Robert C. Amme

Three‐Dimensional Morse‐Potential Calculation of Vibrational Energy Transfer: Application to Diatomic Molecules

A three‐dimensional semiclassical analysis of the vibrational‐energy‐transfer problem is performed for the Morse potential. The resulting equation is compared with the Schwartz—Slawsky—Herzfeld formulation, and with experimental data for a number of diatomic molecules. The particular case of N2 gives a different Morse repulsive index than that obtained from the second virial coefficient. In contrast to the SSH formulation, a single value of the index suffices over a large range of temperature for N2, O2, and CO. Improvement in the theory is found for cases in which e/kT becomes large.

Ion‐Beam Excitation Effects on the Single Charge Transfer between Argon and Nitrogen

Single charge‐transfer cross sections for N2+ in argon and Ar+ in molecular nitrogen and argon have been measured as functions of ion energy and of ion‐source electron energy. Ion energies studied were in the range of 50 to 1000 eV. The ion‐source electron energy was varied from 16 to 24 eV for the N2+ ions, and 16 to 40 eV for Ar+. For the Ar++N2 case, the cross section appeared to be nonresonant and was insensitive to ionizing electron energy. The slightly endothermic charge transfer for the N2++Ar case also appeared to be nonresonant, but was measurably influenced by ion‐source electron energy. This result is attributed to the presence of long‐lived excited states of N2+ reported earlier. Evidence is presented to show that the (N2+)* ions exhibit near‐resonant behavior in charge transfer with argon atoms. This suggests that the effect is due to an increased population of vibrational states v=1, 2 at higher electron energies.

Charge‐Exchange Cross Sections for Argon Ions in H2 and D2 below 1 keV

Charge‐exchange cross sections for Ar+ ions incident on hydrogen and deuterium have been measured over the energy range of 30 to approximately 1000 eV. The argon‐ion beam was formed by electron bombardment and electrostatic acceleration. Ionizing electron energy was nominally 18 eV, although the results appeared to be insensitive to this parameter. The measured cross sections for Ar++H2→Ar+H2+ as a function of ion energy are compared with the results of other investigators, which are in rather poor agreement. The measurements confirm that the cross section exhibits a maximum at approximately 180 eV incident‐ion energy. Theoretical calculations by Gurnee and Magee and by Karmohapatro are discussed for this reaction. The cross section for Ar++D2→Ar+D2+ was also found to exhibit a maximum, but at approximately 85 eV, which is nearly the same center‐of‐mass energy as for the H2 target. The possible influence of ion—molecule reactions on the charge‐transfer cross sections is discussed.

Ionization and Metastable Excitation in Low‐Energy Collisions of Ground‐State Argon Atoms

A low‐energy beam of argon atoms, formed by nonresonant charge transfer of Ar+ in H2, has been used to explore the near‐threshold behavior for ionizing and exciting collisions between argon atoms. For excess center‐of‐mass energies below 12 eV, the ionization data are consistent with the empirical relationship σ− ≈ 1.8 × 10−21(E − 15.8)1.3, where E is the center‐of‐mass energy in electron volts. Target‐gas mixtures of argon and acetylene were used in an effort to observe metastable atoms formed in atom–atom collisions by Penning ionization. In this case, beam energies were restricted to values less than 30 eV to avoid direct ionization of the acetylene (Penning) gas by the ground‐state atom beam. At 14.5 eV c.m. energy, the cross section for metastable production is shown to be (3.6 ± 2.4) × 10−20 cm2, but this value is dependent upon assumed values of the Penning cross section for Ar* + C2H2 collisions.

Charged‐particle production in low‐energy H+H2 and H+He collisions

Absolute cross sections for producing H+, H−, H+2, He+, and e− have been measured for fast hydrogen atom impact on H2 and He targets. The hydrogen atom energy ranged between 50 eV and 3.0 keV. For the H+H2 reaction, the dominant ion‐formation process for hydrogen atom energies below 250 eV was found to be H−+H+2 production. For He targets, production of H+ dominated over the entire hydrogen atom energy range. The results are compared, where possible, with the data of other investigators and are discussed in terms of possible reaction mechanisms.

Generation of a fast atomic hydrogen beam

Apparatus has been developed for producing a beam of fast hydrogen atoms having energies between 10 and 3000 eV. The procedure involves generation of a beam of hydrogen ions (H−), appropriate ion acceleration and trajectory definition, and ion neutralization by a photodetachment process. The magnitude of the resulting neutral atom flux can be determined to within an uncertainty of less than ±3% by a technique described. An atom beam intensity in excess of 1011 atoms/sec at 1000 eV is readily obtainable, with the intensities at other energies scaling approximately inversely with the primary ion velocity. The advantages of the method over other neutral beam formation procedures are discussed, and techniques allowing considerable enhancement of the beam intensity over that presently achieved are suggested.

Vibrational relaxation in H2O vapor in the temperature range 373–946 K

Vibrational relaxation in water vapor has been observed using both ultrasonic velocity dispersion and absorption. The raw data were corrected for real‐gas effects and for classical absorption. Results are compared with those of other investigators. Observed relaxation times appear much too short to be explained by a simple V‐T process, and are interpreted in terms of a V‐R transfer mechanism. Values of kVR range from about 1×108 sec−1 atm−1 at 374 K to 6×108 sec−1 atm−1 at 946 K.

Origin of secondary electrons ejected from gas-covered surfaces by fast neutral beams.

Secondary electron emission from gas covered metal surfaces, discussing results of neutral beam investigations

Degradation of vacuum-exposed SiO2 laser windows

Observations of a damage phenomenon at the surface of fused silica and crystalline quartz windows are presented. Uncoated windows were mounted at Brewsters angle to facilitate the introduction of a vacuum chamber directly into the cavity of an Ar-ion laser (488 - 514 nm). The transmission of these windows, prior to evacuating the chamber to less than 1 Torr, approaches the theoretical value of > 99.9%, remaining constant indefinitely. However in our normal usage, the chamber is evacuated (P < 10-7 Torr), exposing the windows to high vacuum as well as UV borelight from the laser discharge. After several hours of operation, the intracavity power is observed to decrease monotonically (by approximately 15% per hour) accompanied by the development of a red fluorescence on the inside window surface where exposed to the visible laser radiation. Partial rejuvenation of the windows can be accomplished by reintroduction of gas into the vacuum chamber. Possible damage mechanisms are presented.

Excited hydrogen and argon atom production by charge transfer of metastable Ar+ ions in H2 molecules

Observations of excited products from charge‐transfering collisional between Ar ions and molecular hydrogen are reported.(AIP)