Ulrich Schmitt

FTIR-spectroscopy of molecular clusters in pulsed supersonic slit-jet expansions

A new approach to the Fourier transform infrared (FTIR) absorption spectroscopy of molecular clusters in pulsed supersonic jets is developed to the point where it is competitive with high-sensitivity laser absorption techniques for intermediate and large molecular systems. A combination of rapid spectral acquisition and of a buffered jet chamber enables the use of intense gas pulses which cover complete interferometer scans. Applications to (N2O)n, (CH3OH)n and (HCl)n demonstrate the capabilities of this technique. Investigations of the association of bulky alcohols and of clusters within clusters illustrate some ongoing research.

Evidence for the (HF) 5 complex in the HF stretching FTIR absorption spectra of pulsed and continuous supersonic jet expansions of hydrogen fluoride

Abstract Preliminary IR-spectroscopic evidence for the (HF) 5 complex in supersonic jet expansions of hydrogen fluoride (neat and seeded with He) is presented. A wide range of effective temperatures between 230 and 60 K, as calibrated by HF rotational temperature, could be obtained using a standard continuous jet and a novel technique using asynchronously pulsed jets in conjunction with continuous interferogram detection and random averaging. Extrapolating band maxima to 0 K we find νt~ = 3245 cm −1 for the HF stretching band assigned to (HF) 6 and 3300 cm −1 for (HF) 5 . The bandwidths are about 20 cm −1 (fwhm). The results are discussed in relation to ab initio calculations, predictions from our recent HF polymer potential models, matrix spectroscopy data and HF polymer potential models, matrix spectroscopy data and HF vibrational energy transfer.

Ragout-jet FTIR spectroscopy of cluster isomerism and cluster dynamics: from carboxylic acid dimers to N2O nanoparticles

Direct absorption supersonic jet Fourier transform spectroscopy provides a panoramic view of the dynamics of molecular clusters over the entire IR spectral range. The new and generally applicable ragout-jet technique compensates for the sensitivity limits inherent in the incoherent FTIR approach by the use of synchronized giant gas pulses expanding into a large vacuum buffer. A modification based on fragmented interferograms is proposed and demonstrated, by which the spectral resolution can be extended to the limit of the available FTIR spectrometer. The power of the method is illustrated for two classes of compounds. For acetic acid and its isotopomers, the supersonic jet spectra of dimers and oligomers are investigated for the first time, concentrating on the very complex OH/CH stretching domain and on the more regular C=O/C-O stretching range. Issues of cluster isomerism, hydrogen exchange tunneling, anharmonic resonances, intermolecular Franck-Condon sequences, methyl group substitution and cluster coating with argon are explored. For the more weakly interacting nitrous oxide, stretching fundamentals and combination bands of clusters in the 1-3 nm range are studied as a function of composition. Surface vibrations are investigated in detail and modeled quantum mechanically. The semiempirical AM1 approach is found to provide a remarkably accurate description of the cluster structure, energetics and dynamics.

FTIR spectroscopy of hydrogen fluoride clusters in synchronously pulsed supersonic jets. Isotopic isolation, substitution and 3-d condensation☆

Abstract Synchronously pulsed supersonic jet FTIR spectra of HF/DF expansions in rare gases are investigated over a wide range of cluster compositions. Size-resolved (DF) n bands and spectra of (HF) n (DF) m with various n and m are reported and assigned. At higher backing pressures, the spectra are dominated by crystal-like absorptions, which we tentatively assign to 3-d nanometer size solids. The new experimental results support our previous size, structure and vibrational mode assignments and provide new insights into spectra and dynamics of hydrogen fluoride clusters, including intramolecular vibrational redistribution.

High resolution interferometric Fourier transform infrared absorption spectroscopy in a supersonic free jet expansion

The absorption spectrum of CHF3 in a free jet expansion has been recorded at a resolution of 0·004 cm-1 in the 8–9 μm region. The line width is found to be essentially Doppler limited. An approximate rotational temperature of 40K has been determined from the spectrum. Relative intensities and transition moments for the interacting bands are discussed.

Chirality influence on the aggregation of methyl mandelate

The methyl ester of mandelic acid is investigated by a wide range of techniques to unravel its aggregation pattern and the influence of relative chirality of the aggregating monomers. Matrix isolation confirms that a single monomer conformation prevails. The electronic spectrum of the dimers is strongly affected by the relative monomer chirality. Vibrational effects are more subtle and can be explained in terms of the most stable homo- and heteroconfigurational dimer structures, when compared to results of MP2 and DFT-D computations. Selective IR/UV double resonance techniques and wide-band FTIR spectroscopy provide largely consistent spectroscopic fingerprints of the chirality discrimination phenomena. The dominant homochiral dimer has two intermolecular O–H⋯OC hydrogen bonds whereas the more strongly bound heterochiral dimer involves only one such hydrogen bond. This is a consequence of the competition between dispersion and intramolecular or intermolecular hydrogen bonding. Aromatic interactions also play a role in trimers and larger clusters, favoring homochiral ring arrangements. Analogies and differences to the well-investigated methyl lactate system are highlighted. Bulk phases show a competition between different hydrogen bond patterns. The enantiopure, racemic, and 3 : 1 crystals involve infinite hydrogen-bonded chains with different arrangements of the aromatic groups. They exhibit significantly different volatility, the enantiopure compound being more volatile than the racemic crystal. The accumulated experimental and quantum-chemical evidence turns methyl mandelate into a model system for the role of competition between dispersion forces and hydrogen bond interactions in chirality discrimination.

Analysis of the v4 and v1 bands of CF3Cl measured by supersonic free-jet ftir spectroscopy

The first successful detailed analysis of the high-resolution (0.005 cm−1) IR spectrum of the v4 band of CF3CI (

High resolution interferometric fourier transform infrared spectroscopy in supersonic free jet expansions: N2O, CBrF3 and CF3I

v0= 1216.75843(20) cm−1 for 35Cl) is reported on the basis of a measurement in a supersonic free jet. The rotational and vibrational cooling in the jet is evaluated from the spectra of v1 and v4 and their hot bands. Rotational temperatures are 32 ± 7 K and vibrational temperatures of the hot band (v1+v6)-v6 about 240 ±30 K. The results are important in relation to IR laser chemistry and to modelling the absorption of freons in the atmospheric window.

Supersonic jet spectroscopy and high resolution FTIR study of SPF3

Continuous flow supersonic jet expansions of neat nitrous oxide N2O, bromotrifluoro methane CBrF3 and trifluoroiodo methane CF3I have been investigated by FTIR absorption spectroscopy in the 8–9 μm region at high resolutions of 0.004 and 0.0024 cm−1 (fwhm). Rotational temperatures of 26 K for N2O, 45 K for CBrF3 and 50 K for CF3I have been determined from the spectra. For the v1 band of N2O the contribution of background gas to the FTIR spectrum and to the nonlinearity of Boltzmann plots has been evaluated. New spectroscopic parameters have been determined for the v1 and v1 + v6 − v6 bands of C79BrF3 and C81BrF3 with band centers v−01 (C79BrF3) = 1084.7690(1),~ v01 (C81 BrF3) = 1084.5214(1), ~v0166(C79BrF3) = 1083.5292(2) and ~v0166 (C81BrF3) = 1083.2846(2) cm−1 as well as for the v4 band of 12CF3I with band center ~v04 = 1187.6275(1) cm−1. The results are important for IR-multiphoton excitation, laser chemistry, and atmospheric chemistry and spectroscopy.

High-resolution interferometric Fourier transform infrared absorption spectroscopy in supersonic free jet expansions: carbon monoxide, nitric oxide, methane, ethyne, propyne, and trifluoromethane

High resolution infrared FT spectra of the v 2 and v 3 bands of SPF3 have been recorded with linewidths of 2-3×10-3 cm-1 at ambient temperature. The region of the v 1 band centred at 984·92944(1) cm-1 has been studied in a supersonic jet and in a static sample at -80°C. The parallel bands have been rotationally analysed, and standard deviations of about 2 × 10-4 cm-1 have been achieved in the fits. Numerous hot bands, and some bands of 34SPF3, have been identified and analysed, mostly by polynomial methods. Whereas the v 2 and v 3 bands centred at 696·32639(2) and 441·55728(7) cm-1 are unperturbed, the v 1=1 level takes part in a polyad of interacting levels, including the levels v 4,v 2+v 6,v 3+2v 6 and v 5+2v 6. Therefore the parameters obtained for the v 1 = 1 level must be considered effective parameters. Accurate isotope shifts are reported for the fundamentals and are compared with previous results.