W. Urban

The sensitive detection of NO by Faraday modulation spectroscopy with a quantum cascade laser

A spectrometer based on a quantum cascade laser and capable of operating at particular wavelengths in the mid-infrared with very high sensitivity for the detection of open-shell molecules has been developed. It exploits magnetic field modulation in the Faraday rotation configuration. The signals for nitric oxide (NO) that may be observed with this instrument have been studied and their dependence on the J and Ω quantum numbers investigated with a simulation program. It is shown that the Q(3/2) transition of NO in the 2Π3/2 component at 1875.8 cm−1 would provide the greatest sensitivity for detection. The experimental observation of the R(21/2) transition of the Ω = 1/2 component gives a detection limit of 41 ppb of NO in air at a pressure of 25 mbar. Detection of NO through the Q(3/2) transition would provide a detection limit of 4 ppb at this pressure.

Anharmonic vibration-vibration pumping in nitric oxide by resonant IR-laser irradiation

Abstract UV fluorescence and dissociation of NO were observed by irradiating a gas mixture of NO and Ar with a CO laser line that coincides with a line in the NO fundamental. The excitation up to the dissociation limit occurs via anharmonic vibration-vibration pumping. An analysis of the NO first-overtone emission yields a highly non-thermal vibrational population distribution that is characteristic of this excitation mechanism. The time dependence of the IR and UV fluorescence shows that the electronic excitation and dissociation of NO are delayed with respect to the laser pulse by an amount strongly dependent on the NO partial pressure. A set of coupled rate equations is developed to describe the excitation mechanism theoretically. The computer simulations show good qualitative agreement with the experimental results.

Measurement of the first excited bending state of Ar-CO using a new concentration modulation technique in the jet

Using a new modulation scheme: concentration-frequency double modulation, we were able to observe a new band of Ar-CO in the infrared. We assigned 20 lines of the transition from the ground state, K = 0, to the combination band of νCO = 1 and first excited bending, K = 0 state of Ar-CO. The lower state is identical to the state found by McKellar et al., who measured the perpendicular transitions in the CO fundamental branch of Ar-CO. The band origin of this K : 0 → 0 transition is situated at 2154·7460 cm-1. The K = 0 first excited bending state of Ar-CO is found to be 11·9cm-1 higher in energy than the K = 0 ground state. This is the first experiment, which determines the bending energy of the Ar-CO complex. Theoretical predictions were made by Tennyson et al. and Parish et al. They predicted this state to be located at 14·1cm-1 and 13cm-1, respectively, whereas experimentally we found a value of 11·914cm-1. We will present spectroscopic results and details on the new modulation scheme. This technique us...

Mid-infrared laser magnetic resonance using the Faraday and Voigt effects for sensitive detection

Rotation of the linear polarization in a longitudinal (Faraday effect) and a transverse (Voigt effect) magnetic field can be used to increase the sensitivity of laser magnetic resonance spectrometers. Theoretical background and experimental results for these new techniques are presented. The sensitivity and content of information are discussed for the various LMR methods.

A Faraday Laser Magnetic Resonance spectrometer for spectroscopy of molecular radical ions

We describe a mid-infrared Laser Magnetic Resonance (LMR) spectrometer which is based on a cryomagnet and a sealed-off CO-laser. Faraday rotation combined with a multireflection cell is used for polarization sensitive detection of molecular radical ions. Experimental details and first spectroscopic examples are given.

Physics and spectroscopic applications of carbon monoxide lasers, a review

Abstract The gain media in the CO2 laser and the CO laser are compared and the emission structure is discussed on the basis of Patels small-signal gain formula. Some features of a liquid-N-cooled CO laser plasma are discussed with respect to optimization for different wavelength regions. The corresponding resonator conditions are explained. The considerable number of lines available and their distribution over a wide wavelength region make the CO laser an interesting tool for molecular spectroscopy. Examples in the field of laser magnetic resonance (LMR), the possibilities of generating a secondary frequency standard at 5 μm and the potential of the CO laser in photoacoustic spectroscopy (PAS) are discussed.

Treanor pumping of CO initiated by CO laser excitation

Abstract An experiment is discussed in which CO can be excited up to energies of several electronvolts by the absorption of infrared radiation from a relatively low-power CO laser. Furthermore, experimental results are examined through kinetic modelling. In the experiment, the beam of an intracavity-chopped CO laser operating on all lines at 500 mW and containing a few milliwatts of the fundamental ν= 1→0 band component, is focused into an absorption cell containing a mixture of CO and Ar. The absorption of this infrared radiation is monitored by the optoacoustic effect. A second CO laser operating cw and capable of providing 8 W on all lines but not lasing on the ν= 1→0 band component, is then focused into the same volume in the absorption cell. With both lasers simultaneously focused into the absorption cell, strong fluorescence from the irradiated region is detected by a photomultiplier tube. Modulation of the signal intensity with time is observed, and indicates chemical destruction of the CO in the cell. An analysis and kinetic modelling calculation of this experiment shows that it is possible to excite CO up to high vibrational quantum numbers (ν⩾40) at gas temperatures up to 800 K. by means of CO laser irradiation at the fundamental ν= 1→0 band component. One source responsible for the fluorescence signal observed in the experiment is identified as the 4th positive A 1 Π→X 1 Σ + spontaneous emission. Although the present kinetic model does not incorporate the chemical processes that may lead to the production of additional fluorescing species such as C 2 , good agreement is obtained with the observed fluorescence signal characteristics.

Experimental rovibrational populations of CO up to ν = 40 from doppler-limited fourier spectra of the sequences Δν = 1, 2 and 3 emitted by a laser type source

Abstract High-resolution emission Fourier spectra have been recorded from a CON 2 HeO 2 mixture excited by dc discharge and cooled by liquid nitrogen. Intensities of all the observed rovibrational lines belonging to sequences Δν = 1, 2 and 3 of 12 C 16 O and 13 C 16 O, and Δν = 1 of 12 C 16 O, have been measured. The data treatment is described. Accurate determination of the rotational distribution and of the vibrational populations of the three isotopic species is performed. The Δν = 2 and 3 vibrational radiative transition probabilities, versus the Δν = 1 ones, are also given, up to ν = 40.

Vibrational relaxation of NO(X2Π, υ = 1) studied by an IR-UV-double-resonance technique

Abstract Pulses from a mechanically chopped CO-laser were used to optically pump the first vibrational level of NO molecules in their fundamental band near 5.3 μm. The population of NO (υ = 1) was followed by measuring the resonance fluorescence of NO-γ-bands from a microwave discharge lamp in the UV region. Analysis of the first order decays of NO(υ = 1) following the excitation pulses yielded rate constants for VT and VV energy transfer processes in collisions of NO(υ = 1) with ground state NO and added gas molecules He, Ne, Ar, Kr, Xe, H 2 , HD, D 2 , N 2 , O 2 and N 2 O.

Recent progress with the CO-overtone Δv=2 laser

We report the recent progress in the performance of the CO-overtone Δv=2 laser. We were able to increase both the number of lines and the output power by a significant amount. This laser is now a reliable source for spectroscopic applications in the spectral region from 2500–3800 cm−1. The typical parameters of the laser plasma and a table with the observed laser transitions and their frequencies are given.