Diamond & Related Materials 17 (2008) 2029–2036

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Diamond & Related Materials

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Photoinduced H1b and H1c centers in some natural treated diamonds

E. Gaillou a,b,⁎, E. Fritsch b, F. Notari c

a Department of Mineral Sciences, Smithsonian Institution, Washington, DC 20560, USAb Université de Nantes, CNRS, Institut des Matériaux Jean Rouxel (I.M.N.), UMR 6502, 2 rue de la Houssinière, B.P. 32229, Nantes, F-44000 Francec GemTechLab, rue Chantepoulet, 1201 Geneva, Switzerland

⁎ Corresponding author. Department of Mineral ScieWashington, DC 20560, USA.

E-mail addresses: gailloue@si.edu, asteriee@yahoo.fr

0925-9635/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.diamond.2008.06.006

A B S T R A C T

A R T I C L E I N F O

Article history:

Three natural, treated-colo Received 6 March 2007Received in revised form 30 May 2008Accepted 20 June 2008Available online 25 June 2008

Keywords:Natural diamondsPhotoinduced optical centersAbsorptionsDefects

r diamonds present photoinduced H1b/H1c centers after UV exposure. As theH1b/H1c absorptions increase, that of the 595 nm center decreases. Recovery occurs by simple exposure to astandard incandescent 100 W lamp. The centers implied are all related to nitrogen and to the existence of atreatment, the formation of H1b/H1c centers coming from the trapping of the 595 nm center. Thephotoinduced behavior seems to confirm the close relation between the 595 nm center and the H1b/H1ccenters, and might help in gemological identification.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction and background

Photoinduced absorption is absorption created or increasedthrough photon exposure. In diamond, several photoinduced absorp-tions have been previously reported [1–8]. All these authors discussthe “photochromic” effect, even though no change in color occurs,only a change in absorption in ultraviolet–visible (UV–Vis) and/orinfrared (IR) region. However, there are truly photochromic absorp-tions in diamonds [9]. As in our work no change in color is observed,we find the expression “photoinduced absorption” more suitable.

Previous studies of the photoinduced effect have been con-ducted on irradiated and annealed synthetic diamonds [1–6] aswell as HPHT-treated ones [7], limited to diamonds of type IIa andIb. In this work, three natural, type Ia irradiated and annealeddiamonds are examined. H1b and H1c centers are photoinduced, asalready mentioned in preliminary reports [10–13]. Experiments areconducted in the IR and UV–Vis region in order to better under-stand why only a small amount of diamonds demonstrate photo-induced absorptions of the H1b and/or H1c centers. Similarbehaviors were recently observed by De Weerdt [14] on threenatural irradiated and annealed diamonds, after this paper wassubmitted for publication.

In all previous studies, photoinduced absorptions are inducedduring treatment, as experiments were performed before and aftertreatment. They were all present in the absorption spectrum prior to

nces, Smithsonian Institution,

(E. Gaillou).

l rights reserved.

exposure to the excitation radiation, except in the case of a HP–HTtreated stone [7]. In some cases, a heat treatment is needed forrecovery [6,7].

Photoinduced absorptions, when they are interpreted, implynitrogen defects (mainly isolated nitrogen) even in small amounts(only 1 ppm for IIa diamond in [6]). Several authors noticed thatcorrelated changes in absorption peaks can be induced by eitherilluminating or annealing the sample [1–5]. These authors explainedthis effect by a charge transfer between the centers implied. Forexample, Mita et al. [1,2] proposed that the complementaryphotoinduced reaction between H2 and H3 centers, observed insome synthetic treated Ib diamonds, is linked to tunneling ofan electron from a neutral nitrogen atom to an H3 center. Theabsorption of the N3 center increases during exposure to amonochromatic 750 nm light and returns to its initial stateimmediately after exposure. This last phenomenon is explained byelectron transfer from the negative charge state of the N3 center toelectron traps [5].

2. Materials and methods

Our three faceted diamonds were obtained on loan from privateparties. They come from 2 different parcels of diamonds (twenty orso) arrived at the same time for expertise at GemTechLab (Switzer-land). Samples 645 and 648 are cut as round brilliants, whereassample 646 is a marquise cut. Their weight ranges from 0.11 to0.27 ct and their color from brown to yellow (see Table 1 for furtherdetails).

The characterization of samples was done by gemological andphysical methods. Observations with magnificationwere made with a

Table 1Physical description of the three diamonds analyzed for this study

Sampleref.

Dimensions (mm) Weight(ct)

Color LWUV SWUV Lum.a

visibleDiameter Thickness

645 4.30×4.20 2.40 0.27 Orange-yellow

Mediumgreenishyellow

Weakgreenishyellow

Intensegreen

646 6.23×3.60 2.10 0.26 Yellow Veryintenseyellow

Intenseyellow

Mediumgreen

648 3.10×2.90 1.90 0.115 Yellow Veryintenseyellow

Mediumyellow

None

No sample emits phosphorescence.a Luminescence (to visible exposure).

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binocular microscope at a maximum magnification of 40×, mainly inorder to observe inclusions and check homogeneity of color orluminescence for example. Luminescence was observed using astandard A. Krüss longwave (LW) and shortwave (SW) 4 W UVlamp, emitting at 365 and 254 nm respectively.

All experiments were done in sequence: UV–Vis and IR spectrawere first acquired without the diamond being exposed to UV. Then,they were acquired again after diamonds had been exposed to UVlight during 15 min through the open chamber in a darkened room.No light leakage occurred through the transparent lid of the chamberbecause opening around the UV lamp was masked by opaqueadhesive tape during FTIR experiments. Finally, spectra wereacquired after the diamonds were exposed to visible light (fiberoptic lamp described below) to check that the stone returned to itsinitial state.

IR spectra were collected at room temperature, at 4 cm−1

resolution with a Fourier Transform (FT) Nicolet 20SX infraredspectrometer (FTIR) equipped with a commercial Nicolet micro-beam chamber and a DTGS detector. 1000 scans were accumulated inthe 600–6000 cm−1 spectral range.

UV–Vis absorption spectra in the 300–800 nm range wererecorded with a UV–Vis-near IR Varian Cary 5G spectrophotometer.The spectral band width (SBW) ranged from 0.5 to 2 nm, withsampling every 0.5 to 1 nm, and an integration time of 0.5 to 2 s/point,to maintain an acceptable signal/noise ratio. Standard spectra wereacquired at liquid nitrogen temperature, using a home-made, vacuumcold-finger cryogenic unit.

In order to normalize to diamond thickness, spectra were obtainedthrough the girdle of the stone, the beam passing only through twofairly thick segments of the girdle facing each other (geometry similarto that illustrated in [15], obtained with masks). This has long beenrecognized as an adequate way to obtain a good approximation of aparallel window [16]. The pathlength through the girdle was definedby masking the rest of the sample with an opaque paste, so the samevolume was sampled in all experiments. All UV–Vis and IR spectra arenormalized to 1 mm thickness of diamond.

Spectra for photoinduction experiments were obtained at roomtemperature (RT), as it is not possible to submit stone to UV radiationin our cryogenic unit. We present here difference spectra, whichrepresent the change in UV–Vis absorption induced by UV exposure.The observed differences are very small and the signal/noise ratiomay be low in some parts of the difference spectra. For this reason,

Fig. 1. IR absorption spectra with details of the H1b/H1c centers region, a— of sample 645diamonds. Sample 645 presents H1b and H1c center absorptions barely above noise level, copresent in sample 648.

these experiments were conducted at least twice in order to checkthe reproducibility of these weak features.

The recovery process has been examined using differentsettings: for a general overview of this process, we used a Euromexfiber optic light source EK-1, 100 W halogen bulb, but double neckfiber (each fiber has approximately half the total power). Thenmonochromatic lights were used in order to check which specificwavelength bleaches the photoinduced absorptions: the lightsource used is a Bausch and Lomb tungsten lamp coupled with a0.25 m grating H25 Jobin Yvon monochromator. A beam concen-trator (convergent lens) and a mirror were used to focus light on thestone.

3. Results

Table 1 presents general characteristics of the three samples. Allsamples are homogeneous in color, which ranges from brown toyellow. They do not display brown graining often associated with thebrown color in diamonds [17]. Two samples are “green transmitters”(sample 645 and 646) that is, emit a green luminescencewhen excitedwith visible light. Sample 645 presents moreover a zonation ofluminescence.

All samples show fluorescence through UV exposure, alwaysmore intense through LWUV than SWUV (Table 1). They all present ayellow luminescence, moderate (LW) to weak (SW) for sample 645,very intense to intense for sample 646, and very intense tomoderate for sample 648. No phosphorescence is associated withthis fluorescence.

The photoinduced nature of the absorptions was first establishedby chance: IR spectra required for standard gemological analysis wereobtained before and after the observation of the stones under UV light,where it was noticed that the two spectra were different, as describedbelow.

3.1. General sample characteristics

The three samples are type IaAB diamonds (Fig. 1). The mostintense features of the IR spectra are A- and B-aggregate absorptionspeaking at 1282 and 1175 cm−1 respectively. These aggregates are ina high concentration in samples 645 and 646, with spectra lookingvery much alike, as their signals are higher than the intrinsic bandsof diamond (Fig. 1a, b). They are less concentrated in sample 648(Fig. 1c). All samples show platelets absorption at 1365 cm−1. Thesmall peak at 3107 cm−1 reveals the presence of a modest amount ofhydrogen, which is very common in type Ia diamonds. Sample 648shows neither H1c nor H1b in standard conditions (Fig. 1c). Sample645 presents H1b and H1c absorptions but these peaks are barelyabove noise (Fig. 1a), whereas they are distinct for sample 646(Fig. 1b).

Liquid nitrogen temperature UV–Vis spectra of the three samplesare also typical of type IaAB diamonds with nitrogen-related signalsrepresented by H3, H4 and N3 centers at 503, 496 and 415 nmrespectively with their associated vibronic structure (Fig. 2). Sample646 is the only one which is relatively transparent to UV, so thevibronic structure of N3 is clearly visible, as are the absorptions of N4,N5 and N6 (Fig. 2b). There is total absorption of the UV (up to 400 nm)for samples 645 and 648 (Fig. 2a and c). All the samples present theGR1 (741 nm) and the 595 nm centers, which are due respectively toirradiation and irradiation followed by annealing, as is the case forH1b and H1c [e.g. 8,18].

, b— of sample 646, c— of sample 648. All spectra show typical features of type IaABntrary to sample 646 in which these centers are clearly visible. Neither H1b nor H1c is

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Fig. 3. Sample 645. a— IR spectrum before and during SWUV exposure. H1c and H1b absorptions, which are barely above noise in standard conditions, are enhanced by UV exposureb— Difference between the UV–Vis spectra before and after UV exposure. The 595 nm center decreases with UV exposure.

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3.2. Photoinduced absorptions by UV exposure

IR and UV–Vis spectra obtained during and after UV light exposure(respectively) are shown in Figs. 3a–b, 4a–b and 5a–b for samples 645,646, and 648 respectively, and are summarized in Table 2. Only resultsacquired with SWUV exposure are presented here. The sameexperiments conducted with LWUV gave similar results with slightlyless intense photoinduced absorptions.

The IR spectra of diamond 645 before and after exposure to UV aresuperposable, except in the 4900–5200 cm−1 region. Therewas almostno signal in this region before UV light exposure, but after, H1b andH1c centers are clearly photoinduced, with peaks at about 5170 and4936 cm−1 (Fig. 3a). The magnitude of this photoinduced behavior in

Fig. 2. UV–Vis spectra acquired at liquid nitrogen temperature, a— of sample 645, b— of sample 646, c— of sample 648 (SBW: Spectral band width; LT: low temperature). All samplepresent the same features, with absorptions due to nitrogen (H3, H4, N3). The 595 nm and GR1 centers are created during irradiation and annealing. Sample 646 is more transparento UV than the other samples: this is why nitrogen-related absorptions such as N4, N5, N6 and the full vibronic structure of N3 can be seen.

.

this sample remains modest, about 0.0005 A/mm (absorbance unitsper mm). In the UV–Vis region, we noticed small changes induced byUV exposure. In addition to minor variations of the baseline, a peak at595 nm is clearly visible (Fig. 3b) which means that the 595 nm centerdecreases with UV exposure. There is also a large band between 520and 600 nm. To sum up, UV exposure induces a clear increase of H1band H1c centers and a simultaneous decrease of the 595 nm center inthis diamond.

The same phenomenon is observed for sample 646. Indeed, there isa three-fold increase of the H1b and H1c absorptions during UVexposure (H1b/H1c=2 is maintained, Fig. 4a), while the 595 nm centerdecreases (Fig. 4b). The magnitude of the photoinduced effect is about0.0029 A/mm. As for the previous sample, only the difference between

st

Fig. 4. Sample 646. a— IR spectrum before and during SWUV exposure. H1c and H1b absorptions are enhanced by UV exposure. b—Difference between the UV–Vis spectra before andafter UV exposure. The 595 nm center decreases with UV exposure.

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the UV–Vis spectrum before and after UV exposure (at roomtemperature) is presented (Fig. 4b). A peak at 595 nm comes clearlyout of the background.

The IR spectrum of diamond 648, which presented neither H1cnor H1b before UV exposure, shows a UV-induced absorption at4934 cm−1, attributed to H1b (Fig. 5a). The intensity of thephotoinduced absorption is 0.0014 A/mm. UV–Vis spectra on thissmall stone (0.11 ct) were the hardest to obtain. The graph (Fig. 5b) ofthe difference between the UV–Vis spectrum before and after UVlight is the noisiest, but one can observe that this spectrum has asimilar shape than the two previous ones, with possibly a peakcorresponding to the 595 nm as well.

3.3. Recovery process

The photoinduced H1b/H1c centers and the decreased 595 nmcenter remain unchanged when the UV light is stopped and the

samples kept a few hours in the dark, which is how this photoinduc-tion was first noticed. After one night in the dark, all samples stillpresent their photoinduced absorptions, even if they are slightlydecreased. After several days in the dark, UV–Vis and IR spectra returnto their original features. Photoinduced absorptions disappearcompletely after a few seconds exposure to intense visible light, andat the same time the 595 nm absorption increases back to its originalheight. No heat treatment is needed to restore the initial state.

The fact that the experiments were done in sequence proves thatthe signal is real and that no artifact occurs. In order to find the energythat fuels the recovery process, we conducted measurements duringexposure to seven wavelengths: in the violet (400 nm, i.e. 3.10 eV),blue (458 nm, i.e. 2.71 eV), green (514 nm, i.e. 2.41 eV and 550 nm, i.e.2.26 eV), yellow (595 nm, i.e. 2.09 eV) and red (650 nm, i.e. 1.91 eV and741 nm, i.e. 1.67 eV). None of these wavelengths produced a decreaseof the photoinduced absorptions (nor an increase). Two explanationscan be proposed: either the wavelength permitting the recovery

Fig. 5. Sample 648. a— IR spectrum before and during SWUV exposure. The H1b absorption, absent in standard conditions, is created by UV exposure. b—Difference between the UV–Vis spectra before and after UV exposure. Despite the high noise level due to the small size of the sample (0.115 t), one can see that the 595 nm center decreases with UV exposure.

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process is higher than 741 nm (i.e. energy lower than 1.67 eV), or themonochromatic lights we used were not powerful enough. Lasers ofdifferent wavelengths should be used in further experiments.

4. Discussion and conclusion

The three samples examined present an enhancement or adecrease of some known absorptions (H1b/H1c and 595 nm respec-tively) due to UV exposure. Exposure to visible light does provide

Table 2Description of spectroscopic properties of the three samples analyzed for this study

Sample Type Before UV exposure

H1ba H1ca 595b

645 IaAbB Very weak Very weak Medium646 IaAbbB Weak Weak Weak648 IaAbB None None Medium

a Experiments at room temperature.b Experiments at low temperature.

recovery. Recovery is also obtained at room temperature in the darkover long periods of time (several days). Our experiments withmonochromatic lights are not conclusive, lacking probably power, sowe were not able to find the exact energy which makes recoverypossible. It seems that De Weerdt [14] did not try to bleach afterenhancing these absorptions with monochromatic light, but only withannealing (at 500 °C).

All these photoinduced centers are nitrogen-related, as some otherknown photoinduced absorptions [1–7]. Collins et al. [8] have

After or during UV exposure

GR1b H1ba H1ca 595a GR1a

Weak Weak Weak Decreased WeakVery weak Medium Medium Decreased Very weakWeak Medium None Decreased Weak

Fig. 6. Redrawn from [8]. Isochronal annealing curves normalized to 100% at themaximum (experiment done on one sample). The H3 and H1b centers grow withincreasing temperature, compensating the destruction of the GR1 and 595 nm centers.The region in grey corresponds to the possible range of annealing temperature forsamples 645, 646 and 648, as they present all four centers at the same time.

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demonstrated that these centers are linked to one another, even iftheir respective structures are not known: when the annealingtemperature is increased, the absorption of the 595 nm centerdecreases while those of the H1b and H1c centers are enhanced(Fig. 6). These authors proposed that H1b and H1c centers are formedduring the trapping of 595 nm centers at A- and B-aggregatesrespectively. In our case, the absence of the H1c center in sample 648is simply explained by the low B-aggregate concentration.

From this same work of Collins et al. [8], done on one sample, theproportions of GR1 and H3 centers are also temperature-related, asthey are all induced by treatment. De Weerdt [14] observed threesamples (all natural irradiated and annealed) for which H1b and H1ccenters were enhanced by UV light. Only one of them showscorresponding bleaching/enhancement in the visible range: decreaseof the 595 nm center, with moreover an increase of the H3 that we didnot observe (no change of the GR1 center). This sample has beenannealed at a temperature of 800 °C. Assuming that these bands arealso only due to treatment (i.e. not preexistent) in our samples, whichis not unreasonable in the yellow to brown range, the spectrum for oursamples corresponds to annealing temperatures between about 650and 800 °C [8,14] (Fig. 6).

The existence of a relation between the 595 nm and H1b/H1ccenters proposed by Collins et al. [8] seems to be confirmed here.When the stone is exposed to UV, more H1b centers are created byconsuming the 595 nm center. Independent experiments by DeWeerdt also point in this direction, with moreover an increase of theH3 center, which is also consistent with Fig. 6.

This work and that of De Weerdt [14] raise the question of theconditions necessary to obtain photoinduced H1b/H1c and 595 nmcenters. For our samples and one of De Weerdt [14], they occur withthe simultaneous presence of H1b/H1c, 595 nm, GR1 and H3 centers.The presence of optically inactive centers may be important as well,andmight depend on the annealing temperature estimated above. Wecould not prove the presence of an electron transfer process, as oftensuggested in the past for photoinduced absorptions in diamond.

Testing for photoinduced H1b and H1c centers has been useful fora small number of stones tested in gemological laboratories, provingthe existence of treatment by irradiation followed by annealing. Thisbehavior has been noted independently by two teams, indicating thatthis behavior may not be that rare. At this point in time, this procedurehas not been used on a large number of diamonds, as it is time-consuming without attachments devised for this purpose.

Acknowledgment

The authors are grateful to Prof. Serge Lefrant (IMN, Nantes), whohelped with both experiments and discussion. We are also grateful toreviewers who helped improve the clarity of the text.

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