Maarten G. H. Boogaarts

Coherent cavity ring down spectroscopy

Abstract In a cavity ring down experiment the multi-mode structure of a short resonant cavity has been explicitly manipulated to allow a high spectral resolution, which is advantageous for the overall detection sensitivity as well. Coherent cavity ring down spectroscopy is performed around 298 nm on OH in a flame.

Trace gas detection with cavity ring down spectroscopy

Trace gas detection of small molecules has been performed with cavity ring down (CRD) absorption spectroscopy in the near UV part of the spectrum. The absolute concentration of the OH radical present in trace amounts in heated plain air due to thermal dissociation of H2O has been calibrated as a function of temperature in the 720–1125 °C range. Detection of NH3 at the 10 ppb level is demonstrated in calibrated NH3/air flows. Detection of the background Hg concentration in plain air is performed with a current detection limit below 1 ppt. The effect of the laser linewidth in relation to the width of the absorption line is discussed in detail. Basic considerations regarding the use of CRD for trace gas detection are given and it is concluded that CRD spectroscopy holds great promise for sensitive [(sub)‐ppb] and fast (kHz) detection of many small molecules.

High Rydberg states of DABCO: Spectroscopy, ionization potential, and comparison with mass analyzed threshold ionization

Doubly‐resonant excitation/vibrational autoionization is used to accurately determine the ionization potential (IP) of the highly symmetric caged amine 1,4 diazabicyclo[2,2,2]octane (DABCO). The IP of DABCO excited with one quantum of the ν24(e′) vibration lies at (59 048.62±0.03) cm−1, based on fitting 56 components of the npxy Rydberg series (δ=0.406±0.002) to the Rydberg formula. Rydberg state transition energies and linewidths are determined using standard calibration and linefitting techniques. The IP determined from Rydberg state extrapolation is compared with that determined by mass analyzed threshold ionization (MATI). Effects of static electric fields on MATI signals measured for the high Rydberg states are discussed.

A study on the structure and vibrations of diphenylamine by resonance-enhanced multiphoton ionization spectroscopy and ab initio calculations

Laser-desorption jet-cooling has been applied in combination with mass-selective gas-phase spectroscopic techniques to study the structure and low-frequency vibrations of diphenylamine ~DPA!. Two-color ~1118! resonance-enhanced multiphoton ionization has been used to measure the vibrationally resolved excitation spectrum of the S 1 S 0 transition in the 305‐309 nm region. Ion-dip measurements have been performed to determine the vibrational structure in the electronic ground state. The electronic spectra of DPA are dominated by long progressions in low-frequency vibrations involving the motion of the phenyl rings as a whole. For the interpretation of the experimental data ab initio calculations have been performed at the Hartree‐Fock level for the S 0 -state and using single-excitation configuration interaction for the S 1 -state. The DPA molecule is found to change from a pyramidal geometry around the N-atom with unequal torsional angles of the phenyl groups in the S0-state to a planar geometry with equal torsional angles in the S1-state. The two most prominent vibrational motions are the in-phase wagging and the in-phase torsion of the phenyl rings. In addition, the resonance-enhanced multiphoton ionization spectra of the S1 S0 transition in the DPA-Ar, DPA-Kr, and DPA-Xe van der Waals complexes have been measured. From these spectra it is inferred that there is a coupling between the van der Waals modes and the low-frequency intra-molecular modes of DPA.

Measurement of the beam intensity in a laser desorption jet-cooling mass spectrometer

In a laser desorption jet‐cooling molecular beam spectrometer the concentration of translationally and internally cooled laser desorbed organic molecules that can be achieved is experimentally determined. Sensitive direct absorption detection of laser desorbed jet‐cooled diphenylamine (DPA) via cavity ring down (CRD) spectroscopy on the S1←S0 transition around 308 nm is used to measure the line‐integrated absolute absorption of the pulse of laser desorbed DPA molecules. The absolute cross section for the various vibrational bands of the electronic transition that is used, is determined in a separate two‐color ionization experiment. It is concluded that the optimum beam intensity that is obtained with laser desorption is comparable to the beam intensity that is obtained in the same spectrometer by conventional seeding of the desired species at a partial pressure of 10−4.

A demonstration of the Franck-Condon principle upon ionization

Abstract Vibrationally resolved ionization probabilities have been recorded for laser-desorbed jet-cooled diphenylamine via the mass analyzed threshold ionization (MATI) technique. The observed MATI spectra from selected laser-prepared vibrational levels in the first electronically excited singlet state of DPA are a clear demonstration of the Franck—Condon principle upon ionization. The shape of the vibrational wavefunction in the intermediate electronic state is seen directly reflected in the spectra.

Production and characterization of crystalline 13C60

Abstract High-purity crystals of 99.1% 13 C enriched C 60 have been produced and their Raman and fluorescence spectra recorded together with those of C 60 crystals containing the 13 C isotope in natural abundance. The relative frequency shift of the intramolecular Raman-active modes is found to be determined by the square root of the averaged mass of the C 60 molecules. From the observed blue-shift of the electronic origin of the fluorescence spectrum of C 60 upon 12 C → 13 C substitution, it is concluded that the averaged intramolecular vibrational frequency of C 60 decreases slightly upon electronic excitation.

REMPI, SEP and MATI on laser desorbed diphenyl‐amine

Mass‐selective spectroscopic techniques have been applied to unravel the vibrational structure of laser‐desorbed jet‐cooled diphenyl‐amine (DPA) in the first electronically excited singlet state and in the electronic ground state of the neutral molecule as well as in the electronic ground state of the cation. A large change in geometry upon electronic excitation and subsequent ionization is inferred from these spectra.

Trace gas detection via cavity ring down spectroscopy

We report on the use of laser based cavity ring down spectroscopy in the near UV. It is this part of the spectrum that is particulary well-suited for trace gas detection, as many molecules have strong, well characterized absorption bands in this region. We show that the detection limit for e.g. NH3 is in the order of 10 ppb, for OH below 1 ppb and for Hg at the ppt level. We propose a Fourier transform based cavity ring down spectrometer, in which the advantages of cavity ring down detection are combined with the multiplex advantage of a Fourier transform spectrometer. A theoretical description of this spectrometer, as well as a proposal for the construction of a prototype are worked out.