Michael J. Haugh

Formation of electronically excited CO2+ in collisions of H+, H, He+, and Ne+ with CO2

Electronic excitation in collisions of 200 eV–25 keV H+, H, He+ and Ne+ with CO2 has been investigated under single‐collision conditions in an atomic beam experiment. For each of these projectiles, the B 2Σu+ and A2πu states of CO2+ are the dominant electronically excited species emitting in the 200–500 nm wavelength range owing to target excitation. Although dissociative electron capture by He+ and Ne+ to form electronically excited CO+ minimizes the energy defect, no emission is observed from CO+ A2π, and formation of CO+B2Σ+ is of only minor importance. The latter observations are in direct contradiction to predictions derived from the adiabatic criterion and demonstrate that, in the competition among allowed excitation processes, details of the electronic structure of the collision partners override energy considerations. The qualitative features of the emission spectrum excited by H+, H, and He+ are insensitive to relative velocity over the energy range covered. The ratio of the cross sections for ex...

Production of Excited States in the Collision of He+ with HCl and HBr; Selective Production of the v=0 Level in the A2Σ State of the Molecule‐Ion

The emission of light over the 2000–6000‐A wavelength region resulting from the collision of the He+ ions on HBr and HCl has been investigated at ion energies from 0.3 to 4.0 keV. Emission is observed from the A 2Σ → X 2Πi transition of HBr+ and HCl+, from atomic hydrogen, and from Br+ and Cl+ ions. Relative cross sections were determined for the formation of individual H‐atom quantum levels and vibrational levels of the molecular ion. Vibrational populations were proportional to the Franck‐Condon factors for a transition between the neutral ground state and the A 2Σ state. An unusual dependence upon target gas pressure was observed for the intensity of the (0, 0) band of both HCl+ and HBr+. The intensity increases quadratically with target gas pressure in the 10−4‐torr region. Similar studies of other ions impinging on HCl and HBr do not show this effect. The most plausible mechanism satisfying all of the experimental observations is that two molecules of target gas are involved in the excitation process...

Transition moment variation in the A2Σ+ → X2Πi transition of HCl+, DCl+ and HBr+

Abstract Relative emission intensities of sixteen bands of HCl + ( A 2 Σ + - X 2 Π i ), four bands of DCl + ( A 2 Σ + - X 2 Π i ), and 5 bands of HBr + ( A 2 Σ - X 2 Π i ) have been made using both ion-beam excitation and microwave discharge sources. Intensities were determined by comparison with computer-generated spectra. Treatment of the data within the r -centroid approximation shows that in HCl + the electronic transition moment decreases strongly at large r v′v″ [ Re α exp (−3.6 r v′v″ ) for 1.44 A v′v″ A ] but levels off at shorter r v′v″ . DCl + data agree quantitatively with HCl + . The variation in the HBr + moment is similar, with R e α exp [−4.5 r v′v″ ] for 1.58 A r v′v″ A .

Electronically excited states of CO2+ formed in collisions of heavy ions with CO2: Bending vibrations produced in the ? 2Πu state of CO2+

The emission from CO2+ produced by collisions of He+, Ne+, Ar+, and Kr+ has been examined under single collision conditions at 0.1 nm spectral resolution using a beam technique with energies from 0.2 to 4 keV. A fine structure is observed that extends through the 300 to 500 nm region. It has been identified as bending transitions in the ? 2Πu→? 2Πg system of CO2+. The bending transitions in this system have not been analyzed previously. Collisions of the inert gas ions with CO2 produce CO2+ (? 2Πu) with extensive bending vibrations excited, while collisions with H2+, electrons and photons produce the ? 2Πu state with no observable bending. For the heavy ion collisions (except H2+) up to six quanta of bending vibrations are excited. The fraction of ? state produced with bending modes excited range from 0.40 to 0.60 of the total A state produced in the collisions, depending on velocity and particle identity. To explain these observations we propose formation of a quasimolecule analogous to HCO2 which gives ...

Production of metastables in ion‐molecule collisions

A second‐order process observed in the emission from the A 2Σ states of HCl+ and HBr+ produced by collisions of 2.5 keV He+ ions with HBr and HCl is shown to arise from the formation of fast He metastables in the primary collision. These fast metastables react with the molecules to produce molecule‐ions in the A 2Σ state which have a vibrational distribution much different from that expected on the basis of a Franck‐Condon excitation. This vibrational distortion has been observed for thermal He metastable reactions with HBr and HCl but has never been observed for fast metastables and is difficult to reconcile with present theory of Penning ionization. The vibrational distribution in HCl+ produced by the fast He metastables is distorted considerably more than that produced by thermal metastables. The cross section for reaction of fast He metastables with HBr is 90× 10−16 cm2. The Ne+ ions reacting with HBr and HCl also produce fast Ne metastables. These react to produce molecule‐ions with distorted vibrati...