Excimer states in microhydrated adenine clusters.

We present femtosecond pump-probe mass and photoelectron spectra for adenine (A) and microhydrated A(m)(H(2)O)(n) clusters. Three distinct relaxation processes of photoexcited electronic states were distinguished: in unhydrated A, relaxation of the optically bright pipi* state occurred via the dark npi* state with respective lifetimes of <0.1 and 1.3 ps. In microhydrated clusters A(H(2)O)(n), relaxation via the npi* state is quenched by a faster relaxation process, probably involving pisigma* 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(H(2)O)(1)), 8.19 eV (A(H(2)O)(2)), 8.10 eV (A(H(2)O)(3)), 8.18 eV (A(2)), and 8.0 eV (A(2)(H(2)O)(3-5)).

Generation of sub-30 fs ultraviolet pulses by Raman induced phase modulation in nitrogen.

We demonstrate compression of ultrashort light pulses in the ultraviolet (UV) by impulsively excited molecular wave-packets in nitrogen filled in a 25 cm long hollow waveguide of 128 microm diameter. After compression with CaF2 prisms the pulse duration was determined by XFROG to be 23 fs with a time-bandwidth product of 0.50. The advantages of our technique are high efficiency and the possibility to use it also for pulses at wavelength shorter than 200 nm. The experimental observations are explained by a theoretical model.

Fragmentation and ionization dynamics of C60 in elliptically polarized femtosecond laser fields.

Ionization and fragmentation of C60 fullerenes is studied in elliptically polarized, intense fs laser fields at 797 nm [I=(0.5-4.3)x10;{14} W cm;{-2}] and contrasted with Xe+, utilizing time-of-flight mass spectrometry. Very pronounced changes of parent and fragment ion yield as a function of ellipticity are observed. At lower intensities reduction of the ion yield for circular polarization establishes a coherent two-photon process connected with the key role of the LUMO+1(t_{1g}) "doorway state" and multielectron dynamics. Comparison with the behavior at 399 nm corroborates this finding. At high intensities enhanced fragmentation is observed which is tentatively attributed to returning loops of electron trajectories by the combined action of the C60+ field and the circular laser field.

Ultrafast processes in OClO molecules excited by femtosecond laser pulses at 386-409 nm.

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.

Ultrafast deactivation processes in aminopyridine clusters: excitation energy dependence and isotope effects.

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.

Relevance of pi sigma* states in the photoinduced processes of adenine, adenine dimer, and adenine-water complexes.

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.

Ultrafast predissociation dynamics of water molecules excited to the electronic C and D states.

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.

Theoretical study of the hydrogen atom transfer in the heterodimer indole-ammonia and comparison with experimental results.

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.