Photoionization Efficiency Spectroscopy Research Articles

Laser spectroscopic characterization of supersonic jet cooled 2,7-diazaindole.

We report the first gas phase comprehensive study of the electronic spectroscopy of 2,7-diazaindole molecule in the ground and excited states. Single vibronic level fluorescence spectroscopy (SVLF) was performed to determine the ground state vibrations of the molecule, which depicted a large Franck-Condon activity beyond 2600 cm-1. For the excited state characterization, laser-induced fluorescence (LIF) and two-color resonant two-photon ionization spectroscopy (2C-R2PI) were performed. The band origin (000) for S1 ← S0 transition appeared at 33910 ± 1 cm-1 which was red shifted by 718 cm-1 and 1322 cm-1 compared to that of 7-azaindole and indole respectively. The Franck-Condon active vibrational modes in the spectra were seen till the (000) + 1600 cm-1 region. The IR-UV hole burning spectroscopy confirmed the absence of any other isomeric species in the molecular beam. The ionization energy (IE) of the molecule was measured as 8.921 ± 0.001 eV, recorded using photoionization efficiency spectroscopy. The above IE value was significantly higher than that of the related indole derivatives, suggesting the higher photostability of the 27DAI molecule due to N(2) insertion. The ground and excited state N-H stretching frequencies of the molecule were determined using fluorescence-dip infrared spectroscopy (FDIR) and resonant ion-dip infrared spectroscopy (IDIR), and the values are 3523 and 3467 cm-1, respectively. The lower value of νNH in the electronic excited state implied the increased photoacidity of the group. A comparative analysis of the experimental LIF/2C-R2PI spectra was done against Franck-Condon simulated spectra at three different levels of theory. The vibrational frequencies calculated at B3LYP-D4/def2-TZVPP showed the most accurate prediction in comparison with the experimentally detected symmetric modes in the ground state. However, in the excited state, the lower energy asymmetric modes simulated at the B3LYP/def-SVP level of theory provided the best agreement with the experiment. This is most probably due to the distortion observed at the pyrazolyl ring leading to the appearance of asymmetric vibrational modes. The above study highlights the possibility to appropriately tune the excitation wavelengths as well as alter the photostability of the organic chromophores via additional N-insertion in the molecular systems.

Study of photoionization mass spectrum of carbon monoxide from ionization threshold to 575 Å by using synchrotron radiation

The vacuum-ultraviolet photoionization efficiency spectroscopy (PIES) of carbon monoxide in a region 575–890 Å has been investigated with time-of-flight (TOF) photoionization mass spectrometry (PIMS) by using tunable synchrotron radiation (SR). Three photoionization thresholds were determined to be 14.015, 16.550 eV and 19.670 eV for the X2Σ+, A2Π and B2Σ+ states of CO+, respectively, which are in very good agreement with those of previous investigation. Furthermore, the Rydberg series converging to the X2Σ+, A2Π, B2Σ+, D2Π and C2Σ+ states of CO+ were assigned clearly, according to the PIES of CO and calculation with Rydberg series formula. In particular, the Rydberg series (RX-I) (ν+ = 2, 3 and 4) and RX-TTI series (ν+ = 1, 2, 3 and 4) converging to CO+ (X2Σ+), the Rydberg series (RB-V) (ν+ = 1, 2 and 3) converging to CO+ (B2Σ+), and a new vibrational progression (n = 2) converging to CO+ (A2Π) were all identified and listed for the first time. Generally speaking, the Rydberg series identified in this work are in good accordance with those of previous study. Moreover, some of them are extended to higher number, and some new members are classified for the first time.

Using Photoionization Efficiency Spectroscopy and Density Functional Theory to Investigate Charge Transfer Interactions in AuCe3On Clusters.

The characteristics of small cerium oxide and gold-cerium oxide clusters were investigated as models for gold attachment to various defect sites on a ceria surface. Photoionization efficiency (PIE) spectra of gas phase Ce3On (n = 0-4) and AuCe3On (n = 0-3) clusters were recorded and compared to spectral simulations based on DFT calculations. Calculated structures and PIE spectra for the Ce3O5,6 and AuCe3O4-6 clusters are also presented; however, these species were not detected during photoionization experiments. Addition of an Au atom to Ce3 was found to increase the energy of the ionization onset by ∼0.4 eV, whereas addition of one or more oxygen atoms decreases the onset by ∼0.25 eV. The optimized AuCe3On (n = 0-4) cluster geometries correlate with Au atoms adsorbed to oxygen vacancy sites while the AuCe3O5 and AuCe3O6 clusters are consistent with Au adsorption to CeO3 and CeO2 vacancies, respectively. The interactions between the cerium oxide cluster surface and the adsorbed Au atom were found to strongly depend on the nature the of the adsorption site. Au adsorbed to O vacancies are negatively charged with a Ce → Au charge transfer, whereas Au adsorbed to CeO2 and CeO3 vacancies have a reversed Au → Ce charge transfer, resulting in a positively charged Au atom. Au adsorption to the Ce3On clusters has the effect of (i) reducing the differences in the HOMO energies of the AuCe3O4, AuCe3O5, and AuCe3O6 clusters and (ii) lowering the binding energy of oxygen atoms for all AuCe3On (n = 1-6) clusters. Au adsorption appears to have a minimal effect on CeO2 vacancy formation, although CeO2 vacancies were calculated to form more readily than O vacancies on both the Ce3On and AuCe3On clusters. The low energy fragmentation calculated for the Ce3O5,6 and AuCe3O4-6 clusters, via loss of either Au, O, or CeO2, could potentially make photoionization experiments unfeasible since these clusters may simply dissociate when exposed to high energy photons above the ionization threshold.

Investigating Charge Transfer Interactions in AuCe2On Clusters Using Photoionization Efficiency Spectroscopy and Density Functional Theory.

The properties of small cerium oxide and gold-cerium oxide clusters were explored as analogues for gold deposition at defect sites on a cerium oxide surface. Ce2On (n = 0-2) and AuCe2On (n = 0-2) clusters were prepared in the gas phase and investigated using photoionization efficiency spectroscopy complemented by spectral simulations based on DFT calculations; purely theoretical investigations were conducted on the Ce2O3, Ce2O4, AuCe2O3, and AuCe2O4 clusters due to these species not being detected. The optimized AuCe2On (n = 0-3) cluster geometries are consistent with Au adsorption to oxygen vacancy sites while the AuCe2O4 cluster correlates with Au adsorption to a CeO2 vacancy site. The electronic properties of the adsorbed Au atom depend strongly on the nature of the ceria adsorption site: O vacancy-adsorbed Au is negatively charged with a Ce → Au charge transfer occurring at the adsorption interface, whereas Au adsorbed to a CeO2 vacancy is positively charged with an Au → Ce charge transfer. The adsorbed Au atom is proposed to enhance the catalytic properties of the AuCe2On cluster by (i) stabilizing the negatively charged Au atom on reduced AuCe2On clusters to enhance nucleophilicity; (ii) increasing the electron accepting capability of the AuCe2O4 species; (iii) destabilizing the HOMO of the AuCe2O4 cluster; and (iv) facilitating the abstraction of additional surface oxygen atoms by reactants.

Synchrotron radiation VUV double photoionization of some small molecules

The VUV double photoionizations of small molecules (NO, CO, CO2, CS2, OSC and NH3) were investigated with photoionization mass spectroscopy using synchrotron radiation. The double ionization energies of molecules were determined with photoionization efficiency spectroscopy. The total energies of these molecules and their parent dications (NO2+, CO2+, CO2+2, CS2+2, OSC2+ and NH2+3) were calculated using the Gaussian 03 program and Gaussian 2 calculations. Then, the adiabatic double ionization energies of the molecules were predicated by using high accuracy energy mode. The experimental double ionization energies of these small molecules were all in reasonable agreement with their respective calculated adiabatic double ionization energies. The mechanisms of double photoionization of these molecules were discussed based on a comparison of our experimental results with those predicted theoretically. The equilibrium geometries and harmonic vibrational frequencies of molecules and their parent dications were calculated by using the MP2 (full) method. The differences in configurations between these molecules and their parent dications were also discussed on the basis of theoretical calculations.

Onset of Carbon−Carbon Bonding in Ta5Cy (y = 0−6) Clusters: A Threshold Photoionization and Density Functional Theory Study

We have used photoionization efficiency spectroscopy to determine ionization energies (IEs) of the gas-phase tantalum-carbide clusters Ta(5)C(y) (y = 0-6). The structures of the clusters observed in the experiment are assigned by comparing the experimental IEs with those of candidate isomers, calculated by density functional theory. Two competing geometries of the underlying Ta(5) cluster are found to be present in the assigned Ta(5)C(y) structures; either a "prolate" or "distorted oblate" trigonal bipyramid geometry. The onset of carbon-carbon bonding in the Ta(5)C(y) series is proposed to occur at y = 6, with the structure of Ta(5)C(6) containing two molecular C(2) units.

Threshold photoionization and density functional theory studies of bimetallic-carbide nanocrystals and fragments: Ta3ZrCy (y=–4)

Gas-phase bimetallic tantalum-zirconium-carbide clusters are generated using a constructed double ablation cluster source. The Ta(3)ZrC(y) (y = 0-4) clusters are examined by photoionization efficiency spectroscopy to extract experimental ionization energies (IEs). The IE trend for the Ta(3)ZrC(y) cluster series is reasonably similar to that of the Ta(4)C(y) cluster series [V. Dryza et al., J. Phys. Chem. A 109, 11180 (2005)], although the IE reductions upon carbon addition are greater for the former. Complementary density functional theory calculations are performed for the various isomers constructed by attaching carbon atoms to the different faces of the tetrahedral Ta(3)Zr cluster. The good agreement between the experimental IE trend and that calculated for these isomers support a 2x2x2 face centered cubic nanocrystal structure for Ta(4)ZrC(4) and nanocrystal fragment structures for the smaller clusters.

Photoionization efficiency spectroscopy and density functional theory investigations of RhHo2On (n=–2) clusters

The experimental and theoretical adiabatic ionization energies (IEs) of the rhodium-holmium bimetallic clusters RhHo(2)O(n) (n=0-2) have been determined using photoionization efficiency spectroscopy and density functional theory (DFT) calculations. Both sets of data show the IE of RhHo(2)O to be significantly lower than the values for RhHo(2) and RhHo(2)O(2), which are found to be similar. This indicates that there are significant changes in electronic properties upon sequential addition of oxygen atoms to RhHo(2). The DFT investigations show that the lowest energy neutral structures are a C(2v) triangle for RhHo(2), a C(2v) planar structure for RhHo(2)O where the O atom is doubly bridged to the Ho-Ho bond, and a C(2v) nonplanar structure for RhHo(2)O(2), where the O(2) is dissociative and each O atom is doubly bridged to the Ho-Ho bond in the cluster above and below the RhHo(2) trimer plane. Good correlation between the experimental and computational IE data imply that the lowest energy neutral structures calculated are the most likely isomers ionized in the molecular beam. In particular, the theoretical adiabatic IE for the dissociative RhHo(2)O(2) structure is found to compare better with the experimentally determined value than the corresponding lowest energy O(2) associative structure.

Synchrotron radiation VUV double photoionization of CHF 2Cl

VUV double photoionization of CHF 2Cl in an energy region 32–40 eV was investigated with photoionization mass spectroscopy by using synchrotron radiation. The double ionization energy of CHF 2Cl and appearance energies for its main fragment dications (CHCl 2+, CF 2 2+ and CHFCl 2+), were determined with photoionization efficiency spectroscopy for the first time. The single point energies of CHF 2Cl and its parent dication (CHF 2Cl 2+) were calculated using Gaussian 03 program and density functional theory (DFT and B3LYP functional). The vertical double ionization energy of CHF 2Cl was predicted by using B3LYP method and empirical equation. According to our research results, the experimental double ionization energy of CHF 2Cl is in good agreement with the theoretically calculated vertical double ionization energy. The mechanism of double photoionization of CHF 2Cl was discussed based on the comparison of our experimental results with those predicted theoretically.

Threshold Photoionization and Density Functional Theory Studies of the Niobium Carbide Clusters Nb3Cn (n = 1−4) and Nb4Cn (n = 1−6)

We have used photoionization efficiency spectroscopy to determine ionization potentials (IP) of the niobium-carbide clusters, Nb3C(n) (n = 1-4) and Nb4C(n) (n = 1-6). The Nb3C2 and Nb4C4 clusters exhibit the lowest IPs for the two series, respectively. For clusters containing up to four carbon atoms, excellent agreement is found with relative IPs calculated using density functional theory. The lowest energy isomers are mostly consistent with the development of a 2 x 2 x 2 face-centered cubic structure of Nb4C4. However, for Nb3C4 a low-lying isomer containing a molecular C2 unit is assigned to the experimental IP rather than the depleted 2 x 2 x 2 nanocrystal isomer. For Nb4C5 and Nb4C6, interpretation is less straightforward, but results indicate isomers containing molecular C2 units are the lowest in energy, suggesting that carbon-carbon bonding is preferred when the number of carbon atoms exceeds the number of metal atoms. A double IP onset is observed for Nb4C3, which is attributed to ionization from the both the lowest energy singlet state and a meta-stable triplet state. This work further supports the notion that IPs can be used as a reliable validation for the geometries of metal-carbide clusters calculated by theory.

Performance of the atomic and molecular physics beamline at the National Synchrotron Radiation Laboratory

At the National Synchrotron Radiation Laboratory, The University of Science and Technology of China, an atomic and molecular physics beamline with an energy range of 7.5-124 eV has been constructed for studying the spectroscopy and dynamics of atoms, molecules and clusters. The undulator-based beamline, with a high-resolution spherical-grating monochromator (SGM), is connected to the atomic and molecular physics end-station. This end-station includes a main experimental chamber for photoionization studies and an additional multi-stage photoionization chamber for photoabsorption spectroscopy. A mid-photon flux of 5 x 10(12) photons s(-1) and a high resolving power is provided by this SGM beamline in the energy range 7.5-124 eV. The size of the synchrotron radiation beam spot at the sample is about 0.5 mm in the vertical direction and 1.0 mm in the horizontal direction. Some experimental results of photoionization efficiency spectroscopy and photoabsorption spectroscopy of atoms and molecules are also reported.

VUV dissociative photoionization of CHF 2Cl

VUV dissociative photoionization of CHF 2Cl in an energy region ∼12–22 eV was investigated with photoionization mass spectroscopy (PIMS) using synchrotron radiation (SR). The ionization energy of CHF 2Cl and appearance energies for its fragment ions, CHF 2 +, CF +, CHF +, CHFCl +, CF 2 +, Cl +, CFCl +, CCl +, HCl +, CF 2Cl +, H + and CHCl +, were determined with photoionization efficiency spectroscopy (PIES). The dissociation energies of some possible dissociation channels to produce those fragment ions were also determined experimentally. The total energies of CHF 2Cl and its main photofragments were calculated using Gaussian 98 program and Gaussian – 2 calculations. And then, the ionization energy of CHF 2Cl, appearance potentials for its fragment ions, and the dissociation energies to produce them were predicted using high accuracy energy model. The mechanism of photoionization and photodissociation of CHF 2Cl was discussed deeply based on comparison of our experimental results with those predicted theoretically. According to our research results, the experimental dissociation energies were in reasonable agreement with either the calculated dissociation energies ( E d) or the reaction barriers V (G2) of possible photodissociation channels of CHF 2Cl.

Research on photoionization of Ar·NO cluster using synchrotron radiation

A supersonic beam of NO molecules mixed with Ar atoms was ionized by VUV synchrotron radiation from Hefei Light Source(HLS) at National Synchrotron Radiation Laboratory (NSRL). Photoionization efficiency spectroscopy (PIES) measurement was made for NO, Ar and heterogeneous clusters Ar·NO. In PIES spectum of Ar·NO, a strong resonance peak at the position of atomic rare gas resonance lines (11.5—12.0 eV) was observed, and it is shown that the excitation energy of the rare gas atom is transferred to the attached NO molecule within a heterogeneous cluster, leading to ionization of the molecule NO.

Electron-spin multiplicities and molecular structures of neutral and ionic scandium-benzene complexes

Scandium-benzene complexes, Sc-(C6H6)1,2 are produced by interactions between the laser-vaporized scandium atoms and benzene vapor in pulsed molecular beams, and identified by photoionization time-of-flight mass spectrometry and photoionization efficiency spectroscopy. The electron-spin multiplicities and geometries of these complexes and their ions are determined by combining pulsed field-ionization zero electron kinetic-energy spectroscopy and density-functional theory calculations. For scandium-monobenzene, a short-range quartet ground state is determined for the neutral complex, and a low-energy triplet state is probed for the ion. For the dibenzene complex, the neutral ground state is a doublet, and two low-energy ion states are singlet and triplet. The quartet and triplet states of scandium-monobenzene and the triplet state of scandium-dibenzene possess sixfold symmetry, whereas the doublet and singlet of the dibenzene complex have twofold symmetry. Moreover, ionization energies and metal-ring stretching wavenumbers are measured for both complexes.

Ionization Potentials of Tantalum−Carbide Clusters: An Experimental and Density Functional Theory Study

We have used photoionization efficiency spectroscopy to determine the ionization potentials (IP) of the tantalum-carbide clusters, Ta3Cn (n = 1-3) and Ta4Cn (n = 1-4). The ionization potentials follow an overall reduction as the number of carbon atoms increases; however, the trend is not steady as expected from a simple electrostatic argument. Instead, an oscillatory behavior is observed such that clusters with an odd number of carbon atoms have higher IPs and clusters with an even number of carbon atoms have lower IPs, with the Ta4C4 cluster exhibiting the lowest IP. Excellent agreement is found with relative IPs calculated using density functional theory for the lowest energy structures, which are consistent with the development of a 2 x 2 x 2 face-centered nanocrystal. This work shows that IPs may be used as a reliable validation for the geometries of metal-carbide clusters calculated by theory. The variation in IP can also be interpreted qualitatively with application of a simple model based upon isolobal frontier orbitals.

The study of photoionization and fragmentation of CHF 2Cl: experiment and quantum chemical calculation

Photoionization and fragmentation of CHF 2Cl were studied with photoionization mass spectroscopy (PIMS) and photoionization efficiency spectroscopy (PIES) at NSRL. Ionization potential of CHF 2Cl was obtained and major fragment ions CHF 2 +, CHFCl +, CHF +, and CF + were observed with their appearance energies, respectively. Dissociation energies of some possible dissociation channels were deduced. The processes of photoionization and fragmentation of CHF 2Cl are discussed based on the comparison between determined appearance energies and energies predicted by the Gaussian-2 calculations.

Double-resonant photoionization efficiency spectroscopy: A precise determination of the adiabatic ionization potential of DCO

We report the first high-resolution measurement of the adiabatic ionization potential of DCO and the fundamental bending frequency of DCO+. Fixing a first-laser frequency on selected ultraviolet transitions to individual rotational levels in the (000) band of the 3pπ 2Π intermediate Rydberg state of DCO, we scan a second visible laser over the range from 20 000 to 20 300 cm−1 to record double resonance photoionization efficiency (DR/PIE) spectra. Intermediate resonance with this Rydberg state facilitates transitions to the threshold for producing ground-state cations by bridging the Franck–Condon gap between the bent neutral radical and linear cation. By selecting a single rotational state for ionization, double-resonant excitation eliminates thermal congestion. Spectroscopic features for first-photon resonance are identified by reference to a complete assignment of the 3pπ 2Π(000)−X 2A′(000) band system of DCO. Calibration with HCO, for which the adiabatic ionization threshold is accurately known, establishes an experimental instrument function that accounts for collisional effects on the shape of the photoionization efficiency spectrum near threshold. Analysis of the DR/PIE threshold for DCO yields an adiabatic ionization threshold of 65 616±3 cm−1. By extrapolation of vibrationally autoionizing Rydberg series accessed from the Σ+ component of the 3pπ 2Π(010) intermediate state, we determine an accurate rotationally state-resolved threshold for producing DCO+(010). This energy, together with the threshold determined for the vibrational ground state of the cation provides a first estimate of the bending frequency for DCO+ as 666±3 cm−1. Assignment of the (010) autoionization spectrum further yields a measurement of an energy of 4.83±0.01 cm−1 for the (2-1) rotational transition in the Σ+1(0110) state of DCO+.

Ionization potentials and bond energies of TiO, ZrO, NbO and MoO

The adiabatic ionization potentials of TiO, ZrO, NbO, and MoO have been measured using two-color photoionization efficiency (PIE) spectroscopy and mass-analyzed threshold ionization (MATI). From the sharp ionization thresholds in the PIE and MATI spectra the following ionization potentials were derived: IP(TiO)=6.8197(7) eV, IP(ZrO)=6.812(2) eV, IP(NbO)=7.154(1) eV, and IP(MoO)=7.4504(5) eV. These values have been combined with the ionization potentials of the metal atoms and the bond energies of the transition metal oxide cations, D0(MO+) [M. R. Sievers et al., J. Chem. Phys. 105, 6322 (1996)] to derive the bond energies, D0(MO), of the neutral metal monoxides; D0(TiO)=6.87(7) eV, D0(ZrO)=7.94(11) eV, D0(NbO)=7.53(11) eV, D0(MO)=5.44(4) eV. It is argued that these values are more accurate than the currently accepted values and hence are recommended for future work. Experimental evidence suggests that the ground state of MoO+ is the Σ−4 state arising from the δ2σ1 configuration.

Photoionization spectroscopy of dichromium and dimolybdenum: Ionization potentials and bond energies

Photoionization spectroscopy has been used to probe molecular beams of laser-vaporized chromium (Cr2) and molybdenum (Mo2) dimers. Two-color photoionization efficiency spectroscopy has been used to determine the adiabatic ionization potential (IP) of Cr2 and Mo2 to be 56 449±8 cm−1 and 56 042±8 cm−1, respectively. The IP of Cr2 is combined with the IP of Cr [54 575.6±0.3 cm−1, Huber et al., Proc. R. Soc. London, Ser. A 342, 431 (1975)] and the bond energy of Cr2+ [10 200±500 cm−1, Su et al., Chem. Phys. Lett. 201, 199 (1993)] to yield a bond energy of 12 400±500 cm−1 for Cr2. One-color resonant two-photon ionization (R2PI) spectroscopy has been employed to probe the molybdenum dimer molecule in the energy region where its dissociation should occur. The dissociation limit has been ascribed to the threshold observed at 36 100±80 cm−1. This value is combined with the IP of Mo [57 204.3±0.3 cm−1, Rayner et al., J. Opt. Soc. Am. B 4, 900 (1987)] and Mo2 to yield a bond energy of 37 260±80 cm−1 for Mo2+.

Laser spectroscopy of the à 2Π←X̃ 2Σ+ transition of ytterbium monoacetylide

The first spectroscopic identification and characterization of ytterbium monoacetylide (YbCCH) is reported. By combining resonance-enhanced two photon ionization (R2PI), laser-induced fluorescence (LIF), and photoionization efficiency spectroscopy (PIE) with density functional calculations the X̃ 2Σ+ and the à 2Π1/2,3/2 states of YbCCH as well as the X̃ 1Σ+ state of YbCCH+ have been characterized. The à 2Π1/2,3/2–X̃ 2Σ+ system whose 0-0 band for the à 2Π1/2 component lies at around 16 848 cm−1 for YbCCH has been studied at 0.3 cm−1 resolution. The excitation spectra, both R2PI and LIF are characterized by progressions involving the YbCC bending mode (ν5) whose wave number has been determined to be 96 and 103 cm−1 for the X̃ 2Σ+ and à 2Π1/2,3/2 state, respectively. The dispersed fluorescence spectra show a progression in the Yb-C stretching vibration with a wave number of ω(ν3)=328 cm−1. Density functional calculations confirmed the vibrational assignment and yielded a linear geometry for both the X̃ and à state of YbCCH as well as for the X̃ 1Σ+ state of the cation. Photoionization efficiency spectroscopy yielded an adiabatic ionization potential of 47 165(10) cm−1 [5.8477(12) eV]. Rydberg series converging to the 51 and 52 level of YbCCH+ were observed and combined with the appearance potentials led to ω(ν5)=97 cm−1 for the YbCC bending mode of YbCCH+.