers. A more careful measurement with better signal to noise might show that the PI and P2 spectra have slightly different values of T, . Finally, some idea of the relative proportions of isolated nitrogen atoms to paramagnetic nitrogen aggregates, and possibly other paramagnetic centers, can be made by comparing the areas of the PI and central region of spectrum. A ratio of 0.19 results. A comparison of the central region with “weak pitch,” using the unsaturated absorption mode, gives a spin density of about 2 X lOi spins cmm3. Thus the nitrogen atoms causing the Pl spectrum have a density of about 4 X lOI spins cmp3. The spectra described above were representative of many of the diamonds of the original set of 25. However, at least one diamond sample showed broader lines in the P 1 spectrum, having a value of 3.1 G, as against 0.5 G for the more typical spectrum. The relative proportion of isolated to aggregated paramagnetic nitrogen is 0.16. The shape of these broader P 1 lines was Gaussian, Fig. 2b. It is interesting that Samsonenko (11) observed a spectrum of nitrogen in diamond which showed a superposition of the broad and narrow Pl spectra. He ascribed the broadening to weak dipole-dipole interactions arising from the near proximity of other “isolated” nitrogen impurity atoms. REFERENCES 1. J. H. N. LOUBSER AND J. A. VAN WYK, Rep. Prog. Phys. 41, 1201 (1978). 2. C. M. WELBOLJRNE, Solid State Commun. 26, 255 (1978). 3. R. C. BARKLIE AND J. GUVEN, J. Phys. C 14, 3621 (198 1). 4. J. A. VAN WYK, J. Phys. C 15, L981, (1982). 5. J. A. VAN WYK AND J. H. N. LOUBSER, J. Phys. C 16, 1501 (1983 1. 6. C. A. J. AMMERLAAN AND R. VAN KEMP, J. Phys. C 18,2623 (1985). 7. J. H. N. LOUBSER AND A. C. J. WRIGHT, Diamond Rex, 16 (1973). 8. J. S. HYDE, Phys. Rev. 119, 1483 (1960). 9. A. M. PORTIS, Phys. Rev. 100, 1219 (1955). 10. C. MAILER AND C. P. S. TAYLOR, Biochim. Biophys. Acta 322, 195 (1973). I I. N. D. SAMSONENKO, Sov. Phys. Solid State 6, 2460 (1965).
OF MAGNETIC
RESONANCE
74, 155- 157 ( 1987)
Adiabatic Rapid Passage ESR of Natural Diamond I.D. CAMPBELL CSIRO, Division of Materials Science and Technology, Normanby Road, Clayton, Locked Bag 33, Clayton, Victoria 3168, Australia Received December 15, 1986; revised February 24, 1987
A comprehensive review of the ESR spectrum of natural and synthetic diamonds has been published by Loubser and van Wyk (1). Spectra listed include the well-known three line spectrum, PI, due to isolated nitrogen atoms substituting for carbon in the diamond lattice, the Nl and N4 spectra due to pairs of nitrogen atoms, the P2 spectrum due to three nitrogen atoms, and spectra due to more complex configurations. More recent work can be found in Refs. (2-7). Such ESR spectra were obtained by tuning the microwave spectrometer bridge to the low power, in-phase absorption mode. Alternatively, the bridge could have been set to detect the out-of-phase response to record the sample dispersion. Using conventional magnetic field modulation, both absorption and dispersion are recorded as derivatives with respect to the magnetic field sweep. However, at higher microwave power levels, when the in-phase response saturates, Hyde (8) has shown that the absorption can be recorded from the out-of-phase response directly, not as a derivative with respect to the magnetic field sweep, provided certain “adiabatic rapid passage” (ARP) conditions are satisfied. These conditions were used in the present investigation. The spectrometer was a conventional Varian 4502 apparatus equipped with a dual sample cavity. A “strong pitch” sample was used as the standard comparison sample. Since this sample cannot easily be power saturated, the bridge can be unambiguously tuned to either the in-phase or out-of-phase response by observing the signal of the strong pitch sample. All the present diamond spectra were recorded at 293 K. The present work arose from an ESR survey of 25 diamond samples which had also been characterized by a variety of other spectroscopic techniques. Only three typical diamond samples are described here. The incentive to use the ESR ARP mode arose from the fact that all three diamonds saturated in the conventional in-phase absorption mode unless 20 dB of attenuation was applied. Since the average sample weight was only 17 mg, computer signal averaging was necessary to detect the absorption mode with acceptable signal to noise. However the ARP signal was clearly recorded with a signal-to-noise ratio of 50: 1. The diamond samples were obtained from Drukker and Zn., Amsterdam, and classified as type la. ESR spectra due to nitrogen atoms in a variety of configurations were therefore to be expected. A typical ARP signal is shown in Fig. 1. The diamond was carefully aligned with a ( 100) axis parallel to the magnet field Ho. Centered on g = 2.003 is a broad, 15 G 155
0022-2364187 $3.00 Copyright 0 1987 by Academic Ress, Inc. All rights of reproduction in any form reserved.
156
NOTES
,
I -40
I
I 0
1
I
I
I 4oc
RG. 1. Adiabatic rapid passage spectrum of nitrogen in diamond showing the three sharp lines of the Pl spectrum of isolated nitrogen atoms and the central broad line of aggregated nitrogen. A computed Gaussian curve (dotted) is fitted to the broad line. Also shown is the derivative of an absorption spectrum taken from Ref. (7) see text.
line recognizable as the P2 spectrum attributed to three nitrogen atoms substituting for three next nearest carbon atoms in a { 1 1 1 } plane. A computed Gaussian curve (dotted line) gave a good fit to this central broad line. This confirms that the spectrum represents an inhomogeneously broadened line recorded under ARP conditions (8). Also shown is the absorption derivative spectrum obtained by Loubser and Wright (7). The comparison of line positions is good but not exact. Possibly different nitrogen aggregates are present in the two samples. The three sharp lines of the Pl spectrum (centered on g = 2.0024, each 0.5 G wide at half height) due to isolated substitutional nitrogen atoms are also clearly visible. An expanded trace of the low-field line is shown in Fig. 2a. The line profile cannot be fitted by a Gaussian curve but is described rather well by a Lorentzian curve (dotted line). The magnitude of the audio modulation (100 kHz) was estimated as 0.2 G peak-to-peak, well within the overall linewidth, but greater than the individual contributing spin packets, assuming inhomogeneous broadening. -1 I
0 I
1C I
-5 I
0 I
5c 1
FIG. 2. (A) An expanded trace of the low-field line of the Pl spectrum of Fig. I. The satisfactory fit of a superposed Lorentzian curve (dotted) suggests line narrowing due to strong dipole-dipole interactions. (B) An expanded trace of a similar line in another diamond sample, recorded with gain X8. (Note change of horizontal scale.) The fit of a superposed Gaussian curve indicates line broadening due to weak dipoledipole interactions.
The value of T, was estimated by using the relationship
derived by Portis (9)
tan($ - 90”) = w,T,, connecting the observed audio modulation phase lag of the recorded signal, 4, with the modulation frequency w, . As Mailer and Taylor (10) point out this method of determining Tl “does not depend upon the difficult and uncertain determination of the value of Hi .” Experimentally one maximizes the chart record of the diamond by carefully adjusting the lock-in reference phase, &, of the lock-in detector. The modulation cable is then transferred to the strong pitch reference sample and the optimum phase 4, determined. The relative phase lag,
is found to change from 180” to 90” as the modulation frequency is reduced in steps from 100 kHz to 35 Hz. Despite poor signal to noise at the lower frequencies, a value of T, = 1.7 f 0.7 ms was derived. This value is not significantly different from the value 2.7 + 1.2 ms measured by Barklie and Guven (3) for a similar diamond. It is of interest that this value of T, applies to the whole ESR spectrum observed in the present experiment, despite the presence of contributions from a variety of paramagnetic centers. A more careful measurement with better signal to noise might show that the PI and P2 spectra have slightly different values of T, . Finally, some idea of the relative proportions of isolated nitrogen atoms to paramagnetic nitrogen aggregates, and possibly other paramagnetic centers, can be made by comparing the areas of the PI and central region of spectrum. A ratio of 0.19 results. A comparison of the central region with “weak pitch,” using the unsaturated absorption mode, gives a spin density of about 2 X lOi spins cmm3. Thus the nitrogen atoms causing the Pl spectrum have a density of about 4 X lOI spins cmp3. The spectra described above were representative of many of the diamonds of the original set of 25. However, at least one diamond sample showed broader lines in the P 1 spectrum, having a value of 3.1 G, as against 0.5 G for the more typical spectrum. The relative proportion of isolated to aggregated paramagnetic nitrogen is 0.16. The shape of these broader P 1 lines was Gaussian, Fig. 2b. It is interesting that Samsonenko (11) observed a spectrum of nitrogen in diamond which showed a superposition of the broad and narrow Pl spectra. He ascribed the broadening to weak dipole-dipole interactions arising from the near proximity of other “isolated” nitrogen impurity atoms. REFERENCES 1. J. H. N. LOUBSER AND J. A. VAN WYK, Rep. Prog. Phys. 41, 1201 (1978). 2. C. M. WELBOLJRNE, Solid State Commun. 26, 255 (1978). 3. R. C. BARKLIE AND J. GUVEN, J. Phys. C 14, 3621 (198 1). 4. J. A. VAN WYK, J. Phys. C 15, L981, (1982). 5. J. A. VAN WYK AND J. H. N. LOUBSER, J. Phys. C 16, 1501 (1983 1. 6. C. A. J. AMMERLAAN AND R. VAN KEMP, J. Phys. C 18,2623 (1985). 7. J. H. N. LOUBSER AND A. C. J. WRIGHT, Diamond Rex, 16 (1973). 8. J. S. HYDE, Phys. Rev. 119, 1483 (1960). 9. A. M. PORTIS, Phys. Rev. 100, 1219 (1955). 10. C. MAILER AND C. P. S. TAYLOR, Biochim. Biophys. Acta 322, 195 (1973). I I. N. D. SAMSONENKO, Sov. Phys. Solid State 6, 2460 (1965).