Karl-Ludwig Kompa

Molecular dissociation by mid-IR femtosecond pulses

Abstract By focusing a MIR femtosecond laser in a cell containing gas-phase metal carbonyls, the resonant infrared multiphoton dissociation of molecules was observed. Cr(CO)6,Mo(CO)6,W(CO)6, and Fe(CO)5 could easily be dissociated, which requires an excitation to at least v=7 or 8 of the CO stretch vibration. After irradiation with ∼150 fs pulses at 5 μm the metal carbonyl practically disappears in favor of free CO, as detected by the IR spectrum. By comparing the power dependence of the total conversion with a model, we can infer that only few vibrational degrees of freedom are involved in the excitation process.

Getting ahead of IVR: A demonstration of mid-infrared induced molecular dissociation on a sub-statistical time scale

Gaseous diazomethane (CH2N2) has been irradiated with femtosecond laser pulses tuned to the CNN asymmetric stretch at 2100 cm−1 in the mid-infrared. Laser-induced fluorescence detection of 1CH2 [537 nm, b1B1(0u200916u20090)←a1A1(0u200a0u200a0)] confirms infrared (IR) multiphoton-induced scission of the C–N bond on two distinct time scales, 480±70 fs and 36±8 ps. The longer time scale is consistent with a statistical dissociation process; the shorter one is indicative of directed dissociation, which occurs more rapidly than statistical intramolecular vibrational energy redistribution because of direct coupling of the vibrational energy from the IR-excitation mode into the reaction coordinate. Anisotropy measurements demonstrate that the CH2 group bends significantly out of the molecular plane prior to the dissociation.

Programmable amplitude- and phase-modulated femtosecond laser pulses in the mid-infrared.

We present a scheme to produce programmable phase- and amplitude-modulated femtosecond laser pulses in the mid-infrared regime of 3-10mum by difference frequency mixing. The 80-fs signal output of an optical parametric amplifier is shaped with a liquid-crystal mask and mixed in an AgGaS(2) crystal with a temporally stretched idler pulse. Without changing the mechanical alignment, we produce programmable amplitude modulations and chirped pulses at lambda=3mum with energy as high has thas 1muJ . This scheme, further, allows the generation of controllable pulse sequences. The results are in good agreement with theoretical simulations.

Impact dynamics of molecular clusters on surfaces: Fragmentation patterns and anisotropic effects

The fragmentation dynamics of (H2O)1032 clusters colliding with a repulsive surface at incident velocities of 1753 m/s and 2909 m/s, corresponding to kinetic energies of 0.5 and 1.5 times the cluster binding energy, has been examined in a classical molecular dynamics simulations study. The results show a large anisotropy in the energy redistribution inside the cluster upon impact, which leads to asymmetric fragmentation, starting in the leading part of the cluster. The low-mass region of the fragment size distribution can be described by a power law with an exponent close to −1.6, and the range of this region increases with increasing incident velocity. The formed fragments have rather uniform internal temperatures close to the standard boiling point of water, but the translational energy of the monomers formed upon collision is much larger, pointing at the asymmetric energy distribution inside the cluster. The angular distributions of fragment mass and fragment kinetic energy peak at grazing exit angles....

Design of optimal infrared femtosecond laser pulses for the overtone excitation in acetylene

Abstract Optimal laser pulses in the femtosecond region are designed for a state selective excitation of infrared overtone vibrations in acetylene, for which an experiment is actually set up in our institute. To attack the preparation problem the molecules Hamiltonian is reduced to the IR-active modes, which are the twice degenerated cis -bending and the asymmetric CH-stretching mode. For distinct populations we compute the corresponding two-dimensional potential energy surface, dipole moments and eigenfunctions and then use optimal control theory strategies to calculate applicable laser pulses. These optimized pulses are analysed in the time and frequency domain. Ladder climbing is found to be the preferred mechanism for very efficient and highly selective overtone excitation. This mechanism is discussed in detail and a model for the observed transition frequency shift is presented.

Matrix-Free Formation of Gas-Phase Biomolecular Ions by Soft Cluster-Induced Desorption†

All in a ball: Neutral molecular clusters consisting of a few thousand molecules can be seen as tiny snow balls; if they are thrown fast enough onto a surface, they are able to pick up biomolecules such as insulin from that surface. Since they break down and evaporate during and after the collision, bare biomolecular ions are available for mass spectrometry after such an energetic throw.

Molecular logic by optical spectroscopy with output transfer by charge migration along a peptide

Computing on the (sub) nanoscale is discussed and illustrated by a specific example of charge transfer along a molecular frame. The general research program is to implement an entire finite state logic machine on a molecule. It is proposed to do so in stages. The first stage is to implement Boolean logic circuits on a single molecule. This has already been achieved up to the level of a full adder. Our current work seeks to implement even more elaborate circuits, to go beyond Boolean logic gates and to go beyond combinational logic circuits to the level of sequential machines. In the longer run it will be necessary to concatenate logical units so that a molecule-like assembly is needed. Here we show by a concrete experimental example that intramolecular concatenation is possible: The molecular backbone is used to move information between two ends of a short peptide. The experiment is a gas phase laser excitation of a molecule with an aromatic chromophore at one end. The absorption by the chromophore localizes the initial excitation. Different outcomes are possible depending on additional inputs. Specifically, charge can be made to migrate to the other end of the molecule.

The sub-ps lifetime of the 2A state of Z-hexatriene as deduced from its transient absorption spectrum

Abstract Exciting Z-hexatriene in ethanol at its UV absorption band (1B 2 ) gives rise to two types of transient absorption. Below 325 nm the time constants depend on the wavelength; they can be assigned to ground-state processes such as cooling. The (hot) ground state forms within 470 fs as determined from the wavelength-independent decay of the absorption between 325 and 450 nm. This absorption starts from the lowest excited state (2A). This state undergoes internal conversion and Z → E isomerization. The 2A absorption is clearly different from the 2A spectrum of the E isomer. The Z → E and E → Z isomerizations thus do not have a common intermediate.

Enhancement of Raman Modes in Complex Molecules by Coherent Control

Using coherent control the vibrational modes in complex solvated molecules are enhanced by femtosecond pulse shaping compared to Fourier-limited excitation. The crucial role of electronic resonant transitions for the enhancement of selective states is explored.

Two aspects of ultrafast photodynamics: molecular photo- ionization and coherent control of molecular fragmentation and photoreactions involving conical intersections

The paper focuses on two topics of ultrafast photodynamics. The first deals with the ionization and fragmentation of molecules using intense femtosecond laser pulses, while the second addresses ultrafast reaction dynamics of medium-sized organic molecules involving conical intersections. In particular, the dynamics of the photo-initiated unimolecular electrocyclic ring-opening reactions of 1,3,5cyclooctatriene and 7-dehydrocholesterol are examined in a femtosecond pumpprobe experiment in the gas phase using photo-ionization in combination with time-of-flight mass spectrometry for detection. Particular care is taken in the experiment to establish well-defined conditions and, especially, to shed light on the probe step. This is exemplified by the similar-sized molecule toluene, exploring several fundamental aspects of its ionization and fragmentation behaviour. The experimental setup for ionization investigations was reported in previous work [1], so we give only a brief summary here. The apparatus consists of a 1 kHz Ti:sapphire laser system (Spectra-Physics) that delivers 2.0mJ per pulse at 800 nm with pulses of 70 fs duration (shot-to-shot stability 4%) and a full width at half maximum (FWHM) diameter of ð5:37 0:02Þmm, a plano-convex uncoated lens (focal length 500mm or 1000mm) to focus the laser beam in the acceleration zone of the mass spectrometer, a sector filter disk, a neutral density filter with a 79% transmission and a 2mm thick CaF2 optical flat mounted in front of the focusing lens to allow for attenuation of the laser energy in small steps up to two orders of magnitude, a Wiley–McLaren time-of-flight mass spectrometer [2] for mass analysis, and necessary pulse diagnostics tools. These are a standard pulse autocorrelator positioned directly in front of the mass spectrometer, with which we determine an FWHM of ð80 1Þ fs assuming a Gaussian temporal shape of the pulse, a CCD-camera protected with various filters to analyse the shape of the focus outside of the mass spectrometer for each lens independently, and a fibre spectrometer (Ocean Optics SD2000) placed 1m behind the exit window of the mass spectrometer to record the frequency spectrum of the laser beam for exploring the fragmentation behaviour of the toluene molecule (each spectral