Flowing Helium Afterglow Research Articles

Formation of NO(A 2Σ+,C 2Πr,D 2Σ+) by the ion–ion neutralization reactions of NO+ with C6F5Cl−, C6F5Br−, and C6F−5 at thermal energy

The ion–ion neutralization reactions of NO+(X 1Σ+:v″=0) with C6F5Cl−, C6F5Br−, and C6F−5 have been spectroscopically studied in the flowing helium afterglow. The NO(A 2Σ+– X 2Πr,C 2Πr–X 2Πr,D 2Σ+–X 2Πr ) emission systems are observed in the NO+/C6F5Cl− reaction with the branching ratios of 0.96, 0.017, and 0.028, respectively, while only the NO(A–X) emission system is found in the NO+/C6F5Br− and NO+/C6F−5 reactions. The vibrational and rotational distributions of NO(A,C,D) indicate that only 1%–11% of the excess energy is deposited into vibration and rotation of NO(A,C,D) for all the reactions. In the NO+/C6F5X− (X=Cl,Br) reactions, a major part of the excess energy is expected to be partitioned into the relative translational energy of the neutral products and the vibrational energy of C6F5X. A comparison of the observed vibrational and rotational distributions with the statistical prior ones indicates that the reaction dynamics is not governed by a simple statistical theory because of the large impact parameter. The excitation mechanism of NO(A,C,D) in the ion–ion neutralization reactions of NO+ with C6F5X− (X=F,Cl,Br,CF3) and C6F−5 is discussed.

Electronic excitation of NO by the ion/ion neutralization reaction between NO + and C 6F 5CF 3− at thermal energy

The ion/ion neutralization reaction between NO +(X 1 Σ +: v″ = 0) and C 6F 5CF 3 − has been spectroscopically studied in the flowing helium afterglow. The NO(A 2 Σ +-X 2II r, C 2II r-X 2II r, D 2Σ +-X 2II r) emission systems are observed in the NO +/C 6F 5CF 3 − reaction, as in the NO +/C 6F 6 − reaction reported previously. The same electronic state selectivity between the NO +/C 6F 5CF 3 − and NO +/C 6F 6 − reactions suggests that the molecular symmetry of the negative ion is insignificant for the electronic state selectivity in the ion/ion neutralization reaction. The relative formation rates of NO(A), NO(C), and NO(D) in the NO +/C 6F 5CF 3 − reaction are evaluated to be 1.0, 0.41 ± 0.003, 0.060 ± 0.010, respectively. Only the v′ = 0 levels of NO(A,C,D) are formed, indicating that no energy is deposited into the vibration of NO(A,C,D). The rotational distributions of NO(A: v′ = 0), NO(C: v′ = 0), and NO(D: v′ = 0) are expressed by single Boltzmann rotational temperatures of 500 ± 50 K, 300 ± 50 K and 400 ± 50 K, respectively. The average fractions of the total available energy deposited into rotation of NO(A), NO(C), and NO(D) are evaluated to be only 1.8 ± 0.2%, 1.4 ± 0.2%, and 2.5 ± 0.3%, respectively. The observed vibrational and rotational distributions are compared with statistical prior ones in order to obtain information on the dynamical feature of the reaction.

Formation of NO(A 2Σ+, C 2Πr, D 2Σ+) by the ion–ion neutralization reaction between NO+ and C6F6− at thermal energy

The ion–ion neutralization reaction between NO+ (X 1Σ+:v″=0) and C6F−6 has been spectroscopically studied in the flowing helium afterglow. In addition to the NO(A 2Σ+–X 2Πr) emission system, which has been found in the previous studies on the NO+/NO−2 and NO+/SF−6 reactions, the NO(C 2Πr–X 2Πr, D 2Σ+–X 2Πr) emission systems are observed in the NO+/C6F−6 reaction. The relative formation rates of NO(A), NO(C), and NO(D) are evaluated to be 1.0, 0.13±0.04, and 0.24±0.04, respectively. Only the v′=0 levels of NO(A,C,D) are formed, indicating that no energy is deposited into the vibration of NO(A,C,D). The rotational distributions of NO(A:v′=0), NO(C:v′=0), and NO(D:v′=0) are expressed by single Boltzmann rotational temperature of 500±50, 300±50, and 400±50 K, respectively. The average fractions of the total available energy deposited into rotation of NO(A), NO(C), and NO(D) are evaluated to be only 1.5±0.1%, 1.4±0.2%, and 1.9±0.2%, respectively. Most of all excess energy is expected to be partitioned into translation of the products. The observed vibrational and rotational distributions are less excited than statistical prior ones, indicating that the reaction dynamics is not governed by a simple statistical theory. The excitation mechanism of NO(A,C,D) in the NO+/C6F−6 reaction is compared with those in the NO+/NO−2 and NO+/SF−6 reactions, which give only the NO(A) state.

The Orsay polarized electron source from a flowing helium afterglow

Abstract A polarized electron source was designed at Orsay. We have chosen to adapt the flowing helium afterglow source working at Rice University because it provides a very high polarization. We have investigated a new way for the optical pumping of the helium metastables. An 85% electron polarization was reached.

Improved source of polarized electrons based on a flowing helium afterglow

The performance of the Rice source of spin polarized electrons, which is based on an optically pumped flowing helium afterglow, has been substantially improved. He(23S) metastable atoms contained in the afterglow are optically pumped using 1.08 μm 23S1↔23P1 radiation from an LNA laser. Spin conservation in subsequent chemi-ionization reactions with CO2 results in the production of free polarized electrons that are extracted from the afterglow. At low currents, ≲1 μA, polarizations of 80%–90% are achieved. This decreases to ∼75% at 10 μA and to ∼50% near 100 μA. The polarization can be simply reversed (P→−P). The energy spread in the extracted beam is <0.4 eV, and the beam emittance is <4 mrad cm−1 at 270 eV. This source is suitable for use in a wide variety of applications, and is particularly attractive for use with the new generation of high-duty factor electron accelerators that are currently being developed.

Intense source of spin-polarized electrons using laser-induced optical pumping

A source of spin-polarized electrons based on a laser-pumped flowing helium afterglow is described. He(23S) atoms contained in the afterglow are optically pumped using circularly polarized 1.08-μm (23S→23P) radiation provided by a NaF (F2+)* color-center laser. Spin angular momentum conservation in subsequent chemi-ionization reactions with CO2 produces polarized electrons that are extracted from the afterglow. At low currents, ≲1 μA, polarizations of ∼70%–80% are achieved. At higher currents the polarization decreases, falling to ∼40% at 50 μA. The spin polarization can be simply reversed (P→−P) and the source is suitable for use in the majority of low-energy spin-dependent scattering experiments proposed to date.

PN + (B 2Σ +-X 2Σ +) emission produced from penning ionizatton of transient PN (X 1 Σ +) radicals

A new emission spectrum in the 305–370 nm region has been recorded from the reaction of transient PN molecules in a flowing helium afterglow. This band system is assigned to the PN +(B 2Σ +-X 2Σ +) transition on the basis of isotopic and photoelectron spectroscopic data. Molecular constants are estimated for the PN +(B) and PN +(X) states.

Intense source of spin-polarized electrons

Spin angular momentum conservation in chemiionization reactions involving optically oriented He(2(3)S) atoms in a flowing helium afterglow has been exploited to yield a source of spin-polarized electrons. Either transversely or longitudinally polarized electrons can be extracted. Polarized electron beam currents of approximately 2 muA have been realized at 40% polarization. The beam has an effective emittance of approximately 2 mrad/cm over the energy range 100-400 eV, an energy spread of less, similar0.15 eV, and the polarization is readily reversible. The source is relatively inexpensive and appears suitable for the majority of low-energy spin-dependent scattering experiments proposed to date.

Neutral and ionic emission from collisionally excited additives to an helium afterglow

Spatially separated emission from ionic and neutral molecules has been observed when N 2, CO, CO 2, N 2O and other gases were added to a flowing helium afterglow. Emission from neutral molecules is probably the result of ion—electron recombination in the plasma of the afterglow. It was observed that neutral molecular emission was decreased when a magnetic field was applied near the glow.

Study of excitation transfer in a flowing helium afterglow pumped with a tuneable dye laser. III. Transfer and quenching reactions of the He2(3p 3Π) molecule

The collisional transfer of excitation from the He2(3p 3Πg) state to other electronically excited states energetically lying within a few KT is examined by optically pumping a flowing helium afterglow with a tuneable dye laser. Rate coefficients for the transfer of excitation to states conserving spin, particularly 3d 3Πu and 3d 3Δu, are found to be greater than 10−10 cm3/sec, resulting in a rapid thermalization of electronic and rotational energy. Relative probabilities of spontaneous radiation for transitions in the visible wavelength range from the product states are found to be 1 and 1/3 for the 3Πu and 3Δu states, respectively, relative to that from the initial 3Πg state. The absence of significant fluorescence from singlet states tends to confirm the conservation of spin during such reactions.

Intense polarized electron beams from chemi-ionization reactions with optically pumpedHe(2S3)

An optically pumped flowing helium afterglow is evaluated as a potential source of polarized electrons. Spin angular momentum conservation in chemi-ionization reactions involving optically oriented $\mathrm{He}(2^{3}S)$ atoms has been exploited to yield microampere electron beams with 30% polarization and energy spread \ensuremath{\le} 0.5 eV. The beam emittance is on the order of 10 mrad cm at 500 V extraction potential. This source appears well-suited for most spin-dependent scattering experiments that have been proposed.

Optical pumping in a flowing helium afterglow with additive metal impurity atoms

It is demonstrated that in a Penning collision of optically oriented metastable helium atoms with an impurity metal atom such as barium, both the longitudinal and transverse components of spin angular momentum are conserved. This results in a polarization of the ion which may be detected by optical emission in the case of excited ion levels or absorption for the ground states. A series of experiments is described which demonstrates the spin dependence of the Penning-collision process. The magnetic resonance of excited-state ions may be observed directly; from the resonance-linewidth radiative lifetimes may be measured.

Study of Excitation Transfer in a Flowing Helium Afterglow Pumped with a Tuneable Dye Laser. II. Measurement of the Rate Coefficient for the Rotational Relaxation of He2(3p 3Πg)

The rotational relaxation of the He2(3p 3Πg) state is examined by optically pumping a flowing helium afterglow with a tuneable dye laser. The population of the J = 8 rotational state is enhanced by optically saturating the R7 component of the transition connecting this state with the metastable He2(2s3Σu+) molecular state. From the lifetime and yield of the Q7 component, the rate coefficient for the rotational relaxation via the forbidden ΔJ = 1 channel is determined to be of the order of 2× 10−11 cm3/sec. It is found that this represents about half of the total rate of rotational relaxation in this state.

Study of Excitation Transfer in a Flowing Helium Afterglow Pumped with a Tuneable Dye Laser. I. Measurement of the Rate Coefficient for Selected Quenching Reactions Involving He (53P)

A system is described for the measurement of excited state reaction times in the nanosecond range. A flowing afterglow produces large populations of chemically unstable species and a pulsed, tuneable dye laser is used to selectively pump these into the reacting excited state. The transient fluorescence from the populations of reactants and end products is used to determine reaction lifetimes and yields. This radiation is collected with a photon counting system which logs arrival times of spectrally dispersed photons with 10 nsec resolution. With this system the rate coefficient for the reaction He(53P)+He→He2++e has been measured to be 8× 10−11 cm3/sec. Upper limits on the reactions He(53P)+He→He(53D)+He and He(53P)+e→He(53D)+e were determined to be 8× 10−15 cm3/sec and 8× 10−9 cm3/sec, respectively.

Excitation of neutral atomic nitrogen in a helium afterglow

A light source of neutral atomic nitrogen has been developed by mixing traces of molecular nitrogen with a flowing helium afterglow at pressures near one torr. This source produces radiation from energy levels near the ionization limit of the neutral atom but none from excited states of the nitrogen ion. The excited nitrogen is produced by recombination of electrons with ionized nitrogen atoms formed in dissociative charge transfer reactions of ionized helium with molecular nitrogen. Approximately 400 new atomic nitrogen lines were found. In addition, a second mechanism for exciting atomic nitrogen was isolated. It consists of a two step collisional process in which N 2 is dissociated by metastable molecular helium and the excitation is produced by collisions with energetic electrons. The characteristic temperature associated with this level population distribution was about 10 4 K.

Vibrational distributions of the N2first positive system produced in a flowing helium afterglow

(1971). Vibrational distributions of the N2 first positive system produced in a flowing helium afterglow. Molecular Physics: Vol. 20, No. 5, pp. 947-951.

Chemiluminescent Reactions of He(23S) with Nitrogen, Oxygen, Carbon Monoxide, and Carbon Dioxide

Rate coefficients for chemiluminescent reactions of excited helium with N2, O2, CO, and CO2 were determined by measuring the spatial dependence of visible emission in a flowing helium afterglow apparatus at pressures between 1 and 2 mm Hg. Rate coefficients are approximately proportional to the temperature over the range 305°–527°K. The magnitudes extrapolated to 300°K in units of 10−10 cm3 molecule−1·sec−1 are as follows: N2 = 1.0 ± 0.3, O2 = 3.3 ± 0.9, CO = 1.7 ± 0.5, CO2 = 7.9 ± 2.5. The relative concentration of metastable He(23S) was determined by optical absorption measurements as a function of total flow rate (or pressure). A close correlation exists between the resulting curve and the curves for the total emission intensity. These data provide strong evidence that Penning ionization by He(23S) atoms is a major process for populating the emitting states.

Spin Exchange between Optically Oriented Metastable Helium Atoms and Thermal Electrons in a Flowing Afterglow

The effects of spin-exchange collisions between thermal electrons and optically oriented metastable helium (${2}^{3}{S}_{1}$) atoms have been observed in a flowing helium afterglow, and the exchange cross section has been measured. The source of electrons derives from Penning collisions between the 19.8-eV metastable atoms and impurity atoms which are titrated into the flow region. The presence of thermal electrons shortens the metastable-spin-state lifetime and reduces the magnitude of the optical-pumping resonance signal. The electron density is determined from the reduction of the metastable density. The exchange cross section is found to be 1.6\ifmmode\pm\else\textpm\fi{}0.4\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}14}$ ${\mathrm{cm}}^{2}$.

Metastable Measurements in Flowing Helium Afterglow

Microwave and optical techniques have been used to measure the decay of the neutral metastable and the charged species in a flowing helium afterglow. Impurity gases, oxygen and argon, are introduced into the afterglow, and measurements of the resulting ionization and excitation yield values for the triplet metastable diffusion coefficient and three-body combination coefficient of 490 cm2 sec−1 Torr and 0.25 sec−1 Torr−2, respectively. In addition, the ambipolar diffusion coefficient of the molecular helium ion was found to be 724 cm2 sec−1 Torr. The mean helium pressure was in the range of 5 to 35 Torr and the gas nominally at room temperature. Absolute triplet metastable densities early in the afterglow were also measured.

Collisional-Radiative Recombination of He2+ into Dissociative States

Spectroscopic investigation of a flowing helium afterglow revealed the presence of strong 10 830-Å atomic helium emission corresponding to the 23P→23S transition, which could be quenched by the application of a weak microwave field to the afterglow. An examination of the axial variation and pressure dependence of the concentration of 23P atoms established that it resulted primarily from the ion—electron recombination of He2+, rather than He+, at the higher pressures examined. An extension of the theory of collisional-radiative recombination of molecular ions to include dissociating states is postulated to explain these observations.