R. Weinkauf

Anion spectroscopy of uracil, thymine and the amino-oxo and amino-hydroxy tautomers of cytosine and their water clusters

Abstract In this work we investigate different forms of electron binding in the mass-selected and cooled nucleobases uracil, thymine and cytosine and their water clusters. In photodetachment–photoelectron spectra of the pyrimidine nucleobases, sharp structures were found at 86±8 meV (uracil), 62±8 meV (thymine) and 85±8 meV (cytosine), which are due to photodetachment of dipole-bound states. The photodetachment angle dependence of these states shows mostly p-wave detachment, which confirms the predicted predominant s-character of the electronic wave function of dipole-bound states. This anisotropy of electron emission and their sharp photodetachment structures can be taken for dipole-bound state recognition. Water attachment to the nucleobases results in positive valence-bound electron affinity, s-wave detachment and broad spectra, implying that the electron now is trapped inside the π * LUMO of the nucleobases, stabilized by the water dipole. The solvent shifts in dependence on water aggregation are linear and allow by extrapolation an estimation of the monomer electron affinities. All three pyrimidine nucleobases are estimated to have a very similar valence-bound electron affinity in the range of 0–200 meV. In nucleobase·(H 2 O) n clusters, due to the large total dipole moment, dipole-bound states also exist. Resonant excitation of these dipole-bound states with a photon of 1064 nm wavelength causes dissociation of the anion cluster, leading to monomer anions in their dipole-bound state. These monomer anions can be photodetached by a second IR photon. Whereas, for uracil and thymine, one dipole-bound state is detected, for cytosine we find two dipole-bound states (85±8, 230±8 meV) which are attributed to the dipole-bound states of the simultaneously present amino-hydroxy and amino-oxo cytosine tautomers. We also give a possible explanation why the formation of the dipole-bound state of the amino-oxo tautomer at 230 meV is improbable in the supersonic expansion.

Reflectron time-of-flight mass spectrometry and laser excitation for the analysis of neutrals, ionized molecules and secondary fragments

Abstract Reflection time-of-flight mass spectrometry in combination with laser ionization is described in this review. Firstly, options for laser ionization are presented. Then, sources of time-of-flight uncertainties are discussed with special emphasis on the spatial focus of an ion source and on reflection techniques for correction of these time-of-flight uncertainties. Furthermore, the behaviour of spontaneous and induced delayed ion decay in a reflection spectrometer is discussed. Knowledge of this behaviour has been used to develop scanning techniques for laser tandem mass spectrometry. Additional applications of laser reflectron mass spectrometry discussed in this review are the measurement of ion decay times, mass spectrometry combined with laser desorption from surfaces and the laser mass spectrometry of isomers.

Photodetachment photoelectron spectroscopy of mass selected anions: anthracene and the anthracene-H2O cluster

Abstract Photodetachment photoelectron spectroscopy is applied for investigation of mass selected and supersonic jet cooled anthracene and anthracene-water dimers. Electron affinities are directly measured to be 530±5 meV for anthracene and 770±5 meV for anthracene-water. This shift in electron affinity due to H 2 O attachment is small and explained by the delocalization of the surplus electron in anthracene. Whereas for the cluster no vibrational structure is observed, for anthracene a short progression in ν 12 is resolved. The anthracene S 0 –T 1 transition energy in the gas phase is 1.869 eV ± 5 meV.

Laser ion sources for time-of-flight mass spectrometry

Abstract Different selective and non-selective laser induced ion and anion sources are presented as well as their combination with time-of-flight mass spectrometers either of linear or reflectron type. Resonance and non-resonance enhanced laser ionization methods and their features of photofragmentation are discussed, in their combination with inlet systems for neutral gases. Laser induced VUV- and electron ionization and laser desorption—postionization are illustrated. Techniques and applications of multiple laser excitation in the ion source and in the space focus are discussed to achieve synchronicity of two experiments or secondary laser excitation. Examples for laser spectroscopic and mass spectrometric applications are given.

Molecular ion spectroscopy: Mass selected, resonant two‐photon dissociation spectra of CH3I+ and CD3I+

We present a new ion spectroscopic technique by which it is possible to investigate larger molecular ions by scanning an entire electronic state and coincidentally measuring and, hence, fixing the mass of the fragment ion observed. The technique involves primary resonance excitation together with secondary absorption followed by dissociation as detection process, employing the absorption of two photons in the ion. This method accesses the nondissociating vibronic levels of an electronic state rather than just the narrow range of predissociating levels, as in one‐photon dissociation spectroscopy. Spectra of CH3I+ and CD3I+ over a range of 7000 cm−1, beginning at the origin of the A state, were recorded with progression bands up to n=21. These spectra allow for the first time an unambiguous assignment of the A←X transition of CH3I+ and CD3I+. The exact positions of the electronic origin as well as of the origin of several progressions and new values for several vibrational constants of methyl iodide cati...

The electronic spectrum of protonated adenine: Theory and experiment

In this work we present the results of a combined experimental and theoretical study concerned with the question how a proton changes the electronic spectrum and dynamics of adenine. In the experimental part, isolated adenine ions have been formed by electro-spray ionisation, stored, mass-selected and cooled in a Paul trap and dissociated by resonant photoexcitation with ns UV laser pulses. The S(0)-S1 spectrum of protonated adenine recorded by fragment ion detection lies in a similar energy range as the first pipi* transition of neutral 9H-adenine. It shows a flat onset with a broad substructure, indicating a large S(0)-S1 geometry shift and an ultra-short lifetime. In the theoretical part, relative energies of the ground and the excited states of the most important tautomers have been calculated by means of a combined density functional theory and multi-reference configuration interaction approach. Protonation at the nitrogen in position 1 of the neutral 9H-adenine tautomer yields the most stable protonated adenine species, 1H-9H-A+. The 3H-7H-A+ and the 3H-9H-A+ tautomers, formed by protonation of 7H- and 9H-adenine in 3-position, are higher in energy by 162 cm(-1) and 688 cm(-1), respectively. Other tautomers lie at considerably higher energies. Calculated vertical absorption spectra are reported for all investigated tautomers whereas geometry optimisations of excited states have been carried out only for the most interesting ones. The S1 state energies and geometries are found to depend on the protonation site. The theoretical data match best with the experimental onset of the spectrum for the 1H-9H-A+ tautomer although we cannot definitely exclude contributions to the experimental spectrum from the 3H-7H-A+ tautomer at higher energies. The vertical S(0)--> S1 excitation energy is similar to the one in neutral 9H-adenine. As for the neutral adenine, we find a conical intersection of the S1 of protonated adenine with the ground state in an out-of-plane coordinate but at lower energies and accessible without barrier.

Protonation effect on the electronic spectrum of tryptophan in the gas phase

In this work we investigate how the presence of a proton changes the electronic spectrum of tryptophan (Trp). For the S0–S1 transition of protonated Trp gas phase results can be compared to theoretical calculations. Trp ions have been formed by electro-spray ionization, stored in a cooled Paul trap, mass-selected and dissociated by resonant photoexcitation by ns UV laser pulses. The S0–S1 spectrum shows a distinct onset, a gap and a broad rise at higher energies. Interestingly the absorption of protonated Trp is in the same energy range as the S0–S1 transition of neutral Trp. For calculation of the relative tautomer and conformer energies for the ground and the excited states a combined density functional theory/multi-reference configuration interaction approach was used. The proton is put into different positions: The 1 (1-H+–Trp) and the 3 positions in indole and the N terminal (N–H+–Trp). Protonation at the N terminal forms the most stable tautomer. Attaching the proton to position 1 of the indole chromophore results in a very high energy. When the proton is approached in position 3 of the indole chromophore a chemical reaction takes place and a bridged isomer is formed (BI–H+). This isomer lies only 400 cm−1 above the N–H+–Trp form, but the activation barrier for the protonation is expected to be high. We calculated the excited states of the most stable neutral conformer, the neutral precursor conformer for the N–H+–Trp tautomer, the 1-H+–Trp and the N–H+–Trp tautomers and the BI–H+ isomer. The comparison of all S0–S1 transition energies shows that the S1 state, here the Lb state, is not sensitive to the protonation, the site of protonation or even the isomerization. According to theory, the experimentally observed protonated Trp is most probably the N–H+–Trp tautomer. For this tautomer the agreement of theoretical and experimental S0–S1 energies is excellent (3.5 nm). Its energetic position of the La state is blue-shifted and strongly structure-sensitive in all protonated species.

Microsolvation of similar-sized aromatic molecules: Photoelectron spectroscopy of bithiophene–, azulene–, and naphthalene–water anion clusters

We perform a comparison of electron affinities (EA) of the conjugated molecules bithiophene, azulene, naphthalene, and their water clusters. Bithiophene and azulene monomers have positive EAs of +49±5 meV and +790±8 meV, but naphthalene has a negative EA. Despite their different EAs and their different molecular orbital energies the three molecules show very similar microsolvation shifts per water unit. This is explained by similar sizes of the π orbitals in which the surplus electron is delocalized leading to a similar electrostatic water to charge interaction. This qualitative dependence of solvation energy on anion size agrees well with classical solvation concepts. A comparison of our binding energies with previous calculations for other systems shows that formation of a water subcluster can be assumed. For all three molecules the cluster EAs increase nearly linearly with the number of waters. Using a linear approach and a calibration for the error in the first solvation step we extrapolated the napht...

Size dependence of triplet and singlet states of α-oligothiophenes

Energetic positions of singlet and triplet states of small α-oligothiophenes with n=2, 3 and 4 monomer units were measured by photodetachment photoelectron spectroscopy (PD-PES), phosphorescence and time resolved pump–probe spectroscopy. The triplet energy of T1 and T2 were determined experimentally for the first time in the gas phase. The triplet manifold could be extended including the levels observed by triplet–triplet absorption. The transition energies of singlet and triplet states decrease with increasing size n of the oligothiophenes following the extended free electron model (FEMO). The ΔE(n) curves have different slopes for the singlet S1 and triplet T2 states with an intersection at n=2. The mutual positions of S1 and T2 states at equal energy for 2T explain the extremely high triplet quantum yield of 2T and its successive decrease for larger nT. The present measurements, combined with literature data, allow for the first time a complete picture to be drawn of all electronic states which are relevant for photodynamic processes in oligothiophenes with n=1–4.

High-Resolution Laser Mass-Spectrometry

Abstract Multi-photon absorption within the small focus of an intense pulsed laser beam leads to an efficient ionization of the absorbing molecules. The point-like ionization region and the peculiar multi-photon excitation mechanism make for an ideal ion source for a mass spectrometer. It is shown that a reflecting-field time-of-flight configuration in connection with laser multi-photon ionization yields a mass resolution of m/Δ m ∼ 4000.