Michael Hippler

High-resolution cavity ring-down absorption spectroscopy of nitrous oxide and chloroform using a near-infrared cw diode laser

Abstract A high-resolution cavity ring-down spectrometer has been constructed employing a tuneable cw diode lasing around 1300 nm (bandwidth and frequency stability better than 1 MHz) and a frequency-matched cavity. The performance of the spectrometer has been tested on the Σ–Σ combination band ν1+3ν3 of 14 N 2 16 O around 7780 cm−1 at Doppler-limited resolution, for which we report integrated absorption cross-sections, line positions and detection limits. CH-chromophore absorptions of 12 CHCl 3 between 7700 and 7750 cm−1 have also been observed, in agreement with previous predictions with an effective Hamiltonian based on a Fermi-resonance polyad model.

High-resolution Fourier transform infrared and cw-diode laser cavity ringdown spectroscopy of the ν2+2ν3 band of methane near 7510 cm-1 in slit jet expansions and at room temperature

The ν2+2ν3 combination band of 12CH4 near 7510 cm−1 was studied with the recently introduced technique of cavity ring-down spectroscopy employing a cw-diode laser in a pulsed supersonic slit jet expansion and with Doppler-limited Fourier-transform infrared spectroscopy at room temperature. ν2+2ν3 is the strongest absorption band in the high-wave-number region of the N=2.5 icosad of methane. First assignments of the combination band are provided. The vibrational origin of ν2+2ν3 at 7510.3378±0.0010 cm−1, the integrated band strength G=(1.3±0.2)×10−4 pm2 and the vibrational transition moment |μν|=(1.0±0.1)×10−3 D have been determined. The values represent benchmarks to test effective vibrational Hamiltonians and ab initio calculations for methane. Although an isolated band analysis was possible at low J-values, the influence of strong perturbations becomes evident at higher rotational excitation. The F1-component of ν2+2ν3 interacting by a strong Coriolis resonance with the IR-active F2-component appears to...

Cw cavity ring-down infrared absorption spectroscopy in pulsed supersonic jets: nitrous oxide and methane

Abstract We introduce cw cavity ring-down spectroscopy (cw-CRDS) in pulsed supersonic jet expansions employing a tuneable near-infrared cw diode laser and a solenoid slit nozzle. The cavity is mode-matched to the laser wavelength during the gas pulses to achieve highest resolution and lowest noise level. The new technique is characterised by observing very weak rovibrational lines of the ν 1 +3 ν 3 combination band of nitrous oxide near 7780 cm −1 and the ν 2 +2 ν 3 combination band of methane near 7510 cm −1 . We demonstrate the increased spectral resolution due to the reduced Doppler width of rovibrational transitions and the spectral simplification afforded by the rotational cooling.

Quantum chemical study and infrared spectroscopy of hydrogen-bonded CHCl3–NH3 in the gas phase

Molecular association of chloroform with ammonia is studied by high-level quantum chemical calculations including correlated MP2 and CCSD(T) calculations with basis sets up to6-311++G(d,p) and counterpoise corrected energies, geometries, and frequencies. The calculations predict an eclipsed hydrogen-bonded complex of C3v symmetry (ΔE0=−15.07kJmol−1) with 225.4pm intermolecular CH⋯N distance. Intermolecular interactions are analysed by Kitaura-Morokuma [Int. J. Quantum Chem. 10, 325 (1976)] interaction energy decomposition. Compared to the monomer, the C–H bond is elongated, and the CH-stretching fundamental shifts to lower wave numbers and has a marked ∼340-fold increase of its intensity. Based on these predictions, the complex is observed by infrared spectroscopy in the gas phase at room temperature. A subtraction procedure isolates its spectrum, and a dilution series confirms the presence of a 1:1 complex. The CHCl3⋯NH3 complex has an experimental −17.5cm−1 shift of its CH-stretching vibration, and CDCl...

Photodissociation of jet-cooled methyl and t-butyl nitrite near 380 nm

Abstract A one-colour photolysis-ionisation study of the photodissociation of methyl and t -butyl nitrite near 380 nm in a supersonic jet has been performed. The NO photoproduct has been characterised by state-resolved probing by (2 + 1) REMPI via the C→X δ bands of NO. The photodissociation of methyl nitrite yields predominantly vibrationally cold NO, while t -butyl nitrite photodissociation shows a propensity for formation of NO in the first excited vibrational level. The NO fragments are rotationally highly excited in both cases, and the rotational population distributions can be fitted by Gaussian curves in agreement with recent dynamical calculations. The different vibrational energy disposal is attributed to the different excitation of a rapidly predissociating complex. Previous results that were at variance with knowledge of the dynamics of the predissociation of alkyl nitrites may have been affected by experimental problems.

Isotopomer-selective overtone spectroscopy by ionization detected IR+UV double resonance of jet-cooled aniline

Abstract We report the analysis of the NH-stretching overtone absorption of gas-phase aniline C 6 H 5 NH 2 and C 6 H 5 NHD between 6500 and 7100 cm −1 . Spectra of jet-cooled aniline are obtained by a new IR+UV double resonance scheme, where vibrationally excited aniline is ionised by resonantly enhanced two-photon ionisation via hot-bands of the S 1 ←S 0 transition. Ionisation detection in a time-of-flight mass spectrometer allows observing isotopomer-selective overtone spectra (ISOS), for example of C 6 H 5 NHD, which occurs in a mixture with C 6 H 5 NH 2 and C 6 H 5 ND 2 . For C 6 H 5 NH 2 , a strong Fermi resonance between the symmetric NH-stretching and the symmetric NH-bending vibration is identified, with a coupling matrix element W ≈47 cm −1 . We also discuss the interplay of stereomutation on the femtosecond timescale and excitation of high-frequency stretching modes.

The 355 nm laser photolysis of jet-cooled methyl nitrite (CH3ONO). Internal energy distributions of the NO fragment

Abstract The photodissociation of methyl nitrite (CH 3 ONO) at 355 nm in a supersonic free-jet expansion has been studied by state-resolved two-proton laser-induced fluorescence of the NO fragment. The vibrational population distribution of the nascent NO is much colder than reported for the photodissociation of 300 K vapour and can be understood on the basis of the direct electronic dissociation of a very short-lived excited complex. The conformational isomerism of CH 3 ONO and the assignment of the structure of its electronic absorption spectrum prove to be important for the interpretation of the observed dissociation dynamics in the jet and at 300 K.

Cavity-Enhanced Raman Spectroscopy of Natural Gas with Optical Feedback cw-Diode Lasers

We report on improvements made on our previously introduced technique of cavity-enhanced Raman spectroscopy (CERS) with optical feedback cw-diode lasers in the gas phase, including a new mode-matching procedure which keeps the laser in resonance with the optical cavity without inducing long-term frequency shifts of the laser, and using a new CCD camera with improved noise performance. With 10 mW of 636.2 nm diode laser excitation and 30 s integration time, cavity enhancement achieves noise-equivalent detection limits below 1 mbar at 1 bar total pressure, depending on Raman cross sections. Detection limits can be easily improved using higher power diodes. We further demonstrate a relevant analytical application of CERS, the multicomponent analysis of natural gas samples. Several spectroscopic features have been identified and characterized. CERS with low power diode lasers is suitable for online monitoring of natural gas mixtures with sensitivity and spectroscopic selectivity, including monitoring H2, H2S, N2, CO2, and alkanes.

Infrared multiphoton excitation, dissociation and ionization of C60

Abstract We report the observation of mid-infrared multiphoton excitation, dissociation, and ionization of C 60 by shaped CO 2 laser pulses. The results are interpreted in the framework of the general statistical theory of infrared laser chemistry, adding some simple model assumptions. The observations on intensity- and fluence-dependent fragment ion distributions are consistent with a mechanism of vibrational preionization from energies exceeding the threshold for ionization by about a factor of 8.

Detection and probing of nitric oxide (NO) by two-colour laser photoionisation (REMPI) spectroscopy on the A ← X transition

Abstract A simple two-colour (1 + 1′) REMPI scheme for the detection and probing of nitric oxide (NO) via the A 2 Σ + ← X 2 Π transition has been introduced and characterised. Pulsed 226 and 308 nm laser light derived from a XeCl excimer pump laser has been used for resonant excitation and photoionisation. The two-colour (1 + 1′) ReMPI scheme has been found to offer substantially higher sensitivity than the conventional one-colour scheme. The reasons for this improvement and other advantages of the two-colour technique for the state-resolved probing of NO(X) are outlined.