P. Hering

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.

Spectroscopy and ultrafast dynamics of the 2A1 state of Z-hexatriene in gas phase

As in longer polyenes, the strong 1A1→1B2 band in the UV spectrum of 1,3Z,5-hexatriene (Z-hexatriene) has a weak precursor, the 1A1→2A1 transition. It was measured in this work by a conventional spectrometer. Whereas the wave number of the 0–0 transition is by 5400 cm−1 lower than that of the 1B2 origin, the vibrational contour indicates that the vertical transitions of thetwo bands nearly coincide. From the fast decay of the rotational anisotropy in the time-resolved measurements we conclude that this band is perpendicularly polarized. We measured the lifetime of the 2A1 state after pumping it directly by 250 fs pulses and probing the excited molecules by ionizing it by delayed pulses. The lifetimes decreased from several ps to 730 fs, when the excess energy was increased from near 0 to 4000 cm−1 and more. From the temperature dependence we infer a barrier of about 170 cm−1 (2 kJ/mol).

Resonance CARS overtones of NaH in a Na (3p) + H2 gas mixture

Abstract With the CARS technique (coherent anti-Stokes Raman scattering), we found the first, second and third overtone O, Q and S lines of NaH in a Na (3p) + H2 gas mixture, when the CARS pump-laser frequency was tuned close to electronic resonances between X 1Σ+ (ν″=O, J″) and the first excited state A 1Σ+ (ν′=1, J′=J″±1) in NaH. This opens new possibilities in the investigation of the nascent molecular products of the reaction between excited sodium atoms and hydrogen molecules, since resonance CARS can be used with good spatial and temporal resolution. We discuss the possible origin of the NaH CARS lines produced in the Na(3p) + H2 mixture.

CARS spectroscopy of the NaH2 collision complex: the nature of the Na(32P)H2 exciplex — ab initio calculations and experimental results

CARS (Coherent Anti-Stokes Raman Scattering) has been used to analyze the rovibronic state distribution of H2 after collision with Na(32P). New lines, which do not correspond to H2 lines are observed in the CARS spectrum. The experiments point to the formation of a complex of Na(32P)H2 inA2B2 symmetry. Ab initio calculations of theA2B2 potential were performed. On this surface the vibrational spectra of the exciplex are evaluated. The observed lines can be attributed to vibrational transitions in the complex, in which combinational modes are involved. The connection of experimental and theoretical results indicates that a collisionally stabilized exciplex molecule is formed during the quenching process.

Degenerate four-wave-mixing spectroscopy in NaH

We performed DFWM spectroscopy on X−1Σ+−A1Σ+ transitions in NaH produced in an indirect photochemical reaction between Na(3p) and H2 and detected v″=1, 2 and 3 ground state vibrational levels of NaH molecules, whereas with resonance enhanced CARS, we observed v″=0 levels only. This different sensitivity can be explained by considering the Franck-Condon-factors and the relevant damping coefficients for the corresponding transitions in the NaH molecule. Time resolved DFWM spectroscopy showed that NaH(v″=1) molecules effectively live much longer than Na(3p) atoms which merely follow the laser excitation pulse.

P-R doublet enhanced CARS lines of NaH

Abstract We have observed an enhancement of the P and R lines from X1Σ+−A1Σ+ transitions in the NaH molecules through coherent anti-Stokes Raman scattering (CARS). The NaH molecules were photochemically produced in a sodium-hydrogen gas mixture. By changing the pump laser frequency ωpump the location of these P-R enhanced doublet CARS lines shift by the amount of detuning in the usual ωpump-ωstokes representation. However, in the 2ωp-ωs representation these P and R lines remain stationary. Double electronic resonances and upper state Raman overtone CARS lines have been observed. For P5(9,0) and R5(9,0) enhanced CARS lines we tuned the pump laser over the electronic resonance in order to obtain the resonance excitation curve for the double electronic resonance. This excitation curve exhibits a characteristics central dip.

Rotational and vibrational temperature determination by DFWM spectroscopy

Using the DFWM technique we determined rotational and vibrational temperatures of NaH molecules in a dynamical system consisting of NaH, Na, H and H2 for several heat-pipe oven temperatures. We applied a new way to determine the dependence of the DFWM signalIcint on the power of the transition dipole momentµ without previous knowledge of the temperature. The revealed dependence isIcintαµ4. In this case the laser intensities were found to be of the same order as the saturation intensity in accordance with simple DFWM theory. A usual Boltzmann plot was employed for the determination of the relevant rotational temperatures, which displayed higher values than the oven temperature.

CARS investigations of quenching and photochemical reactions in the Na+H2 collision system

SummaryWe have carried out parallel studies of the quenching process in Na(3p)+H2 collisions and the possible reactive process in Na(3p)+H2 (v″=1,2,3) collisions. Rich CARS spectra which were obtained at H2 pressure of 100 mbar and oven temperature of 600 K indicate the presence of vibrationally excited H2 and photochemically produced NaH molecules. Temporal resolution of NaH CARS lines was employed in order to rule out competing collisional processes. We make use of resonantly enhanced CARS methods which enabled us to achieve very high sensitivity for NaH detection.

The role of nuclear spin in inelastic Na (3p) + H2 collisions

Abstract Using the CARS (coherent anti-Stokes Raman spectroscopy) detection technique, we measured the rotational distribution in the v = 3 vibrational level of the para-H2 modification of the hydrogen molecule after inelastic collision with the Na (3p) atom. Para-H2 was used in a flowing-regime cell at a temperature of 523 K and at pressures of para-H2 ranging from 10 to 200 mbar. Our measurements show that the para-H2 modification is conserved in inelastic collisions. The conservation of the nuclear spin was also studied at very low hydrogen pressures, where we obtained the non-thermal rotational distribution at the highest attainable vibrational level v = 3.

Comparison of the Na(4p)+H2 and Na(3p)+H2 reactive/quenching systems studied with CARS, resonance-enhanced CARS, and DFWM

Three different nonlinear optical techniques, CARS (coherent anti-Stokes Raman scattering), resonance-enhanced CARS, and DFWM (degenerate four-wave mixing), were used to compare the reactive and quenching behavior of the two different electronically excited sodium atoms, Na(3p) and Na(4p), in a collision with H2. In the chemical reaction channel both excited sodium states produce NaH molecules, but in the case of Na(3p) it is shown that the reaction is not a direct formation process and involves more than one step. Both sodium states induce a population of the vibrational levels v″=0–3 of NaH. For the Na(3p) excitation the population of the NaH molecules is concentrated at v″=0, whereas for the excitation of the Na(4p) state NaH has its maximum population in the v″=1 level. The differences are attributed to different potential energy surfaces. The quenching investigations were focused on the behavior of the Na(4p) state and the results were compared with the well-known behavior of the Na(3p) state. Becaus...