Tunable Vacuum Ultraviolet Light Source Research Articles

Ionization potentials of metal clusters studied with a broad range, tunable vacuum ultraviolet light source.

In this work, we present an alternative to complex laser setups or synchrotron light sources to accurately measure the ionization potentials of metal clusters. The setup is based on a commercial Xe flash lamp, combined with a vacuum monochromator, and has been applied to determine the ionization potentials of Snn clusters with n = 8-12 atoms. The uncertainty in the determination of the ionization potentials is mainly caused by the bandwidth of the monochromator. The adiabatic ionization potentials (AIPs) are extracted from experimental photoionization efficiency curves. Franck-Condon simulations are additionally used to interpret the shape and onset of the photo-ion yield. The obtained AIPs are (all energies are in eV) Sn8 (6.53 ± 0.05), Sn9 (6.69 ± 0.04), Sn10 (6.93 ± 0.03), Sn11 (6.34 ± 0.05), and Sn12 (IsoI 6.64 ± 0.04 and IsoIII 6.36 ± 0.05). Furthermore, the impact of multiple isomers present in the experiment on the photo-ion yield is addressed and compared with other experimental data in the literature.

Rotational state specific dissociation dynamics of D2O via the C̃(010) state: The effect of bending vibrational excitation.

The rotational state resolved photodissociation dynamics of D2O via the C̃(010) state has been investigated by using the D-atom Rydberg tagging time-of-flight technique combined with a tunable vacuum ultraviolet light source. The D-atom action spectrum of the C̃(010) ← X̃(000) band and the corresponding time-of-flight (TOF) spectra of D-atom photoproducts formed following the excitation of D2O to individual rotational transition have been measured. By comparison with the action spectrum of the C̃(000) ← X̃(000) band, the bending vibrational constant of the C̃ state for D2O can be determined to be v2 = 1041.37 ± 0.71cm-1. From the TOF spectra, the product kinetic energy spectra, the vibrational state distributions of OD products, and the state resolved anisotropy parameters have been determined. The experimental results indicate a dramatic variation in the OD product state distributions for different rotational excitations. This illuminates that there are two distinctive coupling channels from the C̃(010) state to the low-lying electronic states: the homogeneous electronic coupling to the Ã1B1 state, resulting in vibrationally hot OD(X) products, and the Coriolis-type coupling to the B̃1A1 state, producing vibrationally cold but rotationally hot OD(X) and OD(A) products. Furthermore, the three-body dissociation channel is confirmed, which is attributed to the C̃ → 1A2 or C̃ → Ã pathway. In comparison with the previous results of D2O photolysis via the C̃(000) state, it is found that the v2 vibration of the parent molecule enhances both the vibrational and rotational excitations of OD products.

Multiplexed Photoionization Mass Spectrometry Investigation of the O((3)P) + Propyne Reaction.

The reaction of O((3)P) + propyne (C3H4) was investigated at 298 K and 4 Torr using time-resolved multiplexed photoionization mass spectrometry and a synchrotron-generated tunable vacuum ultraviolet light source. The time-resolved mass spectra of the observed products suggest five major channels under our conditions: C2H3 + HCO, CH3 + HCCO, H + CH3CCO, C2H4 + CO, and C2H2 + H2 + CO. The relative branching ratios for these channels were found to be 1.00, (0.35 ± 0.11), (0.18 ± 0.10), (0.73 ± 0.27), and (1.31 ± 0.62). In addition, we observed signals consistent with minor production of C3H3 + OH and H2 + CH2CCO, although we cannot conclusively assign them as direct product channels from O((3)P) + propyne. The direct abstraction mechanism plays only a minor role (≤1%), and we estimate that O((3)P) addition to the central carbon of propyne accounts for 10% of products, with addition to the terminal carbon accounting for the remaining 89%. The isotopologues observed in experiments using d1-propyne (CH3CCD) and analysis of product branching in light of previously computed stationary points on the singlet and triplet potential energy surfaces (PESs) relevant to O((3)P) + propyne suggest that, under our conditions, (84 ± 14)% of the observed product channels from O((3)P) + propyne result from intersystem crossing from the initial triplet PES to the lower-lying singlet PES.

Temperature-dependent absorption cross-section measurements of 1-butene (1-C4H8) in VUV and IR

Vacuum ultraviolet (VUV) and infrared (IR) absorption cross-section measurements of 1-butene (1-C4H8; CH2=CHCH2CH3; Butylene) are reported over the temperature range of 296–529K. The VUV measurements are performed between 115 and 205nm using synchrotron radiation as a tunable VUV light source. Fourier Transform Infrared (FTIR) spectroscopy is employed to measure absorption cross-section and band strengths in the IR region between 1.54 and 25μm (∼6500–400cm−1). The measured room-temperature VUV and IR absorption cross-sections are compared with available literature data and are found to be in good agreement. The oscillator strength for the electronic transition (A1A′→X1A′) around 150–205nm is determined to be 0.32±0.01.The gas temperature has a strong effect on both VUV and IR spectra. Measurements made in the VUV region show that the peak value of the band cross-section decreases and the background continuum increases with increasing gas temperature. This behavior is due to a change in the rotational and vibrational population distribution of 1-butene molecule. Similar changes in rotational population are observed in the IR spectra. Moreover, variation of the IR spectra with temperature is used to measure the enthalpy difference between syn and skew conformations of 1-butene and is found to be 0.24±0.03kcal/mol, which is in excellent agreement with values reported in the literature. The measurements reported in this work will provide the much-needed spectroscopic information for the development of high-temperature quantitative diagnostics in combustion applications and validation of atmospheric chemistry models of extra-solar planets.

Investigations of oxysilanes from the crossed-beam reaction of atomic oxygen with silane using tunable vacuum-ultraviolet ionization

Using a crossed-molecular-beam apparatus and a synchrotron as a tunable vacuum-ultraviolet light source, we investigated oxysilanes formed in the reaction of atomic oxygen with silane. We measured the time-of-flight spectra, photoionization yields, and branching ratios of dissociative ionization of reaction products H 3SiO/H 2SiOH, H 2SiO/HSiOH, HSiO/SiOH and SiO as a function of photoionization energy. The observed ionization thresholds correspond to isomeric products H 2SiOH, HSiOH, and SiOH. The H 2-loss channel has a branching ratio greater than the H-loss channel for the dissociative ionization of H 3SiO/H 2SiOH, whereas this situation is reversed for H 2SiO/HSiOH, which we interpret according to the enthalpies of reactions.

Photofragmentation of Nitryl Chloride in the Ultraviolet Regime and Vacuum Ultraviolet Regime

Photofragmentation of nitryl chloride (ClNO2) is reported in the ultraviolet (UV) (λ = 240 nm and λ = 308 nm) and in the vacuum ultraviolet (VUV) regime (55 nm ≤ λ ≤ 110 nm, corresponding to the photon energy range 11.3 eV ≤ E ≤ 22.5 eV), where pulsed radiation is used to excite the neutral molecule in the gas phase. The neutral photolysis products that are formed upon UV photolysis are subsequently probed by photoionization mass spectrometry by using time-correlated tunable laser-produced plasma VUV radiation. UV-pump/VUV-probe experiments allow us to identify two primary photolysis channels at λ = 308 nm: (i) Cl + NO2 and (ii) O + ClNO. Primary quantum yields for atomic product formation are deduced from photoionization experiments for both channels: γ308 nm(Cl) = 0.93 ± 0.10, and γ308 nm(O) = 0.07 ± 0.01. The yield of Cl formation (N(Cl)) is significantly reduced relative to that of O formation (N(O)) at λ = 240 nm, corresponding to a N(Cl)/N(O) ratio of 1.44 ± 0.15. The atomic oxygen is found to be formed in its 3P ground state at both photolysis wavelengths. The present results are compared to earlier work, and atmospheric implications of the present results are briefly discussed. The tunable VUV light source also allows us to perform photoionization mass spectrometry experiments on nitryl chloride without primary photolysis. These experiments yield the first ionization energy of ClNO2 as well as fragmentation thresholds of ClNO2+.

Tunable pulsed vacuum ultraviolet light source for surface science and materials spectroscopy based on high order harmonic generation

We report here the development of a tunable vacuum ultraviolet light source, providing subpicosecond pulses over the wavelength range from 114 (∼11 eV) to 32 nm (∼39 eV), designed for surface science experiments. The source is based on high order harmonic generation. The experimental setup is described in detail and the harmonic yield as a function of the gas type (xenon or argon) is compared with theoretical descriptions. We address in particular the tunability of the source, desirable for surface science applications. Absolute characterization of the harmonic yield has also been performed and validated with time-resolved fluorescence measurements. The results are extremely promising, with intensities of ∼1010 photons/s, with regard to the use of this coherent source for surface science and spectroscopy.