Tuning and Tracking of Coherent Shear Waves in Molecular Films.

We have determined the time-dependent displacement fields in molecular sub-micrometer thin films as response to femtosecond and picosecond laser pulse heating by time-resolved X-ray diffraction. This method allows a direct absolute determination of the molecular displacements induced by electron-phonon interactions, which are crucial for, for example, charge transport in organic electronic devices. We demonstrate that two different modes of coherent shear motion can be photoexcited in a thin film of organic molecules by careful tuning of the laser penetration depth relative to the thickness of the film. The measured response of the organic film to impulse heating is explained by a thermoelastic model and reveals the spatially resolved displacement in the film. Thereby, information about the profile of the energy deposition in the film as well as about the mechanical interaction with the substrate material is obtained.

Compact 20 W femtosecond laser system based on fiber laser seeder, Yb:YAG rod amplifier and chirped volume Bragg grating compressor.

In this work we presented a compact femtosecond laser system based on Yb doped fiber seed laser and efficient Yb:YAG crystal rod power amplifier. Matched pair of chirped fiber Bragg grating stretcher and chirped volume Bragg grating compressor were used to obtain high fidelity - Strehl ratio 76%, pulses of 764 fs duration, 104 µJ energy at 200 kHz repetition rate at the output of the laser system.

Compact fiber CPA system based on a CFBG stretcher and CVBG compressor with matched dispersion profile.

In this work, a compact fiber chirped pulse amplification system exploiting a tandem of a chirped fiber Bragg grating stretcher and a chirped volume Bragg grating compressor with matched chromatic dispersion is presented. Chirped pulses of 230 ps duration were amplified in a Yb-doped fiber amplifier and re-compressed to 208 fs duration with good fidelity. The compressed pulse duration was fine-tuned by temperature gradient along the fiber Bragg grating stretcher.

Combined Yb/Nd driver for optical parametric chirped pulse amplifiers.

We report on the developed front-end/pump system for optical parametric chirped pulse amplifiers. The system is based on a dual output fiber oscillator/power amplifier which seeds and assures all-optical synchronization of femtosecond Yb and picosecond Nd laser amplifiers operating at a central wavelength of 1030 nm and 1064 nm, respectively. At the central wavelength of 1030 nm, the fiber oscillator generates partially stretched 4 ps pulses with the spectrum supporting a <120 fs pulse duration and pulse energy of 0.45 nJ. The energy of generated 1064 nm pulses is 0.15 nJ, which is sufficient for the efficient seeding of high-contrast Nd:YVO chirped pulse regenerative amplifier/post amplifier systems generating 9 mJ pulses compressible to 16 ps duration. The power amplification stages, based on Nd:YAG crystals, provide 62 mJ pulses compressible to 20 ps pulse duration at a repetition rate of 1 kHz. Further energy scaling currently is prevented by limited dimensions of the diffraction gratings, which, because of the fast progress in MLD grating manufacturing technologies is only a temporary obstacle.

Probing the lifetimes of internally excited amyl nitrite cations.

The photoelectron spectrum shows that multiphoton ionization of amyl nitrite, C(5)H(11)ONO, using ultrafast laser pulses deposits up to 3.7 eV of energy into internal degrees of freedom. As a result, the molecules fragment to produce various daughter ions of masses 87, 71, 60, 57, 41, 30, 29, and 27. Absorption of an additional photon with 3 eV of energy by the ions yields transients with picosecond decay times, revealing the time scale of the decomposition dynamics of the initially prepared parent ion. Each mass peak has a distinct time constant, in the range of 1.2 to 7.9 ps, emphasizing the dependence of the fragmentation mechanism on the ion internal energy.

Excited-state ions in femtosecond time-resolved mass spectrometry: an investigation of highly excited chloroamines.

We have investigated the processes induced by femtosecond laser pulses in chloroamines, with a focus on the generation and observation of a highly reactive radical and on the involvement and general importance of excited-state ions in time-resolved mass spectrometry investigations of gaseous molecules. We have found that 280 nm femtosecond pulses lead to an ultrafast breakage of the N-Cl bond on the repulsive S1 surface, and that resulting radical is long-lived. When exposing the molecule to 420 nm photons a multiphoton ionization takes place to generate ions; these ions can then be excited with a 280 nm photon. The evidence is unambiguous since we observe a distinct temporal evolution of the ion current with no photoelectrons to match. We suggest that the involvement of excited-state ions is a general phenomenon in time-resolved photoionization studies.

Atmospheric photochemical loss of H and H2 from formaldehyde: the relevance of ultrafast processes.

We have performed ab initio calculations to examine the potential energy along the normal modes of ground-state HCHO and along the reaction coordinates for loss of H2 and atomic hydrogen, respectively. This exploration showed that there are no specific features that will lead to reaction on the excited-state surfaces for excitations that are relevant to the troposphere and stratosphere. The calculations did however lead to the localization of a conical intersection point through which a specific loss of H2 could take place. However, the conical intersection lies at 5.4 eV relative to the ground state molecule at equilibrium and is thus inaccessible via single photon excitation at tropospheric and stratospheric wavelengths. In addition to the ab initio investigation we have carried out a femtosecond pump-probe experiment using a 266/400 nm excitation. The results show that the timescale for the internal conversion from the initially prepared high-lying Rydberg states is on the order of a picosecond. This process populates the n --> pi* first excited singlet state which then survives for a substantially longer time before it is depopulated to form hot ground state or triplet-excited molecules that can then decompose.