H. E. Hunziker

Laser desorption jet-cooling of organic molecules

Laser desorption followed by jet-cooling allows wavelength-selective as well as mass-selective detection of molecules desorbed from a surface without fragmentation. The cooling characteristics and detection sensitivity of laser desorption jet-cooling of organic molecules are investigated. From the rotational contour of the electronic origin of the S1 ← S0 transition of laser-desorbed anthracene, rotational cooling to 5–10 K is demonstrated. Vibrational cooling is studied for laser-desorbed diphenylamine, a molecule with low-energy vibrations, and a vibrational temperature below 15 K is found. The absolute detection sensitivity is determined for the perylene molecule. Using two-color (1+1) resonance enhanced multi-photon ionization (with a measured ionization efficiency of 0.25) for detection, it is found that one ion is produced in the detection region for every 2×105 perylene molecules evaporated from the desorption laser spot. A two-color (1+1) REMPI spectrum (400 points) of perylene is recorded using only 30 picogram of material.

A SEARCH FOR C60 IN CARBONACEOUS CHONDRITES

Analysis of interior samples of the Murchison meteorite by two routes yielded an upper limit of 2 ppb for its C60 content, as compared to parts per million levels for individual polycyclic aromatic hydrocarbons (PAHs). Provided the samples contain an interstellar component, which is probable since Murchison hydrocarbons contain excess deuterium, this result argues against the ubiquitous presence of C60 in the interstellar medium. A possible explanation for the absence of C60 was found in experiments showing how PAHs replace fullerenes as stable end products when hydrogen is present during carbon condensation. As a secondary result we found high molecular weight PAHs in the Murchison and Allende meteorites. Coronene and its methyl derivatives are especially interesting since features in the coronene spectrum have been shown to match some of the unidentified interstellar infrared emission bands.

The UV spectra of primary, secondary, and tertiary alkyl radicals

Gas phase absorption spectra have been measured for one primary, three secondary, and two tertiary alkyl radicals in the 195 to 370 nm wavelength range. Radicals were generated by Hg‐photosensitized reactions and observed by modulation spectroscopy. The pattern of bands observed agrees well with ab initio predictions for the 3s, 3p, and 3d Rydberg transitions and can be reproduced by a Rydberg formula using experimental vertical ionization potentials and normal quantum defects. Oscillator strengths and condensed phase blue shifts are consistent with the Rydberg assignment. A similarity with the spectra of primary, secondary, and tertiary amines is noted.

Electronic absorption spectrum of CH3O

The ? 2A1‐? 2E electronic transition of CH3O, which is qualitatively analogous to the ? 2Σ+−? 2Πi transition of OH, has been detected in absorption. It consists of a long series of vibronic bands described by ν= (31 538±9)+v′3(676±5)−v′23(5.7±0.6 cm−1), where ν is the transition wavenumber at the band intensity maxima, and v′3 is the upper state C–O stretching vibrational quantum number. The estimated oscillator strength of the transition, f= (4±2) ×10−4, has the same magnitude as in OH. It is shown that the observed C–O vibrational frequencies and vibronic intensities are consistent with the ab initio SCF geometry of CH3O, and with Franck–Condon factors derived from it. Earlier difficulties in detecting the absorption spectrum are attributed to a low absorption coefficient, and to the marginal thermodynamic stability of the CH3O ground state.

Triplet acetylene: Near infrared electronic absorption spectrum of the cis isomer, and formation from methylene

The electronic absorption spectrum in the gas phase of the metastable 1 3B2 state of acetylene has been observed. Its’ features, including rotational and vibronic structure as well as deuterium isotope effects, agree remarkably well with ab initio theoretical predictions for the 1 3A2–1 3B2 electronic transition of the lowest triplet state in its cis geometry. The metastable species was generated in the Hg‐photosensitized reactions of acetylene, ketene, and diazomethane. In the latter two cases it is probably formed by the reaction 3CH2+3CH2 →C2H2(1 3B2)+H2.

Laser desorption jet-cooling spectroscopy of para-amino benzoic acid monomer, dimer, and clusters

The technique of laser desorption followed by jet cooling allows wavelength‐selective as well as mass‐selective detection of molecules, desorbed from a surface without fragmentation. Resonance enhanced multi photon ionization (REMPI) spectra of the para‐amino benzoic acid (PABA) molecule and its methyl and n‐butyl ester were obtained in this way. The origin of the S1←S0 transition in PABA was found at 34173±2 cm−1. The adiabatic ionization potential of PABA was determined as 7.998±0.001 eV. In addition, jet‐cooled REMPI spectra of the PABA dimer and its ring‐deuterated isotopes were recorded. The dimer is formed by two identical hydrogen bonds between the carboxylic acid groups. The excitation in the dimer is found to be almost completely localized in one monomer unit. Clusters of PABA molecules with molecules seeded in the beam (argon, methanol, water) were resonantly detected as well, using PABA as the chromophore.

Spectroscopy on triphenylamine and its van der Waals complexes

Both vibrationally and rotationally resolved spectra of the S, + So transition in jet-cooled triphenylamine (TPA) around 340320 nm are reported. Medium resolutton spectra (0.5-l .O cm-’ resolution) are recorded using ( 1 + 1 )-resonance enhanced multi photon iomzation (REMPI) with mass selective time-of-flight (TOF) detection in a pulsed molecular beam apparatus. The origin ofthe S,+ So transition is at 29520.7 cm-‘, higher than halfway to the iomzation potential (IP) found at 6.89 eV. A vibrational progression m the symmetric torsion mode ( 1 14 cm-‘) as well as in the symmetric C-N stretching mode (280 cm-‘) is observed m the electronic spectra. The spectrum of the most abundant isomer of the TPA-Ar (TPA-Kr ) complexes is blue-shifted by 2 11 cm- I ( 2 16 cm-’ ) with respect to the spectrum of the free TPA molecule High-resolution (the resolution mamly being determined by the natural linewidth of the transition, i.e. 36 MHz) spectra are recorded using laser Induced fluorescence (LIF) in a cw molecular beam apparatus. Individual rotational transitions are resolved and the spectrum shows unambiguously that TPA is a symmetric top molecule. The rotational constant B” m the So state of TPA is equal to B”=403.7 f0.5 MHz. Upon S,+So excitation both B and C mcrease with 7.4 2 0.1 MHz and 2.8 k 0.1 MHz, respectively. The spectrum of the blue-shifted TPA-Ar isomer is the spectrum of a symmetric top molecule as well, and therefore the Ar atom has to be located on the C3 symmetry axis, either on top of or underneath the umbrella formed by the phenyl rmgs. It appears that when Ar or Kr forms a complex with TPA, the first Ar, Kr. atom goes preferentially in a position on the C, symmetry axis of TPA, a position which causes an abnormal blueshift of the spectrum. With the first rare-gas atom located m this special position, the second rare-gas atom is forced into a “normal” position, i.e. above one of the phenyl rings, causing a normal red-shift with respect to the TPA-Ar complex.

ND4 Schüler band absorption observed by laser FM spectroscopy in a photochemical reaction

Twenty‐one lines of the Schuler band of ND4, previously observed in emission from high pressure ND3 discharges and assigned to a transition between excited states, have been observed in absorption in the Hg‐photosensitized reaction of ND3. In this system ND4 is probably formed by a reaction of the HgND*3 exciplex. The observed transition originates from either the 3s, 2A1 ground state or the 3p, 2F2 excited state, the former being more likely.

Triplet State of Acetylene: Biacetyl Emission from the Mercury Photosensitized Reaction

The question has been studied whether a metastable triplet state of acetylene is formed during its mercury photosensitized reaction, using biacetyl as a probe and employing a modulation‐phase shift technique. While no emission is observable in the mercury photosensitized reactions of acetylene or biacetyl alone, in the presence of both compounds biacetyl phosphorescence is generated whose intensity, under suitable conditions of pressure and flow, approaches closely the one of benzene photosensitized biacetyl emission. The phenomenon is not caused by products such as benzene and can be quenched by increasing the acetylene pressure. A mechanism explaining the emission as energy transfer from a metastable acetylene triplet state to biacetyl is proposed, and rate parameters are evaluated by comparing this model with the experimental results. Self‐quenching of triplet acetylene occurs at 0.083 times the rate of energy transfer to biacetyl. The excitation energy of triplet acetylene, E, was estimated to be 2.6<...

Pulsed laser desorption near a jet orifice: Concentration profiles of entrained perylene vapor

We have investigated the entrainment in a jet expansion of material vaporized with a laser pulse from a surface below but closely adjacent to the jet orifice. Jets of He and Ar were used as carriers, and perylene was the test substance. Its distribution in space and time far from the nozzle was measured by laser-induced fluorescence and one-dimensional imaging with an optical multichannel analyzer. The width of the perylene concentration profile was found to be much narrower than the width of the gas expansion, especially in He. This makes it possible to extract a useful portion of the entrained material through a skimmer. Depending on the values of experimental parameters, the center of the profile first appears above, on, or below the jet axis and then moves lower with time. Effects of several parameters on shape and time-dependent position of the profile were investigated. The results can be used to optimize the overlap of the concentration pulse with a skimmer or other probe.