Electron solvation dynamics in photoexcited anion clusters of I-(D2O)n=4-6 and I-(H2O)4-6 were probed by using femtosecond photoelectron spectroscopy (FPES). An ultrafast pump pulse excited the anion to the cluster analog of the charge-transfer-to-solvent state seen for I- in aqueous solution. Evolution of this state was monitored by time-resolved photoelectron spectroscopy using an ultrafast probe pulse. The excited n = 4 clusters showed simple population decay, but in the n = 5 and 6 clusters the solvent molecules rearranged to stabilize and localize the excess electron, showing characteristics associated with electron solvation dynamics in bulk water. Comparison of the FPES of I-(D2O)n with I-(H2O)n indicates more rapid solvation in the H2O clusters.

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.

The 2-phenylethyl-N,N-dimethylamine (PENNA) radical cation offers two functional groups for a positive charge to reside, the benzyl ring and the substituted amine group. Previously published HeI photoelectron spectra (PES) and our B3LYP data of the cation ground state agree that the amine site has the lowest ionization energy (IE). In this work we present evidence that by resonant laser multiphoton ionization the electron is removed from the phenyl site. B3LYP calculations of the neutral molecule predict that by far the most stable structure is the nonsymmetric unfolded anti conformer and that no other conformer should be significantly populated. This is confirmed by the S0−S1 resonant two-photon ionization (R2PI) spectrum in which one conformer is found to be predominant. The presence of the vibrational fingerprint of the phenyl chromophore and the absolute energetic position of the R2PI spectrum clearly show the local character of the first photoexcitation. Surprisingly, the R2PI mass spectra taken via S1 resonances show strong fragmentation. The parent-to-fragment-ion ratio is about 1:10 and mostly independent of laser intensity. The metastable character of the decay excludes a fragmentation caused by cation photoabsorption. The strong dissociation directly after ionization is explained by (i) a local ionization at the phenyl chromophore, (ii) a fast charge transfer (CT) to the lower-energetic amine site, and (iii) a subsequent metastable dissociation. A first detailed analysis of the ionization process indicates that intersite R2PI ionization between local states is a one-photon two-electron process which is expected to be improbable and that a failure of local ionization only happens in cases of mixed intermediate S1 states or mixed cation states. PENNA with two possible charge sites spaced by a −CH2−CH2− bridge is an ideal model system to study the dynamics of a downhill charge transfer after local ionization to the first excited cation state as presented in a forthcoming paper.

The cation of 2-phenylethyl-N,N-dimethylamine (PENNA) offers two local sites for the charge: the amine group and 0.7 eV higher in energy the phenyl chromophore. In this paper, we investigate the dynamics of the charge transfer (CT) from the phenyl to the amine site. We present a femtosecond resonant two-color photoionization spectrum which shows that the femtosecond pump laser pulse is resonant in the phenyl chromophore. As shown previously with resonant wavelengths the aromatic phenyl chromophore can be then selectively ionized. Because the state "charge in the phenyl chromophore" is the first excited state in the PENNA cation, it can relax to the lower-energetic state "charge in the amine site". To follow this CT dynamics, femtosecond probe photoabsorption of green light (vis) is used. The vis light is absorbed by the charged phenyl chromophore, but not by the neutral phenyl and the neutral or cationic amine group. Thus, the absorption of vis photons of the probe laser pulse is switched off by the CT process. For detection of the resonant absorption of two or more vis photons in the cation the intensity of a fragmentation channel is monitored which opens only at high internal energy. The CT dynamics in PENNA cations has a time constant of 80 +/- 28 fs and is therefore not a purely electronic process. Because of its structural similarity to phenylalanine, PENNA is a model system for a downhill charge transfer in peptide cations.