Reiner P. Schmid

Rotational-state dependent selectivity in the bond fission of C2HD (5ν1)

Abstract We report the first demonstration of rotational-state selectivity in bond fission. C 2 HD is prepared in the 5 ν 1 vibrational-state and photodissociated by ∼ 243.1 nm photons that also probe the H/D fragments. The production of both H and D is greatly enhanced upon rovibrational excitation and is rotational-state dependent. The H/D branching ratio for the photodissociation of C 2 HD (5 ν 1 ) is 1.5 ± 0.2 for J ′ = 2 and 4.9 ± 0.5 for J ′ = 4. Possible mechanisms for the rotational dependence are discussed.

INFRARED SPECTROSCOPY OF ANILINE-X (X = N2, CH4, CHF3, CO) CLUSTERS AND THEIR CORRESPONDING CLUSTER CATIONS IN THE NH2-STRETCHING VIBRATION REGION

Abstract The NH 2 -stretching vibrational modes of four different aniline-X clusters (X = N 2 , CH 4 , CHF 3 , CO) and their corresponding cluster cations were investigated by ion-depletion spectroscopy. The observed frequency shifts of the neutral clusters have been found to be very small, and to be proportional to the red shift of the origin of the S 1 ← S 0 electronic band. This result suggests that the molecules interact with the aromatic ring system of aniline. The shifts of the cation species are larger than those of the neutral species, especially for aniline-CH 4 + . The proton affinity of the interacting molecule has been found to be correlated to the frequency shift except for CHF 3 . The different behavior of the cation species from the neutral ones suggests that the main interaction force in the cation clusters is different from that in the neutral ones. The possibility of the hydrogen bond interaction is discussed.

Combination bands versus overtone stretch excitation and rotational effects in vibrationally mediated photodissociation of acetylene

Rovibrational excitation combined with promotion of C2H2 molecules to the excited electronic trans-bent states A 1Au/B 1Bu and photofragment ionization are used to generate action spectra, H Doppler profiles, and time-of-flight mass spectra. Rovibrational states of C2H2 in the 15 480–15 723 cm−1 region are photodissociated by 243.135 nm photons that subsequently tag the H fragments. The H photofragment yield is greatly enhanced upon rovibrational excitation. In the action spectra, the intensities of the combination bands that involve high stretch and low bend excitation, (1410020) and (2031100), are close to that of the fourth overtone of the C–H stretch, (2030000), while in the absorption spectra the intensities of the combination bands are much weaker. In addition, the effect of rotation on fragment yield and dissociation channels is demonstrated. Several pairs of rotational transitions (sharing similar J′) stand out from the rotational band contour and the R(13) line of (2030000) shows an anomalously ...

Photodissociation of rovibrationally excited C2H2: Observation of two pathways

C2H2 is prepared in the 2030000 (five quanta of C–H stretch) vibrational state and photodissociated by 243.135 nm photons that also probe the H photofragments via (2+1) resonance-enhanced multiphoton ionization (REMPI) in a time-of-flight mass spectrometer. The production of H atoms is greatly enhanced upon rovibrational excitation. The REMPI action spectrum shows the characteristic features of a Σu+–Σg+ band and mimics the absorption spectrum, except that the R(13) line intensity is an order of magnitude higher than that expected for a Boltzmann distribution. The maximum translational energy of the H atoms obtained from dissociation of the regularly distributed rotational states is 0.67±0.10, whereas for R(13) it is 1.34±0.10 eV. The observed intensities and linewidths indicate the existence of two photodissociation pathways following the preparation of C2H2, where the C2H fragment is produced in two different states. In the R(13) pathway an additional bent state is prepared, or an accidental coincidence...

Optical processing on a femtosecond time scale

An all optical full logic unit, operating at femtosecond switching times, is demonstrated, based on transient index gratings from three independent, but mutually coherent, subpicosecond input laser pulses. The output signal is made up by the third harmonic of the input beams, resulting in a unique contrast ratio.

Multiphoton ionization of nitrotoluenes by means of ultrashort laser pulses

Abstract The potential of ultrashort laser pulses in the laser mass spectrometry of photounstable molecules is demonstrated for the case of nitrotoluenes. Nitro compounds tend to quickly dissociate after photoexcitation producing only unspecific fragments under conventional nanosecond multiphoton ionization conditions. The mass spectra of two isomers of mononitrotoluene, two isomers of dinitrotoluene and trinitrotoluene were recorded following multiphoton ionization with 170 fs laser pulses either with a wavelength of 412 or 206 nm. Although even these laser mass spectra are characterized by intense fragmentation they exhibit a clear molecular ion or OH loss signals depending on the substitution positions. Although the two mononitrotoluenes can be distinguished by their mass spectra at both wavelengths the two isomers of dinitrotoluene investigated show characteristic features which allow their clear differentiation only at 412 nm. Keywords: Multiphoton ionization; Mass spectrometry; Ultra short laser pulses; Nitrotoluenes; Explosives

Designing reflectron time-of-flight mass spectrometers with and without grids: a direct comparison

Abstract A direct comparison of a reflectron time-of-flight mass spectrometer equipped with a two stage gridded reflector to one with a gridless was carried out by ion trajectory simulations using a set of starting conditions which represent the different sources of signal broadening in a reflectron. The mass resolution was calculated from the flight time distribution and determined for a broad range of reflector voltages. The results show that the gridded reflector can be adjusted to give higher resolution for all starting conditions and in particular is less affected by ion beam divergence. However, when in addition the transmission is taken into account the gridless reflector is superior as long as the divergence of the ion beam can be kept small. The diameter of the ion beam has a much weaker effect on the instrument’s performance. The ratio of the two reflector voltages resulting in best performance is a constant for both instruments, but while the absolute values have to be adjusted to the ion beam parameters in the case of the gridded reflector, they only weakly depend on diameter and divergence of the ion beam in the case of the gridless reflector.

The van der Waals vibrational frequencies of the aniline-carbon monoxide complex in its S1 state

Abstract A portion of the S1-S0 band of the aniline-CO van der Waals (vdW) complex has been observed near 297 nm, using REMPI/TOF spectroscopy. Compared to the same band in aniline itself, this represents a shift to longer wavelengths of over 300 cm−1, which is one of the largest red shifts so far observed for this type of vdW complex. The spectrum observed consisted of five almost equally intense, relatively sharp sub-bands and we have assigned these as the O00 bands plus four signals arising from transitions to vibrational levels in the S1 state which are associated with the van der Waals bond. In order to account for these four sub-bands, it appears that transitions to a minimum of three different vdW vibrational modes are required. Two signals appear to involve the first and second level of the same vibration, the other two seem to arise from at least a further two different vibrational levels. The large values of both the red shift of the S1-S0 band and the frequencies of the vdW modes indicate that aniline-CO is a relatively strongly bonded complex. It was concluded that the CO molecule probably lies in the symmetry plane above the aromatic ring of the aniline molecule.

Ellipsometric study of molecular orientations of Thermomyces lanuginosus lipase at the air-water interface by simultaneous determination of refractive index and thickness.

Ellipsometric studies of very thin organic films suffer from the low refractive index contrast between layer and bulk substrate. We demonstrate that null ellipsometry can not only provide detailed information about the adsorption kinetics and surface excess values, but in addition on layer thicknesses with submonolayer resolution of a lipase from Thermomyces lanuginosus at the air-water interface. While measuring very close to the Brewster angle, refractive indices and layer-thicknesses can both be determined with a precision that is sufficiently high to make conclusions on the density and orientation of the molecules at the interface. The orientation was found to be concentration- and pH value-dependent. At the isoelectric point, the lipase was almost vertically oriented with respect to the surface, while for pure distilled water and low lipase concentration a rather horizontal alignment was found. Further experiments, varying the size of the interfacial area in a Langmuir trough, confirm the different layer structures.

Competition of Thermomyces lanuginosus lipase with its hydrolysis products at the oil-water interface.

Lipase-catalyzed hydrolysis of triglycerides yields glycerol and free fatty-acids, provided that the enzyme is non-regioselective. For an Sn-1,3 regioselective enzyme, such as lipase from Thermomyces lanuginosus, the final product is no longer glycerol but Sn-2 monoglyceride instead. However, surface active molecules generated by lipolysis may have a detrimental effect on the interfacial biocatalysis since it is known that low molecular weight surfactants can displace proteins from interfaces. By using drop profile analysis tensiometry, we evaluated the interfacial properties of the lipase-generated molecules and their competitive effect on the adsorption behavior of the lipase and on the proceeding lipolysis. Our results show that even at concentration ratios of 8.64×10-4M (Sn-2 monoglyceride) to 2.5×10-7M (lipase), the final interfacial pressure values are very similar as for the system containing the lipase alone (i.e. ∼26 mN/m). This is a strong indication that monoglycerides, as the most interfacially active products generated during regioselective lipolysis, are expelled from the oil-water interface by the lipase. We attribute this effect to intermolecular lipase-lipase interactions, resulting in a low desorption probability of the lipase. For low oleic acid concentrations, the interfacial tension is solely determined by the lipase, while for higher concentrations, lipase and oleic acid both contribute to the tension values. We propose a hypothesis based on the preferential interaction of oleic acid molecules with hydrophobic sites on the lipase. The pH dependence of the adsorption rate and the interfacial activity of the lipase were also investigated.