UV VUV Research Articles

UV-VUV Photofragmentation Spectroscopy of Isolated Neutral Fragile Macromolecules: A Proof-of-Principle Based on a Deprotonated Vancomycin-Peptide Noncovalent Complex.

The gas phase offers the possibility to analyze organic molecules by ultraviolet-vacuum ultraviolet (UV-VUV) spectroscopy without any solvent effect or limitation in terms of spectral range due to absorption by the solvent. Up to now, the size and chemical composition of neutral molecular systems under study have been limited by the use of vaporization methods based on thermal heating. Soft sources of gas-phase thermolabile molecular systems such as electrospray or matrix-assisted laser desorption ionization are appealing alternatives to heating-based techniques, but they lead to the production of ions. In such cases, UV-VUV action spectroscopy is then the method of choice to study the electronic structure and corresponding photodynamics of these gas-phase molecular ions. However, previous investigations have shown that the UV-VUV action spectrum of a given molecular ion depends on the charge state, which in many cases might be a caveat. Here, by means of synchrotron radiation coupled to mass spectrometry and through the test case of the glycopeptide antibiotic vancomycin noncovalently bound to a deprotonated small peptide, we show that the UV-VUV photofragmentation spectrum of neutral thermally fragile organic molecules can be obtained via charge-tagging action spectroscopy.

Electronic states of nitromethane: Experimental and theoretical studies

A comprehensive spectroscopic study of the UV–VUV photoabsorption spectrum of nitromethane in the energy region 5.4–11.8 eV (43,500–95,000 cm−1) using synchrotron radiation is presented; the VUV absorption spectrum in the region > 9 eV being reported for the first time. The observed spectral features are assigned to various valence and Rydberg transitions, supported by quantum chemical calculations using the TDDFT method. The 6 eV region is dominated by a broad, intense absorption band peaking at ∼ 6.2 eV assigned to the π-π* valence transition, followed by rich Rydberg series converging to the first four ionization potentials of nitromethane. Theoretical calculations of orbital energies as well as Rydberg series analysis indicate that the third IP is located at 11.95 eV, in contrast to the earlier estimated value of 11.5 eV. Series of bands attributed to vibrational transitions accompanying the 3s (6a″), 3p(6a"), 3s(10a′) and 3s(5a″) Rydberg transitions are observed in the 60,000–73,000 cm−1 region and are tentatively assigned to progressions involving the NO2 in-plane rock and CH3 asymmetric deform modes. The liquid phase IR spectrum is revisited and assigned with the help of DFT calculations. A few new bands are observed and assigned to overtone and combination modes. Theoretically simulated potential energy curves of the first few excited singlet and triplet states with respect to the CN and NO bond lengths are useful in explaining some of the incompletely understood features of the photodissociation dynamics of nitromethane. It is found that direct dissociation in the Franck Condon region is a possible mechanism for the CH3NO + O channel, while singlet-triplet crossings play an important role in CN and NO bond scissions.

Formation of nitrite and ammonium during the irradiation of nitrate-containing water by VUV/UV

Vacuum ultraviolet/ultraviolet (VUV/UV) has been regarded as an efficient chemical-free advanced oxidation process for degradation of micropollutants, but hazardous byproducts, e.g. nitrite, can be formed during the irradiation of nitrate-containing water by VUV/UV. This study investigated the kinetics of nitrite and ammonium formation by VUV/UV irradiation of nitrate. The results indicate that the formation of nitrite and ammonium followed pseudo zero-order reaction kinetics at an initial nitrate concentration of 10 mg L−1. Direct UV photolysis, VUV photolysis, and indirect VUV photolysis contributed to 24.6 %, 44.6 %, and 30.8 % of nitrite formation, and 0%, 65.0 % and 35.0 % of ammonium formation, respectively, in VUV/UV process. Initial nitrate concentration strongly affected not only the final concentrations of nitrite and ammonium but also the kinetics of their formation by VUV/UV. The presence of nitrate inhibited the degradation of alachlor, a typical herbicide, while alachlor had various influence on the formation of nitrite and ammonium depending on its initial concentration. Inorganic anions inhibited nitrate reduction by competing for VUV, while dissolved oxygen inhibited nitrate reduction by scavenging H and eaq−. The surface water had higher rate constants for the formation of nitrite and ammonium than those of the ground water because the former had lower content of inorganic ions but higher content of natural organic matter. When applying the VUV/UV process for water treatment, the formation of nitrite and ammonium should be taken seriously, especially at the presence of nitrate with high concentration.

The electronic absorption spectrum of Anisole studied by photoabsorption spectroscopy and quantum chemical calculations

Photoabsorption studies on anisole (C6H5OCH3) are performed in the UV–VUV region (4.5–11.8 eV) using synchrotron radiation; the absorption spectrum in the region > 7.8 eV is reported here for the first time. Quantum chemical calculations are carried out on ground and excited states of anisole using the DFT and TDDFT methodologies and compared with experimental spectra in order to interpret the observed features in terms of the transitions involved, their valence/Rydberg nature and relative intensities. In order to validate the ground state spectral parameters, the infrared spectrum is recorded and assigned in the 600–3500 cm−1 region; a few new vibrational bands observed in the 1700–2700 cm−1 region are assigned to overtone and combination modes. The electronic absorption spectrum in the 4.5–5.0 eV region shows an extensive vibrational progression belonging to the first valence (ππ*) transition, while the 5.0–7.0 eV region is dominated by a broad and intense band peaking at ~6.6 eV which is assigned to two strong overlapping valence transitions originating from the HOMO–1 orbital. Additionally, throughout the 5.0–11.8 eV region, diffuse bands are observed which are attributed to Rydberg series (ns, np and nd) converging to the first four IPs of anisole. Theoretically simulated potential energy curves of ground and first few excited singlet and triplet states help in achieving a better understanding of the spectral features and the UV induced photodissociation mechanisms of anisole. The present studies suggest that photon excitation at 248 nm is likely to result in direct dissociation along a repulsive excited state (3s) in contrast to earlier studies which have attributed it to internal conversion to the ground state.

Development of an experimental set-up for low-temperature spectroscopic studies of matrix-isolated molecules and molecular ices using synchrotron radiation.

An experimental set-up for studying photophysics and photochemistry of molecules in an inert gas medium (matrix-isolated) and in the ice phase at low temperatures has been developed and commissioned at the Photophysics beamline, Indus-1 synchrotron radiation source. This end-station uses an in-house-developed closed-cycle cryostat for achieving cryo-temperatures (∼10 K). Synchrotron radiation from the Photophysics beamline is used as the source of UV-VUV photons and the system is equipped with a Fourier transform infrared spectrometer for characterization of the molecular species formed at low temperature. Various individual components of the end-station like closed-cycle cryostat, experimental chamber, gas mixing and deposition systems are tested to ascertain that the desired performance criteria are satisfied. The performance of the composite system after integration with the Photophysics beamline is evaluated by recording IR and UV-VUV photoabsorption spectra of sulfur dioxide at low temperatures (10 K), both in the ice phase as well as isolated in argon matrices. Results obtained are in good agreement with earlier literature, thus validating the satisfactory performance of the system. As an off-shoot of the study, the VUV absorption spectrum of matrix-isolated SO2 in argon matrix up to 10.2 eV is reported here for the first time. This experimental end-station will provide new opportunities to study photon-induced reactions in molecules of environmental, astrochemical and industrial importance. Details of the design, development and initial experimental results obtained are presented.

Luminescence of defect clusters in the disordered anion sublattice of CaF2 crystals

The fast quenching of CaF2 crystals preheated up to 1000 °C allows to stabilize the high-temperature disordering of anion sublattice typical for the superionic state of the crystal. Such procedure results in the formation of structured ultraviolet to vacuum ultraviolet (UV–VUV) absorption with a lengthy long-wavelength tail stretching into the visible spectral range. Such absorption in the quenched crystals cannot be related to any local center and testifies to the presence of nano-sized complexes of displaced anions, the electron structure of which is drastically changed as compared with a regular lattice. Possibly, these clusters are frozen dynamic clusters that were discovered earlier in the superionic state with quasi-elastic diffuse neutron-scattering technique. Four luminescence bands in near UV and visible spectral range correspond to four low-energy absorption bands. Unlike absorption bands that are of zone nature, the luminescence occurs from the localized state that arises due to strong electron-vibration coupling in the clusters. Absorption and luminescence states are divided by a potential barrier that is responsible for the spacing between corresponding excitation and emission bands as well as the temperature quenching of luminescence. The luminescence study reveals the features of the electronic structure of clusters within preheated/quenched CaF2 crystals. The quenching method allows to study the high-temperature superionic state of crystals with fluorite structure using a wide set of experimental techniques.

Time-resolved photoelectron spectroscopy of polyatomic molecules using 42-nm vacuum ultraviolet laser based on high harmonics generation

Time-resolved photoelectron spectroscopy (TRPES) of gaseous polyatomic molecules using 266-nm (4.7eV) pump and 42-nm (29.5eV) probe pulses is presented. A 1-kHz Ti:sapphire laser with a 35fs pulse duration is employed to generate high harmonics in Kr gas, and the 19th harmonic (42-nm) was selected using two SiC/Mg mirrors. Clear observation of the ultrafast electronic dephasing in pyrazine and photoisomerization of 1,3-cyclohexadiene demonstrates the feasibility of TRPES with the UV pump and VUV probe pulses under weak excitation conditions in the perturbation regime.

Vibrationally assisted below-threshold ionization

We perform time-resolved UV pump (4.77 eV) and VUV probe (7.94 eV) measurements of internal conversion of 1,3-cyclohexadiene (CHD). Our measurements reveal a substantial ionization of the ``hot'' ground state following internal conversion despite the fact that our probe photon energy is below the ionization potential (8.21 eV). With the aid of electronic structure calculations, we interpret our results in terms of vibrationally assisted below-threshold ionization, where vibrational energy is converted to electronic energy. The effect relies on both having vibrational modes which allow for this vibrational-electronic coupling and exciting these modes during the internal conversion. We contrast our measurements in CHD with another similar molecule, cis,cis-1,3-cyclooctadiene (cis,cis-COD), for which we do not see the effect.

Optical characteristics of UV–VUV lamps on the electronic‐vibrational transitions of the hydroxyl radical pumped by a nanosecond capacitive discharge

This paper presents the results of a study of UV and VUV low-pressure lamp pumped by nanosecond capacitive discharge. This lamp works on the mixtures of helium and argon with water vapor (H2O) or ‘heavy’ water (D2O). At the capacitive low-pressure lamp emission spectra are mainly narrow band 308 nm OH (A–X) and large band in the VUV spectral range: 140–200 nm OH (C, B, A). Found, that the increasing the partial pressure of H2O and D2O (>130–150 Pa) led to a decrease in the intensity of VUV radiation of hydroxyl radicals. In the transition from He/H2O mixture to He/D2O mixture, the optimal pressure of helium is decreased. Reduced partial pressure of argon led to increased intensity of radiation of hydroxyl in the VUV range. The study of emission characteristics of OD* – lamps on a mixture of He/D2O and the OH* – lamps on a mixture of He/H2O showed that the intensity of the bands OD (X, C, A) is 1.5 times larger than the intensity of the bands OH (X, C, A). The maximum absolute intensity of UV–VUV radiation of the lamp (at a frequency of 1000 Hz) reached 1 W.

The high-resolution absorption spectroscopy branch on the VUV beamline DESIRS at SOLEIL.

A VUV absorption spectroscopy facility designed for ultra-high spectral resolution is in operation as a dedicated branch on the DESIRS beamline at Synchrotron SOLEIL. This branch includes a unique VUV Fourier transform spectrometer (FTS) and a dedicated versatile gas sample chamber. The FTS instrument can cover a large UV-VUV spectral range from 4 to 30 eV, with an ultimate line width of 0.08 cm(-1) on a large spectral window, ΔE/E = 7%, over which all spectral features can be acquired in a multiplex way. The performance can be considered to be a middle ground between broadband moderate-resolution spectrometers based on gratings and ultra-high-spectral-resolution VUV tunable-laser-based techniques over very narrow spectral windows. The various available gaseous-sample-handling setups, which function over a wide range of pressures and temperatures, and the acquisition methodology are described. A selection of experimental results illustrates the performance and limitations of the FTS-based facility.

Photoluminescence properties of CdSrP2O7:Eu2+ blue phosphor for white LED applications

Eu2+-activated CdSrP2O7 phosphor was synthesized by conventional solid-state reaction. The photoluminescence properties of this phosphor were investigated by excitation and emission spectra UV–VUV region. It was found that the synthesized phosphor emitted broad blue emission band centered at 418nm, with both 274 and 131nm excitations. The temperature-dependence luminescence characteristic of CdSr0.99P2O7:0.01Eu2+ phosphor was reported, with the increase of temperature, the emission band exhibited blue shift and decreasing emission intensities. The phosphor CdSr0.99P2O7:0.01Eu2+ possesses good thermal stability. The activated energy was deduced to be 0.227eV. These results indicated that CdSrP2O7:Eu2+ phosphor could be a good candidate for near-UV based white light − emitting diodes (WLEDs).

Time-Resolved luminescence of La2Be2O5 crystals doped with CE3+ and PR3+ under selective UV–VUV–XUV excitation

The electronic states of Ce3+ and Pr3+ impurity centers in lanthanum beryllate crystals upon excitation by synchrotron radiation in the ultraviolet, vacuum ultraviolet (3.4–35 eV), and X-ray (60–610 eV) ranges are investigated by optical and time-resolved luminescence spectroscopy. Energy-band diagrams of the ions are proposed and substantiated; electronic transitions forming the absorption and luminescence spectra are identified. Significant disproportion of the luminescence yield on the excitation energy is found, and a change in the nanosecond luminescence-decay kinetics of Ce3+ centers in the core levels of Be 1s and O 1s is observed, especially in the giant resonance region (110–140 eV).

Intrinsic and defect related luminescence in double oxide films of Al–Hf–O system under soft X-ray and VUV excitation

Low temperature time-resolved luminescence spectra in the region of 2.5–9.5eV under soft X-ray excitation as well as time-resolved luminescence excitation spectra in the UV–VUV region (3.7–12eV) of solid solutions AlxHfyO1−x−y thin films were investigated. The values of x and Al/Hf ratio were determined from X-ray photoelectron srectroscopy data. Hafnia films and films mixed with alumina were grown in a flow-type chemical vapor deposition reactor with argon as a carrier gas. In addition, pure alumina films were prepared by the atomic layer deposition method. A strong emission band with the peak position at 4.4eV and with the decay time in the μs-range was revealed for pure hafnia films. The emission peak at 7.74eV with short nanosecond decay kinetics was observed in the luminescence spectra for pure alumina films. These emission bands were ascribed to the radiative decay of self-trapped excitons (an intrinsic luminescence) in pure HfO2 and Al2O3 films, respectively. Along with intrinsic host emission, defect related luminescence bands with a larger Stokes shift were observed. In the emission spectra of the solid solution films (x=4; 17; 20at%) the intrinsic emission bands are quenched and only the luminescence of defects (an anion vacancies) was observed. Based on transformation of the luminescence spectra and ns-luminescence decay kinetics, as well as changes in the time-resolved luminescence and luminescence excitation spectra, the relaxation processes in the films of solid solution are discussed.

Host sensitization of Tb3+ through Gd3+ in Na3Gd (BO3)2: Tb3+

The spectroscopic properties of undoped and Tb3+-doped Na3Ln (BO3)2 (Ln = Y, Gd) have been investigated in the UV–VUV region. The self-trapped excitons (STEs) emission of undoped-Na3Y (BO3)2 (NYB) was observed upon host excitation at low temperature and the emission intensity decreased with the increase of temperature. When Gd3+ was doped in NYB, the emission intensity of STEs was greatly reduced; instead, strong Gd3+ emission appeared. In Na3Gd (BO3)2: Tb3+, host excitation energy was predominantly transferred to Tb3+ through Gd3+ ions. The energy transfer efficiency from Gd3+ to Tb3+ increased with the increase of Tb3+ content. The results indicate that Gd3+ ions play a crucial intermediate role in host sensitization of Tb3+, which will provide some insights for developing new efficient green-emitting VUV-excited phosphors.

Photoluminescence and thermoluminescence properties of Y3(Al,Ga)5O12:Tb3+ phosphor

A comparative study of the structural, photoluminescence (PL) and thermoluminescence (TL) properties of Y3(Al,Ga)5O12:Tb3+ phosphor powder was performed. The scanning electron microscope images showed that the phosphor particles were agglomerated and irregular in shape. The bigger particles consisted of smaller spherically shaped agglomerated particles ranging in size between 0.5 and 1 μm. The UV–VUV synchrotron radiation was used to study the luminescence properties of the phosphor. The TL study was performed after the sample was irradiated by γ-rays obtained from a 60Co target in the dose range of 10–50 Gy. The effect of heating rate on the TL glow curves and trapping parameters was also calculated. The correlation between PL and TL was explored.

Characteristics and parameters of plasma of a gas-discharge UV–VUV lamp on a system of bands of argon chloride and chlorine molecules

The results of experimental study of the characteristics of a two-wave UV–VUV lamp on a mixture of argon and freon vapors (CCl4) with pumping by a nanosecond barrier discharge are presented. The spectral characteristics of the radiator in the wavelength range of 160–300 nm and the current and voltage oscillograms have been studied. The intensity of the radiation of the 175 nm ArCl (B → X) and 258 nm Cl2 (D' → A') bands were optimized during the variation of pressure and partial composition of the working mixture Ar–CCl4, and also the conditions of the ignition of the nanosecond discharge with two dielectric barriers. The main processes of the formation of the argon chloride molecule have been considered in the barrier discharge on the basis of the Ar–CCl4 mixture.

Photoabsorption and photodissociation studies of dimethyl sulphoxide (DMSO) in the 35,000–80,000 cm−1 region using synchrotron radiation

Photoabsorption and photodissociation studies of dimethyl sulphoxide and its deuterated isotopologue (DMSO-h6 and DMSO-d6) are performed using synchrotron radiation in the 35,000–80,000cm−1 region. In the photoabsorption spectrum, Rydberg series converging to the first three ionization potentials of DMSO at 9.1, 10.1 and 12.3eV corresponding to removal of an electron from the highest three occupied molecular orbitals (14a′, 7a″ and 13a′) are observed. Based on a quantum defect analysis, Rydberg series assignments are extended to higher members as compared to earlier works and a few ambiguities in earlier assignments are clarified. Analysis is aided by quantum chemical calculations using the DFT and TDDFT methodologies. Vibronic structures observed in the spectrum of DMSO-h6 in the regions 7.7–8.1eV and 8.1–8.8eV are attributed to the transitions 7a″→4p at 7.862eV and 14a′→6s/4d at 8.182eV, respectively. Photoabsorption spectra of DMSO-h6 and -d6 recorded using a broad band incident radiation show prominent peaks, which are identified and assigned to electronic and vibronic transitions of the SO radical. This provides a direct confirmation of the fact that DMSO preferentially dissociates into CH3 and SO upon UV–VUV excitation, as proposed in earlier photodissociation studies. An extended vibronic band system associated with the e1Π–X3Σ− transition of the SO radical is identified and assigned. The complete VUV photoabsorption spectrum of DMSO-d6 is also reported here for the first time.

Rydberg and valence excited states of dibromomethane in 35,000–95,000 cm−1 region studied using synchrotron radiation

The UV–VUV photoabsorption spectrum of dibromomethane (CH2Br2) in the energy region 4.3–11.8eV (35,000–95,000cm−1) is investigated using synchrotron radiation. Rydberg series converging to the first four ionization limits at 10.52, 10.74, 11.21 and 11.30eV corresponding to excitations from the 3b1, 2b2, 1a2, and 4a1 orbitals of CH2Br2 are identified and analyzed. Quantum defect values are observed to be consistent with excitation from the bromine lone pair orbitals. Assignments of the ns Rydberg series are revised and the np and nd Rydberg series are assigned for the first time. Observed vibrational features accompanying the 5p and 4d Rydberg states are assigned exclusively to the totally symmetric (a1) –CBr symmetric stretching mode (ν3) in contrast to the earlier assignment to ν3 and –CH2 bending (ν2) modes. The Rydberg and valence transitions observed in the present experiment are found to be in good agreement with the vertical excited states calculated using the TDDFT method. The calculations are further used to infer the valence transitions responsible for the broad intensity pedestals underlying the Rydberg transitions. The assignments are confirmed using isotopic substitution studies on CD2Br2 whose UV–VUV photoabsorption spectrum is reported here for the first time. This work presents a consolidated analysis of the UV–VUV photoabsorption spectrum of dibromomethane.

Luminescence of LaBr3:Ce,Hf scintillation crystals under UV-VUV and X-ray excitation

The present study was carried out by means of the low temperature time-resolved luminescence UV–VUV spectroscopy as well as the optical and thermally activation spectroscopy. The Ce3+ centers in regular lattice sites and located in the vicinity of the point defects of crystal structure were observed. Spectral and kinetic characteristics of the luminescence of these centers were defined. A photoluminescence of new point defects of the crystal structure are also manifested. We evaluated the energy of the interband transitions as Eg ≈ 6.2 eV in LaBr3. At the excitation energies Eexc>13 eV (higher than 2Eg) the effect of the electronic excitations multiplication was detected.

Plasma interactions with aminoacid (l-alanine) as a basis of fundamental processes in plasma medicine

Plasma interactions with l-alaine have been studied as a basis of fundamental processes in plasma medicine. The plasma interactions with l-alaine have been examined for investigations of molecular degradations induced by direct exposures with Ar plasma and exposures with UV–VUV photons emitted from the Ar plasma via chemical bonding states analyses using X-ray photoelectron spectroscopy (XPS). The direct Ar-plasma exposure resulted in significant degradations of COOH group and CNH2 group. Separate experiments via irradiation with photons in UV and VUV regions from the Ar plasma showed that the molecular degradation via irradiation with photons in VUV region was much more significant than via irradiation with photons in UV region. These experiments have indicated that the causality of the molecular degradation of the l-alanine during the Ar plasma exposure is considered to be significant in the following order; ions > VUV photons > UV photons ∼ meta-stable radicals. Furthermore, the exposure with Ar–O2 mixture plasma resulted in insignificant change in the XPS C1s spectra for variation of the exposure time ranging from 30 s to 300 s, indicating that the surface etching process is much more considerable than the chemical degradation process.