Absolute Absorption Cross Sections Research Articles

Revisiting the Ultraviolet Absorption Cross Section of Gaseous Nitrous Acid (HONO): New Insights for Atmospheric HONO Budget.

Nitrous acid (HONO) is an important source of hydroxyl radicals (OH) in the atmosphere. Precise determination of the absolute ultraviolet (UV) absorption cross section of gaseous HONO lays the basis for the accurate measurement of its concentration by optical methods and the estimation of HONO loss rate through photolysis. In this study, we performed a series of laboratory and field intercomparison experiments for HONO measurement between striping coil-liquid waveguide capillary cell (SC-LWCC) photometry and incoherent broadband cavity-enhanced absorption spectroscopy (IBBCEAS). Specified HONO concentrations prepared by an ultrapure standard HONO source were utilized for laboratory intercomparisons. Results show a consistent ∼22% negative bias in measurements of the IBBCEAS compared with a SC-LWCC photometer. It is confirmed that the discrepancies occurring between these techniques are associated with the overestimation of the absolute UV absorption cross sections through careful analysis of possible uncertainties. We quantified the absorption cross section of gaseous HONO (360-390 nm) utilizing a custom-built IBBCEAS instrument, and the results were found to be 22-34% lower than the previously published absorption cross sections widely used in HONO concentration retrieval and atmospheric chemical transport models (CTMs). This suggests that the HONO concentrations retrieved by optical methods based on absolute absorption cross sections may have been underestimated by over 20%. Plus, the daytime loss rate and unidentified sources of HONO may also have evidently been overestimated in pre-existing studies. In summary, our findings underscore the significance of revisiting the absolute absorption cross section of HONO and the re-evaluation of the previously reported HONO budgets.

A new instrumentation for simultaneous terahertz and mid-infrared spectroscopy in corrosive gaseous mixtures.

The correct interpretation of infrared (IR) observations of planetary atmospheres requires an accurate knowledge of temperature and partial and global pressures. Precise laboratory measurements of absorption intensities and line profiles, in the 200-350K temperature range, are, therefore, critical. However, for gases only existing in complex chemical equilibria, such as nitrous or hypobromous acids, it is not possible to rely on absolute pressure measurements to measure absolute integrated optical absorption cross sections or IR line intensities. To overcome this difficulty, a novel dual-beam terahertz (THz)/mid-IR experimental setup has been developed, relying on the simultaneous use of two instruments. The setup involves a newly constructed temperature-controlled (200-350K) cross-shaped absorption cell made of inert materials. The cell is traversed by the mid-IR beam from a high-resolution Fourier transform spectrometer using along a White-cell optical configuration providing absorption path lengths from 2.8 to 42m and by a THz radiation beam (82.5GHz to 1.1THz), probing simultaneously the same gaseous sample. The THz channel records pure rotational lines of molecules for which the dipole moment was previously measured with high precision using Stark spectroscopy. This allows for a determination of the partial pressure in the gaseous mixture and enables absolute line intensities to be retrieved for the mid-IR range. This new instrument opens a new possibility for the retrieval of spectroscopic parameters for unstable molecules of atmospheric interest. The design and performance of the equipment are presented and illustrated by an example of simultaneous THz and mid-IR measurement on nitrous acid (HONO) equilibrium.

Accurate absolute absorption cross-sections of the ozone Wulf bands at 1 µm range: measurements with high-resolution cw-CRDS laser techniques

Ozone plays a major role in atmospheric chemistry and radiative processes with an impact on climate and ecosystems being also considered as a possible biosignature species in exoplanetary observations. This makes important an extension of accurate laboratory data to various spectral ranges. The present work provides absolute absorption cross-sections of ozone accurate to about 3% in the near-IR range around 1 µm using for the first time a continuous-wave cavity-ring-down spectrometer (cw-CRDS) for recording the transitions above the first dissociation threshold of O3. This has permitted high-sensitivity and high-resolution measurements covering the region from 9745 to 10850 cm−1 corresponding to the transitions from the electronic ground state to spin-rovibronic levels of excited triplet electronic states 3A2 and 3B2 that form the Wulf band system. Two complementary methods based on the stationary recording with the quasi-continuous laser tuning and on the pressure ramp measurements at carefully chosen grids of wavenumbers were implemented and provided very well consistent results within 2% on average. The experimental integrated absorption intensity in the range under the study is evaluated as 1.393×10−20 (± 2%) cm/molecule. The comparison with the available published data of continuous recordings shows that our absorption cross-sections are much more accurate, particularly in the region with complex partly resolved spin-rovibronic spectral patterns. This region corresponds to a transparency near-IR window of the water vapor and is thus well suited for atmospheric applications. The theoretical modeling of the measured spectra using eight rovibronic bands is discussed. The experimental absorption cross-sections at the dense grid of wavenumbers are provided in the electronic supplementary files, which could be used as new reference data in the 9745 to 10850 cm−1 range.

Vibrationally Resolved Absorption and Fluorescence Cross Sections of Adamantane in the Far-ultraviolet Spectral Range on an Absolute Scale

High-resolution absorption, dispersed fluorescence emission, and photoionization cross sections are presented for gas-phase adamantane excited by synchrotron radiation in the exciting-photon energy range of 6–30 eV. Relative and absolute absorption cross sections of so-far unmatched resolution of down to 0.27 cm−1 line width in the region from 6.4–28 eV are shown along with newly discovered vibronic substructures around the HOMO–LUMO transition. Absorption line positions are provided with very high accuracy and listed in tabular form to be used as spectral fingerprints for the detection of adamantane in interstellar media, where its column density may be determined via the absolute cross sections. The fluorescence emission lies in the ultraviolet range from 190–250 nm and is excited starting at the HOMO–LUMO transition at 6.49 eV, which corresponds to the highest fluorescence emission energy. Hitherto unreported fluorescence in the same spectral range and relative photoionization cross sections in the exciting-photon energy range up to 30 eV are also presented along with lifetime measurements for differentiation of the involved electronic states.

Formation and consumption of HO2 radicals in ns pulse O2–He plasmas over a liquid water surface

Hydroperoxyl (HO2) radicals are an important precursor in the formation of hydrogen peroxide (H2O2), a key species in plasma-liquid interactions, such that their formation and consumption pathways need to be understood. In this work, the generation and decay of HO2 have been studied in a controlled environment, in ns pulse discharge O2–He plasmas in contact with a liquid water surface. For this, time-resolved, absolute number densities of HO2 in O2–He mixtures excited by a repetitive ns pulse discharge are measured in situ by cavity ring down spectroscopy (CRDS). The discharge cell with external electrodes to generate the plasma and a water reservoir are integrated into the CRDS cavity. The high-reflectivity cavity mirrors are purged with helium to protect them from water vapor condensation. The experimental results are obtained at near room temperature, both during the discharge pulse burst and in the afterglow. The HO2 number density is inferred from the CRDS data using a spectral model exhibiting good agreement with previous measurements of absolute HO2 absorption cross sections. HO2 is generated during the discharge burst and decays in the afterglow between the bursts. The HO2 number density is also measured vs. the O2 fraction in the mixture. Comparison with the kinetic modeling predictions demonstrates good agreement with the data and identifies the dominant HO2 generation and decay processes. HO2 in the plasma is formed predominantly by the recombination of H atoms, generated by the electron impact dissociation of water vapor, with O2 molecules. Reactions with O atoms and hydroxyl (OH) radicals are among the main HO2 decay processes in the afterglow. HO2 is also detected when O2 is not present in the mixture. In this case, it is generated primarily by the recombination of OH radicals, via the formation of H2O2. The results demonstrate that CRDS can also be used for HO2 and other plasma chemical reaction product measurements in atmospheric pressure plasma jets impinging on a liquid water surface in ambient air.

Infrared multiple‐angle incidence resolution spectrometry for vapor‐deposited amorphous water

AbstractInfrared multiple‐angle incidence resolution spectrometry (IR‐MAIRS) is a recently developed spectroscopic technique that combines oblique incidence transmission measurements and chemometrics (multivariate analysis) to obtain both pure in‐plane (IP) and out‐of‐plane (OP) vibration spectra for a thin sample. IR‐MAIRS is established for analyzing the molecular orientation of organic thin films at atmospheric pressure, but it should also be powerful for the structural characterization of vapor‐deposited thin samples prepared in a vacuum. The application of IR‐MAIRS to vapor‐deposited amorphous water is particularly interesting in the fields of physical and interstellar chemistry, because it is a representative model material for interstellar icy dust grains. We recently developed an experimental setup for in situ IR‐MAIRS under low‐temperature, ultrahigh‐vacuum conditions, which thus facilitates measurements of interstellar ice analogs such as vapor‐deposited amorphous water. This review considers the theoretical framework of IR‐MAIRS and our recent experimental results for vapor‐deposited amorphous water. We present spectroscopic signatures for the perpendicular orientation of dangling‐OH bonds for three‐coordinated water molecules at the surface of amorphous water at 90 K. The absolute absorption cross section of the three‐coordinated dangling‐OH bonds is quantitatively measured as 1.0 ± 0.2 × 10−18 cm2 at 3696 cm−1. As IR‐MAIRS can essentially be conducted using only a Fourier‐transform IR spectrometer and an angle‐controllable linear polarizer, it is a useful, low‐cost, and simple spectroscopic technique for studying laboratory analogs of interstellar ices including vapor‐deposited amorphous water.

Time-resolved measurements of HO2 radical in a heated plasma flow reactor

Time-resolved, absolute HO2 number density in diluted H2O2-Ar, CH4O2-Ar, and C2H4O2-Ar mixtures excited by a repetitive ns pulse discharge in a heated plasma flow reactor is measured by Cavity Ringdown Spectroscopy (CRDS). The experimental results are obtained at T = 300–600 K and P = 130 Torr, both during the discharge pulse burst and in the afterglow. The HO2 number density is inferred from the CRDS data using a spectral model exhibiting good agreement with previous measurements of absolute HO2 absorption cross sections. In the room-temperature H2O2 mixture, as well as in CH4O2 and C2H4O2 mixtures over the entire temperature range studied, HO2 is generated only during the discharge burst and decays in the afterglow. However, in the H2O2 mixture at elevated temperatures, T = 400–600 K, HO2 persists in the afterglow up to 10 ms after the discharge burst, comparable with the flow residence time in the reactor. Comparison with kinetic modeling shows that the sustained reactivity after the source of radicals is turned off is due to a chain propagation/ hydrogen oxidation process, which dominates the radical recombination reactions. The kinetic modeling predictions are in good agreement with the relative HO2 number density measured in all three mixtures, although the model underpredicts the absolute number densities in H2O2 at T = 400–600 K by up to a factor of two. Detection of the sustained low-temperature reactivity in H2O2, initiated by the radical generation in the plasma, suggests that the plasma excitation may also affect kinetics of oxidation and reforming of fuels exhibiting low-temperature chemistry below hot ignition point.

Absolute absorption cross section of 2-EHN in IR region

This paper presents the first determination of the absolute absorption cross section of 2-EHN from 850 to 3000 cm−1. The domain of application of our measurement and its accuracy are discussed. DFT calculations were used for band attributions and the identification of the different isomers existing at room temperature.The cross section measurements were used to study the adsorption/desorption processes of the gas that can occur on stainless steel cell walls and the evolution of this equilibrium when introducing 1 atm. of dry air. We also discuss the error made by neglecting those effects in terms of quantitative chemistry and air-broadening measurement.

Absolute Absorption Cross Section and Orientation of Dangling OH Bonds in Water Ice

Abstract The absolute absorption cross section of dangling OH bonds in water ice, a free OH stretch mode by three-coordinated surface water molecules, is derived experimentally as 1.0 ± 0.2 × 10−18 cm2 at 3696 cm−1 for amorphous water at 90 K using infrared multiple-angle incidence resolution spectrometry (IR–MAIRS). The integrated absorption cross section (band strength) of the dangling OH bond at 90 K (1.4 ± 0.3 × 10−17 cm molecule−1 at 3710–3680 cm−1) is found to be more than 1 order of magnitude smaller than those in bulk ice or liquid water. This indicates that a lack of hydrogen-bonding significantly decreases the band strength of dangling OH bonds. The present study also provides average molecular orientations of dangling OH bonds at 10 K and 90 K, because both the surface-parallel (in-plane) and surface-perpendicular (out-of-plane) vibration spectra of dangling OH bonds are quantitatively measured by IR–MAIRS. The intensity ratio of the dangling-OH peaks between in-plane to out-of-plane spectra shows the isotropic nature (random orientation) of the two- and three-coordinated dangling OH bonds in microporous amorphous water prepared at 10 K; however, the three-coordinated dangling OH bonds in nonporous amorphous water prepared at 90 K are dominantly located at the top ice surface and oriented perpendicular to it. These findings provide fundamental insights into the relationship between the structure and optical properties of ice surfaces, and aid quantitative understanding of the surface structure of interstellar ices and their laboratory analogs.

Absorption and emission cross-sections, Stark energy levels, and temperature dependent gain of Yb:QX phosphate glass.

We have obtained the absolute absorption cross-sections of Yb-doped phosphate QX laser glass at 300 K from 800-1100 nm, and generated the emission cross-sections via the standard method of reciprocity. By generating nine detailed high-confidence fitting functions for the absorption cross-sections, we then applied reciprocity to the individual absorption functions and obtain, for the first time to our knowledge, nine corresponding detailed emission functions that when summed, display a more accurate representation of the emission cross-sections as a function of wavelength than the standard method. A deeper mathematical understanding of reciprocity is also presented, along with derived Yb:QX Stark levels.

Absolute Absorption Cross-Section of the Ã←X˜ Electronic Transition of the Ethyl Peroxy Radical and Rate Constant of Its Cross Reaction with HO2

The absolute absorption cross-section of the ethyl peroxy radical C2H5O2 in the Ã←X˜ electronic transition with the peak wavelength at 7596 cm−1 has been determined by the method of dual wavelengths time resolved continuous wave cavity ring down spectroscopy. C2H5O2 radicals were generated from pulsed 351 nm photolysis of C2H6/Cl2 mixture in presence of 100 Torr O2 at T = 295 K. C2H5O2 radicals were detected on one of the CRDS paths. Two methods have been applied for the determination of the C2H5O2 absorption cross-section: (i) based on Cl-atoms being converted alternatively to either C2H5O2 by adding C2H6 or to hydro peroxy radicals, HO2, by adding CH3OH to the mixture, whereby HO2 was reliably quantified on the second CRDS path in the 2ν1 vibrational overtone at 6638.2 cm−1 (ii) based on the reaction of C2H5O2 with HO2, measured under either excess HO2 or under excess C2H5O2 concentration. Both methods lead to the same peak absorption cross-section for C2H5O2 at 7596 cm−1 of σ = (1.0 ± 0.2) × 10−20 cm2. The rate constant for the cross reaction between of C2H5O2 and HO2 has been measured to be (6.2 ± 1.5) × 10−12 cm3 molecule−1 s−1.

Vacuum-Ultraviolet Absorption Spectrum of 3-Methoxyacrylonitrile

The high-resolution absorption spectrum of 3-methoxyacrylonitrile (3MAN) was measured between 5.27 and 12.59 eV using a synchrotron-based Fourier-transform spectrometer. It was related to an absolute absorption cross-section scale. Complementary calculations at the DFT-MRCI/aug-cc-pVTZ level of theory document the vertical transition energies and oscillator strengths toward the first 19 states of both the E and Z geometrical isomers of 3MAN. Comparisons with the experimental absorption spectrum reveal the similarities and differences between 3MAN, a bifunctional molecule, with acrylonitrile and methylvinylether, where only one functional group is present. As in acrylonitrile, several broad valence transitions were observed up to the ionization limit. They are likely associated with the extended π-system induced by the nitrile group but might also involve σσ* transitions close to the ionization limit. As in methylvinylether, Rydberg series converging to the ionization limit are absent. This is attributed to a difference in neutral and cationic geometry due to a 60° rotation of the methyl group.

Unimolecular fragmentation and radiative cooling of isolated PAH ions: A quantitative study.

Time-resolved spontaneous and laser-induced unimolecular fragmentation of perylene cations (C20H12 +) has been measured on timescales up to 2 s in a cryogenic electrostatic ion beam storage ring. We elaborate a quantitative model, which includes fragmentation in competition with radiative cooling via both vibrational and electronic (recurrent fluorescence) de-excitation. Excellent agreement with experimental results is found when sequential fragmentation of daughter ions co-stored with the parent perylene ions is included in the model. Based on the comparison of the model to experiment, we constrain the oscillator strength of the D1 → D0 emissive electronic transition in perylene (fRF = 0.055 ± 0.011), as well as the absolute absorption cross section of the D5 ← D0 excitation transition (σabs > 670 Mb). The former transition is responsible for the laser-induced and recurrent fluorescence of perylene, and the latter is the most prominent in the absorption spectrum. The vibrational cooling rate is found to be consistent with the simple harmonic cascade approximation. Quantitative experimental benchmarks of unimolecular processes in polycyclic aromatic hydrocarbon ions like perylene are important for refining astrochemical models.

High-resolution vacuum ultraviolet absorption spectra of 2,3- and 2,5-dihydrofuran

Using a synchrotron-based Fourier-transform spectrometer, the high-resolution absorption spectra of the C1-symmetric 2,3-dihydrofuran (23DHF) and C2v-symmetric 2,5-dihydrofuran (25DHF) have been measured from 5.5 eV to 9.4 eV with an absolute absorption cross section scale. Oscillator strengths and vertical excitation energies of the lowest 18 states have been computed using the average of the second- and third-order algebraic diagrammatic construction polarization propagator method and the equation-of-motion coupled-cluster method at the level of singles and doubles model. These show that the bright valence transitions of ππ*-character are embedded into Rydberg transitions, whose oscillator strengths are at least one order of magnitude lower. To account for intensity borrowing, the first broad valence transition between 5.5 eV and 6.8 eV was simulated using a nuclear ensemble, and the agreement between experiment and theory is excellent. Whereas 23DHF only exhibits one broad valence transition followed by d/f Rydberg series converging to the ionization energy, the absorption spectrum of 25DHF has four bands, attributed to a valence nπσ → π*-transition, nπσ → 3px,z/3dxz transitions, a second valence nπ → π*-transition followed by d/f Rydberg series converging to the ionization energy, respectively. All Rydberg series converging to the ionization energy have been characterized in terms of their quantum defects.

Cross Sections and Asymmetry Parameters for Formic Acid in the Vacuum-Ultraviolet Energy Range.

We present an experimental and theoretical investigation of the photon interaction with formic acid in the vacuum-ultraviolet energy range. The absolute absorption cross sections and ionization efficiencies were measured in the 11.2-21.4 and 13.5-21.4 eV ranges, respectively, using a double-ion chamber technique. Photoionization and neutral-decay cross sections were derived from these results. From the present ionization cross sections and previously reported ionic dissociative branching ratios, the partial cross sections for dissociating processes were obtained. Theoretically, the photoionization cross sections and the asymmetry parameters of the photoelectron angular distributions for ionization out of the six outermost valence orbitals (10a', 2a″, 9a', 1a″, 8a', and 7a') were obtained in the energy range from near-threshold to 35 eV. For that, the Padé approximant technique along with the single-center partial-wave expansion method was applied to solve the Lippmann-Schwinger equation in the static-exchange-polarization level of approximation. This is the first theoretical investigation concerning the determination of the asymmetry parameters and photoionization cross sections of formic acid in the vacuum-ultraviolet energy range. Comparison is made between our results and the previous ones.

Validity of Judd-Ofelt spectroscopy based on diffuse reflectance spectrum and fluorescence lifetime of phosphor

Validity of Judd-Ofelt spectroscopy based on measured diffuse reflectance spectrum and fluorescence lifetime of a purely radiative electronic transition of rare-earth-ion in a powder material has been demonstrated. Two representative powder samples of Er3+-doped LiNbO3 and Er3+/Yb3+-codoped NaYF4, which have considerably different phonon energies, 880 and 380 cm−1, respectively, have been exemplified to demonstrate the validity. First, diffuse reflectance spectrum was measured for each powder sample and relative absorption spectrum was calibrated from it using Kubelka-Munk relation. Second, fluorescence lifetime of purely radiative 1.5 μm emission of Er3+ was measured and absolute absorption cross-section spectrum was calibrated on the basis of the assumption that the measured 1.5 μm fluorescence lifetime equals the radiative lifetime. Finally, the Judd-Ofelt parameters Ωt (t = 2, 4, 6) are calculated on the basis of the absolute absorption cross-section spectrum. For comparison, Judd-Ofelt analysis was also performed on the Er3+-doped LiNbO3 single-crystal on the basis of the absorption spectrum obtained from Beer-Lambert law. The Ωt result of Er3+-doped LiNbO3 powder sample is compared with that of single-crystal state. The comparison shows that the Ωt result of the powder sample can be considered as the same as that of the single-crystal state within the error, confirming the validity of the method. The study on the Er3+/Yb3+-codoped NaYF4 microcrystal powder allows us to reach the same conclusion.

The absorption spectrum and absolute absorption cross sections of acetylperoxy radicals, CH3C(O)O2 in the near IR

The Ã-X˜ electronic transition of acetylperoxy radicals (CH3C(O)O2) in the near-infrared was measured at 67hPa synthetic air in the spectral ranges from 6094 cm−1 to 6180 cm−1 and 6420 cm−1 to 6600 cm−1. CH3C(O)O2 radicals were generated by the pulsed photolysis of a acetaldehyde/Cl2/O2 mixture at 351 nm and subsequently measured by time-resolved continuous-wave cavity ring-down spectroscopy (cw-CRDS). The absorption cross sections of eight discrete absorption lines were determined relative to the absorption cross section of HO2, which has previously been reported. The strongest absorption cross section was found at 6510.73 cm−1 and was determined to be (4.9 ± 2.5) × 10−20 cm2.

Absolute optical absorption cross-section measurement of Rb atoms injected into superfluid helium using energetic ion beams

We report on a new in situ laser spectroscopy technique using superfluid helium and a highly energetic ion beam. In taking advantage of superfluid helium as a host matrix for laser spectroscopy, we were able to develop an extremely low background detection system. The first demonstration using 85Rb31+ ion beam (66 MeV/nucleon) accelerated at the RIKEN accelerator facility shows improvement of detection limit by two orders of magnitude. A quantitative evaluation of Rb atoms in the observation region enables an analysis of their optical absorption cross-section in superfluid helium from the fluorescence intensity dependence on ion-beam intensity and applied laser power.

Cr L-Edge X-ray Absorption Spectroscopy of CrIII(acac)3 in Solution with Measured and Calculated Absolute Absorption Cross Sections.

X-ray absorption spectroscopy at the L-edge of 3d transition metals is widely used for probing the valence electronic structure at the metal site via 2p-3d transitions. Assessing the information contained in L-edge absorption spectra requires systematic comparison of experiment and theory. We here investigate the Cr L-edge absorption spectrum of high-spin chromium acetylacetonate CrIII(acac)3 in solution. Using a transmission flatjet enables determining absolute absorption cross sections and spectra free from X-ray-induced sample damage. We address the challenges of measuring Cr L absorption edges spectrally close to the O K absorption edge of the solvent. We critically assess how experimental absorption cross sections can be used to extract information on the electronic structure of the studied system by comparing our results of this CrIII (3d3) complex to our previous work on L-edge absorption cross sections of MnIII(acac)3 (3d4) and MnII(acac)2 (3d5). Considering our experimental uncertainties, the most insightful experimental observable for this d3(CrIII)-d4(MnIII)-d5(MnII) series is the L-edge branching ratio, and we discuss it in comparison to semiempirical multiplet theory and ab initio restricted active space calculations. We further discuss and analyze trends in integrated absorption cross sections and correlate the spectral shapes with the local electronic structure at the metal sites.

Direct Determination of Absolute Absorption Cross Sections at the L-Edge of Dilute Mn Complexes in Solution Using a Transmission Flatjet.

The 3d transition metals play a pivotal role in many charge transfer processes in catalysis and biology. X-ray absorption spectroscopy at the L-edge of metal sites probes metal 2p-3d excitations, providing key access to their valence electronic structure, which is crucial for understanding these processes. We report L-edge absorption spectra of MnII(acac)2 and MnIII(acac)3 complexes in solution, utilizing a liquid flatjet for X-ray absorption spectroscopy in transmission mode. With this, we derive absolute absorption cross-sections for the L-edge transitions with peak magnitudes as large as 12 and 9 Mb for MnII(acac)2 and MnIII(acac)3, respectively. We provide insight into the electronic structure with ab initio restricted active space calculations of these L-edge transitions, reproducing the experimental spectra with excellent agreement in terms of shapes, relative energies, and relative intensities for the two complexes. Crystal field multiplet theory is used to assign spectral features in terms of the electronic structure. Comparison to charge transfer multiplet calculations reveals the importance of charge transfer in the core-excited final states. On the basis of our experimental observations, we extrapolate the feasibility of 3d transition metal L-edge absorption spectroscopy using the liquid flatjet approach in probing highly dilute biological solution samples and possible extensions to table-top soft X-ray sources.