K. L. Kompa

Diatomic interhalogen laser molecules: Fluorescence spectroscopy and reaction kinetics

Mixtures of halogen‐containing molecules and rare gases have been excited by a short pulse of high energy electrons. The D′→A′ transitions occurring between an ionically bound upper level and a weakly bound covalent lower level in the diatomic halogens F2, Cl2, and interhalogen compounds ClF, ICl, IF, IBr, BrCl, and BrF formed under these conditions have been studied systematically. Emission wavelengths calculated from a simple model are in good agreement with the experimental data. The processes responsible for the population of the upper level have also been studied. The exchange reaction of an electronically excited atom with a halogen donor molecule appears to be the key step in the kinetic excitation sequence. A rate equation model satisfactorily describes the time development of the observed halogen fluorescence. Based on these results, successful laser experiments have been conducted on several of the interhalogen systems.

Femtosecond CARS on H2

Abstract Time-resolved non-resonant CARS (coherent anti-Stokes Raman spectroscopy) is applied to probe H2 in the gas phase with femtosecond time resolution. The experimental transients show detailed beating structures, reflecting the rotational dynamics of all thermally populated ground state J levels with high fidelity. Modelling of the experimental data yields temperatures and pressures with high resolution. The good agreement with values determined by frequency-domain experiments demonstrates the suitability of this method for the determination of J-dependent collisional-broadening and line-shifting coefficients in gaseous mixtures at high pressures and temperatures, where frequency-domain CARS spectra are difficult to model.

Controlling molecular ground-state dissociation by optimizing vibrational ladder climbing

To achieve large population transfer to high vibrational levels in a selected ground-state mode of a polyatomic molecule [Cr(CO)6], we apply chirped femtosecond mid-infrared laser pulses at 2000 cm−1 to optimize vibrational ladder climbing as an energy deposition mechanism, which in turn controls the outcome of a unimolecular dissociation process. Its dependence on excitation parameters (frequency, intensity, chirp) is investigated and found to be in excellent agreement with a theoretical calculation. In particular, it is shown that optimizing vibrational ladder climbing allows for coherently controlled excitation even in a polyatomic molecule.

FEMTOSECOND PHOTOCHEMICAL RING OPENING OF 1,3-CYCLOHEXADIENE STUDIED BY TIME-RESOLVED INTENSE-FIELD IONIZATION

We found that hydrogen ion formation due to multielectron dissociative ionization by an intense-laser field is much less efficient with 1,3-cyclohexadiene than with its isomer 1,3Z,5-hexatriene (Z-HT). Moreover by suppressing the ionization barrier an intense-laser field ejects electrons most efficiently from molecular states of low ionization potential. After pumping 1,3-cyclohexadiene at 267 nm to its 1B2 state we probe the system by intense-laser field ionization with delayed 800 nm pulses. Monitoring of the parent ion C6H8+, of the main fragment C6H7+ and of H+ allows us to follow the motion from the 1B2 surface to the dark 2A1 state and from there towards the 2A1/1A1 conical intersection to the ground-state surface of the product. The measured 1B2 and 2A1 lifetimes are 43±3 and 77±7 fs, respectively, and the primary photoproduct cZc-HT is produced within 200 fs.

Lasers and Chemical Change

The dictionary defines laser as a. device for the amplification of light. Clearly there is more to it since the acronym itself stands for light amplification by stimulated emission of radiation. Moreover there must be some source for the enhanced light energy coming out. In a chemical laser this energy is provided by a chemical reaction. Chemical lasers combine the physical mechanism of lasing with the versatility and scope of chemical kinetics and spectroscopy. They are the present-day analogues of the electrochemical cells and history again repeated. In addition to their many practical applications chemical lasers have also led to considerable progress in our understanding of the fundamentals of chemical reactions and of bulk systems in disequilibrium.

Collision complex excitation in chlorine-doped xenon

Abstract Single-step optical excitation of XeCl * by absorption of a 158 nm photon in chlorine-doped xenon is reported. The excitation process relies on a radiative chemical reaction process which is induced by an intense laser field and takes place through an electronic transition of the Cl 2 —Xe collision complex. By this method, radiative lifetimes and quenching rate constants are determined for XeCl * (B,C) and Xe 2 Cl * .

Manganese pentacarbonyl bromide as candidate for a molecular qubit system operated in the infrared regime

Our concept for a quantum computational system is based on qubits encoded in vibrational normal modes of polyatomic molecules. The quantum gates are implemented by shaped femtosecond laser pulses. We adopt this concept to the new species manganese pentacarbonyl bromide [MnBr(CO)5] and show that it is a promising candidate in the mid-infrared (IR) frequency range to connect theory and experiment. As direct reference for the ab initio calculations we evaluated experimentally the absorption bands of MnBr(CO)5 in the mid-IR as well as the related transition dipole moments. The two-dimensional potential-energy surface spanned by the two strongest IR active modes and the dipole vector surfaces are calculated with density-functional theory. The vibrational eigenstates representing the qubit system are determined. Laser pulses are optimized by multitarget optimal control theory to form a set of global quantum gates: NOT, CNOT, Pi, and Hadamard. For all of them simply structured pulses with low pulse energies around 1 microJ could be obtained. Exemplarily for the CNOT gate we investigated the possible transfer to experimental shaping, based on the mask function for pulse shaping in the frequency regime as well as decomposition into a train of subpulses.

Ultrafast dynamics of the photochemical ring opening of 1,3-cyclohexadiene studied by multiphoton ionization

Femtosecond time-resolved studies of the ring opening of 1,3-cyclohexadiene are presented. After absorption of a single UV photon in the region 250–270 nm (1A1 → 1B2 transition), the reaction is investigated using time-delayed single- and multiphoton ionization with probe pulses in the region 250–415 nm. Ions are detected by a time-of-flight mass spectrometer. The parent ion is only observed during the time when the pump and probe lasers overlap. The corresponding ionizable state, which we identify as 1B2, has a short lifetime which we estimate to be 60 fs. The only signal detected after a delay is due to C6H7+. It arises from dissociation of vibrationally hot parent ions which are produced by two-photon ionization of vibrationally excited products. The appearance rate constant of the product is as high as 1.7 ± 0.2 ps−1. It is assigned to the transition from the 2A1 to the 1A1 surface via a conical intersection, i.e. to the photochemical ring opening of 1,3-cyclohexadiene to Z-hexatriene. Measuring fragment ions generated by photoionization at suitable wavelengths may be a general method for monitoring vibrationally hot neutral molecules.

Picosecond uv laser-induced multiphoton ionization and fragmentation of benzene

Abstract A 20 ps quadrupled Nd:YAG laser (with intensities up to 400 GW/cm 2 ) is used to reduce the fragmentation in benzene multiphoton ionization. Under these conditions fragmentation was found to be essentially complete at the C + 3 /C + 4 level, while multiple pulse experiments (Δ t = 5 ns) reproduced earlier results obtained with ns excimer lasers. At very high photon fluxes less molecular fragments are seen and atomic ions (even doubly charged) are produced.

A statistical model for the fragmentation of benzene by multiphotoionization

Abstract A statistical products phase space model for the (multi) photon fragmentation/ionization of polyatomic molecules in strong laser fields is proposed and tested on benzene. The mechanism assumes multiple dissociations and branchings starting with energy rich benzene ions. Calculated and experimental fragment mass patterns versus laser fluence are shown to be in good agreement.