Formamidinium lead iodide (FAPbI) perovskite as a superior solar cell material was investigated in two polymorphs at high pressures using in situ synchrotron X-ray diffraction, FTIR spectroscopy, photoluminescence (PL) spectroscopy, electrical conductivity (EC) measurements, and ab initio calculations. We identified two new structures (i.e., Imm2 and Immm) for α-FAPbI but only a structural distortion (in C2/c) for δ-FAPbI upon compression. A pressure-enhanced hydrogen bond plays a prominent role in structural modifications, as corroborated by FTIR spectroscopy. PL measurements and calculations consistently show the structure and pressure dependences of the band gap energies. Finally, EC measurements reveal drastically different transport properties of α- and δ-FAPbI at low pressures but a common trend to metallic states at high pressures. All of these observations suggest strongly contrasting structural stabilities and pressure-tuned optoelectric properties of the two FAPbI polymorphs.

Mass-selected IR plus UV/VUV spectroscopy and mass spectrometry have been coupled into a powerful technique to investigate chemical, physical, structural, and electronic properties of radicals, molecules, and clusters. Advantages of the use of vacuum ultraviolet (VUV) radiation to create ions for mass spectrometry are its application to nearly all compounds with ionization potentials below the energy of a single VUV photon, its circumventing the requirement of UV chromophore group, its inability to ionize background gases, and its greatly reduced fragmenting capabilities. In this review, mass-selected IR plus VUV (118 nm) spectroscopy is introduced first in a general manner. Selected application examples of this spectroscopy are presented, which include the detections and structural analysis of radicals, molecules, and molecular clusters in a supersonic jet.

In present study, photoionization and dissociation of acetic acid dimers have been studied with the synchrotron vacuum ultraviolet photoionization mass spectrometry and theoretical calculations. Besides the intense signal corresponding to protonated cluster ions (CH(3)COOH)(n)·H(+), the feature related to the fragment ions (CH(3)COOH)H(+)·COO (105 amu) via β-carbon-carbon bond cleavage is observed. By scanning photoionization efficiency spectra, appearance energies of the fragments (CH(3)COOH)·H(+) and (CH(3)COOH)H(+)·COO are obtained. With the aid of theoretical calculations, seven fragmentation channels of acetic acid dimer cations were discussed, where five cation isomers of acetic acid dimer are involved. While four of them are found to generate the protonated species, only one of them can dissociate into a C-C bond cleavage product (CH(3)COOH)H(+)·COO. After surmounting the methyl hydrogen-transfer barrier 10.84 ± 0.05 eV, the opening of dissociative channel to produce ions (CH(3)COOH)(+) becomes the most competitive path. When photon energy increases to 12.4 eV, we also found dimer cations can be fragmented and generate new cations (CH(3)COOH)·CH(3)CO(+). Kinetics, thermodynamics, and entropy factors for these competitive dissociation pathways are discussed. The present report provides a clear picture of the photoionization and dissociation processes of the acetic acid dimer in the range of the photon energy 9-15 eV.

Site-selective ionization of ethanol dimer and the subsequent fragmentation were studied by synchrotron vacuum ultraviolet (VUV) photoionization mass spectrometry. With photoionization efficiency spectra measurements and theoretical calculations, the detailed mechanisms of the ionization-dissociation processes of ethanol dimer under VUV irradiation were explored. In 9.49-10.89 eV photon energy range, it was found that the ejection of the highest occupied molecular orbital (HOMO) electron from hydrogen bond donor induces a rapid barrierless proton-transfer process followed by two competitive dissociation channels, generating (C2H5OH)[middle dot]H(+) and CH2O[middle dot](C2H5OH)H(+), respectively. The latter comes from a carbon-carbon bond cleavage in the donor. While the photon energy is 10.9-11.58 eV, the electron of HOMO-1 of the hydrogen bond acceptor, is removed. Besides the dissociation channel to produce C2H5OH and C2H5OH(+), a new channel to generate (C2H5OH)[middle dot]CH2OH(+) is opened, where the cleavage of the carbon-carbon bond occurs in the acceptor. When the photon energy increases to 11.58 eV, the electron from HOMO-2 is ejected.

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BiVO4 exhibits a rich structural polymorphism under high pressure where both fergusonite- and zircon-type BiVO4 transform to scheelite and β-fergusonite structures upon compression.

Discharge and electron-impact excitation lead to the production of metastable helium atoms in two metastable states, 2 1 S 0 and 2 3 S 1 . However, many applications require pure beams of one of these species or at least a detailed knowledge of the relative state populations. In this paper, we present the characterization of an original experimental scheme for the optical depletion of He(2 1 S 0 ) in a supersonic beam which is based on the optical excitation of the 4 1 P 1 ← 2 1 S 0 transition at 397 nm using a diode laser. From our experimental results and from a comparison with numerical calculations, we infer a near unit depletion efficiency at all beam velocities under study (1070 m/s ≤ v ≤ 1750 m/s). Since the technique provides a direct means to determine the singlet-to-triplet ratio in a pulsed supersonic helium beam, our results show that the intrabeam singlet-to-triplet ratio is different at the trailing edges of the gas pulse.

While methyl transfer is well known to occur in the enzyme- and metal-catalyzed reactions, the methyl transfer in the metal-free organic molecules induced by the photon ionization has been less concerned. Herein, vacuum ultraviolet single photon ionization and dissociation of ethanol dimer are investigated with synchrotron radiation photoionization mass spectroscopy and theoretical methods. Besides the protonated clusters cation (C2H5OH)⋅H(+) (m/z = 47) and the β-carbon-carbon bond cleavage fragment CH2O⋅(C2H5OH)H(+) (m/z = 77), the measured mass spectra revealed that a new fragment (C2H5OH)⋅(CH3)(+) (m/z = 61) appeared at the photon energy of 12.1 and 15.0 eV, where the neutral dimer could be vertically ionized to higher ionic state. Thereafter, the generated carbonium ions are followed by a Wagner-Meerwein rearrangement and then dissociate to produce this new fragment, which is considered to generate after surmounting a few barriers including intra- and inter-molecular methyl migrations by the aid of theoretical calculations. The appearance energy of this new fragment is measured as 11.55 ± 0.05 eV by scanning photoionization efficiency curve. While the signal intensity of fragment m/z = 61 starts to increase, the fragments m/z = 47 and 77 tend to slowly incline around 11.55 eV photon energy. This suggests that the additional fragment channels other than (C2H5OH)⋅H(+) and CH2O⋅(C2H5OH)H(+) have also been opened, which consume some dimer cations. The present report provides a clear description of the photoionization and dissociation processes of the ethanol dimer in the range of the photon energy 12-15 eV.