The photodissociation dynamics of HNO3 in the electronic S3 (2 1 A ′ ) state leading to the fragments OH and NO2 was investigated in real time. HNO3 was prepared either in a fluorescence cell at room temperature (LIF probing of OH) or rotationally cold in a molecular beam (probing of NO2 by three-photon ionization). A 2 1 A ′ lifetime of 60–80 fs could be obtained from the experimental results, indicating essentially barrierless dissociation. In addition, secondary dissociation of internally excited nascent fragments NO2  * leading to products NO(X 2 Π) and O(3 P) with a characteristic dissociation time of 2.3 ps was observed. This time is surprisingly long when compared with dissociation lifetimes of NO2 from the literature, obtained after direct photoexcitation. The discrepancy is explained by differences in the preparation conditions of the dissociative state of NO2.

Using the efficient nonlinear conversion scheme which was recently developed in our group [M. Beutler, M. Ghotbi, F. Noack, and I. V. Hertel, Opt. Lett. 134, 1491 (2010); M. Ghotbi, M. Beutler, and F. Noack, ibid 35, 3492 (2010)] to provide intense sub-50 fs vacuum ultraviolet laser pulses we have performed the first real time study of ultrafast, photo-induced dynamics in the electronically excited Ã-state of water clusters (H(2)O)(n) and (D(2)O)(n) , n=2-10. Three relevant time scales, 1.8-2.5, 10-30, and 50-150 fs, can be distinguished which-guided by the available theoretical results-are attributed to H (D)-ejection, OH (OD) dissociation, and a nonadiabatic transition through a conical intersection, respectively. While a direct quantitative comparison is only very preliminary, the present results provide a crucial test for future modeling of excited state dynamics in water clusters, and should help to unravel some of the many still unresolved puzzles about water.

We present femtosecond pump-probe mass and photoelectron spectra for adenine (A) and microhydrated A(m)(H2O)(n) clusters. Three distinct relaxation processes of photoexcited electronic states were distinguished: in unhydrated A, relaxation of the optically bright pi pi* state occurred via the dark n pi* state with respective lifetimes of <0.1 and 1.3 ps. In microhydrated clusters A(H2O)(n), relaxation via the n pi* state is quenched by a faster relaxation process, probably involving pi sigma* states. For the predominantly hydrogen-bonded adenine dimer (A(2)), excited state relaxation is dominated by monomer-like processes. When the adenine dimer is clustered with several water molecules, we observe a nanosecond lifetime from excimer states in pi-stacked clusters. From the electron spectra we estimate adiabatic ionization potentials of 8.32 eV (A), 8.27 eV (A(H2O)(1)), 8.19 eV (A(H2O)(2)), 8.10 eV (A(H2O)(3)), 8.18 eV (A(2)), and 8.0 eV (A(2)(H2O)(3-5)).

The photoinduced dynamics in base pairs of adenine and thymine were analyzed by femtosecond pump-probe spectroscopy. On the short-time scale up to a few picoseconds, the characteristic time constants for the dimers are quite similar to the corresponding values of the monomers. This leads to the conclusion that ultrafast intramolecular relaxation proceeds via ππ∗ and nπ∗ states of one component within the dimer. On the long-time scale, we obtained a novel time constant of roughly 40ps for the thymine dimer and the adenine–thymine base pair. This time constant was never observed in the monomers and is tentatively assigned to an intermolecular relaxation process, possibly via a hydrogen transfer state.

Ultrafast dissociation dynamics in OClO molecules is studied, induced by femtosecond laser pulses in the wavelength region from 386 to 409 nm, i.e., within the wide absorption band to the (approximately)A (2)A(2) electronic state. The decay of the initially excited state due to nonadiabatic coupling to the close lying (2)A(1) and (2)B(2) electronic states proceeds with a time constant increasing from 4.6 ps at 386 nm to 30 ps at 408.5 nm. Dissociation of the OClO molecule occurs after internal conversion within about 250 fs. In addition, a minor channel of direct excitation of the (2)A(1) electronic state has been identified, the lifetime of which increases from a few 100 fs at 386 nm to 2.2 ps at 408.5 nm. Simultaneous excitation of two neighboring vibrational bands in the (approximately)A (2)A(2) state leads to a coherent oscillation of the parent ion signal with the frequency difference of both modes.

During the past decade the understanding of photo-induced ultrafast dynamics in molecular systems has improved at an unforeseen speed and a wealth of detailed insight into the fundamental processes has been obtained.This review summarizes our present knowledge on ultrafast dynamics in isolated molecules and molecular clusters evolving after excitation with femtosecond pulses as studied by pump-probe analysis in real time. Experimental tools and methods as well as theoretical models are described which have been developed to glean information on primary, ultrafast processes in photophysics, photochemistry and photobiology. The relevant processes are explained by way of example - from wave packet dynamics in systems with a few atoms all the way to internal conversion via conical intersections in bio-chromophores. A systematic overview on characteristic systems follows, starting with diatomic and including larger organic molecules as well as various types of molecular clusters, such as micro-solvated chromophore molecules. For conciseness the focus is on molecular systems which remain unperturbed by the laser pulses - apart from the excitation and detection processes as such. Thus, only some aspects of controlling and manipulating molecular reactions by shaped and/or very intense laser pulses are discussed briefly for particularly instructive examples, illustrating the perspectives of this prospering field.The material presented in this review comprises some prototypical examples from earlier pioneering work but emphasizes studies from recent years and covers the most important and latest developments until January 2006.

Fast excited-state relaxation in H-bonded aminopyridine clusters occurs via hydrogen transfer in the excited state. We used femtosecond pump-probe spectroscopy to characterize the excited-state reaction coordinate. Considerable isotope effects for partially deuterated clusters indicate that H-transfer is the rate-limiting step and validate ab initio calculations in the literature. A nonmonotonous dependence on the excitation energy, however, disagrees with the picture of a simple barrier along the reaction coordinate. An aminopyridine dimer serves as a model for Watson-Crick base pairs, where similar reactions have been predicted by theory.

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.

Two-photon excitation with femtosecond laser pulses in the spectral range 240-250 nm was used to prepare vapor phase H(2)O and D(2)O in the C (1)B(1) and D (1)A(1) states. Both states are predissociated via the B (1)A(1) state, forming excited OH/OD(A (2)Sigma(+)) as well as ground state OH/OD(X (2)Pi). We used ultrashort infrared probe pulses (1.65-2.42 microm) to control the ratio between these excited and ground state fragments originating from the dissociation process. Time resolved detection of the OH/OD(A (2)Sigma(+)) --> OH/OD(X (2)Pi) fluorescence allows us to monitor the dynamics of the predissociation. For the heterogeneous predissociation out of the C(1)B(1) state life times of (0.5 +/- 0.1) ps and (1.2 +/- 0.1) ps were found for H(2)O and D(2)O, respectively. The purely homogeneous character of the predissociation out of the D (1)A(1) state was monitored.

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.

We present experimental and theoretical evidence for an excited-state deactivation mechanism specific to hydrogen-bonded aromatic dimers, which may account, in part, for the photostability of the Watson-Crick base pairs in DNA. Femtosecond time-resolved mass spectroscopy of 2-aminopyridine clusters reveals an excited-state lifetime of 65 ± 10 picoseconds for the near-planar hydrogen-bonded dimer, which is significantly shorter than the lifetime of either the monomer or the 3- and 4-membered nonplanar clusters. Ab initio calculations of reaction pathways and potential-energy profiles identify the mechanism of the enhanced excited-state decay of the dimer: Conical intersections connect the locally excited 1ππ* state and the electronic ground state with a 1ππ* charge-transfer state that is strongly stabilized by the transfer of a proton.

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)

Two-photon excitation with femtosecond laser pulses in the spectral range 240–250 nm was used to prepare vapor phase H2O and D2O in the C̃ 1B1 and D̃ 1A1 states. Both states are predissociated via the B̃ 1A1 state, forming excited OH/OD(A 2Σ+) as well as ground state OH/OD(X 2Π). We used ultrashort infrared probe pulses (1.65–2.42 μm) to control the ratio between these excited and ground state fragments originating from the dissociation process. Time resolved detection of the OH/OD(A 2Σ+)→OH/OD(X 2Π) fluorescence allows us to monitor the dynamics of the predissociation. For the heterogeneous predissociation out of the C̃ 1B1 state life times of (0.5±0.1) ps and (1.2±0.1) ps were found for H2O and D2O, respectively. The purely homogeneous character of the predissociation out of the D̃ 1A1 state was monitored.