Geoffrey Duxbury

The 1997 spectroscopic GEISA databank

The current version GEISA-97 of the computer-accessible database system GEISA (Gestion et Etude des Informations Spectroscopiques Atmospheriques: Management and Study of Atmospheric Spectroscopic Information) is described. This catalogue contains 1,346,266 entries. These are spectroscopic parameters required to describe adequately the individual spectral lines belonging to 42 molecules (96 isotopic species) and located between 0 and 22,656 cm-1. The featured molecules are of interest in studies of the terrestrial as well as the other planetary atmospheres, especially those of the Giant Planets. GEISA-97 contains also a catalog of absorption cross-sections of molecules such as chlorofluorocarbons which exhibit unresolvable spectra. The modifications and improvements made to the earlier edition (GEISA-92) and the data management software are described. GEISA-97 and the associated management software are accessible from the ARA/LMD (Laboratoire de Meteorologie Dynamique du CNRS, France) web site: http://ara01.polytechnique.fr/registration.

The Renner effect in a bent triatomic molecule executing a large amplitude bending vibration

The theory is given for a model hamiltonian which combines the effects of large amplitude bending for a bent triatomic molecule with those arising from the coupling of vibrational, electronic and spin angular momenta characteristic of linear molecules. Two numerical examples are given for the application of the theory. (i) The vibronic and spin-orbit coupling between the A, 2 A 1 and [Xtilde], 2 B 1 states of PH2 is discussed in detail in terms of the 2Π u state of linear PH2 with which they correlate. The v 2 and K dependence of the doublet splitting in the A state is well represented in terms of a single spin coupling parameter for the 2Π u state. (ii) The calculations give a quantitative account of the inversion for high v 2 of the K = 0 and K = 1 levels of the bent B 2 component of the a, 3 A 2 state of CS2 arising from its correlation with a 3Δ2u state of linear CS2.

Fast, real-time spectrometer based on a pulsed quantum-cascade laser

We describe a mid-infrared spectrometer that is based on the combination of a multiple-pass absorption cell and a submicrosecond pulsed quantum-cascade laser. The spectrometer is capable of both making sensitive measurements and providing a real-time display of the spectral fingerprint of molecular vapors. For a cell with a path length of 9.6 m, dilution measurements made of the nu9 band transitions of 1,1-difluoroethylene indicate a sensitivity of 500 parts in 10(9), corresponding to a fractional absorbance of 4 x 10(-4).

The Renner effect in a bent triatomic molecule using the adiabatic approach

The consequences of the Renner-Teller effect in molecules executing large amplitude bending motion have been treated using an adiabatic approach (Barrow, Dixon and Duxbury [8]) and using a perturbed linear molecule approach (Jungen and Merer [7]). This paper makes detailed numerical comparisons of these two methods in calculations of the vibronic levels of H2S+, NH2 and CO2 +. It is shown that the two methods are completely equivalent. For molecules in which at least one component of a Renner-Split degenerate state is bent the adiabatic approach is computationally considerably the less demanding of the two. However, for linear molecules the Jungen and Merer approach is more rapidly convergent.

Effects of orbital angular momentum in CH2 The Renner-Teller effect

The vibrational and K-type rotational levels of the a 1 A 1 and the [btilde] 1 B 1 state of CH2 have been fitted by least squares to give a pair of Born-Oppenheimer potential curves for the combining electronic states. An accurate description of the effects of orbital angular momentum on the energy level positions has been obtained. We have been able to relate the force constants used in the calculation of the bond length variation in the semi-rigid bender model to the parameters derived from the deperturbation of the Fermi resonance interaction, and have shown that there is reasonable agreement between them. Finally we have been able to link the amount of unpaired angular momentum ⟨Lz ⟩ produced by the vibronic coupling to the degree of magnetic activity observed in the vibronic sub-bands, and in rotational energy levels of the a, v 2 = 0 state.

Renner-Teller and spin-orbit interactions between the 1A1, 1B1 and 3B1 states of CH2

The [btilde]1B1-a1A1 spectrum of the CH2 radical exhibits singlet-triplet perturbation of the a and the [btilde] states, even though the direct first-order interaction between the [btilde]1B1 and [Xtilde]3B1 states is forbidden. We have derived a model for such interactions that involves both Renner-Teller and spin-orbit coupling. The results of calculations made using this model show that the effect of the Renner-Teller coupling is to generate 1B1-3B1 interactions that are as large as those between the 1A1 and 3B1 states. These effects are much larger than those normally expected when second-order singlet-triplet coupling occurs. From these calculations, we have identified the triplet-state levels responsible for the [btilde]-[Xtilde] perturbations, and have refined the triplet-state potential.

Real-time trace-level detection of carbon dioxide and ethylene in car exhaust gases

A direct-absorption spectrometer, based on a pulsed, distributed feedback, quantum cascade laser with a 10.26-microm wavelength and an astigmatic Herriott cell with a 66-m path length, has been developed for high-resolution IR spectroscopy. This spectrometer utilizes the intrapulse method, an example of sweep integration, in which the almost linear wavelength up-chirp obtained from a distributed feedback, quantum cascade laser yields a spectral microwindow of as many as 2.5 wave numbers/cm(-1). Within this spectral microwindow, molecular fingerprints can be monitored and recorded in real time. This system allows both the detection of carbon dioxide and ethylene and the real-time observation of the evolution of these gases in the exhaust by-products from several cars.

Rapid passage induced population transfer and coherences in the 8 micron spectrum of nitrous oxide

Rapid passage signals showing the effects of molecular alignment have been observed when low pressure samples of nitrous oxide are interrogated by radiation from a pulsed 7.84 µm quantum cascade laser. These effects occur when the sweep rate of the laser through a Doppler broadened absorption line is much faster than the collisional relaxation time, and when the power density of the linearly polarized laser radiation is sufficient to cause optical pumping. Using a laser pulse of duration 1.3 µs, the frequency sweeps approximately 90 GHz. The variation of the laser tuning rate during the laser pulse, from about100 MHz/ns at the beginning to about 20 MHz/ns at the end, allows the relationship between sweep rate and collisional damping to be investigated. It is shown, by comparing the experimental signals with those calculated by coupled Maxwell–Bloch equations, how the rapid passage effects in nitrous oxide are influenced by the number density, transition cross-section and reorientation lifetime.

An intercomparison of laboratory measurements of absorption cross-sections and integrated absorption intensities for HCFC-22

Abstract An intercomparison of measurements of infra-red absorption cross-sections and integrated absorption intensities in HCFC-22 has been carried out. Independent measurements were made by five spectroscopy groups so that their experimental methods and data reduction techniques could be critically examined. The initial results showed a spread in reported values for integrated absorption intensity over the mid infra-red spectral region that were larger than the reported uncertainties. Re-examination of experimental and data reduction methods resulted in consistency of results within the reported errors. It was found that particular attention had to be paid to controlling and characterising errors associated with the non-linear response of MCT detectors and pre-amplifiers, adsorption of the sample on surfaces in the absorption cell, effectiveness of mixing when making measurements on mixtures of absorber gas and a buffer gas, and location of the photometric “full-scale” and “zero” levels.

The Fourier transform infrared spectrum of methyleneimine in the 10 μm region

The infrared spectrum of methyleneimine, CH2NH, has been observed in the gas phase with a resolution of 0.0048 cm−1 using the National Optical Observatory solar Fourier transform spectrometer. The CH2NH was produced by the pyrolysis of CH3NH2 at ∼1000 °C. Vibrational band origins and rotational constants have been determined for the ν7, ν8, and ν9 bands. These were analyzed as a set of three states strongly coupled by Coriolis interaction. More than 2500 transitions were assigned, of which many are ‘‘forbidden’’ transitions, such as qQ branches, induced in the ν9 band by the strong Coriolis interaction between ν7 and ν9. Owing to the high precision of the data, and the high J values of many of the observed transitions, it was necessary to include many of the high order interaction constants in order to achieve a quantitative fit.