F.Z. Chen

Temperature-dependent photoabsorption cross sections in the VUV-UV region. I. Methane and ethane

Abstract Using synchrotron radiation as a continuum light source, we have measured the absolute photoabsorption cross sections of methane (CH4) and ethane (C2H6) from their respective absorption thresholds to 120 nm , with a spectral bandwidth (FWHM) of 0.06 nm and at three different temperatures, i.e., 370, 295, and 150 K . Only moderate temperature effects are observed in the changes of cross-section values of these two molecules and are attributed to their high vibrational frequencies of the ground electronic states and their repulsive potential surfaces of the excited electronic states. When the gas temperature decreases from 360 to 150 K , the percentage changes of cross sections amount to a maximum of ±30% in CH4 at 142.5 nm and ±20% in C2H6 at 142.3 and 155 nm . The well-known vibrational progressions of C2H6 exhibit pronounced temperature effects in their band profiles which become narrower and sharper as the gas temperature decreases. The data presented are an extension of our effort to provide the required data to the planetary atmospheres community and will have an important impact on our understanding of the atmospheres of the giant planets.

Low and room temperature photoabsorption cross sections of NH3 in the UV region

Abstract Using synchrotron radiation as a continuum light source, we have measured the absolute absorption cross sections of NH3 with a spectral bandwidth (FWHM) of 0.5 A. The photoabsorption cross sections of NH3 have been measured from 1750 to 2250 A under temperature conditions of 295, 195, and 175 K. Significant temperature effects in the absorption threshold region which are mainly due to the presence of hot band absorption are observed. The cross section value at peaks and valleys for the vibrational progressions of the (0,0) to (4,0) bands vary between −80% and +40% as the temperature of NH3 changes from 295 to 175 K. In contrast to this, the changes of cross section values, Pc,T, are found to vary less than 20% for the (v′, 0) vibrational progressions with v′ ⩾ 5. The measured separations between the doublet features of the (0,0), (1,0), and (2,0) bands are found to decrease as the temperature of NH3 decreases. The shifts of peak positions of Pc,T with respect to the corresponding room temperature absorption peaks show a sudden change at v′ = 3 which appears to agree with the trend observed in the homogeneous line widths of the vibrational bands of NH3 ( Vaida et al., 1987 ; Ziegler, 1985 ; Ziegler, 1986 ). The unusual behavior of the line widths has been attributed to the A state potential surface which has a dissociation barrier.

High, room and low temperature photoabsorption cross sections of CS2 in the 1800–2300 Å region

Using synchrotron radiation as a continuum light source, we have measured the absolute photoabsorption cross sections of CS2 with a spectral bandwidth (FWHM) of 0.8 A. The absorption cross sections of CS2 have been measured from 1800 to 2300 A at temperatures of 383 and 295 K and from 1800 to 2150 A at 203 K. Significant temperature effects and hot bands are observed, particularly in the long wavelength region. The apparent continuum cross section underlying the vibrational features in the 2200–2300 A region increases by about an order of magnitude when the gaseous CS2 temperature increases from 295 K to 383 K. The new temperature-dependent laboratory cross section data should be useful in the determination of the CS2 abundance in Jupiters atmosphere after the impact of Comet Shoemaker-Levy 9.

Temperature-dependent photoabsorption cross sections of selected molecular systems

Abstract Using synchrotron radiation as a continuum light source, we have measured the absolute absorption cross sections of several planetary atmospheric gases under various temperature conditions. In general, significant temperature effects are observed in the absorption threshold region which are mainly due to the presence of hot band absorption. Apparent continuum cross sections underlying the vibrational features may increase by orders of magnitude when the gaseous temperature increases. Temperature effects on the linewidth, lineshape, and rovibronic population are discussed. Significant changes in a perturbed spectral region may be possible since perturbation is strongly J-dependent and rotational population depends on temperature. Examples are given to illustrate these temperature effects.

Fluorescence of excited photofragments correlated with electronic states of multiply charged CO produced through photoexcitation of CO in the 28–100 eV region

Abstract Fluorescence excitation functions produced through photoexcitation of CO using synchrotron radiation in the range 28–100 eV have been studied. Two broadband detectors were employed to simultaneously monitor fluorescence in the 1080–3000 A and 3000–7000 A regions. Broad fluorescence peaks are observed and are interpreted in terms of dissociative ionization excitation of multiple electronic states, ion states and Rydberg states converging to multiply charged CO (i.e. CO m + for m =1–3).