L. C. Lee

Photoabsorption cross sections of CH4, CF4, CF3Cl, SF6, and C2F6 from 175 to 770 Å

Using synchrotron radiation as a continuum background, the photoabsorption cross sections of CH4, CF4, CF3Cl, SF6, and C2F6 were measured photoelectrically in the 175–770 A region. Discrete structure was observed and attributed to the Rydberg states converging to the known states of the ions: In CH4, Rydberg states converging to the (2a1)−1 ion state with vibrational frequencies varying from 1800 to 2200 cm−1 were observed in the 563–627 A region. In CF4, Rydberg states converging to the (4a1)−1 and (3t2)−1 ion states with vibrational frequencies varying from 550 to 730 cm−1 were observed in the 559–611 A region. ’’Window’’ type Rydberg states converging to the (4a1)−1 ion states were also observed. In CF3Cl, discrete structure with energy spacings varying from 540 to 700 cm−1 was observed in the 614–666 A region. The assignment of this structure to Rydberg states was not attempted because the vibrational structure of the CF3Cl ion states is not known. In SF6, structure was observed in the 630–676 A regio...

Electronic Transition Moments for the A → X, B→ X, and B→ A Transitions in CO+ and the A← X and B← X Moments for the CO→ CO+ Systems; Absolute Cross Sections for the Absorption Processes

The A→ X, B→ X, and B→ A bands of CO+ have been excited using monochromatic photons and the band intensities measured. Using such data the variation of the electronic transition moments for these above emission bands as well as for the absorption bands CO(X 1Σ+, v″=0) → CO+ (A 2πi, v′ = 0–8) and CO+ (B2Σ+, v′ = 0, 1) have been determined. Further, the specific cross sections for the absorption processes have been determined by measuring the total emission intensity in the band system through which the upper state decays. The band intensity measurements and the derived results are presented in the ensuing discussion.

CS(A 1Π→X 1Σ+) fluorescence from photodissociation of CS2 and OCS

The CS(A 1Π→X 1Σ+) fluorescence resulting from photodissociation of CS2 and OCS by vacuum ultraviolet emission lines extending from λλ686–1239 A has been investigated. With the assumption that the electronic transition moment is constant, the Franck–Condon factors for the emission system and the populations of the vibrational levels of the CS(A 1Π) electronic state have been determined. The population data are approximately represented by a Poisson distribution, which is predicted from a theoretical model. A population inversion between the v=0 and 1 levels of the CS(A 1Π) state is found. The cross sections for the production of fluorescence are also presented.

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.

Vacuum ultraviolet fluorescence from photodissociation fragments of CO and CO2

The vacuum ultraviolet fluorescence from the atomic photofragments of O2 and N2 produced by extreme ultraviolet excitation has been investigated and their production cross sections measured. The processes were studied using synchroton radiation, in the λλ175–780 A region, to excite the gases. The nature of the fluorescence spectrum in the wavelength range λλ1050–1800 A was studied by using a combination of optical and gaseous filters. The dependence of the intensity of the fluorescence radiation on the gas pressure was studied in order to determine the relative contributions from direct excitation and photoelectron excited emissions. Direct excitation processes were verified to be the primary contributor, particularly at low pressures. In the incident wavelength range λλ200–350 A, the cross sections for the production of fluorescence for each gas shows a broad band, which is presumably produced by dissociative ionization processes. In the longer wavelength region, discrete structure was observed at the po...

Cross sections for the production of the N2+ (B→X), CO+ (B→X) and N2O+ (A→X) fluorescence by photoionisation

The cross sections for the production of the N2+(B 2 Sigma u+ to X 2 Sigma g+), CO+(B 2 Sigma + to X 2 Sigma +) and N2O+(A 2 Sigma + to X 2 Pi ) fluorescence in the 3000-6000 AA region are measured with synchrotron radiation from their fluorescence thresholds to 175 AA. The technique used to correct the second-order effect is described. The present results are compared with previous data. Near 540 AA the cross section for the production of the N2+ fluorescence shows structure at the positions of the Rydberg states converging to the N2+(C 2 Sigma u+) state. The cross section for the production of the CO+ fluorescence shows a broad band with its peak at 400 AA. The cross sections for the production of the N2O+ fluorescence shows structure at the positions of the Rydberg series VI-IX.

The absorption cross sections of H2 and D2 from 180 to 780 Å

Abstract The absolute photoabsorption cross sections of H2 and D2 have been measured photoelectrically in the wavelength region from 180 to 780 A using synchrotron radiation. At photon wavelengths shorter than 650 A, the absorption cross section was found to be structureless and to decrease montonically with decreasing wavelength. In this spectral region, both gases have nearly equal absorption cross sections, varying only within the experimental error. At photon wavelengths greater than λ 650 A, structure is evident in the cross section of both gases and presumably corresponds to the Rydberg States reported by various authors.

Co+(A2Π to X2Σ +) and CO2+(A2 Π u to X2 Π g) fluorescence produced by vacuum ultraviolet radiation between 175-750 AA

The CO+(A2 Pi to X2 Sigma +) and CO2+(A2 Pi u to X2 Pi g) fluorescence produced by impacting CO and CO2 with synchrotron radiation of wavelength in the 175-750 AA region was investigated and the production cross sections were measured. For primary photon wavelengths longer than 630 AA, the fluorescence cross sections show structure at the positions of the Rydberg series for both gases. For primary photon wavelengths shorter than 500 AA, the cross sections decrease monotonically with the wavelengths. The present measurements are compared with the various results reported by previous authors.

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.

Dispersed fluorescence observations of the CO(A 1Π→X 1Σ+) transitions from photodissociation of CO2

Spectra from 1500 to 1800 A have been obtained showing CO(A 1Π→X 1Σ+) fluorescence arising from photodissociation of CO2. A line emission source provided the incident photons at 15 selected wavelengths from 449 to 955 A. Analysis of the spectra confirms that the CO(A 1Π→X 1Σ+) band system dominates the vuv fluorescence at incident photon wavelengths from 700 to 923 A. Vibrational populations of the CO(A 1Π) electronic state for v=0, 1, and 2 are seen to follow Poisson distributions. The onset of dissociation to CO(A 1Π)+O(1D) at 801 A, the threshold for that process, is apparent from the relative increase in the A 1Π (v=0) population. At the minimum energy required to produce CO(A 1Π)+O(1S), no increase in the v=0 population is observed. Absolute partial cross sections for production of the CO(A→X) bands at incident photon wavelengths of 901.2 and 923.2 A are reported. The data also show the electronic transition moment to decrease linearly with the r centroid to ∼1.25 A, consistent with previously establ...