Ab initio calculations and time-resolved photoionization spectroscopy were carried out to characterize the role of the lowest two pi sigma(*) excited states for the photoinduced processes in the adenine monomer, adenine dimer, and adenine-water clusters. The calculations show-with respect to the monomer-a stabilization of 0.11-0.14 eV for the pi sigma(*) states in different isomers of adenine dimer and an even bigger stabilization of 0.14-0.36 eV for isomers of adenine-(H2O)(1) and adenine-(H2O)(3). Hence, the stabilized pi sigma(*) states should play an important role in the excited-state relaxation of partially or fully solvated adenine. This conclusion is supported by experimental results: In the adenine monomer, strong n pi(*) state signals are observed. Those signals are reduced in adenine dimer and vanish in water clusters due to the competing relaxation via the pi sigma(*) states.

The excited-state dynamics of adenine and thymine dimers and the adenine-thymine base pair were investigated by femtosecond pump-probe ionization spectroscopy with excitation wavelengths of 250-272 nm. The base pairs showed a characteristic ultrafast decay of the initially excited pi pi* state to an n pi* state (lifetime tau(pi pi*) approximately 100 fs) followed by a slower decay of the latter with tau(n pi*) approximately 0.9 ps for (adenine)2, tau(n pi*) = 6-9 ps for (thymine)2, and tau(n pi*) approximately 2.4 ps for the adenine-thymine base pair. In the adenine dimer, a competing decay of the pi pi* state via the pi sigma* state greatly suppressed the n pi* state signals. Similarities of the excited-state decay parameters in the isolated bases and the base pairs suggest an intramonomer relaxation mechanism in the base pairs.

Intense (approximate to80 GW cm(-2)) ultrashort (approximate to100 fs) infrared (IR) laser pulses may be employed for excitation of a high frequency (approximate to3500 cm(-1)) local mode vibration in a molecule. Subsequently, an intense (16-256 GW cm(-2)), ultrashort visible (VIS) laser pulse yields electronic excitation with near adiabatic transfer of the vibrational energy, which has been accumulated by the IR pulse. The net result of these sequential IR+VIS laser pulses may be the breaking of a strong molecular bond close to the pre-excited one. In contrast, exclusive excitation by just a visible laser pulse breaks a competing weak bond. The effects of IR+VIS laser pulse control may be considered as an extension of vibrationally mediated chemistry, from ns pulses or continuous wave (cw) excitations to sub-ps laser pulses, and from direct vibrational pre-excitation of the bond to be broken to a neighboring bond, thus exploiting intramolecular vibrational redistribution (IVR) from the pre-excited local mode to the bond to be broken in the electronic excited state. The mechanism is demonstrated by quantum simulations for the model system BaFCH3, where BaF-, FC- and CH3 play the roles of the weak and strong bonds to be broken, and the vibrationally pre-excited CH3 stretch. The theoretical predictions are confirmed experimentally. Various extensions of the control by IR+VIS laser pulses include the control of the branching ratio of weak versus strong bond breaking, as well as isotopomer selectivity depending on the vibrational pre-excitations.

To understand the fundamental photochemical processes in biologically relevant molecular systems, prototype molecules, such as,phenol or indole - the chromophores of the amino acids tyrosine respective trypthophan - embedded in clusters of ammonia or water molecules are an important object of research. Numerous studies have been performed concerning the dynamics of photo-induced processes in phenol-ammonia or phenol-water clusters. As a main result a hydrogen transfer reaction is clearly indicated in phenol(NH 3 ) n clusters, whereas for phenol(H 2 O) n complexes no signature for such a reaction has been found. According to a general theoretical model, a similar behavior is expected for the indole molecule surrounded by ammonia or water clusters. As the primary step an internal conversion from the initially excited ΠΠ * state to a dark Πσ * state is predicted which may be followed by the H-transfer process on the Πσ * potential energy surface. Ab initio, calculations of the indole-water potential energy surfaces are under way now, to elucidate this process in the hetero-cluster and to understand the difference with respect to the indole-ammonia complex.

The ultrafast dynamics of the electronically excited ethylene molecule has been studied in pump–probe experiments with the pump wavelength of 198 nm and the probe wavelength of 790 nm. The lifetime measured for the initially excited ππ∗ state is about 10 fs for C2H4 as well as for C2D4. No secondarily populated states of ethylene can be directly observed despite the high intensity of the probe pulses used. However, the observed time dependence of the fragment ions C2H3+ (C2D3+) and C2H2+ (C2D2+) suggests that they are formed by dissociative ionization via a secondary electronic state of ethylene. This result was confirmed by the time-resolved photoelectron spectra.

The dynamics of the H atom transfer reaction in indole-(NH3)(n), clusters has been studied in femtosecond pump-probe experiments as a function of the pump wavelength (250-282 nm), i.e. for different vibrational excess energies in the excited electronic states. Typical changes of the corresponding time constants in the sub-ps and ps time region were observed for the small clusters sizes (n = 1-3) and compared with those for the larger clusters (n = 4-6). For the heterodimer indole.(NH3)1 the energetic threshold of the reaction has been found at excitation energies of about 4.4 eV. In the sub-ps region we observed no isotope effect for all pump wavelengths used while on the ps time scale the rates for the deuterated clusters d-indole-(ND3)(n) are smaller by a factor up to 4 in comparison to the nondeuterated complexes.

Applying femtosecond pump–probe spectroscopy at different wavelengths we demonstrate that for the indole molecule an efficient coupling does not exist between the initially excited ππ* state and the adjacent higher lying πσ* state. This result which is in contrast to corresponding theoretical studies is represented by a stable population of the ππ* state. The analysis of the time-dependent photoelectron spectra reveals further information about the energetics of the pump and probe processes.

In recent years, the understanding of the photochemical processes in biologically relevant molecules in the gas phase has gained broad interest. In particular, aromatic biomolecules with an OH (hydroxy) or an NH (azine) group such as phenol, indole, pyrrole, or adenine were studied in detail. These molecules are important building blocks of naturally abundant compounds, such as proteins, porphyrins, or nucleic acids. As ab initio electronic-structure calculations and spectroscopic investigations suggested (see, e.g. , ref. [1]), the key role of the photophysics in such molecules is played by an excited ps singlet state, which is characterized by a repulsive potential curve with respect to the O H or N H stretching coordinate. Generally, the dark ps state is not excited directly but is populated through neighboring pp states. The coupling between the initially excited pp state and the ps state, as well as the shape of the conical intersection of the ps state with the electronic ground state upon elongation of the X H bond (X=O,N), determines the dynamics and energetics of the photochemical processes in these molecules. The existence and properties of the repulsive ps state were demonstrated recently for different biomolecular systems, for example, for indole—the chromophore of the amino acid tryptophan—and its derivatives, on the basis of spectroscopic measurements. The H-atom transfer reactions in phenol(NH3)n [3,4] or indole(NH3)n clusters, [5, 6] which were studied experimentally and theoretically, have also been interpreted on the basis of the ps state. The transfer of the H atom (from the OH group in phenol or the NH group in indole) to the NH3 molecules of the cluster proceeds on the ps potential surface. The detection of neutral (NH3)n 1NH4 radicals proves that a Htransfer reaction, followed by fragmentation, occurs. However, until very recently, it was not clear whether the repulsive state could be confirmed only indirectly (for example, by the detection of the reaction products) or whether the Hatom elimination could also be observed directly. The experiments reported by the group of F. Temps demonstrated, for the first time, the H-atom detachment from pyrrole in the excited ps state using photofragment velocity map imaging. Pyrrole is the photoreactive center of indole and represents one of the simplest heterocyclic aromatic ring molecules with a p-electron system. The lowest electronic state (at around 240 nm) has been assigned to the 1A2(3s) Rydberg state, [8] which has, according to the calculations performed by Sobolewski and Domcke, a ps character due to a Rydberg-to-valence transformation. The antibonding s orbital is located on the N H bond, which leads to a repulsive potential with respect to the N H coordinate. As the transition from the ground state to the 1A2(ps ) state is electric-dipole-forbidden, its excitation around 240 nm is only possible by vibronic coupling with the nearby pp states (2A1 or 1 B2) induced by out-of-plane vibrations. This behavior represents a Herzberg–Teller coupling similar to that observed, for example, in the ffiB2u !X1A1g system of benzene. At larger N H distances the ps state of pyrrole crosses the electronic ground state by a symmetry-allowed conical intersection. This leads to an ultrafast internal conversion to the ground state and should be the reason for the missing fluorescence of pyrrole in this wavelength region. In the one-color experiments of Wei et al. at 243.1 nm the photodissociation of the pyrrole molecule is initiated by a onephoton excitation, whereas the produced H atoms are detected by (2+1) resonance-enhanced multiphoton ionization (REMPI) spectroscopy. The authors found a dominant contribution ( 76%) of fast H atoms with a narrow kinetic-energy distribution, which was assigned to a rapid, direct N H bond dissociation, in agreement with the repulsive character of the excited ps state. This observation was also confirmed by the corresponding anisotropy parameter indicating the vibronic coupling between pp and ps. The second contribution is characterized by H atoms that exhibit a broad kinetic-energy distribution with a maximum at smaller velocities. This contribution is attributed to the fragmentation of the molecule in the electronic ground state after an internal conversion. Replacing the H atom of the NH group in pyrrole by a methyl group leads to the disappearance of the fast component; these results confirm that the fast component is, in fact, due to the direct dissociation of pyrrole in the ps state along the N H bond. Quite similar conclusions were drawn in a previous study performed using photofragment translational spectroscopy, where three dissociation channels were observed at 248 nm. Each channel involved a H-atom elimination. The dissociation from an excited electronic state ( 47%) and the N H bond cleavage after an internal conversion to the ground state ( 42%) are the main contributions. The third channel ( 11%) was attributed to a C H bond cleavage in the excited electronic state. In the present work we report on the results of a two-color pump–probe experiment with femtosecond laser pulses. The pyrrole molecules were excited at 250 nm and the products were probed by (2+1) REMPI at 243.1 nm. We were indeed able to detect H atoms using a femtosecond laser, despite the inefficient excitation of a narrow atomic resonance with the spectrally broad laser pulse. Thus, we were able to observe an ultrafast H-atom elimination on the excited ps state of a biomolecule directly for the first time. The analysis of the time-dependent H signal reveals two time constants (t1 0.1 ps, t2 [a] H. Lippert, Dr. H.-H. Ritze, Prof. Dr. I. V. Hertel, Prof. Dr. W. Radloff Max Born Institute, Max-Born-Strase 2A 12489 Berlin (Germany) Fax: (+49)30-6392-1259 E-mail : radloff@mbi-berlin.de [b] Prof. Dr. I. V. Hertel Freie Universit6t Berlin, Physics Department Arnimallee 14, 14195 Berlin (Germany)

Ab initio calculations on the heterodimer C8H6NH...NH3 are carried out for its ground, the excited pisigma*, and the ground cationic electronic states, enabling the description of hydrogen or proton transfer, respectively. Two-dimensional quantum-dynamical computations on the pisigma* potential surface help one to understand the mechanism and the time scale of the hydrogen transfer. Subsequent decay processes are discussed depending on the vibrational excitation of the ammonium constituent. Finally, the theoretical results obtained are used for the interpretation of the time-dependent signals observed in femtosecond pump-probe experiments.

Applying femtosecond laser pulses at the wavelength of 250 nm the photoinduced processes in indole(H2O)n and d-indole(D2O)n clusters have been studied. An ultrafast (∼50 fs) decay of the initially excited ππ∗ state is tentatively attributed to the internal conversion to the dark πσ∗ state. Subsequent processes are characterized for n=1,2 by a single-exponential signal decay on the ps time scale, while for larger clusters (n⩾3) no significant time dependence is observed. The strong differences with respect to the photophysics of indole(NH3)n clusters are discussed.

The photoinduced H-atom-transfer reaction in indole(NH3)(n) clusters has been analyzed by femtosecond time-resolved photoelectron-photoion coincidence spectroscopy. The different contributions to the measured time-dependent ion and electron signals resulting from ionization by one and two probe photons can be discriminated and analyzed separately. In particular, the distinctively different dynamical behavior observed for clusters with small (n = 1-3) and larger (n greater than or equal to 4) numbers of ammonia molecules is elucidated. For the small clusters an ultrafast process with a time constant of about 150 fs is identified and attributed to internal conversion from the initially excited pipi* state to the pisigma* state. In contrast, for the larger clusters (n greater than or equal to 4) such an initial ultrafast process is not observable probably for Franck-Condon reasons, while a structural rearrangement mechanism after the H transfer on a time scale of 10 ps is clearly recognized.

The femtochemistry of the indole molecule which represents the chromophore of the amino acid trypophan is of particular interest because of its biological relevance. Recent studies (e.g. [1]) of indole in aqueous solution have shown that the photoexcitation leads to a fast charge separation process characterized by the formation of a solvated electron. The microscopic mechanism of the solvation process can be elucidated by gas phase studies of indole-solvent clusters with polar molecules like water or ammonia. Recent ab initio investigations of indolewater clusters have stressed the crucial role of the nonadiabatic coupling between the optically excited ππ* and a low-lying dark πσ* state for the electron transfer process [2]. In first pump-probe experiments on indole (NH3)n clusters with ns laser pulses a H atom transfer reaction has been stated [3].

The intracluster reaction dynamics initiated by femtosecond laser pulses at 263 nm has been studied in nondeuterated and deuterated indole–ammonia clusters. No isotope effect is observed on the sub-ps timescale. On the long-timescale (of few 100 ps), however, the structural reorientation dynamics of the parent as well as the product clusters is prolongated (up to about 3 times) for the deuterated complexes. This effect is interpreted as due to the difference of the torsion modes and due to low energetic barriers between different isomers of both isotopic complexes.

The hydrogen atom transfer reaction in indole (NH3)(n) clusters excited at 263 nm to the S-I(pipi*) state is studied in pump-probe experiments with femtosecond laser pulses. For small clusters the reaction is characterized by two successive processes on distinct time scales: the time constant of the primary process is in the sub-ps region, whereas the secondary decay time growing with the cluster size reflects a relaxation process within 25 to 150 ps. A preliminary model of the H-transfer reaction is discussed.

The H atom transfer reaction in electronically excited indole(NH 3 ) n clusters is studied in pump-probe experiments with femtosecond laser pulses. By applying different probe photon energies we are able to detect the dissociation products (NH 3 ) n - 1 NH 4 for n = 1-6. Furthermore we show that the analysis of the corresponding ion signals is not distorted by contributions from larger cluster ions due to evaporation of NH 3 molecules. The formation times of the products are ca. 140ps for n = 2-4 and about 80ps for n = 5, 6.