Calculation of photoionization cross sections of small metal clusters

In this paper we present photoionization cross sections of the Cu and Al dimers and tetramers. The local spin density method is applied to calculate the electronic structure and the ground state potential. The cross sections are calculated using the continuum multiple scattering method, and a basic analysis of the photoionization process is given within the independent particle picture.     

Near threshold photoionization of the ground and first excited states of C(2)

Calculations using the multichannel Schwinger configuration-interaction method are presented for the photoionization from the ground and the first excited states of the C(2) molecule. Both single channel and multichannel calculations are presented in a photon energy range from the threshold to about 50 eV of photon energy. For the ground state, inclusion of both intrinsic and dynamical correlation effects is seen to strongly alter the picture of the photoionization process inferred from single-channel frozen-core Hartree-Fock calculations. Furthermore, the photoionization study of the first excited state of molecular carbon has revealed the presence of strong interchannel coupling between the 3sigma(g)-->ksigma(u) channel and the photoionization channels leading to the A (4)Pi(g) and f (2)Pi(g) ionic states in the near threshold region.     

FLASH PHOTOLYSIS OF AQUEOUS TRYPTOPHAN, ALANYL TRYPTOPHAN AND TRYPTOPHYL ALANINE

Abstract— Tryptophan has been flash photolysed in deoxygenated and air equilibrated solutions. The kinetics of the subsequent reactions of the tryptophyl radical and the hydrated electron have been elucidated. Oxygen has been found to reduce the primary photoionization yield, indicating a triplet state precursor, but no evidence has been found for a biphotonic process. Appreciable differences in the photoionization quantum yields have been found for tryptophan, alanyl tryptophan and tryptophyl alanine.     

Time-dependent photoionization of azulene: competition between ionization and relaxation in highly excited states

Pump-probe photoionization has been used to map the relaxation processes taking place from highly vibrationally excited levels of the S(2) state of azulene, populated directly or via internal conversion from the S(4) state. Photoelectron spectra obtained by 1+2(') two-color time-resolved photoelectron imaging are invariant (apart from in intensity) to the pump-probe time delay and to the pump wavelength. This reveals a photoionization process which is driven by an unstable electronic state (e.g., doubly excited state) lying below the ionization potential. This state is postulated to be populated by a probe transition from S(2) and to rapidly relax via an Auger-like process onto highly vibrationally excited Rydberg states. This accounts for the time invariance of the photoelectron spectrum. The intensity of the photoelectron spectrum is proportional to the population in S(2). An exponential energy gap law is used to describe the internal conversion rate from S(2) to S(0). The vibronic coupling strength is found to be larger than 60+/-5 microeV.     

Polarity effects on wavelength dependence of the fluorescence quantum yield for indole

The change in fluorescence quantum yield for indole as a function of excitation wavelength between 250 and 220 nm is found to vary with the static dielectric constant of various alcohol—water mixtures at 296 K. The supports the intermediacy of a CTTS state in the photoionization process.     

EXCITED STATES IN PHOTODIMERIZATION OF AQUEOUS TYROSINE AT ROOM TEMPERATURE

Abstract— The photoionization of tyrosine in aqueous solution in the liquid state was studied at room temperature by analyzing the kinetics of formation of bityrosine upon irradiation of tyrosine, and the external heavy-atom effect on the formation of bityrosine. The relative number of bityrosine molecules was determined by measuring the fluorescence intensity at 400 nm. A kinetic model for the formation of bityrosine was formulated on the assumption that the rate constants were first-order for production of radicals and for recombination with ejected electrons. The applicability of this model to the present case was confirmed by the experimental data. On the basis of the model, we found that the electronic process of photoionization of tyrosine at room temperature is different in acidic and alkaline media. In acidic media a tyrosine molecule absorbs one light quantum and photoionizes through a singlet excited state, while in alkaline media a tyrosinate ion photoionizes after absorption of two light quanta. The intermediate product that absorbs the second photon is in a triplet excited state.     

THE STEPWISE BIPHOTONIC PHOTOIONIZATION OF CHLORPROMAZINE AS SEEN BY LASER FLASH PHOTOLYSIS

It is generally accepted that both promazine (PZ) and chlorpromazine (CPZ) photionize monophotonically to their respective cation radicals and the corresponding hydrated electrons. It is also supposed that this photoinization has a role in the phototoxic effects of these drugs. However, using laser flash photolysis, we have observed that photoionization of CPZ during S1 excitation (lambda greater than 300 nm) is a stepwise biphotonic process. In the case of PZ our flash photolysis results are less clearcut, but are consistent with stepwise biphotonic photoionization for S1 excitation. We demonstrate, using computer simulation of the intramolecular kinetics, that the estimated triplet state lifetime of CPZ is sufficiently long (23 ns at room temperature) to account for the apparent monophotonic photoionization that has been observed by others at high light intensities and short pulse times. Our laser flash photolysis results also suggest that the photo-ionization mechanism of PZ and CPZ is wavelength-dependent. Both drugs exhibit apparent monophotonic photoionization when they are excited at 266 nm under conditions of laser pulse width and intensity similar to those at 355 nm. We suggest that photoionization is not an important mechanism in the observed phototoxic and photoallergic effects of PZ and CPZ in sunlight.     

Calculation of photoionization cross sections of Na2–8 and K2–8 clusters

Analysis of free metal clusters studied with photoionization mass spectrometry or photoelectron spectroscopy requires theoretical predictions of the photoionization cross sections to gain a deeper physical understanding. Calculated energy-dependent photoionization cross sections of Na_2–8 and K_2–8 clusters are presented in this study. The ground state electronic structure of the clusters are calculated using the Local Spin Density method (LSD) which is also the starting point for the cross section calculation with the continuum multiple scattering method. A basic analysis of the photoionization process is given within the independent electron picture. Strong resonances are predicted in the UV cross sections (5–10 eV) of K_3–8 but not for Na_3–8, interpreted as shape resonances, i.e. quasibound states in which electrons are trapped by a potential barrier. Unfortunately experimental data are only known close to the ionization threshold and a comparison between our values and experimental data in a broad energy range is not possible.     

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.     

Time-energy mapping of photoelectron angular distribution: application to photoionization stereodynamics of nitric oxide

The time-energy mapping of the photoionization integral cross section and laboratory-frame photoelectron angular distribution is used to study photoionization stereodynamics of a diatomic molecule. The general theoretical formalism [Y. Suzuki and T. Suzuki, Mol. Phys., 2007, 105, 1675] is simplified for application to a diatomic molecule, and a high-resolution photoelectron imaging apparatus is used to determine the transition dipole moments and phase shifts of photoelectron partial waves in near-threshold and non-dissociative photoionization of NO from the A(2)Σ(+) state. The transition dipoles and phase shifts thus determined are in reasonable agreement with those by state-to-state photoionization experiment and Schwinger variational calculations. The difference of the phase shifts from those expected from the quantum defects of Rydberg states suggests occurrence of weak hybridization of different l-waves, in addition to the well-known s-d super complex. The circular dichroism in photoelectron angular distribution is also simulated from our results.     

One and two-photon photochemistry in rigid solutions of durene in 3-methylpentane at 77°K

A study of the photochemistry of a dilute rigid solution of durene in 3-methylpentane at 77°K reveals the presence of an independent one photon process in competition with the biphotonic process for β-bond cleavage at energies corresponding to excited vibrational levels of the first excited singlet state. In contrast, the photoionization of durene remains a two photon process over the entire energy range examined.     

Photochemical inactivation of trypsin

Abstract —The quantum yield for inactivation of aqueous trypsin fits the expression φ_f=Σ_rf_rφ‘_r, where f_r, is the fraction of incident light absorbed by residues of type r and the φ’_r are constants. The values φ‘_trp= 0.012, φ_tyr= 0.005 and φ’_eys= 0.10, obtained at pH 3 in the wavelength range 240–290 nm, are attributed to independent events by comparing with quantum yields of the initial photochemical products and permanent residue destruction. The proposed inactivating processes are photoionization of one essential tryptophyl residue, photolysis of one essential cystyl residue, and splitting of an essential cystyl residue induced by light absorption in a nearby tyrosyl residue. The initial photochemical process from pH 3–7 identified by flash photolysis is the ejection of electrons from approximately two tryptophyl residues, leading to the formation of the disulfide bridge electron adduct and the hydrated electron. It is proposed that one photoionized tryptophyl residue is permanently disrupted and the other is restored through a back reaction that leads to a damaged, active enzyme form. An enhanced inactivation quantum yield at flash photolysis light intensities is attributed to a biphotonic process. A model based on one-photon photoionization of tryptophan from a short-lived precursor of the fluorescent state and the biphotonic photoionization of tryptophan via the triplet state is consistent with the experimental results.     

Primary photophysical properties of ofloxacin

Steady-state fluorescence has been used to study the excited singlet state of ofloxacin (OFLX) in aqueous solutions. Fluorescence emission was found to be pH dependent, with a maximum quantum yield of 0.17 at pH 7. Two pKa*s of around 2 and 8.5 were obtained for the excited singlet state. Laser flash photolysis and pulse radiolysis have been used to study the excited states and free radicals of OFLX in aqueous solutions. OFLX undergoes monophotonic photoionization from the excited singlet state with a quantum yield of 0.2. The cation radical so produced absorbs maximally at 770 nm with an extinction coefficient of 5000 +/- 500 dm3 mol-1 cm-1. This is confirmed by one-electron oxidation in the pulse radiolysis experiments. The hydrated electron produced in the photoionization process reacts with ground state OFLX with a rate constant of 2.0 +/- 0.2 x 10(10) dm3 mol-1 s-1, and the anion thus produced has two absorption bands at 410 nm (extinction coefficient = 3000 +/- 300 dm3 mol-1 cm-1) and at 530 nm. Triplet-triplet absorption has a maximum at 610 nm with an extinction coefficient of 11,000 +/- 1500 dm3 mol-1 cm-1. The quantum yield of triplet formation has been determined to be 0.33 +/- 0.05. In the presence of oxygen, the triplet reacts to form both excited singlet oxygen and superoxide anion with quantum yields of 0.13 and < or = 0.2, respectively. Moreover, superoxide anion is also formed by the reaction of oxygen with the hydrated electron from photoionization. Hence the photosensitivity due to OFLX could be initiated by the oxygen radicals and/or by OFLX radicals acting as haptens.     

Photoabsorption and S 2p photoionization of the SF6 molecule: resonances in the excitation energy range of 200-280 eV

Photoabsorption and S 2p photoionization of the SF(6) molecule have been studied experimentally and theoretically in the excitation energy range up to 100 eV above the S 2p ionization potentials. In addition to the well-known 2t(2g) and 4e(g) shape resonances, the spin-orbit-resolved S 2p photoionization cross sections display two weak resonances between 200 and 210 eV, a wide resonance around 217 eV, a Fano-type resonance around 240 eV, and a second wide resonance around 260 eV. Calculations based on time-dependent density functional theory allow us to assign the 217-eV and 260-eV features to the shape resonances in S 2p photoionization. The Fano resonance is caused by the interference between the direct S 2p photoionization channel and the resonant channel that results from the participator decay of the S 2s(-1)6t(1u) excited state. The weak resonances below 210-eV photon energy, not predicted by theory, are tentatively suggested to originate from the coupling between S 2p shake-up photoionization and S 2p single-hole photoionization. The experimental and calculated angular anisotropy parameters for S 2p photoionization are in good agreement.     

A FLASH PHOTOLYSIS STUDY OF 5-METHOXYINDOLE

Abstract— The microsecond flash photolysis of 5-methoxyindole in aqueous solutions has been studied at γ_exc≥ 290 nm. Transients identified in this time realm in neutral solutions are: e_aq^-, the 5-methoxyindole radical cation (γ_max≅ 440 nm), the neutral transient with γ_max≅ 530 nm) and an unidentified oxygen sensitive transient with γ_max≅ 435 nm. Radical cations and e^-_aq are shown to be produced in equal amounts consistent with a photoionization process as the only source of both transients. H⁺ quenching of fluorescence and radical cation production gives equivalent Stern-Volmer constants indicating that photoionization occurs from the fluorescent state. The unidentified oxygen sensitive transient exhibits a pK _a of2–2.5 and is quenched at lower pH values indicating that it also has a fluorescent state precursor.     

Transient photoelectron spectroscopy of the dissociative Br2(1Piu) state

Photodissociation of bromine on the Br2(1Piu) state is probed with ultrafast extreme ultraviolet (53.7 nm) single-photon ionization. Time-resolved photoelectron spectra show simultaneously the depletion of ground state bromine molecules as well as the rise of Br(2P3/2) products due to 402.5 nm photolysis. A partial photoionization cross-section ratio of atomic versus molecular bromine is obtained. Transient photoelectron spectra of a dissociative wave packet on the excited state are presented in the limit of low-power-density, single-photon excitation to the dissociative state. Transient binding energy shifts of "atomic-like" photoelectron peaks are observed and interpreted as photoionization of nearly separated Br atom pairs on the Br2(1Piu) state to repulsive dissociative ionization states.     

Unraveling Vibrational Wavepacket Dynamics using Femtosecond Ion Yield Spectroscopy and Photoelectron Imaging

Time-resolved photoionization is a powerful experimental approach to unravel the excited state dynamics in isolated polyatomic molecules. Depending on species of the collected signals, different methods can be performed: time-resolved ion yield spectroscopy (TR-IYS) and time-resolved photoelectron imaging (TR-PEI). In this review, the essential concepts linking photoionization measurement with electronic structure are presented, together with several important breakthroughs in experimentally distinguishing the oscillating wavepacket motion between different geometries. We illustrate how femtosecond TR-IYS and TR-PEI are employed to visualize the evolution of a coherent vibrational wavepacket on the excited state surface.     

Ionization energy reductions in small 2,5-dihydroxybenzoic acid-proline clusters

The photoionization of (pro)(n)DHB (pro = proline, DHB = 2,5-dihydroxybenzoic acid, n = 0, 1, 2 or 4) clusters was studied both experimentally and computationally. Experimentally the (pro)(n)DHB clusters are generated in the gas phase by laser desorption and supersonic jet entrainment. The photoionization thresholds are then determined by the mass-selective measurement of both one- and two-color photoionization efficiency curves. These experiments demonstrate that the ionization energies (IEs) of the (pro)(n)DHB clusters are substantially reduced in comparison with the IE of free DHB. Computational studies of the (pro)(n)DHB clusters provide insights into the mechanism of IE reduction. For the (pro)DHB system the IE reduction results from spin delocalization in the ion state of the cluster. In contrast, for the (pro)(2)DHB and (pro)(4)DHB clusters the IE reduction results from an inductive delocalization of electron density from pro to DHB in the ground state of the cluster. This latter effect, which is a result of the specific hydrogen-bonding interactions occurring in the mixed clusters, leads to IE reductions of >1 eV. Finally, determination of the energetics of the (pro)(2)DHB radical cation demonstrate that the DHB-to-proline proton transfer reaction is a barrierless, exoergic process in the ion state and that energetic demands for cluster dissociation to protonated (pro)(2) plus a deprotonated DHB radical are substantially lower than those for cluster dissociation to (pro)(2) plus DHB(+*). Cumulatively, these studies provide new energetic and mechanistic insights into both primary and secondary MALDI ionization processes.     

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.