C. Y. Robert Wu

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.

Photoabsorption and direct dissociation cross sections of C2H2 in the 1530–1930 Å region: A temperature dependent study

Using synchrotron radiation as a continuum light source, the absolute photoabsorption and direct dissociation cross sections of C2 H2 in the 1530–1930 A region have been measured at 295 and 155 K. In comparison with cross sections obtained at room temperature, the cross sections at absorption peaks typically increase by 10%–40% while those at absorption valleys decrease by as much as 30% at 155 K. Further, the absorption band profiles become narrower as the gas temperature decreases. In the presently studied wavelength region, there are at least three electronic states, namely, the A 1 Au , the B 1 Bu , and a continuum. Using the present low temperature data we have constructed the repulsive potential energy curve of the direct dissociation state according to the Franck–Condon principle. The potential energy curve thus constructed can be more accurate than that constructed by using room temperature cross section data because only the absorption cross section from the ν’=0 level contributes. The present ...

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.

Photoabsorption cross sections of CH3F, CHF3, CH3Cl, and CF2Cl2 from 175 to 760 A

Using synchrotron radiation as a continuum background, the absorption cross sections of CH3F, CHF3, CH3Cl, and CF2Cl2 were measured in the 175–760 A region using a double ionization chamber. Discrete structure was observed in CHF3 and CH3Cl and attributed to the Rydberg states converging to the known ion states. In CHF3, Rydberg states convergin to the (3e)−1 and (5a1)−1 ion states with averaged vibrational frequency of 910 cm−1 were observed in the 610–640 A region. Another Rydberg state converging to the (4a1)−1 ion state with an averaged vibrational frequency of 1080 cm−1 was also observed in the 530–540 A region. In CH3Cl, a Rydberg state converging to the (3a1)−1 ion state with a vibrational frequency of 2330 cm−1 was observed in the 580–620 A region. No vibrational structure was observed in CH3F and CF2Cl2.

PHOTOABSORPTION AND PHOTOIONIZATION CROSS SECTIONS OF NH3, PH3,H2S, C2H2, AND C2H4 IN THE VUV REGION

Abstract Using synchrotron radiation as a continuum light source, the photoabsorption and photoionization cross sections of NH3, PH3, H2S, C2H2, and C2H4 have been measured from their respective ionization thresholds to 1060 A. The vibrational constants associated with the v2 totally symmetric, out-of-plane bending vibration of the ground electronic state X2A1 of PH+3 have been obtained. The cross sections and quantum yields for producing neutral products through photoexcitation of these molecules in the given spectral regions have also been determined. In the present work, autoionization processes were found to be less important than dissociation and predissociation processes in NH3, Ph3, and C2H4. Several experimental techniques have been employed in order to examine the various possible systematic errors critically.

Measurements of temperature-dependent absorption cross sections of C2H2 in the VUV-UV region

Using a synchrotron radiation as a continuum light source, we have carried out laboratory measurements of the temperature-dependent photoabsorption cross sections of C2H2 from 120 to 230 nm region at 295 K and from 120 to 215 nm region at 150 K with a spectral resolution of 0.007 nm. In the presently studied spectral region the cross section values vary by 6 orders of magnitude, and the absorption features vary from sharp rovibronic structure, shaded bands, to diffuse bands. High-temperature cross sections of C2H2 in the 120–140 nm region has also been measured for the purpose of identifying possible hot bands. The previous assignments of several hot bands in the 130–140 nm region were crucial in the determination of the electronic structure of a valence E state. However, we find that the previously assigned hot bands at 135.09 and 136.24 nm are not hot bands at all because they are insensitive to temperature change from 150 to 370 K. The Galileo spacecraft has made incredibly successful observations of the Jovian system including the prominent C2H2 auroral features, and the Jupiter flyby of the Cassini spacecraft is yet to come. Equipped with higher-resolution spectrometers, the Cassini mission promises to provide higher scientific returns regarding the Jovian auroral observations. Solar and stellar occultation experiments at Saturn by the Cassini spacecraft will provide more detailed observations of the Saturnian upper atmosphere than ever before. The data presented in the present work are an extension of our effort to provide the required data to the scientific community and will have a significant impact on our understanding of the atmospheres of Jupiter and Saturn, in particular, and other outer planets and satellites, in general.

Assignments of autoionization states of O2

Abstract Autoionization states of O2 leading to the b 4Σg-, B 2Σg- and c 4Σu- states of O+2 have been well established, while those leading to the a 4∏u and A 2∏u are not certain. Further, some of the assignments of the autoionization states and the determinations of effective quantum numbers and quantum defects are open to questions. In view of a recent study of isotopic oxygen molecules, the vibrational assignments for the autoionization states leading to the a 4∏u and A 2∏ucan be unamiguosly assigned. We have systematically examined the known Rydberg series and have suggested new assignments and interpretations for several autoionization states leading to the various states of O2+.

Study of sulfur-containing molecules in the EUV region. II - Photoabsorption cross section of COS

Using synchrotron radiation as a continuum background, the absorption cross sections of COS have been measured using a double ion chamber. The cross sections range from 7 to 50 Mb in the 175–760 A region. The window resonance series VII (and VII′) and IX (and IX′) are tentatively assigned as nsσ and npπ converging to the C 2Σ+ ion state of COS+. A dip at about 846 A, which possibly corresponds to the n = 3 member of the VII series, is found in the high resolution photoionization curves previously published [Eland and Berkowitz, J. Chem. Phys. 70, 5155 (1979); Ono et al., J. Chem. Phys. 74, 1645 (1981), and Delwiche et al., J. Chem. Phys. 74, 4219 (1981)]. Broad structures observed in the 580–650 A region may be Rydberg series converging to the highly excited ion states which are yet to be resolved and analyzed.

Photoabsorption cross section of acetylene in the EUV region

Using synchrotron radiation as a continuum background, the absolute total absorption cross section of C2H2 has been measured using a double ionization chamber. The cross sections range from 2.5 to a maximum of 35 Mb in the 175–740 A region. Two new Rydberg series have been identified and apparently converge to the B 2Σ+u state of C2H+2 at 18.71 eV. The observed Rydberg states are tentatively assigned to (2σu)−1 nsσg 1Σ+u and (2σu)−1 ndσg 1Σ+u, respectively. The present cross section data have been used in the analysis of various sum rules including the TRK sum rule.

Lyman‐α and Balmer‐series fluorescence from hydrogen photofragments of H2O vapor

Atomic hydrogen Lyman‐α and Balmer‐series (3–9→2) fluorescence have been observed from photodissociation of H2O, and the production cross sections have been measured. A line emission source provided the primary photons at wavelengths from 400–900 A. The maximum fluorescence production cross sections for Lyman‐α and Balmer‐α are 1.72±0.34 Mb at 703 A and 0.58±0.17 Mb at 672 A, respectively. The photodissociation is mainly a direct process following absorption of a photon into the continuum state(s). The repulsive potential energy curves corresponding to the respective pseudodiatomic OH–H(n) molecule are constructed according to the Franck–Condon principle. The efficiency for converting H(2S) to H(2P) by H2 collisions, following photodissociation of H2, is determined to be 56±10%, while that by H2O, following photodissociation of H2O, is estimated to be nearly 100%. The present data are important for further understanding of cometary photochemistry.