Two-color resonantly enhanced multiphoton ionization and zero-kinetic-energy photoelectron spectroscopy of jet-cooled indan

We report studies of supersonically cooled indan using two-color resonantly enhanced multiphoton ionization and two-color zero-kinetic-energy photoelectron spectroscopy. With the aid of ab initio and density-functional calculations, vibrational modes of the first electronically excited state of the neutral species and those of the cation have been assigned, and the adiabatic ionization energy has been determined to be 68458 +/- 5 cm(-1). Similar to the ground state and the first electronically excited state of the neutral molecule, the ground state of the cation is also proven to be nonplanar, with an estimated barrier of 213 cm(-1) and a puckering angle of 15.0 degrees. These conclusions will be discussed in comparison with a previous study of an indan derivative 1,3-benzodioxole.     

Excited state dynamics of liquid water: insight from the dissociation reaction following two-photon excitation

The authors use transient absorption spectroscopy to monitor the ionization and dissociation products following two-photon excitation of pure liquid water. The primary decay mechanism changes from dissociation at an excitation energy of 8.3 eV to ionization at 12.4 eV. The two channels occur with similar yield for an excitation energy of 9.3 eV. For the lowest excitation energy, the transient absorption at 267 nm probes the geminate recombination kinetics of the H and OH fragments, providing a window on the dissociation dynamics. Modeling the OH geminate recombination indicates that the dissociating H atoms have enough kinetic energy to escape the solvent cage and one or two additional solvent shells. The average initial separation of H and OH fragments is 0.7+/-0.2 nm. Our observation suggests that the hydrogen bonding environment does not prevent direct dissociation of an O-H bond in the excited state. We discuss the implications of our measurement for the excited state dynamics of liquid water and explore the role of those dynamics in the ionization mechanism at low excitation energies.     

DNA excited-state dynamics: ultrafast internal conversion and vibrational cooling in a series of nucleosides

To better understand DNA photodamage, several nucleosides were studied by femtosecond transient absorption spectroscopy. A 263-nm, 150-fs ultraviolet pump pulse excited each nucleoside in aqueous solution, and the subsequent dynamics were followed by transient absorption of a femtosecond continuum pulse at wavelengths between 270 and 700 nm. A transient absorption band with maximum amplitude near 600 nm was detected in protonated guanosine at pH 2. This band decayed in 191 +/- 4 ps in excellent agreement with the known fluorescence lifetime, indicating that it arises from absorption by the lowest excited singlet state. Excited state absorption for guanosine and the other nucleosides at pH 7 was observed in the same spectral region, but decayed on a subpicosecond time scale by internal conversion to the electronic ground state. The cross section for excited state absorption is very weak for all nucleosides studied, making some amount of two-photon ionization of the solvent unavoidable. The excited state lifetimes of Ado, Guo, Cyd, and Thd were determined to be 290, 460, 720, and 540 fs, respectively (uncertainties are +/-40 fs). The decay times are shorter for the purines than for the pyrimidine bases, consistent with their lower propensity for photochemical damage. Following internal conversion, vibrationally highly excited ground state molecules were detected in experiments on Ado and Cyd by hot ground state absorption at ultraviolet wavelengths. The decays are assigned to intermolecular vibrational energy transfer to the solvent. The longest time constant observed for Ado is approximately 2 ps, and we propose that solute-solvent H-bonds are responsible for this fast rate of vibrational cooling. The results show for the first time that excited singlet state dynamics of the DNA bases can be directly studied at room temperature. Like sunscreens that function by light absorption, the bases rapidly convert dangerous electronic energy into heat, and this property is likely to have played a critical role in life's early evolution on earth.     

Ultrafast dynamics in thiophene investigated by femtosecond pump probe photoelectron spectroscopy and theory

A hybrid of a time-of-flight mass spectrometer and a time-of-flight "magnetic-bottle type" photoelectron (PE) spectrometer is used for fs pump-probe investigations of the excited state dynamics of thiophene. A resonant two-photon ionization spectrum of the onset of the excited states has been recorded with a tunable UV laser of 190 fs pulse width. With the pump laser set to the first intense transition we find by UV probe ionization first a small time shift of the maxima in the PE spectrum and then a fast decay to a low constant intensity level. The fitted time constants are 80+/-10 fs, and 25+/-10 fs, respectively. Theoretical calculations show that upon geometry relaxation the electronic state order changes and conical intersections between excited states exist. We use the vertical state order S1, S2, S3 to define the terms S1, S2, and S3 for the characterization of the electron configuration of these states. On the basis of our theoretical result we discuss the electronic state order in the UV spectra and identify in the photoelectron spectrum the origin of the first cation excited state D1. The fast excited state dynamics agrees best with a vibrational dynamics in the photo-excited S1 (80+/-10 fs) and an ultrafast decay via a conical intersection, presumably a ring opening to the S3 state (25+/-10 fs). The subsequently observed weak constant signal is taken as an indication, that in the gas phase the ring-closure to S0 is slower than 50 ps. An ultrafast equilibrium between S1 and S2 before ring opening is not supported by our data.     

Acid-Base Properties of the Ground and Excited States of Ruthenium(II) Bis(2,2'-bipyridine) 3-Carboxyl-2,2'-bipyridine

Acid-base properties for ruthenium(II) bis(2,2'-bipyridine) 3-carboxyl-2,2'-bipyridine reveal a ground state pK(a) of 0.82 +/- 0.07 and an excited state pK(a) of 2.31 +/- 0.05, a 1.5 pH unit increase from the ground state. The excited state pK(a) is temperature independent while the ground state pK(a)(0) increases with temperature and has DeltaH(0) and DeltaS(0) values of -990 +/- 149 cm(-)(1) and -4.57 +/- 0.48 cm(-)(1) K(-)(1), respectively. The acidic form of the complex emits at lower energy than the basic form at both 296 and 77 K. The emission energy maxima are solvent dependent and decrease in energy when the solvent changes from 4:1 (v/v) 2-MeTHF-CH(2)Cl(2) to water and when the pH decreases. Changes in excited state lifetimes with emission energy follow the energy gap law with an intercept of 49 +/- 1 and a slope of (2.11 +/- 0.09) x 10(-)(3). Emission quantum yields for protonated and deprotonated species in 4:1 (v/v) 2-MeTHF-CH(2)Cl(2) are 0.023 +/- 0.001 and 0.110 +/- 0.002, respectively. The temperature dependence of the emission lifetimes gives energy barriers of 270 cm(-)(1) for the complex in aqueous solution at pH -0.5, and 990 cm(-)(1) in aqueous solution at pH 4.5, and 1920 cm(-)(1) in 4:1 (v/v) 2-MeTHF-CH(2)Cl(2.)     

Charge exchange ionization in collision cells as a method to detect the presence of long-lived excited electronic states of polyatomic ions

Charge exchange ionization in collision cells installed in a double focusing mass spectrometer with reversed geometry has been used to detect the presence of a long-lived excited electronic state of benzene ion. In particular, the first collision cell located between the ion source and the magnetic sector was modified to serve as an ion source for the reagent ion generated by charge exchange with the primary ion. Strong reagent ion signals were observed when the ionization energies of the reagents (1,3-C4H6, CS2, CH3Cl) were lower than the recombination energy (approximately 11.5 eV) of the excited state benzene ion, while the signals were negligible for reagents (CH3F,CH4) with higher ionization energy. The fact that a strong signal is observable only for electronically exoergic charge exchange is useful for detecting the presence of a long-lived electronically excited state.     

A theoretical study of AmOn and CmOn (n = 1, 2)

Americium and curium oxides AmOn and CmOn (n = 1, 2) were studied using state-of-the-art multiconfigurational, relativistic, quantum chemical methods. Spectroscopic properties for the ground state and several excited states of the four target compounds were determined. The computed dissociation energy of AmO (4.6 eV) agrees fairly well with estimates derived from experimental studies (5.73 +/- 0.37 eV) while the computed dissociation energy of CmO (7.1 eV) agrees well with the experimental value (7.5 eV). The computed ionization energy of AmO (6.3 eV) is in good agreement with the current experimental value (5.9 +/- 0.2 eV).     

Ultrafast dynamics of acetylacetone (2,4-pentanedione) in the S2 state

The dynamics of the enolic form of acetylacetone (E-AcAc) was investigated using a femtosecond pump-probe experiment. The pump at 266 nm excited E-AcAc in the first bright state, S2(pi pi*). The resulting dynamics was probed by multiphoton ionization at 800 nm. It was investigated for 80 ps on the S2(pi pi*) and S1(n pi*) potential energy surfaces. An important step is the transfer from S2 to S1 that occurs with a time constant of 1.4 +/- 0.2 ps. Before, the system had left the excitation region in 70 +/- 10 fs. An intermediate step was identified when E-AcAc traveled on the S2 surface. Likely, it corresponds to an accidental resonance in the detection scheme that is met along this path. More importantly, some clues are given that an intramolecular vibrational energy relaxation is observed, which transfers excess vibrational energy from the enolic group O-H to the other modes of the molecule. The present multistep evolution of excited E-AcAc probably also describes, at least qualitatively, the dynamics of other electronically excited beta-diketones.     

Spectroscopy and metastability of CO2 2+ molecular ions

The spectroscopy and metastability of the carbon dioxide doubly charged ion, the CO(2) (2+) dication, have been studied with photoionization experiments: time-of-flight photoelectron photoelectron coincidence (TOF-PEPECO), threshold photoelectrons coincidence (TPEsCO), and threshold photoelectrons and ion coincidence (TPEsCO ion coincidence) spectroscopies. Vibrational structure is observed in TOF-PEPECO and TPEsCO spectra of the ground and first two excited states. The vibrational structure is dominated by the symmetric stretch except in the TPEsCO spectrum of the ground state where an antisymmetric stretch progression is observed. All three vibrational frequencies are deduced for the ground state and symmetric stretch and bending frequencies are deduced for the first two excited states. Some vibrational structure of higher electronic states is also observed. The threshold for double ionization of carbon dioxide is reported as 37.340+/-0.010 eV. The fragmentation of energy selected CO(2) (2+) ions has been investigated with TPEsCO ion coincidence spectroscopy. A band of metastable states from approximately 38.7 to approximately 41 eV above the ground state of neutral CO(2) has been observed in the experimental time window of approximately 0.1-2.3 mus with a tendency towards shorter lifetimes at higher energies. It is proposed that the metastability is due to slow spin forbidden conversion from bound excited singlet states to unbound continuum states of the triplet ground state. Another result of this investigation is the observation of CO(+)+O(+) formation in indirect dissociative double photoionization below the threshold for formation of CO(2) (2+). The threshold for CO(+)+O(+) formation is found to be 35.56+/-0.10 eV or lower, which is more than 2 eV lower than previous measurements.     

Photodissociation yield spectroscopy of vinyl bromide cation generated by mass-analyzed threshold ionization: vibrational spectroscopy and decay dynamics in the (~)B state

A new technique [mass-analyzed threshold ionization (MATI)-photodissociation yield spectroscopy] to probe bound excited states of a cation was developed, which measures photodissociation yield of the cation generated by mass-analyzed threshold ionization. A vibrational spectrum of vinyl bromide cation in the (~)B state was obtained using this technique. Optical resolution in the low vibrational energy range of the spectrum was far better than in conventional MATI spectra. The origin of the (~)B state was found at 2.2578+/-0.0003 eV above the first ionization onset. Almost complete vibrational assignment was possible for peaks appearing in the spectrum. Analysis of time-of-flight profiles of C(2)H(3) (+) product ion obtained with different laser polarization angles suggested that photoexcited vinyl bromide cation remained in the (~)B state for several hundred picoseconds prior to internal conversion to the ground state and dissociation therein.     

Mass-analyzed threshold ionization study of vinyl bromide cation in the first excited electronic state using vacuum-ultraviolet radiation generated by four-wave mixing in Hg

The vibrational spectrum of the vinyl bromide cation in the first excited electronic state A 2A' was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. The use of an improved vacuum-ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for MATI signals. From the MATI spectrum, the ionization energy to the A 2A' state of the cation was determined to be 10.9150+/-0.0006 eV. Nearly complete vibrational assignments for the MATI peaks were possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the density-functional theory (DFT) and time-dependent DFT/B3LYP levels with the 6-311+G(df,p) basis set.     

Energy-transfer processes in neon-hydrogen mixtures excited by electron beams

Energy- and charge-transfer processes in neon-hydrogen mixtures (500-1400 hPa neon and 0.001-3 hPa hydrogen partial pressures) excited by a pulsed low-energy (approximately 10 keV) electron beam were investigated using time-resolved spectroscopy. Time spectra of the hydrogen Lyman-alpha line, neon excimer emission (second continuum), and neon atomic lines (3p-3s transitions) were recorded. The time-integrated intensity of the Lyman-alpha emission was measured for the same range of gas mixtures. It is shown that direct energy transfer from Ne*2 excimers and neon atoms in the four lowest excited states as well as recombination of H3+ ions are the main channels populating atomic hydrogen in the n=2 state. A rate constant of (4.2+/-1.4)x10(-11) cm3 s(-1) was obtained for the charge transfer from Ne2+ ions to molecular hydrogen. A lower limit for the depopulation rate constant of Ne*2 excimers by molecular hydrogen (combination of energy transfer and ionization) was found to be 1.0 x 10(-10) cm3 s(-1).     

Zero kinetic energy photoelectron spectroscopy of p-amino benzoic acid

We report studies of supersonically cooled p-amino benzoic acid using one-color resonantly enhanced multiphoton ionization and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state S(1) of the neutral species and those of the cation have been assigned, and the adiabatic ionization potential has been determined to be 64 540+/-5 cm(-1). A common pattern involving the activation of five vibrational modes of the cation is recognizable among all the ZEKE spectra. A propensity of Deltav=0, where v is the vibrational quantum number of the intermediate vibronic state from S(1), is confirmed, and the origin of this behavior is discussed in the context of electron back donation from the two substituents in the excited state and in the cationic state. A puzzling observation is the doublet splitting of 37 cm(-1) in the ZEKE spectrum obtained via the inversion mode of the S(1) state. This splitting cannot be explained from our density functional calculations.     

Photodissociation dynamics of acetoxime in gas phase

The dynamics of photodissociation of acetoxime at 193 nm, leading to the formation of (CH3)2C=N and OH fragments, has been investigated. The nascent OH radicals, which are both rotationally and vibrationally excited, were probed by laser photolysis-laser induced fluorescence technique. OH fragments in both v" = 1 and v" = 0 vibrational states were detected with a ratio of population in the higher to lower level of 0.07+/-0.01. The rotational temperatures of v" = 0 and 1 levels of OH radicals are 2650+/-150 K and 1290+/-20 K, respectively. More than 30% of the available energy, i.e., 115+/-21 kJ mol(-1) is partitioned into the relative translational energy of the fragments. The results of excited electronic state and transition state calculations at the configuration interaction with single electronic excitation level suggest that the dissociation takes place with an exit barrier of approximately 126 kJ mol(-1) at the triplet state (T2) potential energy surface, formed by internal conversions/intersystem crossing from the initially populated S2 state. Using the calculated transition state geometry and its energy, the observed energy distribution pattern can be reproduced by the hybrid model within experimental uncertainties. The presence of an exit barrier is further supported by the observation of N-OH dissociation upon 248 nm excitation, where the relative translational energy of the fragments is found to be approximately 96 kJ mol(-1). The photodissociation dynamics of acetoxime is compared with C-OH dissociation in enols and carboxylic acid and N-OH dissociation in nitrous acid. The observed emission (lambda(max)=430 nm) and the N-OH dissociation dynamics indicate crossing of the initially populated state to an emissive state of acetoxime, which is different from the dissociative state.     

Study of acyl group migration by femtosecond transient absorption spectroscopy and computational chemistry

The primary photophysical and photochemical processes in the photochemistry of 1-acetoxy-2-methoxyanthraquinone (1a) were studied using femtosecond transient absorption spectroscopy. Excitation of 1a at 270 nm results in the population of a set of highly excited singlet states. Internal conversion to the lowest singlet npi* excited state, followed by an intramolecular vibrational energy redistribution (IVR) process, proceeds with a time constant of 150 +/- 90 fs. The 1npi* excited state undergoes very fast intersystem crossing (ISC, 11 +/- 1 ps) to form the lowest triplet pipi* excited state which contains excess vibrational energy. The vibrational cooling occurs somewhat faster (4 +/- 1 ps) than ISC. The primary photochemical process, migration of acetoxy group, proceeds on the triplet potential energy surface with a time constant of 220 +/- 30 ps. The transient absorption spectra of the lowest singlet and triplet excited states of 1a, as well as the triplet excited state of the product, 9-acetoxy-2-methoxy-1,10-anthraquinone (2a), were detected. The assignments of the transient absorption spectra were supported by time-dependent DFT calculations of the UV-vis spectra of the proposed intermediates. All of the stationary points for acyl group migration on the triplet and ground state singlet potential energy surfaces were localized, and the influence of the acyl group substitution on the rate constants of the photochemical and thermal processes was analyzed.     

Mass-analyzed threshold ionization study of vinyl chloride cation in the first excited electronic state using vacuum ultraviolet radiation in the 107-102.8 nm range

Vibrational spectrum of vinyl chloride cation in the first excited electronic state, A2 A', was obtained by one-photon mass-analyzed threshold ionization (MATI) spectroscopy. Use of an improved vacuum ultraviolet radiation source based on four-wave sum frequency mixing in Hg resulted in excellent sensitivity for the MATI signals. From the MATI spectrum, the ionization energy to the A2 A' state of the cation was determined to be 11.6667 +/- 0.0006 eV. Nearly complete vibrational assignment for the MATI peaks was possible by utilizing the vibrational frequencies and Franck-Condon factors calculated at the DFT and TDDFT/B3LYP levels with the 6-311++G(3df,3pd) basis set. Geometry of the cation in the A2 A' state was determined by Franck-Condon fitting of the MATI spectrum.     

Resonantly enhanced two photon ionization and zero kinetic energy spectroscopy of jet-cooled 4-aminopyridine

We report studies of supersonically cooled 4-aminopyridine (4-AP) using two-color resonantly enhanced multiphoton ionization (REMPI) and two-color zero kinetic energy (ZEKE) photoelectron spectroscopy. With the aid of ab initio and density functional calculations, vibrational modes of the first electronically excited state (S1) of the neutral species and those of the cation have been assigned, and the adiabatic ionization potential has been determined to be 62291+/-6 cm(-1). The REMPI spectrum of the S1 state is dominated by ring deformation modes and the inversion mode of the amino group, while the ZEKE spectra demonstrate a strong propensity of Deltav=0, where v is the vibrational quantum number of the intermediate vibronic state from S1. In addition, the ZEKE spectra obtained via different vibrational levels of the S1 state contain four common features, corresponding to the activation of four different vibrational modes of the cation. These observations are explained in terms of the structural changes from the ground state to S1 and further to the cation. The vibrational mode distributions in both the REMPI and the ZEKE spectra, the excitation energy of the S1 state, and the ionization potential of 4-AP, are remarkably similar to those of aniline, suggesting that the electronic activity is centered on the ring.     

Spectroscopy of the UO+2 cation and the delayed ionization of UO2

Vibronically resolved spectra for the UO+2 cation have been recorded using the pulsed field ionization zero electron kinetic energy (PFI-ZEKE) technique. For the ground state, long progressions in both the bending and symmetric stretch vibrations were observed. Bend and stretch progressions of the first electronically excited state were also observed, and the origin was found at an energy of 2678 cm(-1) above the ground state zero-point level. This observation is consistent with a recent theoretical prediction [Infante et al., J. Chem. Phys. 127, 124308 (2007)]. The ionization energy for UO2, derived from the PFI-ZEKE spectrum, namely, 6.127(1) eV, is in excellent agreement with the value obtained from an earlier photoionization efficiency measurement. Delayed ionization of UO2 in the gas phase has been reported previously [Han et al., J. Chem. Phys. 120, 5155 (2004)]. Here, we extend the characterization of the delayed ionization process by performing a quantitative study of the ionization rate as a function of the energy above the ionization threshold. The ionization rate was found to be 5 x 10(6) s(-1) at threshold, and increased linearly with increasing energy in the range investigated (0-1200 cm(-1)).     

Ab initio study of the structure, bonding, vibrational spectra, and energetics of XBS+ (where X=H, F, and Cl)

High level ab initio electronic structure calculations at the CCSD(T) level with augmented correlation-consistent basis set extrapolated to complete basis set limit have been performed on XBS and XBS+ for X=H, F, and Cl. The geometries have been optimized up through the aug-cc-pV5Z level and the vibrational frequencies have been calculated with the aug-cc-pVQZ basis sets. Analysis of the bonding in XBS and XBS+ using natural bond orbital analysis shows that the BS bond in XBS is a triple bond, while in XBS+ it is a double bond. The energetic properties of XBS cation and its first excited state are reported. The calculated adiabatic ionization potential is 11.11+/-0.01 eV as compared to the experimental value of 11.11+/-0.03 eV for HBS. The adiabatic ionization potentials for FBS and CIBS are 10.89+/-0.01 and 10.57+/-0.01 eV, respectively.     

Appearance of interatomic Coulombic decay in Ar, Kr, and Xe homonuclear dimers

Interatomic Coulombic decay (ICD) is observed in the rare gas homonuclear dimers Ar2, Kr2, and Xe2 with photoion spectroscopy techniques. Inner valence ionization of the outer ns shell of these systems is known to create a metastable state that dissociates to form a ground state ion and a neutral excited fragment. Inner valence ionization to form ns satellite states leads to similar dissociations, but the neutral fragment gets all the more excited as the internal energy of the ns satellite state increases. When enough excitation energy is transferred to reach the ionization potential, ICD occurs. ICD threshold is observed to coincide with the position of the A+A+ ground state in the Franck-Condon region.