Photoionization Yield Research Articles

Simultaneous multi-photon ionization of aromatic molecules in polymer solids with ultrashort pulsed lasers

Radical cations of perylene, terthiophene, N, N, N′, N′-tetramethylbenzidine, N, N, N′, N′-tetramethyl- p-phenylenediamine, and N-ethylcarbazole were formed in poly(methyl methacrylate) solids by the direct excitation of each absorption band with a 355-nm picosecond (ps) laser and a 400-nm femtosecond (fs) laser and also by the indirect excitation with a 800-nm fs laser. The photoionization yields of perylene with ps and fs lasers were independent of the concentration of tetracyanobenzene, indicating that the relaxed singlet excited state of perylene is not involved in the photoionization. These results show that radical cations are formed through simultaneous multi-photon ionization enhanced by resonance with the lowest singlet excited state.

Characterization of the transient species generated by the photoionization of Berberine: A laser flash photolysis study

Using 266 nm laser flash photolysis it has been demonstrated that Berberine (BBR) in aqueous solution is ionized via a mono-photonic process giving a hydrated electron, anion radical that formed by hydrated electron react with steady state of BBR, and neutral radical that formed from rapid deprotonation of the radical cation of BBR. The quantum yield of photoionization is determined to be 0.03 at room temperature with KI solution used as a reference. Furthermore utilizing pH changing method and the SO 4 − radical oxidation method, the assignment of radical cation of BBR was further confirmed, the p K a value of it was calculated, and the related set up rate constant was also determined.

Production of multiply charged ions from the 4d photoionization of Cs+

Using an ion-photon merged-beam apparatus, photoion yield spectra of Cs2+ and Cs3+ from the 4d photoionization of Cs+ have been measured as a function of photon energy in the range 80-140 eV. Above 90 eV, the 4d giant resonance peak is observed clearly for Cs3+ spectrum. Below 90 eV, some prominent discrete peaks are observed for both Cs2+ and Cs3+. These discrete peaks can be attributed to the 4d-np and 4d-nf transitions.

Ion Dissociation of Hydrazoic Acid Investigated by Synchrotron-Radiation-Based Photoionization Mass Spectrometry

We report mass resolved photoionization yield spectra of a molecular-beam cooled sample of HN(3) using photoionization mass spectrometry based on high-resolution monochromatized synchrotron radiation. Spectra are reported at mass-to-charge ratios of 14(N(+)), 15(NH(+)), 29(N(2)H(+)), 42(N(3)(+)), and 43(HN(3)(+)) in the region of each ionization threshold. The thresholds observed here are all lower than the previously reported ones obtained with electron impact ionization, which were the only ones available in the literature prior to this work. The appearance energies can be used to evaluate four key thermochemical quantities of relevance to HN(3): D(0)(H-N(3)), D(0)(N-N(2)), D(0)(N-H), and IE(NH). We observe the appearance energy of the parent ion (HN(3)(+)) to be 10.56 +/- 0.02 eV, somewhat below the reported ionization energies derived from photoelectron spectroscopy. Great care was taken to evaluate the importance of vibrational hot bands to the photoionization yield spectra. This experiment also provides a lower limit to the proton affinity of N(2) allowing us to bracket this quantity with improved certainty: 119.3 kcal/mol </= PA(N(2)) </= 121.4 kcal/mol. We also derive an upper limit to the ionization energy of N(2)H (IE(N(2)H) </= 7.92 eV), a molecule that has yet to be observed. We hope that knowledge of this ionization energy might help in future attempts to detect this interesting radical. We also take this opportunity to review the status of the thermodynamics of many molecules and ions containing N and H within the context of these new results and make new recommendations. In particular, we recommend a new value for Delta(f)H degrees (0)(HN(3)), nearly 5 kcal/mol larger than prior evaluations.

Laser flash photolysis of aqueous acetaminophen solutions

The N-acetyl-4-aminophenoxyl radical, a supposed intermediate of the enzymatic oxidation of acetaminophen in living organisms, was prepared and studied by means of nanosecond laser flash photolysis. A number of important spectral-kinetic parameters of this species were determined, namely, the absorption coefficient at 440 nm ((4.2±0.2)×103 l mol−1cm−1), the quantum yield of acetaminophen photoionization at 266 nm (φ= 0.03), and the rate constants for recombination (2k= (2.4±0.3))×109 l mol−1s−1) and the reaction with the superoxide radical (k= (9±2))×109 l mol−1s−1).

Angle-resolved photoion yield and resonant Auger spectroscopy for the doubly excited Rydberg states above the C1sthreshold of CO

Doubly excited core-hole states of carbon monoxide in the photon energy region of $300--305\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, i.e., directly above the C $1s$ ionization threshold, have been studied using both angle-resolved ion-yield and high-resolution resonant Auger spectroscopies. The leading configurations of the most prominent doubly excited Rydberg states are assigned by careful analysis of the ion-yield spectra and the final-state spectra to C $1{s}^{\ensuremath{-}1}$ $(5{\ensuremath{\sigma}}^{\ensuremath{-}1}2{\ensuremath{\pi}}^{1}S=1)$ $3s\ensuremath{\sigma}$ $({v}^{\ensuremath{'}}=0,1,2)$, C $1{s}^{\ensuremath{-}1}$ $(5{\ensuremath{\sigma}}^{\ensuremath{-}1}2{\ensuremath{\pi}}^{1}S=0)$ $3s\ensuremath{\sigma}$ $({v}^{\ensuremath{'}}=0,1,2)$, and C $1{s}^{\ensuremath{-}1}$ $(5{\ensuremath{\sigma}}^{\ensuremath{-}1}2{\ensuremath{\pi}}^{1}S=1)$ $4s\ensuremath{\sigma}$ $({v}^{\ensuremath{'}}=0,1)$, which can only be populated via a conjugate shake-up process. Analysis of the resonant Auger spectra provides an assignment of several two-hole--one-electron $(2h\text{\ensuremath{-}}1e)$ final states.

Investigations of Silicon−Nitrogen Hydrides from Reaction of Nitrogen Atoms with Silane: Experiments and Calculations

Using a quadrupole mass filter and vacuum-ultraviolet ionization, we measured the time-of-flight spectra of species at mass-to-charge ratios of m/ z = 45-42 from the reaction of N + SiH 4 in crossed molecular beams. Species with m/ z = 44 and 43 correspond to reaction products HSiNH/SiNH 2 and HSiN/HNSi, respectively; species with m/ z = 45 and 42 are assigned to isotopic variants and daughter ions, respectively, of those two reaction products. We measured the photoionization yields and branching ratios for dissociative ionization of reaction products as a function of photoionization energy. The ionization thresholds of products HSiNH/SiNH 2 and HSiN/HNSi were determined to be 6.7 and 9.2 eV, respectively. Furthermore, we calculated the equilibrium structures, electronic energies, and vibrational wavenumbers of various silicon-nitrogen hydrides H x SiNH y ( x + y = 0-3) using quantum-chemical methods. SiNH 2 (X (2)B 2) and HNSi (X (1)Sigma (+)) are more stable than HSiNH (X (2)A') and HSiN (X (1)Sigma (+)) by 0.82 and 2.81 eV, respectively. SiNH 2 (X (2)B 2), HSiNH (X (2)A'), HNSi (X (1)Sigma (+)), and HSiN (X (1)Sigma (+)) have adiabatic ionization energies of 6.81, 8.19, 10.21, and 10.23 eV, respectively. These experimental and calculated results indicate that SiNH 2 (X (2)B 2) and HNSi (X (1)Sigma (+)) are dominant among isomeric products in the reaction of N + SiH 4. This work presents the first observation of products from the reaction of N + SiH 4 in crossed beams and extensive calculations on pertinent silicon-nitrogen hydrides.

Photoionization Cross Section ofXe+Ion in the Pure5p5P3/22Ground Level

Coupling an ion trap with synchrotron radiation is shown here to be a powerful approach to measure photoionization cross sections on ionic species relaxed in their ground state. The photoionization efficiency curve of Xe+ ions stored in a Fourier transform ion cyclotron resonance ion trap was recorded at ELETTRA in the 20-23 eV photon energy range. Absolute cross sections were derived by comparison of the photoionization yield of Xe+ with measurements from the ASTRID merged-beam experiment. Multiconfiguration Dirac-Fock calculations were performed for the interpretation of these new data.

VUV photoionisation of free azabenzenes: Pyridine, pyrazine, pyrimidine, pyridazine and s-triazine

Photoionisation mass spectrometry was used to obtain the fragmentation pathways of pyridine, pyridazine, pyrimidine, pyrazine and s-triazine molecules upon absorption of 23.0, 15.7 and 13.8 eV synchrotron photons. The ionic fragments observed vary from molecule to molecule, however C 2H 2 +, HCN +and HCNH + are common to all five molecules at the three photon energies. Furthermore, the presence of C 2H 2N 2 +, C 3H 3N + and C 4H 4 + in the spectra of some of the molecules suggests dissociation pathways via loss of HCN moieties. The respective parent cations, m / q = 79 , 80 and 81 have a greater yield at low photon energies when compared to the most intense fragment peak in each spectra. We recorded two of the fragment cation yields, as well as the parent photoion yield curves of pyridine, pyridazine, and pyrimidine in the 8–30 eV range. The formation of abundant cation fragments show a strong propensity of the molecules for dissociation after the absorption of VUV photons higher than 14 eV. The differences in relative fragment yields from molecule to molecule, and when changing the excitation energy, suggest significant bond rearrangements and nuclear motion during the dissociation time. Thus, bond cleavage is dependent on the photon energy deposited in the molecule and on intramolecular reactivity. With the aid of photoion yield curves and energy estimations we have assigned major peaks in the spectra and discussed their fragmentation pathways.

The 4 d–4 f dipole resonance of the Pr atom in an endohedral metallofullerene, Pr@C 82

The photoion yield spectra of an endohedral metallofullerene Pr@C 82 were measured in the photon energy range of 100–150 eV by using time-of-flight mass spectrometry. Parent ions Pr@C 82 +, Pr@C 82 2+ and Pr@C 82 3+ were observed in the mass spectra. The photoion yield spectra of Pr@C 82 2+ showed a broad peak at 120–140 eV that was assigned to the 4 d–4 f giant dipole resonance of the encapsulated Pr atoms. Absolute photoabsorption cross sections of Pr@C 82 were evaluated from the photoion yield spectra to be 37±12 Mb at 110 eV (off-resonance) and 52±13 Mb at 130 eV (on-resonance). These cross sections of Pr@C 82 were compared with the results of Ce@C 82, the only metallofullerene whose photoionization properties have ever been studied near the 4 d edge of the encapsulated metal atom. The enhancement of photoabsorption due to the giant resonance was found to be similar in Pr@C 82 and Ce@C 82, but there are marked differences in the peak shapes, which can be explained as due to interference effects between the fullerene cage and the encapsulated metal atoms.

The interplay of wave packet dephasing, optimization efficiency, and target state population in optimally controlled isotope selective photoionization

The mutual dependencies of characteristic quantities for an isotope selective photoionization, namely optimization efficiency, target state population, and wave packet dephasing are presented for the NaK dimer. A pre-optimized pulse shape obtained from the maximization of the isotopomer ratio 23Na 39K/ 23Na 41K for the first excited electronic state serves as an initial guess for the subsequent optimization. For the ionization it provides almost vanishing population of the heavier ionic isotopomer and an ionic isotopomer ratio which is significant higher than the findings obtained from former investigations. The wave packet motions on the first excited state of the neutral molecule are in phase for both isotopomers. The optimization procedure, i.e. the simultaneous maximization of the 23Na 39K + photoionization yield and the minimization of the 23Na 41K + photoionization yield for the electronic ground state is based on optimal control theory and leads to a rise of ionic ground state population for both isotopomers related to a significant wave packet dephasing. Upon optimization the isotopomer ratio of the yields falls due to incidental resonances of vibronic transitions. The interplay of the observables are discussed and compared with the values obtained from an optimization which was started from a pure Gaussian pulse shape.

Photoionization of the endohedral fullerene ions Sc3N@C80+ and Ce@C82+ by synchrotron radiation

The first experimental results on photoionization of mass-selected endohedral Sc3N@C80+ and Ce@C82+ fullerene ions are reported. The merged-beams technique was employed to measure photo-ion yield spectra as well as absolute cross sections. Comparing the results of endohedral fullerenes with those obtained using "empty" C80+ and C82+ ions provides insight into the mutual influence of the fullerene cage and the encapsulated atom or molecule on one another in photoabsorption processes.

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.

Dissociative photoionization of ClN 3 using high-resolution synchrotron radiation: The N–Cl bond energy in ClN 3

High-resolution synchrotron-radiation-based photoionization mass spectrometry was applied to study the dissociative photoionization of ClN 3 under collision-free molecular beam conditions at ionization energies between 10 and 17 eV. No parent ion (ClN 3 +) could be detected under our experimental conditions. This suggests that the ground and excited states of ClN 3 + are weakly bound or repulsive, a conclusion supported by electronic structure calculations also reported here. We recorded photoionization yield spectra at m/ z = 49, 42, 35 and 14 from which we extracted the appearance potentials for NCl +, N 3 +, Cl +, and N +. The appearance potential of NCl + (10.17 ± 0.02 eV) observed here is close to the previously reported ionization potential of ClN 3 obtained form photoelectron spectroscopy. Using the theoretically calculated binding energy of ClN 3 + (0.2 eV), we derive an estimate of the adiabatic ionization potential of ClN 3 = 9.97 ± 0.02 eV. The measured appearance potentials for N 3 +, Cl +, and N + provide three independent determinations of the Cl–N bond energy in ClN 3, which agree within their respective error limits. The observations of this work are consistent with a new value of the N–Cl bond energy in ClN 3, D 0(Cl–N 3) = 1.86 ± 0.05 eV, 0.3 eV lower than previously reported values, which are however experimentally derived upper limits. The bond energy reported here is consistent with high level ab initio (CCSD(T)) electronic structure calculations extrapolated to the complete basis set limit, which yield a value: D 0(Cl–N 3) = 1.87 eV.

1s(2s2pP1)P2nl,1s(2s3sS3,1)S2np, and1s(2s3pP1)P2nsK-shell photoexcited Rydberg series of beryllium atoms

The inner-shell photoexcitation of the Rydberg series of Be atoms is explored from both experimental and theoretical points of view. We used synchrotron radiation to photoionize Be atoms and the time-of-flight method to record charge-separated photoion yield spectra. Many new autoionizing resonance peaks were observed in the ${\mathrm{Be}}^{2+}$ and ${\mathrm{Be}}^{+}$ spectra. They were categorized as Rydberg series members converging to the $1s(2s2p\phantom{\rule{0.2em}{0ex}}^{1}P^{o})^{2}P^{o}$, $1s(2s3s\phantom{\rule{0.2em}{0ex}}^{3}S)^{2}S$, and $1s(2s3s\phantom{\rule{0.2em}{0ex}}^{1}S)^{2}S$ series limits. Furthermore, numerical calculations using multiconfiguration Dirac-Fock (MCDF) method enabled us to assign unknown resonance peaks as the $1s(2s3p\phantom{\rule{0.3em}{0ex}}^{3}P^{o})^{2}P^{o}ns$ series members. The calculation of the ground state of Be atoms revealed that the admixture of the $1{s}^{2}2{p}^{2}$ into the ground state amounts to 8.67%.

Photodissociation dynamics of vinyl fluoride (CH2CHF) at 157 and 193nm: Distributions of kinetic energy and branching ratios

Using photofragment translational spectroscopy and tunable vacuum-ultraviolet ionization, we measured the time-of-flight spectra of fragments upon photodissociation of vinyl fluoride (CH2CHF) at 157 and 193 nm. Four primary dissociation pathways--elimination of atomic F, atomic H, molecular HF, and molecular H2--are identified at 157 nm. Dissociation to C2H3 + F is first observed in the present work. Decomposition of internally hot C2H3 and C2H2F occurs spontaneously. The barrier heights of CH2CH --> CHCH + H and cis-CHCHF --> CHCH + F are evaluated to be 40+/-2 and 44+/-2 kcal mol(-1), respectively. The photoionization yield spectra indicate that the C2H3 and C2H2F radicals have ionization energies of 8.4+/-0.1 and 8.8+/-0.1 eV, respectively. Universal detection of photoproducts allowed us to determine the total branching ratios, distributions of kinetic energy, average kinetic energies, and fractions of translational energy release for all dissociation pathways of vinyl fluoride. In contrast, on optical excitation at 193 nm the C2H2 + HF channel dominates whereas the C2H3 + F channel is inactive. This reaction C2H3F --> C2H2 + HF occurs on the ground surface of potential energy after excitation at both wavelengths of 193 and 157 nm, indicating that internal conversion from the photoexcited state to the electronic ground state of vinyl fluoride is efficient. We computed the electronic energies of products and the ionization energies of fluorovinyl radicals.

Competition between ultrafast relaxation and photoionization in excited prefluorescent states of tryptophan and indole

The quantum yield of photoionization of TrpH and IndH from the nonrelaxed prefluorescent state S* increases with the temperature decrease. This effect is attributed to the competition between temperature independent ionization and ultrafast thermal relaxation S* --> S1. The rate constant of the relaxation does not depend on the solvent and on the presence of the amino acid side chain: the temperature dependences of photoionization quantum yield, obtained for TrpH and IndH in different solvents, practically coincide. The activation energy for the relaxation rate constant Er approximately 4.5 kJ/mol probably corresponds to intramolecular process or to the formation of the vibronically excited transient complex between photoexcited molecule and solvent molecules.

Laser Flash Photolysis Studies on Gallic Acid

AbstractThe transient species of gallic acid (GA) have been studied by 266 nm nanosecond laser flash photolysis in aqueous solution and acetonitrile. The intermediate with absorption at 320 nm was identified as excited triplet state (3GA*), the decay rates of which were obtained in aqueous solution and acetonitrile respectively. Energy transfer from 3GA* to β‐carotene was observed and the energy transfer rate constant kent was determined to be 2.2×109mol−1·L·s−1. GA underwent photoionization during photolysis and the quantum yield of photoionization was determined to be 0.12 at room temperature with KI as a reference.

Photoionization of helium nanodroplets doped with rare gas atoms

Photoionization of He droplets doped with rare gas atoms (Rg=Ne, Ar, Kr, and Xe) was studied by time-of-flight mass spectrometry, utilizing synchrotron radiation from the Advanced Light Source from 10 to 30 eV. High resolution mass spectra were obtained at selected photon energies, and photoion yield curves were measured for several ion masses (or ranges of ion masses) over a wide range of photon energies. Only indirect ionization of the dopant rare gas atoms was observed, either by excitation or charge transfer from the surrounding He atoms. Significant dopant ionization from excitation transfer was seen at 21.6 eV, the maximum of He 2p 1P absorption band for He droplets, and from charge transfer above 23 eV, the threshold for ionization of pure He droplets. No Ne+ or Ar+ signal from droplet photoionization was observed, but peaks from HenNe+ and HenAr+ were seen that clearly originated from droplets. For droplets doped with Rg=Kr or Xe, both Rg+ and HenRg+ ions were observed. For all rare gases, Rg2+ and HenRgm+ (n,m> or =1) were produced by droplet photoionization. Mechanisms of dopant ionization and subsequent dynamics are discussed.

Photoionization of hot radicals: C2H5,n-C3H7, and i-C3H7

The combination of ion-imaging and vacuum-ultraviolet (vuv) single-photon ionization is used to study the internal energy dependence of the relative photoionization yields of the C(2)H(5),n-C(3)H(7), and i-C(3)H(7) radicals following the 266 nm photodissociation of the corresponding alkyl iodides. The comparison of the ion images obtained by vuv photoionization of the radical with those obtained by two-photon-resonant, three-photon ionization of the complementary I (2)P(32) and I*(2)P(12) atoms allows the extraction of the internal energy dependence of the cross sections. Factors influencing the appearance of the ion images in the different detection channels are discussed, including the secondary fragmentation of the neutral radicals, Franck-Condon factors for the photoionization process, and the unimolecular fragmentation of the parent photoions.