Deep UV Excitation Research Articles

Rare earth element assessment in Jezero crater using the Planetary Instrument for X-ray Lithochemistry on the Mars 2020 rover Perseverance: A case study of cerium

The “Planetary Instrument for X-ray Lithochemistry” (PIXL) X-ray spectrometer conducts in situ geochemical analyses of martian rocks and regolith interrogated by the Mars 2020 rover, Perseverance. In addition to quantifying primary rock-forming elements, PIXL can quantify trace elements that in turn can provide additional constraints on the geologic history of Mars. Accurate quantifications of trace elements can require additional analytical techniques to mitigate experimental, background, and crystalline effects within PIXL spectra. In this study, we focus on reducing the impact of these effects and investigate the potential presence of rare earth elements (REEs). The study specifically investigates cerium given its typical relative abundance in many geologic materials compared to other REEs and its potential to mimic fluorescence features produced by organics under deep UV excitation. A detailed analysis of PIXL targets analyzed through the first 887 martian days of the Perseverance mission did not produce any conclusive Ce detections. Phosphorus-enriched materials analyzed by PIXL are estimated to contain sub-675 ppm Ce and sulfate-enriched materials sub-450 ppm Ce. The method presented can help constrain limits on the abundance of additional trace elements of interest that also face a similar analytical burden. PIXL's potential to assess REE abundances, outside of yttrium, is limited for expected concentrations in surface materials. Determining most REE concentrations in materials interrogated by Perseverance will therefore likely require terrestrial analyses.

Deep-Ultraviolet Photoexcitation of Aqueous Urea Forms Carbamic Acid/Carbamate in Less Than One Picosecond.

Urea is believed to have been essential to the synthesis of prebiotic nucleotides and thereby the RNA or DNA of the first lifeforms. Models suggesting that life began in wet-dry cycles around shallow aquatic ponds imply that reactants such as urea were exposed to deep ultraviolet irradiation from the young sun. Detrimental photodissociation of urea induced by deep UV excitation potentially challenges these models. We here follow the primary deep ultraviolet photochemistry of aqueous urea. The data show that urea is barely excited at 200 nm due to weak ultraviolet absorption. The likelihood of photodissociation is further reduced by strong intra-molecular coupling of the CN and CO stretch vibrations accompanied by an efficient dissipation of the excitation energy to the surrounding water molecules mitigated by urea-water hydrogen bonds. We find that 54±5 % of the excited urea molecules dissociate. Reactions between the photoproducts and surrounding solvent molecules form carbamic acid or the carbamate anions within 0.6 ps. The molecules that do not dissociate return to the electronic ground state in 2 ps. Interestingly, the photodissociation processes of urea in the aqueous phase is different from earlier reported reactions observed following the VUV photolysis of urea in noble gas matrices and highlight the potential influence of water on the prebiotic photochemistry.

Aqueous pyruvate partly dissociates under deep ultraviolet irradiation but is resilient to near ultraviolet excitation

The deep ultraviolet photochemistry of aqueous pyruvate is believed to have been essential to the origin of life, and near ultraviolet excitation of pyruvate in aqueous aerosols is assumed to contribute significantly to the photochemistry of the Earth’s atmosphere. However, the primary photochemistry of aqueous pyruvate is unknown. Here we study the susceptibility of aqueous pyruvate to photodissociation by deep ultraviolet and near ultraviolet irradiation with femtosecond spectroscopy supported by density functional theory calculations. The primary photo-dynamics of the aqueous pyruvate show that upon deep-UV excitation at 200 nm, about one in five excited pyruvate anions have dissociated by decarboxylation 100 ps after the excitation, while the rest of the pyruvate anions return to the ground state. Upon near-UV photoexcitation at a wavelength of 340 nm, the dissociation yield of aqueous pyruvate 200 ps after the excitation is insignificant and no products are observed. The experimental results are explained by our calculations, which show that aqueous pyruvate anions excited at 200 nm have sufficient excess energy for decarboxylation, whereas excitation at 340 nm provides the aqueous pyruvate anions with insufficient energy to overcome the decarboxylation barrier.

Fluorescence Microscopy with Deep UV, Near UV, and Visible Excitation for In Situ Detection of Microorganisms.

We report a simple, inexpensive design of a fluorescence microscope with light-emitting diode (LED) excitation for detection of labeled and unlabeled microorganisms in mineral substrates. The use of deep UV (DUV) excitation with visible emission requires no specialized optics or slides and can be implemented easily and inexpensively using an oblique illumination geometry. DUV excitation (<280 nm) is preferable to near UV (365 nm) for avoidance of mineral autofluorescence. When excited with DUV, unpigmented bacteria show two emission peaks: one in the near UV ∼320 nm, corresponding to proteins, and another peak in the blue to green range, corresponding to flavins and/or reduced nicotinamide adenine dinucleotide (NADH). Many commonly used dyes also show secondary excitation peaks in the DUV, with identical emission spectra and quantum yields as their primary peak. However, DUV fails to excite key biosignature molecules, especially chlorophyll in cyanobacteria. Visible excitation (violet to blue) also results in less mineral autofluorescence than near UV, and most autofluorescence in the minerals seen here is green, so that red dyes and red autofluorescence of chlorophyll and porphyrins are readily distinguished. The pairing of DUV and near UV or visible excitation, with emission across the visible, represents the most thorough approach to detection of labeled and unlabeled bacteria in soil and rock.

Signatures of Conical Intersections in Extreme Ultraviolet Photoelectron Spectra of Furan Measured with 15 fs Time Resolution.

Ultrafast internal conversion of furan upon deep UV excitation at 200 nm is studied by using extreme ultraviolet time-resolved photoelectron spectroscopy with a time resolution of 15 fs. Ballistic nuclear wavepacket motion from the 1B2(ππ*) state to the ground state is fully observed using 21.7 eV probe pulses. Through the performance of a comparison with the results of electronic structure calculations at the MS(3)-CASPT2(10,10)/cc-pVTZ level of theory, the photoelectron signals from the conical intersection regions are identified.

100 Mev Ag and 25 Kev He Induced Defects in 4H-SiC

We demonstrate the formation of radiative defects, known as color centres, in 100 MeV Ag and 25 keV He irradiated semi-insulating 4H-SiC using low-temperature (~77 K) photoluminescence (PL). In PL, the luminescence resulting from radiative recombination of photo-excited carriers (electrons and holes) is studied. In our work, a 266 nm deep UV excitation source is utilized for performing PL measurements. In the range, 850-1150 nm, the photoluminescent peaks ascribed to silicon vacancies (V Si) and unknown defect-related bands (UD3 and UD4) are observed after both ion irradiations. It has also been observed that till an optimum irradiation fluence there is an enhancement of photoluminescence from the ion irradiation-induced defects but further increase in irradiation fluence results in accumulation of photo-absorbing defects, providing pathways for non-radiative processes. In addition, ion irradiation has been shown to suppress the undesirable luminescence in the range 300-600 nm caused by the intrinsic defects in as-grown 4H-SiC.

Raman Diagnostics of Cathode Materials for Li-Ion Batteries Using Multi-Wavelength Excitation

Lithium-ion batteries have been commonly employed as power sources in portable devices and are of great interest for large-scale energy storage. To further enhance the fundamental understanding of the electrode structure, we report on the use of multi-wavelength Raman spectroscopy for the detailed characterization of layered cathode materials for Li-ion batteries (LiCoO2, LiNixCo1−xO2, LiNi1/3Mn1/3Co1/3O2). Varying the laser excitation from the UV to the visible (257, 385, 515, 633 nm) reveals wavelength-dependent changes in the vibrational profile and overtone/combination bands, originating from resonance effects in LiCoO2. In mixed oxides, the influence of resonance effects on the vibrational profile is preserved but mitigated by the presence of Ni and/or Mn, highlighting the influence of resonance Raman spectroscopy on electronic structure changes. The use of UV laser excitation (257, 385 nm) is shown to lead to a higher scattering efficiency towards Ni in LiNi1/3Mn1/3Co1/3O2 compared to visible wavelengths, while deep UV excitation at 257 nm allows for the sensitive detection of surface species and/or precursor species reminiscent of the synthesis. Our results demonstrate the potential of multi-wavelength Raman spectroscopy for the detailed characterization of cathode materials for lithium-ion batteries, including phase/impurity identification and quantification, as well as electronic structure analysis.

Luminescence from color centres induced by oxidation and ion irradiation in 4H–SiC

We report on the generation of radiative defects, the so-called color centres, in 25 keV He irradiated and thermally oxidized 4H–SiC <0001> surface investigated by photoluminescence spectroscopy utilizing 266 nm deep-UV excitation. Ion irradiation showed the generation of photoluminescent peaks ascribed to silicon vacancies (VSi) and unknown defect-related bands (UD3 and UD4). Thermal treatment (200–800 C) resulted in a significant increase of luminescence at an optimal temperature, depending on the initial density of emitters. While thermal oxidation resulted in several other high-brightness color centres, emitting photons even at room temperature. In addition to the defects in the crystalline 4H–SiC lattice, the difference in photoluminescence after oxidation indicates the presence of various other complex surface and interfacial defects.

Size evaluation of gold nanoparticles using transient grating method with deep-UV excitation pulses

Transient grating (TG) method was applied for evaluating the size of gold nanoparticles (AuNPs) in aqueous solution. Using nanosecond deep-UV pulses for excitation, AuNPs were reduced to smaller particles, making it feasible to determine their diffusion coefficients and hydrodynamic diameters. The diffusion signals of AuNPs were enhanced significantly by probing at the plasmon resonant wavelength, enabling the data acquisition within a few seconds. The determined hydrodynamic diameters were in agreement with those by other experimental techniques, indicating that the TG method is applicable for size evaluation of these nanoparticles in solution at short time.

Ultraviolet and Deep-Ultraviolet Excitation Photothermal Heterodyne Interferometer Combined with Semi-Micro HPLC.

A detection system consisting of a photothermal heterodyne interferometer (PHI) combined with semi-micro HPLC (high-performance liquid chromatography) has been designed and investigated. An ultraviolet (UV) or deep-UV laser emitting at 375 or 213 nm, respectively, was used for the excitation of nitro-polycyclic aromatic hydrocarbons (NPAHs) and amino acids. A photothermally induced change in the refractive index of the solvent causes an optical phase difference between two arms of the interferometer, one beam passing through the photoexcited region and another used as a reference, which was sensitively detected with the PHI. The separation and detection of NPAHs and amino acids were successfully demonstrated via semi-micro HPLC with the PHI and a UV detector. The detection limits of the UV-excitation PHI for NPAHs were 1.2 - 5.2 times better than that of the commercial UV detector, although the first demonstration of deep-UV excitation suffered from significant baseline fluctuation.

Luminescence of 5d–4f transitions of Pr3+ in natural fluorite CaF2, anhydrite CaSO4 and apatite Ca5(PO4)3F

Natural fluorite, anhydrite and apatite under deep UV excitation exhibits several narrow emission bands in the spectral range of 225–300 nm with the strongest peaks at 230–240 nm and 260–290 nm and with half-widths of 20–30 nm. They are characterized by narrow excitation band peaking at 220 nm and very short decay times of 20–50 ns. Those spectral-kinetic parameters enable their interpretation as 4f5d–4f transitions in Pr3+ luminescence center. Such kind of luminescence has been studied extensively over the past several decades in synthetic phosphors, including those which are analogous to natural compounds, but it is the first time 5d–4f luminescence transitions of Pr3+ is being proved in minerals.

Deep-UV excitation fluorescence microscopy for detection of lymph node metastasis using deep neural network

Deep-UV (DUV) excitation fluorescence microscopy has potential to provide rapid diagnosis with simple technique comparing to conventional histopathology based on hematoxylin and eosin (H&E) staining. We established a fluorescent staining protocol for DUV excitation fluorescence imaging that has enabled clear discrimination of nucleoplasm, nucleolus, and cytoplasm. Fluorescence images of metastasis-positive/-negative lymph nodes of gastric cancer patients were used for patch-based training with a deep neural network (DNN) based on Inception-v3 architecture. The performance on small patches of the fluorescence images was comparable with that of H&E images. Gradient-weighted class activation mapping analysis revealed the areas where the trained model identified metastatic lesions in the images containing cancer cells. We extended the method to large-size image analysis enabling accurate detection of metastatic lesions. We discuss usefulness of DUV excitation fluorescence imaging with the aid of DNN analysis, which is promising for assisting pathologists in assessment of lymph node metastasis.

Mobility of pectin methylesterase in pectin/cellulose gels is enhanced by the presence of cellulose and by its catalytic capacity

The pectin methylesterase action is usually studied in a homogeneous aqueous medium in the presence of a large excess of soluble substrate and water. However in the cell wall, the water content is much lower, the substrate is cross-linked with itself or with other polymers, and the enzyme has to diffuse through the solid matrix before catalysing the linkage breakdown. As plant primary cell walls can be considered as cellulose-reinforced hydrogels, this study investigated the diffusion of a fungal pectin methylesterase in pectin/cellulose gels used as cell wall-mimicking matrix to understand the impact of this matrix and its (micro) structure on the enzyme’s diffusion within it. The enzyme mobility was followed by synchrotron microscopy thanks to its auto-fluorescence after deep-UV excitation. Time-lapse imaging and quantification of intensity signal by image analysis revealed that the diffusion of the enzyme was impacted by at least two criteria: (i) only the active enzyme was able to diffuse, showing that the mobility was related to the catalytic ability, and (ii) the diffusion was improved by the presence of cellulose in the gel.

Phenolic distribution in apple epidermal and outer cortex tissue by multispectral deep-UV autofluorescence cryo-imaging

Phenolic compounds in fruit are involved in responses to biotic and abiotic stresses and are responsible for organoleptic properties. To establish the distribution of these secondary metabolites at the tissue and sub-cellular scales, mapping of fluorescence in apple epidermis and outer cortex tissue in cryogenic condition was performed after deep-UV excitation at 275 nm. Douce Moën and Guillevic cider apple varieties were sampled and frozen after harvest, after 30 days at 4 °C and after 20 days at room temperature. Image analysis of fluorescence emission images acquired between 300 and 650 nm allowed the assignment of fluorescence signals to phenolic compound families based on reference molecules. Emission attributed to monomeric and/or condensed flavanol was localized in whole tissue with major fluorescence in the cuticle region. Hydroxycinnamic acids were found predominantly in the outer cortex and appeared in the cell wall. Fluorescent pigments were mostly found in the epidermis. The distribution of flavanols in the sub-cuticle and phenolic acids in the outer cortex distinguished apple varieties. Storage conditions had no impact on phenolic distribution. The proposed fluorescent imaging and analysis approach enables studies on phenolic distribution in relation to fruit development, biotic/abiotic stress resistance and quality.

Photo-Enhanced Acid Chemical Etching of High-Quality Aluminum Nitride Grown by Metal-Organic Chemical Vapor Deposition

Photo-enhanced chemical (PEC) etch of wurtzite aluminum nitride (AlN) was investigated to overcome its low etch rate. Epitaxial AlN grown on Al2O3 substrate by metal-organic chemical vapor deposition method was immersed in phosphoric acid at different temperatures under 185 nm deep-UV excitation. An inverted pyramid structure with {10-1-1} faces initially appeared, after chemical etching, evolving into hexagonal columns with {1-100} m-planes. The PEC etch rate of m-plane AlN was six-fold higher than that of non-PEC one because the reaction was enhanced by the electron-hole pairs photo-generated by 185 nm wavelength. The decrease of the activation energy from 68.5 kJ/mol (non-PEC etch) to 48.1 kJ/mol (PEC etch) was extracted, implying a reaction-limited etching process. The PEC etch mechanism of wurtzite AlN in phosphoric acid was proposed, including the initiation and growth of the etch pit and the agglomeration of the hexagonal columns. Our results propose a facile method to control the surface morphology of AlN-based electronic and optoelectronic devices.

Multimodal Analysis of Central Nervous System Tumor Tissue Endogenous Fluorescence With Multiscale Excitation

The primary therapeutic approach for high-grade brain tumor is surgical resection. However, identifying tumor margins in vivo remains a major challenge. Biopsy analysis remains the standard diagnostic technique on tumor margins. This ex-vivo analysis is time consuming and delays treatment. The aim of this study is tissue discrimination using label free autofluorescence and application in intraoperative optical probe for optical biopsy. Biopsy samples from fifty-one patients who underwent brain tumor surgery (21 metastasis tumors, 17 glioblastoma tumors, GBM, and 13 control samples) were included in this study. The samples underwent a multiscale and multi-contrast optical analysis. The excitation were performed with a deep-UV synchrotron beam, at 275nm, and a near-infrared Ti:sapphire pulsed laser, from 690nm to 1040nm. The detection modalities were fluorescence imaging, spectroscopy and fluorescence lifetime. Using deep-UV excitation, and combining three molecular ratios (tyrosin-tryptophan, tryptophan-collagen, tryptophan-NADH) resulted in discrimination with a sensitivity of 90% and a specificity of 73%. Using a two-photon excitation, and combining average lifetime, NADH-FAD ratio and Porphyrin-NADH ratio, resulted in discrimination with a sensitivity of 97% and a specificity of 100%. A multiscale algorithm resulted in an overlap of only 1.8% between control and tumor samples.

Sensitized deep-UV excitation of Tm3+ ions in strontium phosphate glasses with Sb3+ co-doping

The novel Tm3+/Sb3+ co-doped phosphate glasses are studied in response to an improvement of blue light emitting source. The analysis of J-O intensity parameters suggests that the present strontium phosphate glass is an appropriate matrix for Tm3+ doping to obtain an enhanced blue emission. The 5 s2-type Sb3+ ion serves as a sensitizer to transfer its strong allowed excitation in the deep UV region to Tm3+ effectively. PL and PLE spectra show that by Sb3+ co-doping, the blue emission of the Tm3+ doped strontium phosphate glass is enhanced greatly, while the excitation band of Tm3+ extends largely to the deep UV region. The emission decay curves of Sb3+ demonstrate the existence of the energy transfer (ET) process from Sb3+ to Tm3+ with the maximum efficiency and probability of ET as 20.43% and 9.20%, respectively. The emission color of these glasses can be adjusted from warm to cold blue via modulating excitation wavelengths. The present work suggests that Sb3+/Tm3+co-doped strontium phosphate glasses are promising for applications in blue emitters and plasma display panels.

Ultrafast excited-state dynamics of 2,5-dimethylpyrrole.

The ultrafast excited-state dynamics of 2,5-dimethylpyrrole following excitation at wavelengths in the range of 265.7-216.7 nm is studied using the time-resolved photoelectron imaging method. It is found that excitation at longer wavelengths (265.7-250.2 nm) results in the population of the S1(1πσ*) state, which decays out of the photoionization window in about 90 fs. At shorter pump wavelengths (242.1-216.7 nm), the assignments are less clear-cut. We tentatively assign the initially photoexcited state(s) to the 1π3p Rydberg state(s) which has lifetimes of 159 ± 20, 125 ± 15, 102 ± 10 and 88 ± 10 fs for the pump wavelengths of 242.1, 238.1, 232.6 and 216.7 nm, respectively. Internal conversion to the S1(1πσ*) state represents at most a minor decay channel. The methyl substitution effects on the decay dynamics of the excited states of pyrrole are also discussed. Methyl substitution on the pyrrole ring seems to enhance the direct internal conversion from the 1π3p Rydberg state to the ground state, while methyl substitution on the N atom has less influence and the internal conversion to the S1(πσ*) state represents a main channel.

Sensitizing effects of Sn2+ ions on Tm3+ emissions for deep UV excitation in phosphate glasses

In the present work, Tm3+-doped strontium phosphate glasses modified by Sn2+ are investigated. Sn2+ and Tm3+ can be co-excited with the deep UV light to emit the tunable blue light. The reinforced and purified blue emission is obtained at the excitation bands toward the deep-UV part of the spectrum. Photoluminescence emission and excitation spectra, and emission decay curves indicate consistently that the improved Tm3+ emission is in association with the presence of Sn2+ in the host glass where energy transfer from Sn2+ to Tm3+ occurs. Relevant mechanism is discussed.

Deep UV Raman spectroscopy for planetary exploration: The search for in situ organics

Raman spectroscopy has emerged as a powerful, non-contact, non-destructive technique for detection and characterization of in situ organic compounds. Excitation using deep UV wavelengths (< 250nm), in particular, offers the benefits of spectra obtained in a largely fluorescence-free region while taking advantage of signal enhancing resonance Raman effects for key classes of organic compounds, such as the aromatics. In order to demonstrate the utility of this technique for planetary exploration and astrobiological applications, we interrogated three sets of samples using a custom built Raman instrument equipped with a deep UV (248.6nm) excitation source. The sample sets included: (1) the Mojave Mars Simulant, a well characterized basaltic sample used as an analog for Martian regolith, in which we detected ∼0.04 wt% of condensed carbon; (2) a suite of organic (aromatic hydrocarbons, carboxylic acids, and amino acids) and astrobiologically relevant inorganic (sulfates, carbonates, phosphates, nitrates and perchlorate) standards, many of which have not had deep UV Raman spectra in the solid phase previously reported in the literature; and (3) Mojave Mars Simulant spiked with a representative selection of these standards, at a concentration of 1 wt%, in order to investigate natural 'real world' matrix effects. We were able to resolve all of the standards tested at this concentration. Some compounds, such as the aromatic hydrocarbons, have especially strong signals due to resonance effects even when present in trace amounts. Phenanthrene, one of the aromatic hydrocarbons, was also examined at a concentration of 0.1 wt% and even at this level was found to have a strong signal-to-noise ratio. It should be noted that the instrument utilized in this study was designed to approximate the operation of a 'fieldable' spectrometer in order to test astrobiological applications both here on Earth as well as for current and future planetary missions. It is the foundation of SHERLOC, an arm mounted instrument recently selected by NASA to fly on the next rover mission to Mars in 2020.