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The dissociative photoionization of quinoline was studied by photoionization mass spectrometry and ion yield measurements over a synchrotron photon excitation energy range 7–26 eV. The ionic and neutral products were identified with the aid of thermochemical calculations that, in some cases, led to deeper understanding of photodissociation pathways and the determination of upper limits of heats of formation of ionic and neutral dissociation products. A detailed comparison between the 20 eV photon excitation and 70 eV electron impact mass spectra, coupled with estimation of thermochemical appearance energies, leads to assignment of the dissociative ionization cation and neutral products for each detected ion. Reaction schemes for formation of these products are proposed in a number of cases. Ion intensities in the photon and electron impact mass spectra were used to consider extending a rule of charge retention in simple bond cleavage to more complex cases of dissociative ionization.

The mechanism of the phase transition of supercooled water to ice Ih at −36 ºC is discussed on a molecular level. The structure of liquid water is described by the interstitial model first proposed by Samoilov. It is consistent with the results of X-Ray and neutron scattering and the experimental results on the diverse properties of liquid water. In order to illustrate our reflections several numerical calculations have been performed using exclusively experimental data. From the temperatur dependence of the nucleation rate the minimum number of water molecules, which constitute the critical germ, and the volume of the critical germ are determined. The occupation numbers of the regular tridymite lattice and the vacancies of the interstitial lattice are evaluated. The negative entropy of nucleation is discussed. It can be split in two contributions. They refer to the change of the statistics by the the transition from the protonucleus to the nucleus and the changes of the degrees of freedom of motion. Considering the simple model, the calculated value fits well to the experimental value of the nucleation entropy. The transition state, which is a key point in the classical nucleation theory, is discussed on a molecular level. It has a lifetime of 0.57 × 10−12 s. The nucleation volume is of the order nm3. One can expect that in this small volume density fluctuations exist in which especially the molecules in interstitial positions are involved. They are rich in energy and easily moveable. Statistical calculations deliver the deviation ±5 from the average value 23 of the number of molecules at interstitial positions in the nucleation volume. Corresponding density fluctuations have been observed by Huang et al. by very fast measurements of small angle X-ray scattering.

The underpotential deposition (UPD) of Copper on Au(111) in nonaqueous acetonitrile/perchlorate electrolyte has been investigated. The deposition mechanism is thereby strongly influenced by coadsorbed acetonitrile molecules and the stability of the solvation shell around the Cu(I) ions. The UPD mechanism obeys a two step mechanism even in presence of only none specifically adsorbing perchlorate ions. Thereby the first sub monolayer of copper is stabilized by coadsorbed acetonitrile molecules. However already 5% vol water content changes the structure within the double layer and consequently degrades the copper sub monolayer. At about 20% vol water content the Cu(I) ion/solvent complex is still stable. The reduction of Cu(I) ions from this complex is much faster compared to the reduction from the respective Cu(II) water complex.

The formation of ice in a droplet of supercooled water is an irreversible process. It has been investigated experimentally by measuring the Number Nl of droplets of liquid water of volume V at time t out of a total of N0 liquid droplets at t = 0. The plot ln (Nl/N0) vs. Vt shows a decreasing straight line at short times, which turns downward with increasing t. In order to describe the experimental results, two concepts of irreversible thermodynamics were applied. The first concept is based on the nucleation theory of crystallization and deals with the statistics of the formation of a critical germ, followed by rapid growth to an ice droplet. The binomial and Poisson statistics were used and extended to irreversibility by introducing the time t. In that way it was possible to describe the linear range of the above mentioned plot. From the slope the intensive nucleation rate constant of nucleation J = 2.1 · 105 cm− 3s− 1 was determined at 237 K. The linear range is equally valid for mono-, di- and oligo-nucleation as far as the rate constants J are identical for all types of nucleation. The deviations from the linear plot are due to differences in the rate constants J. Extending the Poisson distribution by some kind of perturbation calculation leads to a new statistics, which completely agrees with the experimental results, both at short times and increased times also. The second concept is based on the theorem of the shortest way, which is a transcription of Hamilton's principle of classical mechanics and the Euler–Lagrange equation of motion to irreversible thermodynamics. It allows the temporal evolution of the entropy S(t) during crystallization of a droplet of supercooled liquid water. Applying this formalism one exactly obtains the same results as nucleation statistics at short times t. This is astonishing, because neither nucleation nor other mechanistic ideas were used for the derivation of the formalism, and moreover the basic concepts of both versions of irreversible thermodynamics are completely different. According to the rate constant of crystallization, the lifetime of a droplet of supercooled liquid is τ = 15 s at 237 K. The rate constant J strongly depends on the temperature T. The experimental results can be reproduced by the phenomenological Arrhenius equation. The plot lnJ vs. 1/T shows an ascending straight line. The slope gives the activation energy Ea = − 2.65 · 10− 18 J at 237 K. In contrast to chemical kinetics, the activation energy is negative, Ea

International audience; A photoionization mass spectrometric study of N-methyl formamide and N,N'-dimethyl formamide has been made using synchrotron radiation over the photon energy range 8-20 eV. Photoion yield curves were measured for the parent ion in both species and for six fragment ions in N-methyl formamide. Assignments of the fragment ions and the pathways of their formation by dissociative photoionization were made on the basis of ion appearance energies in conjunction with thermochemical data and the results of electron impact mass spectral studies. Our results illuminate aspects of the effects of methylation of formamide on ionization energies and dissociative ionization channels, as well as on the relative order of analogous molecular orbitals. The principal dissociative ionization process in both compounds involves HCO-loss. This neutral product may be formed in an electronic excited state. A comparison between the ionization properties, in particular the heats of formation of the cations, of the methyl derivatives of formamide and acetamide, shows that the ionization energies of the latter require re-investigation.

A photoion mass spectrometry study of the prebiotic species formamide was carried out using synchrotron radiation over the photon energy range 10-20 eV. Photoion yield curves were measured for the parent ion and seven fragment ions. The ionization energy of formamide was determined as IE (1(2)A') = 10.220 +/- 0.005 eV, in agreement with a value obtained by high resolution photoelectron spectroscopy. The adiabatic energy of the first excited state of the ion, 1(2)A'', was revised to 10.55 eV. A comparison of the ionization energies of related formamides, amino acids, and polypeptides provides useful information on the varied effects of methylation and shows that polymerization does not substantially alter the ionization properties of the amino acid monomer units. Assignments of the fragment ions and the pathways of their formation by dissociative photoionization were made on the basis of ion appearance energies in conjunction with thermochemical data and the results of earlier electron impact mass spectral studies. Some of the dissociation pathways are considered to involve coupling between the 1(2)A' ground state and the low-lying 1(2)A'' excited state of the cation. Heats of formation are derived for all ions detected and are compared with literature values where they exist. Formation of the HNCO(+) ion occurs by two separate paths, one involving H(2) loss, the other H + H. In the conclusion a brief discussion is given of some astrophysical implications of these results.

The vacuum UV photoabsorption spectrum of CH3I has been investigated between 5 eV and 20 eV. Numerous vibronic transitions are observed. In the high 10–20 eV photon energy range weak to very weak diffuse bands are observed and ascribed to electronic transitions from 3a1, 1e and 2a1 to Rydberg orbitals. In the 6–10.5 eV photon energy range more than 200 sharp and strong to weak lines have been observed. Several photon energy ranges were explored under high resolution conditions allowing us to observe many series up to high values of the principal quantum number n. They are assigned to vibrationless Rydberg transitions and classified into two groups converging to the two components of the spin–orbit split X ˜ 2 E state of CH3I+. These two groups consist of six different Rydberg series, i.e., nsa1, npa1, npe, nda1, nde and nf. A very close correlation has been established between the term values of the Rydberg states in CH3I and in Xe for ns, np, nd and nf Rydberg transitions. For the first time, the same measurements have been performed and the interpretation has been proposed for the photoabsorption spectrum of CH2DI and CD3I in the 6–10.5 eV photon energy range. For these two species, ionization energies are deduced: for the X ˜ 2 E3/2 and X ˜ 2 E1/2 states of CH2DI, these are IEad = 9.544 eV and IEad = 10.168 eV, respectively, and the corresponding energies are 9.552 eV and 10.173 eV in CD3I.

The vacuum UV photoabsorption spectrum of C2H3F has been examined in detail between 6 eV and 25 eV photon energy by using synchrotron radiation. The analysis of the data is supported by ab initio quantum mechanical calculations applied to valence and Rydberg excited states of C2H3F. At 7.6 eV the π → π∗ and the 2a″ → 3s transitions are observed. An analysis is proposed and applied to the mixed fine structure belonging to these transitions. For the π → π∗ transition one single long vibrational progression is observed with hcωe = 95 ± 7 meV (766 ± 56 cm−1) and its adiabatic excitation energy is 6.892 eV (55 588 cm−1). The 2a″ → 3s transition is characterized by a single short progression with hcωe = 167 ± 10 meV (1350 ± 80 cm−1) starting at 6.974 eV (56 249 cm−1). From the present ab initio calculations these two wavenumbers best correspond to the vibrational modes v9 (CH2 rock in-plane, FCC-bend) and v6 (CH2 rock in-plane, CF stretch) calculated at 615 cm−1 in the π∗ state and 1315 cm−1 in the (2A″)3s Rydberg state respectively. The C C stretching could not be excluded. The dense structured spectrum observed between 8.0 eV and 10.5 eV has been analyzed in terms of vibronic transitions to Rydberg states all converging to the C 2 H 3 F + ( X ∼ A ″ ) ionic ground state. An analysis of the associated complex fine structure of the individual Rydberg states has been attempted providing average values of the wavenumbers, e.g., for the (2A″)3p Rydberg state hcω9 = 60 ± 1 meV (or 484 ± 8 cm−1), hcω7 = 151 ± 7 meV (or 1218 ± 60 cm−1), hcω4 = 191 ± 3 meV (or 1540 ± 24 cm−1). The assignment of hcω = 105 ± 5 meV (or 823 ± 40 cm−1) is discussed. These experimental values are in good agreement with the theoretical predictions for C2H3F+ [R. Locht, B. Leyh, D. Dehareng, K. Hottmann, H. Baumgartel, Chem. Phys. (in press)]. Above 10.5 eV and up to 25 eV several broad and strong bands are tentatively assigned to transitions to valence (V–V) and/or Rydberg (V–R) states converging to excited ionic states of C2H3F.

The photoinduced ion yield curve of O- from gaseous sulfur dioxide shows remarkable structures in the energy range between 14 and 20 eV. Their analysis reveals that they image different vibrationless and vibrationally excited Rydberg states converging to the ionic states C, D, E, F. In addition S-, SO- and SO2 - have been observed. Besides ion pair formation small contributions from dissociative electron attachment are recognized.

International audience; We review our recent results on the vacuum ultraviolet (VUV) photochemistry of small biomolecules. The experimental techniques used, mass spectrometry and photofragment fluorescence spectroscopy, are described. Emphasis is laid on our mass spectrometric results obtained for five nucleic acid bases and five amino acids. Ionisation and appearance energies are determined from photoionisation mass spectrometry, many for the first time. From this, fragmentation pathways following 6-22 eV photoexciation are derived. The adiabatic ionisation energies of the biomolecules studied lie between 8.2 eV (adenine) and 9.6 eV ( alpha-amino-isobutyric acid). We show that the nucleic acid monocations, and chemically related molecular cations, do not fragment even when formed with large internal energies ( Eint) ranging from 1.80 to 5.35 eV. In contrast, amino acid monocations are unstable and rapid fragmentation occurs via rupture of the C sbnd C(OOH) bond, except for beta-alanine, where rupture of the bond between the alpha-C and beta-C is the lowest lying ionic dissociation channel. The VUV photochemistry of the prebiotic species formic acid, acetic acid and methylformate, studied in more detail previously by several techniques, including fluorescence spectroscopy, is also reviewed. Astrophysical implications of our work are discussed in the conclusion.

The threshold photoelectron (TPES) and the photoionization mass spectrometric study of CH3Br in the 8–20 eV photon energy range is presented. The interpretation and assignments are supported by ab initio calculations. The TPES shows several new discrete features in the Jahn–Teller split ground state X ˜ 2 E ( 2 A ′ – 2 A ″ ) of CH3Br+. An additional continuous band starts at about 11.8 eV. These observations are both correlated with direct ionization and autoionizing transitions. This is supported by constant ion state (CIS) spectroscopy. A large enhancement of the transitions to the A ˜ 2 A and B ˜ 2 E states is ascribed to important autoionizing contributions. Based on the present calculations, the weak to very weak bands in the 17.5–22.0 eV photon energy range were mainly assigned to 2 a 1 - 1 ionization and to double excitations described essentially by the 2 e - 2 4 a 1 1 and 1 e - 1 2 e - 1 4 a 1 1 configurations. The photoionization mass spectrometric study allowed us to investigate in detail the ionization and dissociation of CH3Br+ leading to CH 2 + , CH 3 + , Br+ and CH2Br+ from threshold up to 20 eV photon energy. The experimental data are compared to ab initio dissociation energies. At the onset, the CH 3 + and CH2Br+ fragment ion production is correlated with the ground state of CH3Br+ and both fragment ions have to appear through dissociative autoionization from the ( 3 a 1 1 / 1 e 3 ) 6 s or 5s Rydberg state. This interpretation is supported by the photoabsorption spectrum measured recently in the same photon energy range. At higher energies, beside a likely direct (pre)dissociation of the A ˜ 2 A 1 and B ˜ 2 E states of CH3Br+, autoionization also contributes to the fragmentation in all decay channels. Avoided crossings in a manifold of 2A′ states are likely to be involved. This is supported by ab initio calculations. For CH 3 + the photoion-pair process is analyzed and detailed assignments are proposed on the basis of our latest VUV photoabsorption spectroscopic data.

Rates of homogeneous nucleation of H2O droplets in a temperature range from 236.37 to 237.91 K and of D2O droplets from 241.34 to 242.33 K were measured. The single microdroplets consisted of pure H2O or D2O and were levitated in an electrodynamic balance. In comparison to H2O, D2O shows a stronger tendency to nucleate. Over the investigated temperature interval, D2O droplets need to be supercooled less by 1.1 K compared to H2O droplets in order to arrive at the same nucleation rate. This is in good agreement with the higher degree of intermolecular association in liquid D2O, a fact which has been well established previously both from theory and experimental studies.

The S 1(1 B 2) ← S 0(1 A 1) electronic transition of anisole and the anisole/CO2 aggregate cooled in a supersonic free jet has been investigated in detail using REMPI spectroscopy and quantumchemical model calculations. The 42 intramolecular modes of anisole in the 1 S 1 state are assigned. Some previous assignments of modes of anisole in the 1 S 1 state have been improved, some of the assignments are still tentative. The origin of the corresponding electronic transition in the 1:1-aggregate is blueshifted by 117cm−1 versus the 0-0 transition of anisole, the origine in the 1:2-agreggate is redshifted by 216cm−1. Probably a second conformer of the 1:1-aggregate is formed in the molecular beam. 36 fundamental modes of the anisole/CO2 1:1-cluster out of possible 46 intramolecular modes are assigned. Spectral shifts of the fundamental modes in the 1 S 1 state of anisole/CO2 due to the aggregation have been observed. The intermolecular modes and their binary combinations with intramolecular modes have been analyzed.

The nucleation kinetics of n-alkane droplets with carbon numbers of 15 and 17 was observed in an electrodynamic balance. Changes in the elastic light scattering pattern of the single levitated microdroplets indicate the phase transition liquid to solid. Measurements of the nucleation rates gave information on the dynamics of the nucleation process and allow to propose a new mechanism for the nucleation. Large induction times observed for the C15H32 droplets indicate that the nucleation starts after a solid layer has been built up at the surface.

The nucleation behavior of n-alkane droplets with carbon numbers ranging from 14 to 17 was observed in an electrodynamic balance. Changes in the elastic light-scattering pattern of the single levitated microdroplets indicate the phase transition from liquid to solid. Cooling/heating experiments showed larger supercooling temperatures than expected for alkane droplets with an alkane/air interface. Measurements of the nucleation rates of C15H32 and C17H36 gave additional information about the dynamics of the nucleation process and allowed us to distinguish homogeneous from heterogeneous nucleation.

We report on the adsorption of adenine as well as on the coadsorption of adenine and thymine at Au(111). Adenine is chemisorbed in two different states. Mutual interaction between adenine and thymine could be detected only at negative potentials where both molecules are oriented with their plane parallel to the surface. This interaction depends on the concentration of thymine, the pH value, the temperature and the roughness of the surface. At positive potentials where both molecules are oriented perpendicular to the electrode surface, from our experiments no hints to their interaction could be found.

He I photoelectron spectra of four isotopologues of formic acid, HCOOH, HCOOD, DCOOH and DCOOD have been measured, mainly with an electron kinetic energy resolution of 15 meV. Quantum chemical calculations of geometries, vibrational mode frequencies and the potential energy distributions characterising each normal mode were made for the neutral 1 1 A ′ ground state, the cation ground state 1 2 A ′ and first excited ion state 1 2 A ″ of the four isotopologues. The results were used to analyse observed vibronic structure in the two PES bands in the 11.3–13.5 eV energy region and this analysis provided values for several vibrational mode frequencies of the ion states. The calculated structure and internal dynamics of the 1 2 A ′ ground state of the ion provide satisfactory agreement with experiment but for the 1 2 A ″ excited state a more refined theoretical treatment, permitting greater structural flexibility, is required. The PES first ionization energy of HCOOH agrees well with that obtained from Rydberg series in absorption spectra, and values more precise than hitherto were obtained for the first and second ionization energies of the four isotopologues. Analysis is also made of PES features concerning the higher energy states of the ions between 12.2 and 21 eV. The adabatic ionization energies of the 2 2 A ′ , 2 2 A ″ , 3 2 A ′ and 4 2 A ′ states in this energy region were determined. Vibrational frequencies were obtained for the 3 2 A ′ ion state of the isotopologues and new assignments were made concerning the energy and structure of the 4 2 A ′ ion state, whose adiabatic energy was found to lie about 60 meV below the 3 2 A ′ state.

Vacuum-UV (VUV) photodissociation of gaseous formic acid (HCOOH) has been studied in the 6−23 eV range using photofragment fluorescence spectroscopy and synchrotron radiation excitation. Previous s...

Absorption spectra of HCOOH were measured between 6 and 22 eV at a maximum resolution of 3 meV. Previous measurements had a spectral limit of 11.7 eV. Analysis and band assignment were aided by data from theoretical calculations on valence and Rydberg states and from photoelectron spectroscopy. Five valence transitions and the different types of Rydberg transitions converging to the ground and the first excited electronic state of HCOOH+ are discussed and assigned in the spectral region below 12.3 eV. Our assignments differ considerably in many aspects from those of previous studies. Observation, analysis and assignment of absorption features between 12 and 22 eV were carried out for the first time. Rydberg bands converging to five expected ionization limits were not observed as discrete features, except for the 3 2A′ ion state, corresponding to promotion of an 8a′ electron. The Rydberg bands converging to the other ionization limits are broad and merge to form broad absorption features. Assignments are made for npa′ ← 8a′ and nda′ ← 8a′ Rydberg transitions, which exhibit discrete absorption bands. It is shown that Rydberg states in the 15.7–16.8 eV region undergo autoionization with a rate kai ≈ 7.5 × 1013 s−1 and that this autoionization is probably dissociative.

Excited state photoelectron spectra of anisole have been measured using different excitation pathways. They have been chosen according to vibrational levels in the S1 state as observed in the REMPI spectrum. The combination of extended ab initio calculations with the spectroscopic results leads to the frequencies and assignment of the 42 normal modes of the anisole cation and neutral anisole. A strong influence of the excitation pathway on the appearance of the photoelectron spectra has been observed. With increasing excitation energies the dominant signals in the spectrum are shifted to higher energy. The analysis of the spectra reveals that the major contribution to the intensity is caused by the population of vibrational combination states in the ion. Increasing the energy of the intermediate S1 state changes the population of these final states selectively. In addition, the population of the ν9b state is strongly supported by changes of the geometry in the S1 intermediate state.

He I photoelectron spectra (PES) of HCOOH and DCOOD have been measured with an electron kinetic energy resolution of 15 meV. Analysis of observed vibronic structure in the two PES bands in the 11.3–13.5 eV energy region provided values for several vibrational modes of the X 2 A ′ and A 2 A ″ electronic states of HCOOH+ and DCOOD+. This analysis is based on the implications of structural changes expected to occur on ionization, using literature values for calculated geometries of these states. The vibronic assignments of both PES bands were consistent with isotopic effects on vibrational mode frequencies and were helped by considerations on relations between carbon–oxygen bond lengths and stretch mode frequencies. The analysis of the second PES band indicates that the calculated structure of the A 2 A ″ state of the ion requires more refined theoretical treatment. The PES first ionization energy of HCOOH agrees well with that obtained from Rydberg series in absorption spectra, and values more precise than hitherto were obtained for the first and second ionization energies of DCOOD, 11.335±0.002 eV and 12.380±0.002 eV respectively, and the second ionization energy of HCOOH, 12.378±0.002 eV.

The threshold photoelectron spectroscopic (TPES) and the photoionization mass spectrometric (PIMS) study of methyl chloride (CH3Cl) in the 8–20 eV photon energy range is presented. The interpretation and assignments are supported by ab initio calculations. The detailed analysis of the TPES shows numerous features in the Jahn–Teller split ground X 2 E ( 2 A ′ – 2 A ′′ ) state of CH3Cl+ and a new continuous band peaking at 13.4 eV. These were assigned to both direct ionization and autoionizing transitions. The transitions to the A 2 A and to the B 2 E states undergo a large enhancement ascribed to important autoionizing contributions. Based on the present calculations, the weak to very weak signals in the 19–26 eV photon energy range were mainly assigned to 2a1−1 ionization and to double excitations described essentially by the 2e−2, 4a11 and 1e−12e−14a11 configurations. The PIMS study allowed us to investigate in detail the ionization and dissociation of CH3Cl+ into CH2+, CH3+, CHCl+ and CH2Cl+ from threshold up to 20 eV photon energy. At the onset, the CH3+, CHCl+ and CH2Cl+ fragment ion production is correlated to the ground state of CH3Cl+ and all fragment ions have to appear through dissociative autoionization from the 2e → 3p Rydberg state. This interpretation is supported by the photoabsorption spectrum (PAS) measured recently in the same photon energy range. At higher energies, beside direct dissociation of the A 2 A 1 and B 2 E states of CH3Cl+ autoionization also contributes to the fragmentation in all decay channels. For CH3+ the photoion-pair process is analyzed and detailed assignments are proposed on the basis of recent PAS data.

The kinetics of unimolecular fragmentation reactions of energy selected fluoroethene and 1,1-difluoroethene ions has been investigated by the threshold photoelectron photoion coincidence (TPEPICO) technique. A systematic comparison of kinetic data recorded in a linear and a reflecting time-of-flight mass spectrometer is presented. From the analysis of the TPEPICO-TOF spectra complete k(E) curves have been derived over a range from 104 to 107 s-1. The current work complements previous work aimed at the kinetic energy released in the elementary reactions of the title molecules (Guthe, F.; Locht, R.; Leyh, B.; Baumgartel, H.; Weitzel, K.-M. J. Phys. Chem. A 1999, 103, 8404).

The vacuum-UV absorption and fluorescence spectroscopy of CF\(_2\)X\(_2\) (X=H,Cl,Br) in the range 190-690 nm is reported. Tunable vacuum-UV radiation in the range 8-22 eV from synchrotron sources at either Daresbury, UK or BESSY1, Germany is used to excite the titled molecules. Fluorescence excitation spectra, with undispersed detection of the fluorescence, were recorded at Daresbury with a resolution of 0.1 nm. VUV absorption spectra at a resolution of 0.08 nm, and dispersed emission spectra with an optical resolution of 8 nm were recorded at BESSY1. Action spectra, in which the VUV energy is scanned with detection of the fluorescence at a specific wavelength, were also recorded at BESSY1 with a resolution of 0.3 nm ; appearance energies for production of a particular emitting state of a fragment are then obtained. Using the single-bunch mode of BESSY1, lifetimes of all emitting states that fall in the range ca. 3-80 ns have been measured. The peaks in the VUV absorption spectra of CF\(_2\)X\(_2\) are assigned to Rydberg transitions. For CF\(_2\)H\(_2\) below 11 eV, there is good agreement between the absorption and the fluorescence excitation spectra, whereas above 11 eV and for the whole range 8-22 eV for CF\(_2\)Cl\(_2\) and CF\(_2\)Br\(_2\) there is little similarity. This suggests that photodissociation to emitting states of fragment species represent minor channels. In the range 8-15 eV, emission is due mainly to CF\(_2\) A\(^1\)B\(_1\) - X\(^1\)A\(_1\)1 and weakly to CFX A\(^1\)A” - X\(^1\)A’. These products form by photodissociation of Rydberg states of CF\(_2\)X\(_2\), and the thresholds for their production therefore relate to energies of the Rydberg states of the parent molecule. For CF\(_2\)H\(_2\) below 11.8 eV CF\(_2\) A\(^1\)B\(_1\) can only form with H\(_2\), whereas for CF\(_2\)Cl\(_2\) and CF\(_2\)Br\(_2\) it is not possible to say whether the other products are 2X or X\(_2\). For energies above ca. 15 eV, emission is due to diatomic fragments ; CF B \(^2\Delta\) and A \(^2\Sigma\)\(^+\), CCl A \(^2\Delta\), CH B \(^2\Sigma\)\(^-\) and A \(^2\Delta\), Cl\(_2\) and Br\(_2\) D’ 2 \(^3\Pi_g\), and possibly CBr A \(^2\Delta\). From their appearance energies, there is evidence that, with the exception of CF B \(^2\Delta\) / CF\(_2\)H\(_2\) where the ground state of HF must form , the excited state of CF, CCl or CH forms in association with three atoms. Our results yield no information whether the three bonds in CF\(_2\)X\(_2\)* break simultaneously or sequentially. We suggest that the anomalous behaviour of CF\(_2\)H\(_2\), in forming H-H or H-F bonds in unimolecular photofragmentation processes, relates to the small size of the hydrogen atom, and hence the unimportance of steric effects in the tightly-constrained transition state. In no cases is emission observed from excited states of either the CF\(_2\) X free radical or the parent molecular ion, CF\(_2\)X\(_2\)X2\(^+\) .

In this paper, we report the photoabsorption and the constant ionic state spectroscopy of vinyl bromide (C 2H3Br). The photoabsorption spectrum was measured using synchrotron radiation and was investigated in detail between 5.0 and 12.0 eV photon energy revealing many previously unobserved structures. These features were analyzed in terms of valence to virtual valence transitions and Rydberg series. The examination of the three Rydberg series converging towards the first ionization threshold (2a" 1 ns, 2a" 1 np and 2a" 1 nd) leads to wave numbers of 335 ± 30, 690 ± 30 and 1305 ± 30 cm -1 . The vibrational wave numbers of the progressions belonging to Rydberg series converging towards the second ionization threshold are 485 ± 30 and 1145 ± 50 cm 1 . Ab initio calculations helped the assignment of the valence transitions and of the observed vibrational wave numbers in the Rydberg series. Constant ionic state spectra were recorded for vibronic states corresponding to the first and the second electronic states of the ion. Their fine structures are assigned to the autoionization of Rydberg states. The autoionization decay of these Rydberg states is analyzed qualitatively in terms of the vibrational nature of the final ionic state.

The photoelectron spectra of anisole and its van der Waals aggregates with argon (1:1 and 1:2) or carbon dioxide (1:1) have been measured using resonant two-photon laser ionization and a magnetic bottle spectrometer. The influence of the aggregation toward the intramolecular vibrations is discussed. The evaluation of the photoelectron spectra suggests that two different isomers of the anisole–argon (1:2) aggregate exist in the supersonic beam.

The vacuum UV photoabsorption spectrum of CH 3 F has been recorded between 7 and 24 eV and has been analyzed in detail. Broad and structureless peaks are observed over the entire photon energy range. They are all assigned to transitions to Rydberg states, members of series converging successively to the 2e −1 , 5a −1 and 1e −1 ionization limits. These features are underlying very long series of narrow and weak structures in the range of 10–13.2 eV. Through a continuum subtraction procedure, about 70 lines could clearly be identified. These have been assigned to long vibrational progressions belonging to Rydberg states corresponding to the 2e→3p and 2e→6s/5d configurations. These states are split by a Jahn–Teller distortion. Ab initio calculations lead to a Jahn–Teller stabilization energy of about 0.84 eV. The main features observed in the two bands are assigned to one single long vibrational progression of hcω ≃0.160 eV (1 290 cm −1 ) related to the C–F Jahn–Teller inactive stretching vibration. The remaining structure is assigned to the low excitation of the Jahn–Teller active vibrational modes, i.e. the bending modes ν 5 and ν 6 characterized by hcω 5 ≃0.120 eV (970 cm −1 ) and hcω 6 ≃0.082 eV (660 cm −1 ). Leaning on the present results and assignments previously reported photoelectron spectroscopic data on CH 3 F + [L. Karlsson, R. Jadrny, L. Mattsson, F.T. Chau, K. Siegbahn, Phys. Scripta 16 (1977) 225] have been reconsidered and reinterpreted.

The uptake dynamics of HCl by single sulfuric acid microdroplets under stratospheric conditions is investigated. The droplets are typically 30−70 μm in diameter and weakly charged so that they can be stored in an electrodynamic trap. The gas uptake of the droplets is monitored either by measuring their size by angle resolved Mie- scattering patterns or by electrostatic balancing of the droplets in the trap. At low temperatures and high sulfuric acid concentration (T < 190 K for 48 wt % H2SO4 and T < 195 K for 56 wt % H2SO4, respectively), liquid-phase diffusion inside the droplet is the rate-limiting step in the overall uptake process. In this regime, the diffusion coefficients Dliq of HCl in supercooled sulfuric acid solutions are found to increase strongly with temperature and H2O concentration. The results are discussed with respect to diffusion models that have been proposed recently. In contrast, at higher temperatures and lower sulfuric acid concentrations (30−40 wt % H2SO4, 185−207 K) gas-phase dif...

In this paper, we report the He(I) photoelectron spectrum (PES) and the threshold-photoelectron (TPES) spectrum of C2H3Br. The fine structure observed in the first two ionic states in the He(I) spectrum is assigned to progressions belonging partially to previously unobserved vibrational normal modes. The TPES has been measured between 9.0 and 25.0 eV, and the photon energy range of 9.8–12.0 eV has been investigated in more detail. Extensive calculations with the gaussian set of programs have been performed to help in the assignment of the observed features. Furthermore, a conical intersection between the A 2 A ′′ and the B 2 A ″ states was found to take place along the C–Br stretching coordinate. Intramolecular dynamics of the B 2 A ′′ state, initially prepared in the Frank–Condon region, was probed by the Fourier transform of the spectrum. The B 2 A ′′ state is almost readily depleted, most probably due to a very effective internal conversion through the conical intersection.

Supercooled sulfuric acid solution droplets are a major constituent of the stratospheric background aerosol. We report the rate of homogeneous freezing and the crystallization dynamics of single levitated sulfuric acid solution droplets with 0 to 57.6 wt.% H2SO4. The energy of formation of the critical nucleus and the self-diffusion activation energy are deduced from the experiments as a function of temperature and concentration. The results are compared to both theoretical and experimental work of other groups.

Homogeneous nucleation rates are determined for micrometer sized water droplets levitated inside an electrodynamic Paul-trap. The size of a single droplet is continuously measured by analyzing the angle-resolved light scattering pattern of the droplets with classical Mie theory. The freezing process is detected by a pronounced increase in the depolarization of the scattered light. By statistical analysis of the freezing process of some thousand individual droplets, we obtained the homogeneous nucleation rate of water between 236 and 237 K. The values are in agreement with former expansion cloud chamber measurements but could be determined with considerably higher precision. The measurements are discussed in the light of classical nucleation theory in order to obtain the size and the formation energy of the critical nucleus.

The unimolecular dissociation of energy-selected fluoroethene and 1,1-difluoroetheneions has been investigated by the threshold photoelectron photoion coincidence (TPEPICO) technique. The breakdown...

Experiments are described in which UV and visible fluorescence is observed following vacuum-UV photoexcitation of a range of polyatomic molecules in the gas phase. Tunable radiation in the energy range 8–25 eV from synchrotron sources at Daresbury, UK and BESSY 1, Germany is used as the photoexcitation source. Non-dispersed fluorescence excitation spectra and dispersed fluorescence spectra are recorded at Daresbury and BESSY 1, respectively. The experiments are sensitive to Rydberg states of molecules that photodissociate to an excited state of a fragment that fluoresces, and to valence states of the parent molecular ion that fluoresces. Using single-bunch mode of BESSY 1, lifetimes of the emitters in the range ca. 3–100 ns are measured. Examples are taken from work either published or in press in SiF 4 and GeF 4 , PF 3 , MCl 4 (M=C, Si, Ge), and CXCl 3 (X=H, F, Br).

Photoionization mass spectrometry of four chlorine oxides (Cl 2 O, Cl 2 O 4 , Cl 2 O 6 , and Cl 2 O 7 ) is reported. Photoionization efficiency curves of parent and fragment ions are measured in the energy regime between 10 and 21 eV using monochromatic synchrotron radiation. Ionization energies (IE) of the chlorine oxides and fragmentation appearance energies (AE) are obtained from photoion yield curves of mass-selected cations. Specifically, we obtain IE(Cl 2 O) = 10.88 ± 0.02 eV, IE(Cl 2 O 4 ) = 11.23 ± 0.05 eV, IE(Cl 2 O 6 ) = 12.66 ± 0.05 eV, and IE(Cl 2 O 7 ) = 12.15 ± 0.05 eV. The experimental results are used to derive numerous thermochemical stability data of the neutral and ionic chlorine oxides and their fragments, as well as plausible cation fragmentation mechanisms. Resonant excitation with tunable vacuum ultraviolet radiation is applied in the case of Cl 2 O to obtain further insight into autoionization processes. Three series of Rydberg excitations are assigned converging to the C ( 2 A 2 ) ionization energy at 12.74 eV.

The fluorescence spectroscopy of CCl3X (X=F, H, Br) in the range 200–700 nm is reported, using vacuum-UV radiation in the range 8–30 eV from a synchrotron as a tunable photoexcitation source. Excitation spectra, with undispersed detection of the fluorescence, have been recorded at the Daresbury UK source with a resolution of 0.1 nm, corresponding to an average energy resolution of ca. 0.015 eV. Dispersed emission spectra in the range 200–700 nm have been recorded at the BESSY 1 Germany source with an optical resolution of 8 nm, following photoexcitation at the energies of the peaks in the excitation spectra. Action spectra, in which the vacuum-UV energy is scanned with detection of the fluorescence at a specific wavelength, have also been recorded at BESSY 1 with a resolution of 0.3 nm; thresholds for production of a particular excited state of a fragment are then obtained. Using single-bunch mode, lifetimes of all the emitting states that fall in the range ca. 3–100 ns have been measured. For photon energies in the range 8–12 eV, emission is due to both CCl2 A1B1–1A1 and CXCl A1A″–1A′. These products form by photodissociation of low-lying Rydberg states of CCl3X, and the thresholds for their production therefore relate to energies of the Rydberg states of the parent molecule. It is not possible to say whether the other products form as two halogen atoms or a diatomic molecule. For energies in the range 13–17 eV, emission is due to diatomic fragments; CCl A2Δ, CF B2Δ, CH B2Σ- and A2Δ, CBr A2Δ, and Cl2 D′ 23Πg. From their threshold energies, there is now accumulated evidence that the excited state of CCl or CX forms in association with three isolated atoms. Our results yield no information on whether the three bonds in CCl3X* break simultaneously or sequentially. In the range 13–17 eV, Cl2* almost certainly forms in conjunction with ground-state CX+Cl. This ion-pair state of Cl2 also forms at higher excitation energies around 20 eV, probably with atomic products C+X+Cl. In no cases is emission observed from excited states of either the CCl3 radical or the parent molecular ion, CCl3X+.

The fluorescence processes following vacuum-UV (VUV) photoexcitation of BF3 and BCl3 in the energy range 9-22 eV have been studied using synchrotron radiation from the BESSY 1, Berlin source. The fluorescence is dispersed in a secondary monochromator, whereby information on the nature of the emitting species is obtained. For excitation energies in the range 13-17 eV, photodissociation of Rydberg states of BF3 produces a substantial branching ratio into two, possibly three, excited valence states of the BF2 free radical, which fluoresce to the ground state. The dispersed spectra are compared with predictions from ab initio calculations of the electronic spectroscopy of this radical. Fluorescence from BCl3 photoexcited in the range 9-22 eV is more complicated, with emissions having already been observed and mostly assigned in BCl2, BCl, BCl3+, BCl2+ and B. Using the single-bunch, pulsed mode of the BESSY 1 source, the radiative lifetimes of emitters from BCl3 lying in the range 3-100 ns have been measured. Our results for BCl3 are compared with data from other techniques. Where more than one emitter is excited at a particular VUV energy, the dispersion of the induced fluorescence through a monochromator means that the lifetimes of the different emitters can be differentiated.

The vacuum–ultraviolet (UV) and visible spectroscopy of MCl4 (M=C, Si, Ge) using fluorescence excitation and dispersed emission techniques is reported. Fluorescence excitation spectra have been recorded following photoexcitation with monochromatised synchrotron radiation in the vacuum–UV energy range 9–25 eV with an average resolution of ca. 0.06 eV. All the peaks in the Rydberg spectra that photodissociate to a fluorescing state of a fragment have been assigned. The Rydberg states arise predominantly from electron excitation from one of the Cl 3pπ non-bonding molecular orbitals of MCl4, and quantum defects for the Rydberg states characteristic of a chlorine atom are obtained. Dispersed emission spectra in the UV/visible region have been recorded with an optical resolution of ca. 4–8 nm at the energies of the peaks in the excitation spectra. Five different decay channels are observed: (a) CCl2 A 1 B 1 – X 1 A 1 fluorescence at ca. 420–600 nm from CCl4 excited in the range 9–12 eV, (b) SiCl2/GeCl2 a 3 B 1 – X 1 A 1 and A 1 B 1 – X 1 A 1 fluorescence from SiCl4/GeCl4 excited in the range 9–14 eV, (c) SiCl + 4 /GeCl + 4 C 2 T 2 – X 2 T 1 and C 2 T 2 – A 2 T 2 fluorescence at ca. 350–700 nm from SiCl4/GeCl4 for photon energies above the adiabatic ionisation energy of the C 2 T 2 state of SiCl+4/GeCl+4 (15.0 and 14.6 eV, respectively), (d) CCl A2Δ–X2Π fluorescence at ca. 276–280 nm from CCl4 excited in the range 14–18eV, and (e) Si* and Ge* atomic emission lines at wavelengths below 310 nm from SiCl4/GeCl4 excited in the range 19–25 eV. These assignments are confirmed by action spectra in which the energy of the vacuum–UV radiation is scanned with detection of the fluorescence at a fixed, dispersive wavelength. By using the single-bunch mode of synchrotron radiation, lifetimes of all the emitting states that fall in the range ca. 4–100 ns have been measured. The MCl ∗ 2 products are formed by photodissociation of low-lying Rydberg states of MCl4; the thresholds for their production therefore relate to energies of Rydberg states of the parent molecule. The CCl*, Si* and Ge* products are formed by sequential, multi-step photodissociation of MCl ∗ 2 ; the thresholds for these emissions correspond to the thermodynamic thresholds to form the emitting product with either three (in the case of CCl*) or four (in the case of Si*/Ge*) chlorine atoms.

The photoelectron spectra of van der Waals dimers of toluene with rare gases or diethylether have been measured using resonant two-photon laser ionization and a magnetic bottle spectrometer.

The role of the potential of zero charge (PZC) and of the potential of maximum adsorption Em during the adsorption of neutral molecules at the electrode surface is not yet well understood. The phase transition of the adsorbed molecules to a condensed state adds further complications. But under certain circumstances this process can be utilized to obtain additional information about the adsorbate system and the electrochemical double layer. On these grounds a new method for the determination of the potential of maximum adsorption in condensed layers is proposed. This method is based purely on a qualitative analysis of the shape of current−time transients, which change their sign at the potential of maximum adsorption and become inverted. From this datum and the corresponding capacity−potential curves of the adsorbate system and of the pure electrolyte one can construct the true charge−potential characteristics of the system and obtain the value of the PZC. This method was applied to the system thymine/merc...

Using synchrotron radiation in the 10–25 eV energy range, the vacuum–UV spectroscopy of GeF4 using fluorescence excitation and dispersed emission techniques is reported. Rydberg states that photodissociate to a fluorescing fragment are assigned, and dispersed emission spectra are recorded at the peak energies of the excitation spectrum. Three channels are observed: GeF2 a 3 B 1 – X 1 A 1 phosphorescence, GeF2 A 1 B 1 – X 1 A 1 fluorescence, and emission from the GeF4+ D 2 A 1 state. Both GeF2 a 3 B 1 and A 1 B 1 probably form with F2, and not 2F, as the other product. Using single-bunch mode, lifetimes of the GeF2 A 1 B 1 and GeF4+ D 2 A 1 states are measured to be 9.3±0.1 and 5.02±0.01 ns, respectively.

We have studied the ionization of Rydberg-excited xenon atoms in THz-laser fields and by quantum dynamical calculations. The experimental threshold laser field strength for 10% ionization probability follows an n*-1.68 (\(\)1.04 THz) dependence (n* effective principal quantum number) with additional weak resonance structures and shows that ionization does not occur by a Landau-Zener mechanism. At scaled frequencies of \(\) to 5.6 the simulated threshold fields for ionization in oscillatory fields show a dependence on the principal quantum number n of n-4.1 to n-1.35.

We have investigated the adsorption of thymine on Au(111) with in-situ infrared spectroscopy. Using IR spectroscopy we have been able to probe the structure and bonding of the ‘chemisorbed’ thymine phase. IR bands in the 1800–1550 cm −1 region, which have a major contribution from stretching vibrations of the carbonyl groups of thymine, exhibit large spectral shifts between thymine in aqueous solution and the chemisorbed phase. By contrast, ‘physisorbed’ thymine shows no IR bands in the spectral region studied, which is consistent with a flat-lying adsorbate. In-situ IR spectroscopy provides compelling evidence for the orientational change of adsorbed thymine occurring concomitantly with the most pronounced anodic current peak in the voltammogram. We have compared the in-situ IR spectra for chemisorbed thymine with IR spectra of metal co-ordination complexes of thymine and uracil. These comparisons provide evidence that chemisorbed thymine bonds to the Au(111) surface through both carbonyl functionalities and a deprotonated N3.

In the high-resolution He(I) photoelectron spectrum of NH3 and its isotopomer ND3, the complete vibrational structure of the X 2 A ″ 2 state of NH3+ and ND3+ is examined in detail. Three vibrational progressions are identified. The well-known strongest progression, already unambiguously assigned to the v2 out-of-plane bending mode, is observed from v=0–17 in NH3+ and from v=0–20 in ND3+. For NH3+ this vibration could be characterized not only by its energy hcω2=0.109±0.001 eV (or ω2=878±7 cm−1), but also by its first anharmonicity constant hcω22x22=−(16.2±1.2)×10−4 eV (or ω22x22=−13.0±1.0 cm−1). The best fit of the experimental data required the introduction of a second anharmonicity constant, i.e. hcω22y22=−(30.7±4.2)×10−6 eV (or ω22y22=−0.248±0.034 cm−1). The earlier reported weak progression, assigned to the vibrational combination v1+nv2 transitions has been reexamined. Suitable handling of the data leads to two possible energies for the v1 degenerate NH stretching vibrational normal mode, i.e. hcω1=0.306±0.006 or 0.422±0.005 eV. Several arguments are brought to favour the value of hcω1=0.422 eV (or ω1=3404 cm−1). Finally a third weak progression, reported for the first time, is assigned to v4+nv2 transitions where the v4 in-plane bending mode is optically forbidden. This vibrational normal mode is characterized by an energy hcω4=0.186±0.010 eV (ω4=1500±80 cm−1). In agreement with theoretical predictions, this transition becomes allowed through a strong Coriolis vibro-rotational coupling between the v4 and the v2 vibrational normal modes. The same measurements and the isotope effect on the molecular constants are investigated in ND3+ too.