Richard Engeln

CAVITY ENHANCED ABSORPTION AND CAVITY ENHANCED MAGNETIC ROTATION SPECTROSCOPY

It is experimentally demonstrated that a narrow band continuous wave (cw) light source can be used in combination with a high-finesse optically stable cavity to perform sensitive, high-resolution direct absorption and optical rotation spectroscopy in an amazingly simple experimental setup, using ideas from the field of cavity ring down spectroscopy. Light from a scanning narrow band cw laser is coupled into the cavity via accidental coincidences of the laser frequency with the frequency of one of the multitude of modes of the cavity. The absorption and polarization rotation information is extracted from a measurement of the time-integrated light intensity leaking out of the cavity as a function of laser wavelength.

Phase shift cavity ring down absorption spectroscopy

Abstract Cavity ring down absorption spectroscopy with a continuous light source is used to measure the transition frequencies and absolute absorption coefficient of the weak b1Σg+(v′ = 2) ← X 3Σg−(v″ = 0) transition of 18O2. The absorption spectrum is extracted from a measurement of the magnitude of the phase shift that an intensity modulated continuous light beam experiences upon passing through an unstabilized optical cavity.

A Fourier transform cavity ring down spectrometer

We present a pulsed multiplex absorption spectrometer in which the sensitivity of the cavity ring down absorption detection technique is combined with the multiplex advantage of a Fourier Transform spectrometer. A description of the Fourier transform cavity ring down (CRD) spectrometer—substantiated with first experimental results on the atmospheric band of molecular oxygen—is given. It is shown that as in the case of normal CRD spectroscopy, the measurement is independent of light intensity fluctuations provided the spectral intensity distribution of the light source is known and is constant during the measurement.

POLARIZATION DEPENDENT CAVITY RING DOWN SPECTROSCOPY

both techniques. The b 1 S g (v852) X 3 Sg (v950) transition of molecular oxygen around 628 nm is used to demonstrate the possibility to selectively measure either the polarization-dependent absorption or the resonant magneto-optical rotation of gas-phase molecules in the appropriate setup. Just as in CRD absorption spectroscopy, where the rate of absorption is measured, in the here presented polarization-dependent CRD ~PDCRD! detection scheme the rate of polarization rotation is measured, which enables the polarization rotation to be quantitatively determined. Apart from studying electro-optic and magneto-optic phenomena on gas-phase species, the PDCRD detection scheme is demonstrated to be applicable to the study of magneto-optical rotation in transparent solid samples as well.

Evidence for a gauche minor conformer of 1,3-butadiene

Abstract We present experimental Raman data on torsional transitions that prove that the less stable conformer of 1,3-butadiene has a gauche structure. It is shown that in order to predict the relative intensities of the “gauche” transitions with respect to the “trans” transitions, one has to include a second-order term in the Fourier expansion of the molecular polarizability.

Cavity ring down spectroscopy on solid C60

The light absorption of a solid sample in the 8.5 μm region is measured via cavity ring down (CRD) absorption spectroscopy, using a free electron laser (FEL) as a source of widely tunable infrared (IR) radiation. A 3 mm thick zinc-selenide (ZnSe) window is used as a substrate for a 20–30 nm thick C60 film. On top of the structureless absorption due to ZnSe (<340 ppm), one of the well-known fundamental IR absorption lines of C60 is measured with monolayer sensitivity.

Cavity ring down spectrocopy with a free-electron laser

Abstract A cavity ring down (CRD) absorption experiment is performed with a free-electron laser (FEL) operating in the 10–11 μm region. A short infrared pulse of approximately 20 ns, sliced from the much longer FEL pulse, is used to measure CRD spectra of ethylene in two different ways. First, “ordinary” CRD spectra with a resolution determined by the bandwidth of the FEL (≈ 5 cm−1) are recorded. Second, Fourier transform (FT) CRD spectra with a resolution that is in principle determined by the FT-spectrometer are obtained by analyzing the light exiting the ring down cavity with a FT-spectrometer while the FEL is operated in broadband mode.

Methyl torsion in C2H4-n (CH3) n, n = 1 or 2

Abstract Torsional Raman spectra of trans-2-butene, cis-2-butene, 2-methylpropene and propene are recorded. the molecular parameters describing the torsional motions are determined. Especially, the torsional barrier parameters, V3, and the reduced moments of inertia, F, of the tops are compared with the values determined from MW data. We conclude that one has to analyse together MW and Raman data in order to obtain a reliable description of internal motions.

Internal motion of two-top molecules: propane and dimethylamine

Abstract Two molecules with two internal rotors each (CH 3 group) were investigated by spontaneous Raman scattering. The torsional (overtone-) spectra yielded the relevant potential parameters for the internal motions. The potentials show characteristic differences due to the C 2v symmetry of propane and C s symmetry of dimethylamine.

Torsional motion of the CH3 groups of propane studied by Raman overtone spectroscopy

Abstract The Raman spectrum of propane is recorded between 300 and 900 cm −1 ; first and third overtones are observed with an Ar + laser intracavity setup. This spectrum is analyzed by means of a model with two hindered and coupled methyl rotors. Since the levels probed extend to the region above the torsional barrier ( V 3 =1353 cm −1 ), all the relevant molecular parameters, in particular the coupling between the methyl rotors, could be accurately determined. By means of a classical approach we have obtained further insight into the qualitative structure of the torsional spectrum.