A. Hopkirk

A study of the absolute photoabsorption cross section and the photoionization quantum efficiency of nitrous oxide from the ionization threshold to 480 Å

Abstract The absolute photoabsorption cross section and the photoionization quantum efficiency of nitrogen have been measured using a double ion chamber and monochromated synchrotron radiation. In the wavelength range encompassed by the present study, several well-established Rydberg series give rise to prominent structure in the absorption spectrum. A molecule excited into a Rydberg state may decay either by autoionization or by predissociation into neutral products, and this competition between de-excitation processes causes variations to be observed in the photoionization quantum efficiency. The results indicate that many of the Rydberg states undergo rapid predissociation. A sum rule analysis has been carried out by combining the present absolute photoabsorption cross section with similar data covering the remaining wavelength regions.

A study of the absolute photoabsorption, photoionization and photodissociation cross sections and the photoionization quantum efficiency of oxygen from the ionization threshold to 490 Å

Abstract The absolute photoabsorption, photoionisation and photodissociation cross sections and the photoionisation quantum efficiency of sulphur dioxide have been measured using a double ion chamber and monochromated synchrotron radiation. The absorption spectrum exhibits extensive vibrational structure extending from the ionisation threshold to approximately 750 A. Many of the features can be arranged into vibrational progressions with spacings characteristic of the symmetrical stretching mode, in agreement with previous interpretations. The photoionisation quantum efficiency increases rapidly to reach a value of approximately unity around 880 A, but at shorter wavelengths local minima are observed around 760, 820 and 870 A. Some of the variations in the photoionisation quantum efficiency appear to be correlated with the absorption vibrational structure. A sum rule analysis has been carried out by combining the present absolute photoabsorption measurements with similar data covering the remaining wavelength regions.

Decay pathways of excited electronic states of Group IV tetrafluoro and tetrachloro molecular ions studied with synchrotron radiation

This paper describes experiments to probe the dynamics and decay pathways of the C 2T2 and D 2A1 excited electronic states of Group IV tetrahalide molecular ions MX+4 (M=C, Si, Ge; X=F, Cl) in the gas phase. Tunable vacuum UV radiation from a synchrotron source is used to ionize MX4 into these electronic states of MX+4. Flouorescence from ions initially produced or from fragments is monitored undispersed by a suitable pm tube. When the synchrotron is operated cw, such synchrotron‐induced fluorescence spectra give energy thresholds for fluorescence, and in favorable cases an estimate of the fluorescence quantum yield (of MX+4 C or D) can be made. When the synchrotron is operated pulsed, radiative lifetimes can be measured as a function of excitation energy. The fluorides and chlorides display very different decay properties. The dynamical behavior of these states is rationalized (a) with respect to their spectroscopic properties, and (b) with respect to the dissociation channels energetically ‘‘open’’ ...

Vacuum ultraviolet fluorescence excitation spectrum of I2

A detailed study of the vacuum ultraviolet fluorescence excitation spectrum of I2 in the region 110–210 nm is reported. Both absorption and fluorescence excitation spectra were recorded simultaneously using continuously tunable synchrotron radiation. At long wavelengths (176–210 nm) the fluorescence excitation spectrum is dominated by the D(0+u) ion‐pair state. Perturbations between the D(0+u) state and isoenergetic levels of the c6 Rydberg state lead to pronounced resonances (dips) in the fluorescence excitation spectrum. Weaker absorption to the F(0+u) and F’(0+u) ion‐pair states is identified at 170 and 150 nm, respectively. Below 149 nm electronically excited iodine atoms (I 6s 4P5/2 and I 6s 2P3/2) are formed by predissociation of high‐lying Rydberg and ion‐pair states.

Electronic emission spectroscopy of Group IV tetrachloro molecular ions

Two broad continuous bands are observed in the visible region following electron impact ionization of a He‐seeded molecular beam of SiCl4 or GeCl4. By using tunable vacuum UV radiation from a synchrotron source to measure the threshold energy at which the fluorescence bands occur, it is shown that the bands are related to the initial formation of the C 2T2 state of the parent ion SiCl+4/GeCl+4. By comparison with photoelectron data, the bands are assigned to bound–free transitions in SiCl+4/GeCl+4 C 2T2–A 2T2 and X 2T1. The C state of CCl+4 does not fluoresce. The results are compared with the electronic emission spectra of the three tetrafluoro molecular ions.

Threshold photoelectron spectroscopy of I2

The threshold photoelectron spectrum of molecular iodine has been measured at a resolution of 6–8 meV using synchrotron radiation in the region 9–14 eV and a penetrating-field electron spectrometer. The spectrum reveals resolved vibrational structure in each spin-orbit component of the X (2Πg,i) state of I2+, which arises as a result of both direct photoionization and autoionization processes. In addition, extensive resolved vibrational structure has been observed in the A (2Πu,i) state of I2+ for the first time in any electron spectroscopic study. The average vibrational spacings were as follows: 220 (8) cm −1 in X (2Πg,32), 214(8) cm−1 in X (2Πg12), and 120(8) cm−1 averaged over both spin-orbit components in A (2Πu,i).

Threshold photoelectron spectroscopy of CF4 up to 60.5 eV

Abstract The threshold photoelectron spectrum of CF4 up to 60.5 eV has been recorded using synchrotron radiation and a penetrating-field electron spectrometer of high resolution. Our results confirm the broad spectral features of the X 2T1, A2T2 and B 2E state bands of CF4+ reported previously with He I PES. The influence of autoionizing super-excited states of CF4 has been observed as an extension of the high-energy tail of the B 2E state, as a structured band in the energy region between the B 2E and C 2T2 states, and as additional vibrational structure extending the D 2A1 state band. Improved spectra of the C 2T2 and D 2A1 states have been obtained as a consequence of the high resolution of this study. The overall electronic state band profiles observed here up to about 26eV agree with a recently published low-resolution threshold photoelectron spectrum of CF4 [Creasey et al., Chem. Phys., 174 (1993) 441]. Between 26 and 60.5 eV a continuum-like background was observed with some broad-band features superimposed on it. One broad peak, centered on 40.3 eV, is tentatively assigned to the formation of the Ẽ 2T2 and F 2A1 states of CF4+, but the formation of the double-charged ion, CF42+, may also be involved.

Vacuum ultraviolet absorption and fluorescence excitation spectra of Br2

Abstract Absorption and fluorescence excitation spectra of Br 2 in the region 125–170 nm have been recorded using tunable synchrotron radiation. Computer simulations of the absorption and dispersed fluorescence spectra have allowed identification of the upper electronic state responsible for the main fluorescence excitation system (150–167 nm), as the D(0 + u ) ion-pair state. A potential function for this state is presented which accounts for vibrational levels up to ν′ ≈ 170 and a pronounced inflection on the inner wall of this potential, due to an avoided crossing, is identified at T e + 15000 cm −1 . The mean radiative lifetime of the D(0 + u ) state has been determined as ≈ 9 ns. An analysis of the 320–360 nm structured continuum fluorescence, from the D(0 + u ) state to a lower repulsive state, is also given.

High-resolution threshold photoelectron spectroscopy of hydrogen iodide

The threshold photoelectron spectrum of hydrogen iodide has been recorded at high resolution (4–30 meV) using synchrotron radiation and a penetrating-field electron spectrometer over the photon energy range 10.2–31.0 eV. Extended vibrational structure in the first Franck-Condon gap region between the X (2Πi) and A (2Σ+) states of HI+ has been observed and analyzed to yield improved spectroscopic constants for the X (2Πi) state. The Rydberg states responsible for the production of this structure, through resonance autoionization, are identified as [A (2Σ+]nsσ 1Σ+ for n = 6–8. A vibrational progression in the A (2Σ+) band system in HI+ leading up to the dissociation continuum of this state has been identified for the first time, yielding spectroscopic constants for the state. The A (2Σ+) state of HI+ appears to be formed by an avoided crossing between the diabatic A (2Σ12+) and 4Π12 potentials. Spectroscopic constants are derived for the ‘main’ band system for 5sσ−1 inner-valence ionization of HI.

High-resolution threshold photoelectron spectroscopy of molecular fluorine

Abstract The threshold photoelectron spectrum of molecular fluorine has been recorded in the 5.6–21.9 eV photon energy range, at resolutions ranging from 3 to 12 meV, using synchrotron radiation and a penetrating-field electron spectrometer. In addition to observing the three known band systems of F 2 + at higher resolution than previously achieved with conventional photoelectron spectroscopy, extensive vibrational structure is found in the Franck-Condon gaps between the main electronic systems of F 2 + . This extended vibrational structure is attributed to resonance autoionization of neutral Rydberg states.