Vacuum Ultraviolet Single Photon Research Articles

Proton Transfer in Nitromethane-Ammonia Clusters under VUV Single-Photon Ionization Explored by Infrared Spectroscopy and Theoretical Calculations.

It is known that the acidity and reactivity of the CH bond can be enhanced after ionization. Also, this property plays a pivotal role in proton transfer reaction and in the formation of new molecules. Herein, infrared spectroscopy and high-precision quantum chemical calculations are used to study the neutral and cationic clusters of nitromethane-ammonia (CH3NO2-NH3). It is found that in the neutral cluster, CH3NO2 and NH3 are mainly bonded by three intermolecular hydrogen bonds, in which electrostatic contribution plays a major role. After vacuum ultraviolet (VUV) single-photon ionization of CH3NO2-NH3, the positive charge redistributes from the ionized nitrogen atom of NH3 to the CH3NO2 molecule immediately. Then, the proton of CH3NO2 transfers to NH3 to form a proton-transferred type structure CH2NO2-NH4+, without any effective energy barrier, due to the positive hyperconjugation of cationic nitromethane. A closed loop of positive charge transfer takes place in the CH3NO2-NH3 cluster after VUV ionization. The present work demonstrates that both the proton transfer reaction and charge transfer process have occurred in the ionized CH3NO2-NH3 cluster. Moreover, it is found that the proton transfer reaction is a result of the highly acidic CH bond caused by hyperconjugation between the σ (CH) bond and π orbital.

Studying Interfacial Dark Reactions of Glyoxal and Hydrogen Peroxide Using Vacuum Ultraviolet Single Photon Ionization Mass Spectrometry

Aqueous secondary organic aerosol (aqSOA) formation from volatile and semivolatile organic compounds at the air–liquid interface is considered as an important source of fine particles in the atmosphere. However, due to the lack of in situ detecting techniques, the detailed interfacial reaction mechanism and dynamics still remain uncertain. In this study, synchrotron-based vacuum ultraviolet single-photon ionization mass spectrometry (VUV SPI-MS) was coupled with the System for Analysis at the Liquid Vacuum Interface (SALVI) to investigate glyoxal dark oxidation products at the aqueous surface. Mass spectral analysis and determination of appearance energies (AEs) suggest that the main products of glyoxal dark interfacial aging are carboxylic acid related oligomers. Furthermore, the VUV SPI-MS results were compared and validated against those of in situ liquid time-of-flight secondary ion mass spectrometry (ToF-SIMS). The reaction mechanisms of the dark glyoxal interfacial oxidation, obtained using two different approaches, indicate that differences in ionization and instrument operation principles could contribute to their abilities to detect different oligomers. Therefore, the mechanistic differences revealed between the VUV SPI-MS and ToF-SIMS indicate that more in situ and real-time techniques are needed to investigate the contribution of the air–liquid interfacial reactions leading to aqSOA formation.

Vacuum Ultraviolet Single-Photon Postionization of Amino Acids

In this study, ultraviolet (UV) laser desorption and vacuum UV single-photon (VUV SP) postionization were performed to ionize and successfully analyze 20 common amino acids. The analytical merit and efficiency of the ionization was compared with those of conventional UV matrix-assisted laser desorption ionization (UV-MALDI). A VUV light source (118 nm) was generated from the ninth harmonic of a Q-switched Nd:YAG laser, and the photon number was determined to be larger than 1012 for each laser pulse in the ionization region. In general, the detection sensitivity of VUV-SP-postionization was 10–100 times higher than that of conventional UV-MALDI. In particular, the ion signal from VUV-SP-postionization was considerably larger than that from UV-MALDI for analytes with low proton affinity such as glycine. However, some fragmentation of intact ions was observed in VUV-SP-postionization. Quantitative analysis performed using a glycine/histidine mixture and tryptophan/phenylalanine mixture revealed that the dynamic range of VUV-SP-postionization was one order of magnitude larger than that of UV-MALDI, indicating that VUV-SP-postionization is suitable for the quantitative analysis of amino acids.

Femtosecond photoelectron and photoion spectrometer with vacuum ultraviolet probe pulses

We describe a femtosecond photoelectron and photoion spectroscopy setup using a simple, novel way to produce a vacuum ultraviolet (VUV) probe pulse. We isolate the femtosecond VUV pulse from a high harmonic continuum by using the bandpass characteristics of a thin indium (In) metal filter. The filter transmits the 9th harmonic of the 800nm laser corresponding to a wavelength of 89nm (hν=14eV). The 9th harmonic is obtained with high conversion efficiencies and has sufficient photon energy to access the complete set of valence electron levels in most molecules. Compared to using grating monochromators, the filter strategy does not change the length of VUV pulse. The setup also allows for direct comparison of VUV single-photon probe with 800nm multi-photon probe without influencing the delay of excitation and probe pulse or the beam geometry. We use a magnetic bottle spectrometer with high collection efficiency for electrons, serving at the same time as a time of flight spectrometer for ions. Characterization measurements on Xe reveal the spectral width of the 9th harmonic to be 190±60meV and a photon flux of ∼1×107photons/pulse after spectral filtering. As a first application, we investigate the S1 excitation of perylene using time-resolved ion spectra obtained with multiphoton probing and time-resolved electron spectra from VUV single-photon probing. The time resolution extracted from cross-correlation measurements is 65±10fs for both probing schemes and the pulse duration of the 9th harmonic is found to be 35±8fs.

Vacuum ultraviolet single-photon and ultraviolet nonresonant two-photon pulsed field ionization photoelectron study of CH 3SCH 3

The vacuum ultraviolet (VUV) single-photon and ultraviolet nonresonant two-photon (N2P) pulsed field ionization photoelectron (PFI-PE) spectra for CH 3SCH 3 have been obtained in the energy range of 69 500–72 500 cm −1. Vibrational structures observed in the VUV-PFI-PE and N2P-PFI-PE spectra are similar. Guided by the ab initio theoretical harmonic frequencies, we have assigned the vibrational bands resolved in these spectra. Using a semiempirical simulation scheme, together with ab initio theoretical rotational constants for CH 3SCH 3 and CH 3SCH 3 +, we have also obtained a good fit to the contours of rotational branches resolved in the origin band of the VUV-PFI-PE spectrum. Taking into account the uncertainty of the simulation model used, we obtain a value of 70 097.3 ± 2.0 cm −1 (8.69096 ± 0.00016 eV) for the adiabatic ionization energy of CH 3SCH 3.