Room-Temperature Phosphorescence and Lifetime of Fossil Resins (Amber) from Dominican Republic, Mexico, Baltic Sea, Myanmar, and Fushun, China

INTRODUCTION

Amber is a well-known fossil resin produced from tens of millions of years ago by kinds of higher plants: angiosperms and conifers, which sporadically generated resins in the Triassic and Cretaceous (Bray & Anderson, 2009). Paleogene and Neogene are the most abundant accumulation periods (Drzewicz et al., 2016). Fresh-liquid resin underwent a variety of geological processes to deposit in the sea sediments or rocks. The presence of amber has been reported around worldwide, except Antarctica. They contain the primary deposit symbiosis with lignite and the secondary redeposit after fluvial transport (Langenheim, 2003). Because of the impact on the burial environment (temperature, pressures, etc.), the appearances of amber vary from golden transparent to brown translucent, to black opaque (Langenheim, 2003; Seyfullah et al., 2018).

For decades, with various advanced technologies assistant, amber’s physical and chemical properties have been well deciphered. Chemical compositions and structures are mainly analyzed via gas-chromatography-mass spectrometer (GC-MS)(Wang et al., 2017), infrared spectroscopy (IR) (Beck et al., 2007), Raman spectroscopy(Brody et al., 2001), and nuclear magnetic resonance spectroscopy (NMR)(Lambert et al., 2015). Since paleontologists are interested in kinds of inclusions, the optical microscope, scanning electron microscope (SEM) (Néraudeau et al., 2017), and computed tomography (CT)(Xing et al., 2016)are well performed. Other properties, like the density, hardness, electrical conductivity, and refractive index, also have been reported (Drzewicz et al., 2016).

As a natural resin, amber consists of complex organic compounds. When contains some fluorescent substances(Bellani et al., 2005; Chekryzhov et al., 2014; Matuszewska et al., 2002; Zhang, 2021), amber can emit bright fluorescence (Zhang, 2020), sometimes phosphorescence (Jiang et al., 2020; Liu et al., 2014). Liu et al. (2014) have explained that the extreme blue emission contributes to Dominican blue amber’s unique appearance. Fluorescence of fossil resin has been widely noted recently, presented as a three-dimensional contour of excitation wavelength vs.emission wavelength vs. fluorescence relative intensity (Zhang et al., 2021; Li et al., 2022; Lucyna et al., 2023). Once the phosphorescence of amber at room temperature was observed in Burmite (Jiang et al., 2020; Bai et al., 2020), this non-distructive method gradually becomes another way to distinguish amber from different geographic origins (Zhang, 2021).

Therefore, our group devoted into the phosphorescence of amber at room temperature. We have noticed that the strong bright yellow RTP of Burmite can last up to several seconds (Jiang et al., 2020). However, besides Burmite, amber from other deposits have not been reported so far. In this study, we selected 20 pieces of non-blue amber from 5 deposits: Baltic sea, Dominican Republic, Mexico, Myanmar, and Fushun, China. They all have visible RTP for us to study their spectra and lifetimes in details.

EXPERIMENT SETTINGS AND DATA ANALYSIS

Experiment Settings

In Gemmological Institute of China University of Geosciences(Wuhan), we prepared the samples slices with two-side polished, then captured samples’ photos and luminescence images under a hand-hold LED 365 nm flashlight in a dark room. Phosphorescence spectra were collected by an FP8500 fluorescence spectrometer (JASCO, Japan) with an 150 W xenon lamp. The bandwidth of excitation and emission were fixed at 5 nm. The chopping period lasted 400 ms. The integration time started from 108 ms to 290 ms with a two times accumulation.

Time-resolved phosphorescence spectra were recorded using an Edinburgh Instruments FLS980 equipped with a microsecond flash lamp μF2 and an R928P single-photon-counting PMT detection system in Hubei Key Laboratory of Low Dimensional Optoelectronic Materials and Devices. We used a handheld LED 365 nm flashlight to irradiate slices for 1 min before the test, then set the μF2 flash lamp source at 365 nm with a 20 nm slit width and a 0.1 Hz frequency, and the detection time range up to 8 seconds.

Data Analysis

All normalized phosphorescence spectra were analyzed with a nonlinear iterative procedure using Gaussian, once performed on the fluorescence spectra in various fields (Subhash & Mohanan, 1997; Meira et al., 2014; Mysiura et al., 2017; Jiang et al., 2017). Based on phosphorescence decay spectra, calculate the lifetime via single- or multi-exponential functions. The coefficient of determination (R²) and reduced α² determine the quality of data analysis.

RESULTS AND DISCUSSION

Fig. 1a lists the appearance of amber from 5 producing areas. Their fluorescence and phosphorescence images were irradiated by a 365 nm ultraviolet lamp. Obviously, as in Fig. 1b, Baltic amber shows greenish-yellow (some described as yellow-white) fluorescence, Dominican and Mexican ones have blue emission, while Burmite and Chinese ones glow violetish blue. These are consistent with previous literature(Zhang et al., 2020). Furthermore, after one-minute irradiated by the same 365 nm light, Myanmar and Fushun amber both have an intense bright yellow phosphorescence for a long time(Fig. 1c). Mexican and Dominican amber show a weaker yellow-green phosphorescence with a short duration. However, Baltic amber phosphorescence is also yellow, but it lasts too short to be captured.

Figure  1.  The magnification of the photos (a), fluorescence images (b), and phosphorescence images (c) of some samples are adjusted for optimally showing the appearance. Photo by Zhiqing Zhang.

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Fig. 2a shows all phosphorescence spectra. These curves of the same locality samples are similar in shape but various in exact peak and intensity.Baltic amber phosphorescence (samples 9^#-12^# in Fig. 2b) is generally weaker than that of other samples, with a peak center close to 455 nm and a bulge trailing at 650 nm towards to longer wavelength. Dominican samples (1^#-4^#) have two peaks centered at 468 nm and 542 nm. The former 468 nm is stronger in samples 1^# and 2^#, oppositely, weaker in 3^# and 4^#. Mexican samples (5^#-8^#) also have a 468 nm peak, with a bulge centered at 530 nm. Compared with Baltic, Dominican and Mexican counterparts, Fushun amber (13^#-16^#) emit a stronger phosphorescence with a peak close to 540 nm and a weaker bulge in the shortwave region around 410 nm. The maximum value of Burmite phosphorescence locates at 550 nm in sample 17^#, not 530 nm in samples 18^#-20^#. As for Fushun amber, they also have a weak bulge in the violet-blue (380-450 nm) region.

Figure  2.  Phosphorescenc spectra of 5 origins are drawn in black (1^#-4^#: Dominican Rep.), red (5^#-8^#: Mexico), green (9^#-12^#: Baltic sea), blue (13^#-16^#: China) and purple (17^#-20^#: Myanmar), with solid (-), dash (- - -), dash-dot (- · -) and short dot (…) for each one.

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At the same time, we are aware that the thicknesses of samples 9^# to 11^# are 3.60 mm, 2.40 mm and 2.80 mm, but their actual phosphorescence intensities increased as the order of 9^# < 10^# < 11^#. This suggests that amber phosphorescence may be independent of their thickness. While Mysiura et al. (2017) have proposed that the fluorescence is related to the sample thickness.

A Gaussian function is performed in the phosphorescence spectrum analysis. It gives out the peak center (λ /nm), the full width at half maximum (FWHM /nm), the normalized amplitude (Nor. Amp.), and the normalized area (Nor. A.) of each Gaussian component, as well as the best fit R² value. These are summarized in Table 1. It is obviously that Dominican samples have five Gaussian luminous centers, Mexicans also have four or five, but Baltic ones decrease to three or four, while Burmite and Fushun samples increase to four to six.

Table  1.  Results of Gaussian curve-fitting on the phosphorescence spectra of amber from Dominican Republic (1^#-4^#), Mexico (5^#-8^#), Baltic sea (9^#-12^#), China (13^#-16^#) and Myanmar (17^#-20^#)

Sample No.	λ/nm	FWHM	Nor. Amp.	Nor. A.	R2 value	α2 value
1#	428	81.0	0.30	23.63	0.999 00	0.000 117 00
	445	19.2	0.08	1.67
	472	49.3	0.36	18.96
	511	21.5	0.06	1.40
	544	84.4	0.99	89.03
2#	428	68.9	0.21	14.62	0.999 75	0.000 029 00
	452	27.4	0.18	5.22
	475	24.4	0.17	4.52
	536	94.6	0.97	97.40
	560	28.7	0.11	3.28
3#	424	79.6	0.49	37.25	0.999 87	0.000 020 9
	448	21.6	0.31	7.20
	472	27.3	0.39	11.47
	528	98.8	0.99	103.88
	562	27.0	0.09	2.48
4#	414	61.0	0.19	11.16	0.999 95	0.000 005 87
	449	24.5	0.30	7.91
	470	30.4	0.39	12.62
	509	115.3	0.76	92.52
	557	37.4	0.11	4.20
5#	399	38.5	0.06	2.30	0.999 95	0.000 005 94
	447	21.5	0.12	2.78
	466	39.3	0.56	23.20
	520	107.1	0.84	95.26
	562	30.1	0.10	3.10
6#	400	35.8	0.15	5.27	0.999 46	0.000 086 50
	464	54.7	0.86	50.13
	511	28.4	0.10	2.96
	537	85.2	0.95	85.90
7#	422	79.7	0.48	36.09	0.999 93	0.000 010 10
	448	26.6	0.30	8.46
	473	29.4	0.36	11.30
	528	99.8	0.99	105.01
	561	29.8	0.08	2.59
8#	408	42.7	0.24	10.22	0.999 82	0.000 023 60
	458	57.0	0.76	45.91
	488	46.4	0.14	6.93
	529	93.1	0.72	71.03
9#	423	52.9	0.56	30.65	0.999 50	0.000 066 40
	443	14.2	0.11	1.71
	466	31.5	0.29	9.85
	501	115.9	0.78	95.21
10#	469	33.1	0.18	6.45	0.999 80	0.000 017 60
	428	66.9	0.70	47.76
	501	109.8	0.69	80.23
11#	443	87.6	0.85	75.87	0.999 88	0.000 014 70
	468	22.0	0.09	2.11
	524	100.4	0.50	53.76
	560	25.8	0.05	1.24
12#	466	36.0	0.32	12.37	0.999 77	0.000 027 40
	427	67.9	0.62	42.10
	511	103.6	0.71	78.21
13#	398	42.9	0.10	3.88	0.999 93	0.000 007 25
	480	92.7	0.37	36.15
	532	54.1	0.48	27.80
	563	88.6	0.45	42.71
	563	32.5	0.21	7.25
14#	408	56.2	0.14	7.14	0.999 52	0.000 055 30
	455	37.3	0.23	8.98
	480	21.4	0.07	1.57
	536	96.0	0.97	99.50
	560	25.8	0.08	2.24
15#	401	43.9	0.07	2.74	0.999 97	0.000 003 58
	516	132.9	0.45	62.82
	535	56.9	0.49	29.60
	564	35.8	0.31	11.91
	595	51.5	0.14	7.47
16#	410	59.3	0.15	8.38	0.999 89	0.000 005 87
	452	32.4	0.17	5.76
	486	45.2	0.45	21.80
	521	39.9	0.61	25.76
	556	50.2	0.87	46.55
	603	54.7	0.20	11.59
17#	414	61.1	0.18	10.58	0.999 91	0.000 009 36
	535	45.4	0.23	10.90
	545	99.4	0.74	78.65
	564	29.3	0.18	5.67
18#	408	50.2	0.17	8.47	0.999 30	0.000 079 60
	445	30.4	0.09	2.84
	479	19.5	0.12	2.56
	531	94.2	0.98	98.47
	564	16.7	0.06	0.99
19#	413	60.3	0.27	15.44	0.999 35	0.000 071 60
	447	32.5	0.12	4.30
	477	26.6	0.17	4.87
	533	92.7	0.95	93.68
	562	141.3	0.06	8.79
20#	409	50.2	0.44	21.31	0.999 77	0.000 027 60
	442	26.9	0.17	4.87
	483	59.3	0.75	47.18
	522	39.0	0.54	22.44
	556	53.0	0.73	41.41
	612	47.5	0.17	8.37

 | Show Table

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We drew the colorful bubble chart(Fig. 3) to present each Gaussian luminous center and their normalized intensity (also Nor. Amp.) in everyone. The biggest bubbles distribute around 550 nm (the gray area in Fig. 3). However, their normalized intensity and specific normalized area are slightly vary with the samples. These differences indicate a great diversity of amber’s luminous properties and the complexity of amber’s chemical compositions.

Figure  3.  This bubble scatter illustrates the centers and normalized intensity of each Gaussian component in every sample. The bubble color presents samples’origins; the size shows the normalized intensity.

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The spectral similarity between Dominican and Mexican amber may be related to their similar derived-plant Hymenaea (Fabaceae), approximate geological age in Miocene (ca. 28 Ma. -13 Ma. years ago), and same category: Class Ic in Anderson’s amber classification system (Bray & Anderson, 2009; Anderson et al., 1992; Iturralde-Vinent et al., 2001).Fushun and Burmite are also similar, probably owing to their older geological ages. As for Baltic amber, their weakest phosphorescence potentially results from their young age, high volatile contents, as well as multiple placer deposition through the quaternary glaciation(Wang, 2019).

Besides color and intensity, we are also interested in the exactly lasting time of the phosphorescence. We selected the samples 3^#, 7^#, 12^#, 13^#, and 18^#, because they last longer phosphorescence than other samples from same origin, then did the phosphorescence decay spectra at their strongest Gaussian phosphorescence center at 550 nm±10 nm (Fig. 3). The results show phosphorescence life time of the five origins samples as the following order: Myanmar>Fushun>Mexico≈Dominican Republic > Baltic sea, corresponding to our naked-eye’s observation.

Exact lifetimes (τ) were calculated by an exponential function(Joseph, 2006). In this process, we considered former Gaussian fitting results to determine how many lifetimes may exist. Within 550 nm±10 nm, samples 3^# and 7^# held two Gaussian centers at 528 nm and 561 nm, but the latter is too weak to contribute to the total lifetime. Sample 12^# only has one Gaussian component. Thus, a single exponential function was performed in these 3 samples for their phosphorescence lifetimes. However, the decay curves of samples 13^# and 18^# visibly present two possible lifetimes, so that multiple exponential functions are proper. Fig. 4 shows all calculated results. Samples from Dominican Republic, Mexico, and Baltic sea have a single calculated lifetime as τ=0.229 s (3^#), τ=0.234 s (7^#), and τ=0.151 s (12^#), respectively, in Fig. 4(a, b, c). Fushun amber and Burmite both have two calculated lifetimes as τ₁=0.126 s/ τ₂=0.708 s (13^#) and τ₁=0.155 s/τ₂=0.985 s (18^#), in Fig. 4(d and e). The higher values (τ₂) mainly contribute to the longer phosphorescence occurring in Fushun amber and Burmite, comparing to the other 3 samples. This phosphorescence lifetime results of Burmite are also consistent with that in 0.134 s-1.396 s we have reported before(Jiang et al., 2020).

Figure  4.  These diagrams show the phosphorescence decay spectra at the strongest Gaussian center (550 nm±10 nm) in samples 3^#(a), 7^#(b), 12^#(c), 13^#(d), and 18^#(e), after one-minute 365 nm light irradiation. Red curves are the results of the exponential fittings. Red numbers in the exponential functions present each calculated decay time (τ).

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Amber is a natural material composed of various hydrocarbon compounds and its chemical compositions keep change and volatilization in long-time underground maturation. The maturation of Burmites and Fushun amber is the highest among all samples with a less volatile fraction from literature(Anderson et al., 1992; Langenheim, 2003), but they last longer-time phosphorescence in our study. Therefore, we are inclined to hypothesize that amber’s phosphorescence may be derived from their non-volatile component(Lucyna et al., 2023; Menor-Salvan et al., 2010). However, the exact luminescent substances still need further chemical investigation.

CONCLUSIONS

In this study, we have studied and proved the room-temperature phosphorescence occurring in amber through our visual observation and spectroscopic tests. For the first time, induced by 365 nm ultraviolet light, we measured the amber phosphorescence spectra from the Dominican Republic, Mexico, Myanmar, Baltic sea, and Fushun in China. These spectra suggest that the same origin amber has a similar shape with some differences in the intensity and the emission maxima position. The fact is that all samples emit yellow phosphorescence, but Baltic amber phosphorescence is much weaker than others. The spectral Gaussian function fitting calculated results reveal that the prominent emission peak locates at a wavelength near 550 nm. At this Gaussian center region, samples from different origins have different lifetimes (τ), calculated via the exponential function. The value is about 0.230 s in Dominican and Mexican amber, close to 0.151 s in Baltic amber, up to 1 second in Burmite and Fushun fossil resins. This variation is relevant to the geological ages and geographic deposits areas of amber. It indicates that older fossil resin emits brighter and longer lifetime yellow phosphorescence, as well as the terrestrial geological deposition environment possibly reserves the phosphorescence agents in amber.Moreover, the Burmite and Fushun amber may be valuable as a long lifetime RTP material.