Abstract: Absorption, fluorescence, and fluorescence excitation spectra of 12-hydroxy-1-azaperylene (HAP) and 1-azaperylene were studied in n-alkane matrices at 5 K. Two stable tautomers of HAP, each of them in n-nonane embedded in two sites, were identified and attributed to the enol and keto forms. Theoretical calculations of the energy and vibrational structure of the spectra suggest that tautomer A, with the (0, 0) transition energy at 18 980 ± 10 cm–1 (and 19 060 ± 10 cm–1 in the high energy site), should be identified as the keto form, whereas tautomer B, with the (0, 0) energy at 19 200 ± 20 cm–1 (19 290 ± 20 cm–1), as the enol form. Observation of absorption and fluorescence of both tautomeric forms and lack of large Stokes shift of fluorescence of the keto form classify HAP as the limiting case of the excited-state intramolecular proton transfer system.
Irena Deperasińska, Daniel T. Gryko, Elena Karpiuk, Bolesław Kozankiewicz, Artur Makarewicz, and Joanna Piechowska

Abstract: We studied absorption and fluorescence spectra and decays of pentacene (Pc) in Shpol'skii matrices of n-heptane (C7), n-nonane (C9), n-decane (C10), n-dodecane (C12), n-tetradecane (C14) and n-hexadecane (C16) in the temperature range 1.7–200 K. The fluorescence quantum yields of Pc in C12, C14 and C16 at 1.7 K were determined to be 20±6%, 26±6% and 22±6%, respectively. The kinetic analysis provided values of the radiative, internal conversion and intersystem crossing rate constants for the relaxation channels of the S1 state of Pc in different matrices. Calculated fully saturated emission rates of single molecule were in very good relation with previous observation of single Pc molecules in Shpol'skii matrices.
M. Banasiewicz, I. Deperasińska, D. Fabjanowicz, B. Kozankiewicz

Abstract: We describe a simple and inexpensive method, which corrects the astigmatism of a Czerny–Turner spectrometer. Initial characterization of the astigmatism for a particular Czerny–Turner spectrometer was performed and the design of the corrective optic is described. The optic is a thin piece of glass, which is used as a one-dimensional waveguide between the light source and the spectrometer such that the sagittal and tangential focal planes are brought to the same position. This method is demonstrated to work well between 360 and 900 nm for an f / 4.7 spectrometer. With appropriate materials, corrections for longer and shorter wavelengths should also be possible. When using an inexpensive glass plate, light intensity lost with this method is approximately 12%. Improved surface finish should reduce this loss.
C. Chrystal, K. H. Burrell, and N. A. Pablant

Abstract: We report graphitic carbon growth on crystalline and amorphous oxide substrates by using carbon molecular beam epitaxy. The films are characterized by Raman spectroscopy and X-ray photoelectron spectroscopy. The formations of nanocrystalline graphite are observed on silicon dioxide and glass, while mainly sp 2 amorphous carbons are formed on strontium titanate and yttria-stabilized zirconia. Interestingly, flat carbon layers with high degree of graphitization are formed even on amorphous oxides. Our results provide a progress toward direct graphene growth on oxide materials.
Sahng-Kyoon Jerng, Dong Seong Yu, Jae Hong Lee, Christine Kim, Seokhyun Yoon, Seung-Hyun Chun

Abstract: The use of lithium-coated plasma facing components for plasma density control is studied in the National Spherical Torus Experiment (NSTX). A recently installed liquid lithiumdivertor (LLD) module has a porous molybdenum surface, separated by a stainless steel liner from a heated copper substrate. Lithium is deposited on the LLD from two evaporators. Two new spectroscopic diagnostics are installed to study the plasma surface interactions on the LLD: (1) A 20-element absolute extreme ultraviolet (AXUV) diode array with a 6 nm bandpass filter centered at 121.6 nm (the Lyman-α transition) for spatially resolved divertor recycling rate measurements in the highly reflective LLD environment, and (2) an ultraviolet-visible-near infrared R=0.67 m imaging Czerny–Turner spectrometer for spatially resolved divertor D I, Li I-II, C I-IV, Mo I, D2, LiD, CD emission and ion temperature on and around the LLD module. The use of photometrically calibrated measurements together with atomic physics factors enables studies of recycling and impurity particle fluxes as functions of LLD temperature, ion flux, and divertor geometry.
V. A. Soukhanovskii1, A. L. Roquemore, R. E. Bell, R. Kaita and H. W. Kugel

Abstract: The new wavelength calibration system of the grating monochrometer was described to research the wavelength characteristic of Model 207 monochrometer which was used in the spectral responsivity measurement.The automatic parts of the facility were all connected with the computer by RS232 or IEEE-488.The wavelength calibration could be carried out automatically,and the monochrometer's performance was also tested.At last the method of processing acquisition data was introduced in the program.The experiment results show that the accuracy of the monochrometer wavelength is 0.02 nm.
Yang Jiajian, Lu Xiaofeng, Feng Xiaodong, Yuan Zundong

Abstract: We present a newly designed soft plasma ionization (SPI) source developed for mass spectrometric study of organic compounds in this study. The SPI cell having a relatively small size consists of a hollow anode and a hollow mesh cathode. The voltage–current characteristic depending on the pressure was investigated, indicating that it has similar characteristics to conventional hollow cathode glow discharges. To investigate the emission characteristics of the SPI source, some molecular band emission spectra (N2, N2+ and OH+) were measured by using argon and helium discharge gases. The SPI source was installed to a commercially used quadrupole mass analyzer for analyzing organic compounds. To demonstrate the SPI source, the mass spectra of some organic compounds (methylene chloride, toluene, benzene, cyclohexane and chloroform) were measured. The organic compounds were ionized with good stability in the plasma, and the fragmentation depended on the applied current. When helium and argon gases were used as the discharge gas, the helium plasma was more suitable for SPI-MS rather than argon because the argon plasma not only suffers from spectral interference but also has lower sensitivity.
Hyunkook Parka, Ickhee Leeb, Kyu Seong Choib, Kazuaki Wagatsumaa, Sang Chun Leeb

Abstract: Plasma electrolytic oxidation of zirconium in citric acid was investigated using optical spectroscopy. A rich emission spectrum consisting of about 360 zirconium and 170 oxygen atomic and ionic lines was identified in the spectral regions 313-320, 340-516, and 626-640 nm. It was shown that the remaining features observed in the spectrum could be ascribed to various molecular species, which involve zirconium, oxygen, hydrogen, and carbon. The temperature of the plasma core (T = 7500 ± 1000 K) was determined using measured Zr line intensities, and the temperature of peripheral plasma zone (T = 2800 ± 500 K) was estimated from the intensity distribution within a part of an OH spectrum. The composition of the plasma containing zirconium, oxygen, and hydrogen, under assumption of local thermal equilibrium, was calculated in the temperature range up to 12,000 K and for pressure of 10(5) and 10(7) Pa, in order to explain the appearance of the observed spectral features
Stojadinović S1, Radić-Perić J, Vasilić R, Perić M

Abstract: Neutral beams based on positive ion source technology are a key component of contemporary fusion research. An accurate assessment of the injected beam species mix is important for determining the actual plasma heating and momentum input as well as proper interpretation of beam-based diagnostics. On DIII-D, the main ion charge-exchange spectroscopy system is used to extract well-resolved intensity ratios of the Doppler-shifted D(α) emission from the full, half, and third energy beam components for a variety of beam operational parameters. In conjunction with accurate collisional-radiative modeling, these measurements indicate the assumed species mix and power fractions can vary significantly and should be regularly monitored and updated for the most accurate interpretation of plasma performance. In addition, if stable active control of the power fractions can be achieved through appropriate source tuning, the resulting control over the deposition profile can serve as an additional experimental knob for advanced tokamak studies, e.g., varying the off axis beam current drive without altering the beam trajectory
Thomas DM, Grierson BA, Muñoz Burgos JM, Van Zeeland MA

Abstract: The neutral-beam induced D(α) emission spectrum contains a wealth of information such as deuterium ion temperature, toroidal rotation, density, beam emission intensity, beam neutral density, and local magnetic field strength magnitude |B| from the Stark-split beam emission spectrum, and fast-ion D(α) emission (FIDA) proportional to the beam-injected fast ion density. A comprehensive spectral fitting routine which accounts for all photoemission processes is employed for the spectral analysis. Interpretation of the measurements to determine physically relevant plasma parameters is assisted by the use of an optimized viewing geometry and forward modeling of the emission spectra using a Monte-Carlo 3D simulation code
Grierson BA, Burrell KH, Chrystal C, Groebner RJ, Kaplan DH, Heidbrink WW, Muñoz Burgos JM, Pablant NA, Solomon WM, Van Zeeland MA

Abstract: To improve poloidal rotation measurement capabilities on the DIII-D tokamak, new chords for the charge exchange recombination spectroscopy (CER) diagnostic have been installed. CER is a common method for measuring impurity rotation in tokamak plasmas. These new chords make measurements on the high-field side of the plasma. They are designed so that they can measure toroidal rotation without the need for the calculation of atomic physics corrections. Asymmetry between toroidal rotation on the high- and low-field sides of the plasma is used to calculate poloidal rotation. Results for the main impurity in the plasma are shown and compared with a neoclassical calculation of poloidal rotation
Chrystal C, Burrell KH, Grierson BA, Groebner RJ, Kaplan DH

Abstract: Surface enhanced Raman spectroscopy (SERS) has been intensively investigated during the past decades for its enormous electromagnetic field enhancement near the nanoscale metallic surfaces. Chemical enhancement of SERS, however, remains rather elusive despite intensive research efforts, mainly due to the relatively complex enhancing factors and inconsistent experimental results. To study details of chemical enhancement mechanism, we prepared various low dimensional semiconductor substrates such as ZnO and GaN that were fabricated via metal organic chemical vapor deposition process. We used three kinds of molecules (4-MPY, 4-MBA, 4-ATP) as analytes to measure SERS spectra under non-plasmonic conditions to understand charge transfer mechanisms between a substrate and analyte molecules leading to chemical enhancement. We observed that there is a preferential route for charge transfer responsible for chemical enhancement, that is, there exists a dominant enhancement process in non-plasmonic SERS. To further confirm our idea of charge transfer mechanism, we used a combination of 2-dimensional transition metal dichalcogenide substrates and analyte molecules. We also observed significant enhancement of Raman signal from molecules adsorbed on 2-dimensional transition metal dichalcogenide surface that is completely consistent with our previous results. We also discuss crucial factors for increasing enhancement factors for chemical enhancement without involving plasmonic resonance.
Jayeong Kim, Yujin Jang, Nam-Jung Kim, Heehun Ki2, Gyu-Chul Yi, Yukyung Shin, Myung Hwa Kim, Seokhyun Yoon

Abstract: The interplay between free and bound charges in two-dimensional (2D) semiconductor/ferroelectric oxide structures is responsible for the unique opto-electrical properties of these structures. In this study, we vertically combined the 2D layered semiconductors MoS2 (n-type) and WSe2 (p-type) with a ferroelectric oxide (PbTiO3) and found that a ferroelectric polarization induced accumulation or depletion in the layered materials. The heterostructures exhibited polarization-dependent charge distribution and pinched hysteresis. We show that polarization at the interface promoted efficient charge separation of photo-generated carriers in the 2D layers. Optical control of electrical transport was effectively achieved in the MoS2 layers. This study potentially opens up new applications for semiconductor/ferroelectric systems in electronic devices.
Hye-Jin Jin, Jayeong Kim, Yejin Kim, Seokhyun Yoon, Yangjin Lee, Kwanpyo Kimb, William Jo

Abstract: Three new donor–acceptor (D–A) compounds, positional isomers of phenoxazine-substituted acridone, namely 1-phenoxazine-N-hexylacridone (o-A), 2-phenoxazine-N-hexylacridone (m-A) and 3-phenoxazine-N-hexylacridone (p-A), were synthesized. The synthesized compounds showed interesting, isomerism-dependent electrochemistry. Their oxidation was reversible and their potential (given vs. Fc/Fc+) changed from 0.21 V for o-A to 0.36 V for p-A. In contrast, their reduction was irreversible, isomerism-independent and occurred at rather low potentials (ca. −2.25 to −2.28 V). The electrochemical results led to the following values of the ionization potentials (IPs) and electron affinities (EAs): 5.03 eV and −2.14 eV, 5.15 eV and −2.20 eV, and 5.20 eV and −2.28 eV for o-A, m-A and p-A, respectively. The experimentally obtained values were in very good agreement with those predicted by DFT calculations. All three isomers readily formed single crystals suitable for their structure determination. o-A and p-A crystallized in P[1 with combining macron] and P21/n space groups, respectively, with one molecule per asymmetric unit, while m-A crystallized in the P21/c space group with two molecules in the asymmetric unit accompanied by disordered solvent molecules. The UV-vis spectra of the studied compounds were isomerism and solvent independent, yielding absorption maxima in the vicinity of 400 nm. Their photoluminescence spectra, in turn, strongly depended on isomerism and the used solvent showing smaller Stokes shifts for the emission bands registered in toluene as compared to the corresponding bands measured in dichloromethane. The photoluminescence quantum yields (ϕ) were systematically higher for toluene solutions reaching the highest value of 20% for p-A. For all three isomers studied, stationary and time-resolved spectroscopic investigations carried out in toluene at different temperatures revealed spectral features indicating a contribution of thermally activated delayed fluorescence (TADF) to the observed spectroscopic behaviour. The measured photoluminescence quantum yields (ϕ) were higher for solid state films of pure compounds and for their dispersions in solid matrices (zeonex) than those recorded for toluene and dichloromethane solutions of the studied phenoxazine-N-hexylacridone isomers. The obtained experimental spectroscopic and structural data were confronted with theoretical predictions based on DFT calculations.
Irena Kulszewicz-Bajer, Malgorzata Zagorska, Marzena Banasiewicz, Piotr A. Guńka, Petr Toman, Boleslaw Kozankiewicz, Gabriela Wiosna-Salyga, Adam Pron

Abstract: Absorption and emission spectra of single crystals of 2,3-dibromonaphthalene (23DBN) were measured at 5K. Stokes shift between the absorption edge and the onset of weak fluorescence indicated intermolecular charge transfer (CT) character of the electronic transition between the singlet states. Theoretical analysis performed with the aid of quantum chemistry methods showed that CT operated between 23DBN molecules belonging to two subgroups within the crystal unit cell. Structured phosphorescence, enhanced by the heavy-atom (Br) effect, had a local character. Vibronic structure of this emission, in the first approximation, was described by the spectrum calculated for an isolated 23DBN molecule. Simulation of the spectrum details required more advanced computations taking into account the whole crystal unit cell with 8 molecules of 23DBN.
Irena Deperasińska, Marzena Banasiewicz, Bolesław Kozankiewicz

Abstract: Methylammonium lead trihalide perovskites CH3NH3PbX3 (X = Cl, Br, and I) have recently attracted huge attention as a promising candidate for highly efficient solar cell absorber materials. To understand the physical properties of halide perovskites, we investigated the CH3NH3PbCl3 single crystal by Raman scattering spectroscopy from 80 K to room temperature. Benchmarking the phonon modes and their Raman activities obtained by density functional calculations, we successfully assign the molecular vibrations of methylammonium in the frequency range from 400 to 3300 cm–1. In the temperature-dependent Raman scattering spectra, the internal vibrational modes of the CH3NH3+ cation are observed in the frequency range above 400 cm–1 and a number of peaks among them show characteristic changes that reflect the phase transition occurring at about 160 K in CH3NH3PbCl3. A noticeable Raman peak at 2900 cm–1 persists in a wide range of temperature, suggesting the existence of microcrystalline methylammonium chloride possibly left on the crystal surface from the growth process or spontaneously formed after synthesis.
Trang Thi Thu Nguyen, Yejin Kim, Soungmin Bae, Maryam Bari, Hye Ri Jung, William Jo, Yong-Hoon Kim, Zuo-Guang Ye, and Seokhyun Yoon

Abstract: Environment-friendly Cu(In,Ga)Se2 (CIGS) solar cells requires the replacement of Cd-containing buffers with non-toxic materials. Zn(O,S) buffers have been developed and yielded even better efficiency than CdS-buffered CIGS thin-film solar cells [23.35%, Ref. [6]]. In this work, we studied band offsets of Zn(O,S) and CIGS interfaces. The Cd-free buffer layers were deposited with 1.0%, 1.3%, and 1.6% oxygen (O2) gas partial pressure during the deposition. Effects of the oxygen partial pressure on the structure and electronic properties of the devices were investigated by micro-Raman scattering spectroscopy and Kelvin probe force microscopy, respectively. We achieved depth-profiling of spatial work function mapping across the interface between the absorbers and the buffers. The best efficiency sample, grown using 1.3% of oxygen, showed 80 mV spike-like band offsets. We propose that the efficiency can be improved through tailoring of the band offsets at the interface as well as improving the absorber and the buffer materials.
Juran Kim, Jayeong Kim, Eunji Ko, Ha Kyung Park, Seokhyun Yoon, Dae-Hyung Cho, Woo-Jung Lee, Yong-Duck Chung, William Jo

Abstract: The dual nature of the magnesium acceptor in gallium nitride results in dynamic defect complexes. Europium spectator ions reveal switching between two spectrally unique metastable centers, each corresponding to a particular acceptor state. By ion co-implantation of europium and oxygen into GaN(Mg), we produce, in addition, an anchored state system. In doing so, we create an abundance of previously unidentified stable centers, which we denote as “Eu0(Ox).” We introduce a microscopic model for these centers with oxygen substituting for nitrogen in the bridging site.
D. Cameron, K. P. O'Donnell, P. R. Edwards, M. Peres, K. Lorenz, M. J. Kappers, and M. Boćkowski

Abstract: Absorption and fluorescence from single molecules can be tuned by applying an external electric field – a phenomenon known as the Stark effect. A linear Stark effect is associated to a lack of centrosymmetry of the guest in the host matrix. Centrosymmetric guests can display a linear Stark effect in disordered matrices, but the response of individual guest molecules is often relatively weak and non‐uniform, with a broad distribution of the Stark coefficients. Here we introduce a novel single‐molecule host‐guest system, dibenzoterrylene (DBT) in 2,3‐dibromonaphthalene (DBN) crystal. Fluorescent DBT molecules show excellent spectral stability with a large linear Stark effect, of the order of 1.5 GHz/kVcm−1, corresponding to an electric dipole moment change of around 2 D. Remarkably, when the electric field is aligned with the a crystal axis, nearly all DBT molecules show either positive or negative Stark shifts with similar absolute values. These results are consistent with quantum chemistry calculations. Those indicate that DBT substitutes three DBN molecules along the a‐axis, giving rise to eight equivalent embedding sites, related by the three glide planes of the orthorhombic crystal. The static dipole moment of DBT molecules is created by host‐induced breaking of the inversion symmetry. This new host–guest system is promising for applications that require a high sensitivity of fluorescent emitters to electric fields, for example to probe weak electric fields.
Amin Moradi, Dr. Zoran Ristanović, Prof. Dr. Michel Orrit, Dr. Irena Deperasińska, Prof. Dr. Bolesław Kozankiewicz

Abstract: We studied surface-enhanced Raman spectroscopy (SERS) in 4-mercaptopyridine (4-Mpy) deposited on zinc oxide (ZnO) nanostructures, by using resonance Raman scattering covering a range of incident photon energies from 1.7 to 5.7 eV. We investigated all primary routes of the energy-specific resonances that are associated with the electronic transitions between the ZnO valence band (VB) to the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) to the ZnO conduction band (CB), respectively. Two resonances at 5.55 and 5.15 eV in the ultraviolet (UV) spectral range can be associated with transitions into the CB and most importantly into an excitonic-related state below the ZnO CB, respectively. The energy difference between the UV resonances is 0.4 eV corresponding to the excitonic binding energy as a result of excitonic quantum confinement in the 10–20 nm thick ZnO nanowalls. The observed excitonic SERS resonance enhancement of the ring-breathing mode of 4-Mpy is about 15 times stronger than for the VB resonance observed at 2.43 eV and free of luminescence background. Hence, we outline new pathways of improving the detectability of molecules by chemical SERS due to tuning of the quantum confinement in the excitonic resonance enhancement.
Jayeong Kim, Tomke E. Glier, Benjamin Grimm-Lebsanft, Sören Buchenau, Melissa Teubner, Florian Biebl, Nam-Jung Kim, Heehun Kim, Gyu-Chul Yi, Michael Rübhausen, and Seokhyun Yoon

Abstract: We report highly efficient Cu2ZnSn(S,Se)4 (CZTSSe) thin films with a power conversion efficiency (PCE) of 12.3% at their surface and interface. The structural and electrical properties were locally investigated, using scanning probe microscopy and micro‐Raman scattering, to improve the performance of kesterite solar cells. Interestingly, this research reports quite different results from the conventional kesterite solar cells, owing to the observance of undesirable voids and secondary phases. Nonetheless, the solar cells exhibit a high PCE of over 12%. Thus, we probe the kesterite solar cells as a function of the depth and introduce a mechanical dimple‐etching process. The relatively low melting temperature of the pure‐metal precursors results in the unique properties within the solar cell materials. Understanding these phenomena and their effects on carrier behavior enables the achievement of a higher PCE and better performance for kesterite solar cells.
Juran Kim, Jayeong Kim, Eunji Ko, Seokhyun Yoon, Jun Hyoung Sim, Kee Jeong Yang, Dae Hwan Kim, Jin Kyu Kang Yu Jin Song, Chan Wook Jeon, William Jo

Abstract: Transparent conductor SrVO3 thin films were grown on (LaAlO3)0.3(Sr2AlTaO6)0.7 (LSAT), SiO2/Si, LaAlO3, and sapphire substrates using RF magnetron sputtering deposition with commercial SrVO3 targets at temperatures as low as 400 °C. Considering the complex phases of SrVO3 material systems, the growth temperature and sputtering gases were optimized and precisely controlled to yield a transparent and conductive SrVO3 phase. The authors used a mixed gas atmosphere of Ar and H2 during growth for reduction. Structural and morphological properties of all SrVO3 films were investigated using x-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and scanning electron microscopy. XRD and HRTEM showed a highly crystalline cubic phase of SrVO3 films. In addition, HRTEM showed that a superstructure along the [100] direction could be formed due to Jahn-Teller distortion in the cubic phase of SrVO3 films. The authors obtained a resistivity of 0.2 × 10−3 Ω cm, mobility of 1.82 cm2/(V s), and carrier concentration of 1.57 × 1022 cm−3 for SrVO3/LSAT films. Optical transmittance was measured as 88% at a photon wavelength of 633 nm for 39-nm-thick SrVO3 films. Using x-ray photoemission spectroscopy (XPS) and its depth profile analysis, the authors investigated chemical compositions and binding energies of Sr, V, and O atoms in SrVO3 films, and their depth profiles. The authors found a correlation between the resistivities and XPS binding energy spectra for SrVO3 films as functions of film thickness and substrates.
Dae Ho Jung, Hyeon Seob So, Hosun Lee

Abstract: In this work, copper indium gallium selenide (Cu(In,Ga)Se2; CIGS) absorbers were grown on polyimide (PI)/molybdenum substrates by a three-stage co-evaporation process at various temperatures, film formation was systemically studied using various advanced characterization methods such as transmission electron microscopy, micro-Raman spectroscopy, Kelvin probe force microscopy, and atom probe tomography. The CIGS films on PI were found to exhibit considerable physical and electrical variations with respect to the process temperature of three-stage co-evaporation. In particular, when the process temperature reached 400 °C, the CIGS absorber on PI began to exhibit controlled microstructure and intergrain properties. By adjusting the microstructure and intergrain properties of the absorber films by means of the process temperature of three-stage co-evaporation, flexible CIGS solar cells on PI with an efficiency of 16.7% (with anti-refection coating) were achieved.
Kihwan Kim, Juran Kim, Myeng Gil Gang, Se-Ho Kim, Soomin Song, Yunae Cho, Donghyeop Shin, Young-Joo Eo, Inyoung Jeong, Seung Kyu Ahn, Ara Cho, Jayeong Kim, Seokhyun Yoon, Pyuck-Pa Choi, William Jo, Jin Hyeok Kim, Jihye Gwak, Jae Ho Yun,

Abstract: Absolute measurements of photoluminescence are commonly performed using an integrating sphere setup, as this allows the collection of all emitted photons independent of the spatial characteristics of the emission. However, such measurements are plagued by multiple reflection effects occurring within the integrating sphere that make the sample illumination and sphere throughput sample dependent. To address this problem, we developed a matrix theory for integrating spheres with photoluminescent surfaces. In conjunction with a bispectral luminescence data set, this model allows for multiple reflection effects to be fully accounted for. The bispectral data is obtained by mounting both the sample and a non-luminescent reference on the sphere and permuting their positions in order to compare direct and diffuse sample illumination conditions. Experimental measurements of a photoluminescent standard confirm the validity of the method.
Luke J. Sandilands, Joanne C. Zwinkels