Tunable Vacuum Ultraviolet Light Research Articles

Determination of absolute photoionization cross-sections of nitrogenous compounds

The absolute photoionization cross-sections of 24 nitrogenous compounds were measured in the photon energy range from their respective ionization thresholds to 11.5 eV, including n-propylamine, iso-propylamine, n-butylamine, 2-butylamine, iso-butylamine, tert-butylamine, diethylamine, dibutylamine, triethylamine, aniline, benzylamine, n-methylaniline, cyclohexanamine, nitromethane, nitroethane, nitropropane, nitrobenzene, pyrrole, pyridine, pyrrolidine, morpholine, N, N-dimethylethanolamine, benzonitrile and benzeneacetonitrile. The experiments were performed by photoionization mass spectrometry with tunable synchrotron vacuum ultraviolet (VUV) light as an ionization source. Binary-liquid-mixtures of investigated and standard species were used in this measurement. As a good solvent, benzene was chosen as a calibration standard, since its photoionization cross-section had been well know. Moreover, photodissociative fragments were also presented.

Rotational state specific dissociation dynamics of D2O via the C̃ electronic state

The rotational state resolved photodissociation dynamics of D(2)O via the C state has been investigated using the D-atom Rydberg "tagging" time-of-flight technique, with a tunable vacuum ultraviolet photolysis light source. The photodissociation action spectrum of the D(2)O C<--X (3p(z)<--1b(1)) band has been recorded. The linewidths of rotational transitions have been determined by the Lorentzian profile simulation. Product kinetic energy distributions and angular distributions for individual rotational lines have been measured. From these distributions, the internal state distributions of the OD radical product as well as the state resolved angular anisotropy parameters for each rotational transition have been obtained. The dramatic variation of the OD product state distributions from different rotational excitations has been observed. These results suggest that there are two distinctive coupling channels from the C state to the lower electronic states: homogenous electronic coupling to the A((1)B(1)) state, resulting in a vibrationally hot OD(X (2)Pi) product and Coriolis-type coupling between the C((1)B(1)) state and the B((1)A(1)) state. Through the second mechanism, OD(X (2)Pi) as well as OD(A (2) summation operator(+)) products are produced. The OD(X (2)Pi) products are mainly populated in the vibrationally cold but extremely rotationally excited states. The comparison between the D(2)O and H(2)O results illustrates that the C((1)B(1))-->B((1)A(1)) pathway is relatively larger in the D(2)O photodissociation as a consequence of the isotopic effect.

Determination of absolute photoionization cross-sections of oxygenated hydrocarbons

The near-threshold absolute photoionization cross-sections of 12 oxygenated hydrocarbons, including 1-butanol, 2-butanol, iso-butanol, tert-butanol, 1-pentanol, tert-pentanol, iso-pentanol, methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), furan, 2-methylfuran, 2,5-dimethylfuran, were measured in the photon energy range from respective ionization thresholds to 11.5 eV. The experiments were performed with photoionization mass spectrometry and tunable synchrotron vacuum ultraviolet (VUV) light. Binary-liquid-mixtures of the investigated sample and benzene were used in the measurements where benzene acts as a calibration standard, due to its well known photoionization cross-section at the photon energies from its ionization energy (9.24 eV) to 11.5 eV. Photodissociative fragments from the molecules were also observed, and their photodissociation cross-sections are also presented.

Chemical dynamics, molecular energetics, and kinetics at the synchrotron

Scientists at the Chemical Dynamics Beamline of the Advanced Light Source in Berkeley are continuously reinventing synchrotron investigations of physical chemistry and chemical physics with vacuum ultraviolet light. One of the unique aspects of a synchrotron for chemical physics research is the widely tunable vacuum ultraviolet light that permits threshold ionization of large molecules with minimal fragmentation. This provides novel opportunities to assess molecular energetics and reaction mechanisms, even beyond simple gas phase molecules. In this perspective, significant new directions utilizing the capabilities at the Chemical Dynamics Beamline are presented, along with an outlook for future synchrotron and free electron laser science in chemical dynamics. Among the established and emerging fields of investigations are cluster and biological molecule spectroscopy and structure, combustion flame chemistry mechanisms, radical kinetics and product isomer dynamics, aerosol heterogeneous chemistry, planetary and interstellar chemistry, and secondary neutral ion-beam desorption imaging of biological matter and materials chemistry.

Imaging with Mass Spectrometry: A Secondary Ion and VUV-Photoionization Study of Ion-Sputtered Atoms and Clusters from GaAs and Au

A new mass spectrometry surface imaging method is presented in which ion-sputtered neutrals are postionized by wavelength-tunable vacuum ultraviolet (VUV) light from a synchrotron source. Mass spectra and signal counts of the photoionized neutrals from GaAs (100) and Au are compared to those of the secondary ions. While clusters larger than dimers are more efficiently detected as secondary ions, certain species, such as As(2), Au, and Au(2), are more efficiently detected through the neutral channel. Continuously tuning the photon wavelength allows photoionization efficiency (PIE) curves to be obtained for sputtered As(m) (m = 1,2) and Au(n) (n = 1-4). From the observed ionization thresholds, sputtered neutral As and Au show no clear evidence of electronic excitation, while neutral clusters have photoionization onsets shifted to lower energies by approximately 0.3 eV. These shifts are attributed to unresolved vibrational and rotational excitations. High-spatial resolution chemical imaging with synchrotron VUV postionization is demonstrated at two different photon energies using a copper TEM grid embedded in indium. The resulting images are used to illustrate the use of tunable VUV light for verifying mass peak assignments by exploiting the unique wavelength-dependent PIE of each sputtered neutral species. This capability is valuable for identifying compounds when imaging chemically complex systems with mass spectrometry-based techniques.

Infrared spectroscopy for acetone and its dimer based on photoionization detection with tunable coherent vacuum-ultraviolet light

Infrared (IR) spectroscopy based on vacuum-ultraviolet (VUV) one-photon ionization detection is carried out for bare acetone and its dimer by use of tunable coherent VUV light. This is the first application of tunable VUV light to IR spectroscopy of a size-selected cluster, which has no suitable chromophore for conventional detection schemes. The advantage of this spectroscopic scheme is demonstrated. The observed IR spectrum of acetone dimer indicates formation of a C 2h symmetric structure, where the acetone molecules stack with an antiparallel orientation of the carbonyl groups.

Dynamics of the reaction of atomic oxygen with ethene: Observation of all carbon-containing products by single-photon ionization

We measured time-of-flight (TOF) spectra of products from the reaction O( 3P/ 1D) + C 2H 4 at collision energy 6.4 kcal mol −1 using a quadrupole mass filter and tunable vacuum-ultraviolet light for ionization. All carbon-containing products from five exit channels – CH 2CHO + H, CH 2CO + H 2, CH 3 + HCO, CH 2 + HCHO, and CH 2CO + 2H – were identified. Product channels CH 2CHO + H and CH 2CO + 2H associate with 3P and 1D atomic oxygen reactants, respectively. Both 3P and 1D oxygen reactants might be responsible for the other reactions. The ionization threshold of nascent vinoxy radicals is 9.3 ± 0.1 eV.

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.

Coupling a versatile aerosol apparatus to a synchrotron: Vacuum ultraviolet light scattering, photoelectron imaging, and fragment free mass spectrometry

An aerosol apparatus has been coupled to the Chemical Dynamics Beamline of the Advanced Light Source at Lawrence Berkeley National Laboratory. This apparatus has multiple capabilities for aerosol studies, including vacuum ultraviolet (VUV) light scattering, photoelectron imaging, and mass spectroscopy of aerosols. By utilizing an inlet system consisting of a 200μm orifice nozzle and aerodynamic lenses, aerosol particles of ∼50nm–∼1μm in diameter can be sampled directly from atmospheric pressure. The machine is versatile and can probe carbonaceous aerosols generated by a laboratory flame, nebulized solutions of biological molecules, hydrocarbon aerosol reaction products, and synthesized inorganic nanoparticles. The sensitivity of this apparatus is demonstrated by the detection of nanoparticles with VUV light scattering, photoelectron imaging, and charged particle detection. In addition to the detection of nanoparticles, the thermal vaporization of aerosols on a heater tip leads to the generation of intact gas phase molecules. This phenomenon coupled to threshold single photon ionization, accessible with tunable VUV light, allows for fragment-free mass spectrometry of complex molecules. The initial experiments with light scattering, photoelectron imaging, and aerosol mass spectrometry reported here serve as a demonstration of the design philosophy and multiple capabilities of the apparatus.

Synchrotron Radiation Based Aerosol Time-of-Flight Mass Spectrometry for Organic Constituents

A synchrotron radiation based aerosol time-of-flight mass spectrometer using tunable vacuum-ultraviolet (VUV) light is described for real-time analysis of organic compounds in ultrafine and large aerosol particles. Particles are sampled from atmospheric pressure and are focused through an aerodynamic lens assembly into the mass spectrometer. As the particles enter the source region, they impinge on a cartridge heater and are vaporized. The particle vapor expands back into the source region and is softly ionized with tunable, quasicontinuous VUV light generated with synchrotron radiation. The radiation can be tuned to an energy close to the ionization energy of the sample molecules, thus minimizing the complications resulting from ion fragmentation. Photoionization efficiency scans (photon scans) can be readily collected, which permit measurement of the molecule's ionization energy and fragmentation onsets. Four high molecular weight, low vapor pressure organic compounds of importance in atmospheric aerosols are analyzed and their ionization energies measured with uncertainties of +/-60 meV. These are oleic acid (8.68 eV), linoleic acid (8.52 eV), linolenic acid (8.49 eV), and cholesterol (8.69 eV).

A new horizon in secondary neutral mass spectrometry: post-ionization using a VUV free electron laser

A new time-of-flight (TOF) mass spectrometer incorporating post-ionization of sputtered neutral species with tunable vacuum ultraviolet (VUV) light generated by a free electron laser (FEL) has been developed. Capabilities of this instrument, called SPIRIT, were demonstrated by experiments with photoionization of sputtered neutral gold atoms with 125 nm light generated by the VUV FEL located at Argonne National Laboratory (ANL). In a separate series of experiments with a fixed wavelength VUV light source, a 157 nm F 2 laser, a useful yield (atoms detected per atoms sputtered) of about 12% and a mass resolution better than 1500 were demonstrated for molybdenum.

Development of a coherent vacuum ultraviolet source at 104–108 nm generated by four-wave sum frequency mixing in Hg vapor

A continuously tunable vacuum ultraviolet (VUV) light source in the 104–108 nm range has been developed by utilizing four-wave sum frequency mixing in Hg vapor. The windowless Hg cell designed uses a conelike glass capillary which transmits most of VUV generated while it restricts the flow of Hg and buffer gas. Usable power of about 109 photons/pulse (2 nJ/pulse) was obtained across the spectral region covered. The light source could be run for at least a month without maintenance and did not seem to contaminate the main apparatus.

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.

Discrimination of product isomers in the photodissociation of propyne and allene at 193 nm

The photodissociation dynamics of propyne and allene are investigated in two molecular beam/photodissociation instruments, one using electron impact ionization and the other using tunable vacuum ultraviolet (VUV) light to photoionize the photoproducts. The primary dissociation channels for both reactants are C3H3+H and C3H2+H2. Measurement of the photoionization efficiency curves on the VUV instrument shows that the C3H3 product from propyne is the propynyl (CH3CC) radical, whereas the C3H3 product from allene is the propargyl (CH2CCH) radical. The dominant C3H2 product from both reactants is the propadienylidene (H2CCC) radical. We also observe a small amount of secondary C3H2 product from photodissociation of the C3H3 radicals in both cases.

Flame sampling photoionization mass spectrometry of dichloroethenol

A newly developed flame-sampling laser-ionization mass spectrometer has been used for the detection of combustion intermediates in a low-pressure methane/oxygen flame seeded with trichloroethylene. Among the most prominent intermediates are 1,1-dichloroethylene and 2,2-dichloroethenol. Near-threshold photoionization efficiency curves, produced with a tunable vacuum ultraviolet (VUV) laser light source, are used for isomerically selective detection of these species. The previously unmeasured ionization potential for 2,2-dichloroethenol is placed at 9.09±0.02 eV.

Mass-resolved VUV laser spectroscopy of XeAr: Two competing predissociation pathways in the C1 state

Mass-resolved excitation spectra of the C1–X0+ band of XeAr were measured under jet-cooled conditions using a tunable coherent vacuum ultraviolet (VUV) light (λ∼130 nm) as an excitation light source and a high-resolution (m/Δm∼1100) reflectron time-of-flight mass spectrometer. The rotational structure of the C1–X0+ (v′,0) (v′=2–6) vibronic bands were recorded by monitoring the parent Xe40132Ar+ ions and the fragment Xe+132 ions. From the analysis of the rotational structures, the existence of the two dissociation pathways from the C1 state was identified for the first time; i.e., (i) the rotationally dependent predissociation caused by an electron-rotation interaction (L- and/or S-uncoupling) with the dissociative state with 0− symmetry correlating with the Xe 6s′(1/2)0 level, and (ii) the rotationally independent predissociation caused by the electrostatic interaction with a repulsive state with Ω=1 symmetry correlated either with the Xe 6s(3/2)1 level or the Xe 6s(3/2)2 level. From the least-squares fit to the partially resolved rotational structures, the dissociation rates for these two pathways as well as the band origins and the rotational constants, were derived for the v′=2–6 levels in the C1 state.

Flame sampling photoionization mass spectrometry of CH 3PO 2 and CH 3OPO 2

Laser photoionization mass spectrometry has provided the first detection of the isolated dioxophosphoranes, CH 3PO 2 and CH 3OPO 2. These molecules are found to be key intermediates in the combustion of dimethyl methylphosphonate (DMMP). Near-threshold photoionization efficiency curves, produced with a tunable vacuum ultraviolet (VUV) laser light source, yield ionization potentials of 9.24 ± 0.01 and 8.97 ± 0.02 eV for CH 3PO 2 and CH 3OPO 2, respectively. A newly developed flame-sampling laser-ionization mass spectrometer provided measurements of concentration profiles of these species throughout the flame zone of a premixed, low pressure hydrogen/oxygen/argon flame, seeded with DMMP.

Rydberg and ion-pair state mixing in the 1 1Σ+u and 2 3Πu vibronic states of Cl2 as studied by vacuum ultraviolet laser spectroscopy

Fluorescence excitation and resonantly enhanced photoionization spectra of jet-cooled Cl2 in the 71 000–77 500 cm−1 region are measured using a tunable vacuum ultraviolet light generated by four-wave difference frequency mixing of two lasers in Kr. For the 1 1Σ+u state, vibrational term values for 35Cl2, 35Cl37Cl, and 37Cl2 and rotational constants for 35Cl2 and 35Cl37Cl are determined with high precision by the measurement of the 1 1Σ+u –X 1Σ+g transitions. It is shown clearly that the v′=39 vibrational wave functions of the 1 1Σ+u state of 35Cl2 and 35Cl37Cl are localized in the Rydberg well while the vibrational wave functions for the v′&amp;lt;39 levels are delocalized in the ion-pair well and those for the v′&amp;gt;39 levels are between the inner Rydberg wall and the outer ion-pair wall. A double minimum potential composed of the Rydberg and the ion-pair states explains well the observed vibrational and rotational structures. The molecular constants for the Rydberg well of 2 3Π(0+u) determined from the rovibrational structures of the 2 3Π(0+u)–X 1Σ+g transitions are ωe=647.3(33) cm−1, ωexe=3.37(59) cm−1 and re=1.833(20) Å, respectively.

Interatomic potentials of the C1 and D+ states of XeNe, XeAr, and XeKr as studied by tunable vacuum ultraviolet laser spectroscopy

Fluorescence excitation and resonantly enhanced photoionization spectra of the C–X and D–X bands of XeNe, XeAr, and XeKr, formed in a supersonic free jet are measured by use of a tunable vacuum ultraviolet light generated by four-wave difference frequency mixing of two lasers in Kr. The potential parameters of the heteronuclear rare gas dimers for the two excited state potentials, C1 and D0+, correlated with the Xe6s′(1/2)1 state are derived by an analysis of rovibronic structures of the C–X and D–X bands. The dissociation energies De for the C and D states are, respectively, determined to be 244 and 112 cm−1 for XeAr and are 1445 and 54 cm−1 for XeKr.