Luminescence in near-thermal charge exchange. III. CH(+) and CD(+) A → X emission from He+ C2H2 and He+ + C2D2

A pulsed ICR cell fitted with synchronous photon counting equipment is used to investigate the emission produced between 185 and 500 nm by near-thermal charge exchange between He⁺ and C₂H₂ (C₂D₂). The emission bands observed are A ²Δ → X²π and (weakly) B²Σ^? → X²π in CH(CD) and A ¹π → X¹Σ in CH⁺(CD⁺). Wavelength measurements on the bandheads of the (0,0) and (0,1) bands of CD⁺ A → X are used to evaluate vibrational constants of CH⁺(CD⁺) X¹Σ⁺. The results are (in cm^?1): ω_e = 2869 ± 27 (2106 ± 20); ω_eχ_e = 65 ± 13 (35 ± 7). These constants are used to calculate Morse-potential Franck—Condon factors and vibrational branching ratios for CH⁺ and CD⁺ A → X emission. The spectral distributions and the (relatively low) absolute emission rates produced by He⁺/C₂H₂(C₂D₂) charge exchange are briefly discussed in the light of presently available information on the charge transfer reaction and on the excited states of C₂H_2?⁺     

Vibrational branching ratios for the A3Πi → X3Σ− system of OH+ and OD+

Near-thermal charge exchange between He⁺ and H₂(D₂O) is used as a source of OH⁺(OD⁺) A³H_i→ X³Σ^? emission. A comparison between experimental emission branching ratios and those calculated in the r-centroid approximation suggests that the electronic transition moment varies as a function of the r-centroid.     

Excitation and decay of the C2Σ+u state of N+2 following collisions of He+ ions with N2 isotopes

A quantitative analysis is made of the N⁺₂ “2nd negative” emission (“2N”: C²Σ⁺_u → X²Σ⁺_g) produced by the impact of 500 eV to 25 keV He⁺ beams on ¹⁴N₂, ¹⁴N¹⁵N and ¹⁵N₂. Above about 5 keV, the relative 2N emission rates from the various vibrational levels of the C state are the same as those observed for ? 2 keV Ne⁺, or > / 90 eV electron-impact. These limiting distributions are compared to those predicted for a Franck-Condon excitation of the C state, modified by configuration interaction. The weakening in 2N emission at the vibrational levels ν′ > / 3 is ascribed to spontaneous C-state predissociation. The data fully confirm recent reports that this predissociation extends over a wide range of ν′ and that it is subject to a strong isotope effect. The ratios of the rates of C-state predissociation to 2N emission are obtained for the levels ν′ = 3 to 8 of each nitrogen isotope. By means of these data it is shown that near-resonant charge transfer dominates the distribution of vibrational excitation probabilities only at energies below about 10 eV. A comparison is made of absolute cross-sections for C-state emission with those for N⁺ and N⁺₂ production in He⁺/¹⁴N₂ collisions at energies between 5.5 eV and 25 keV.     

Sensitization of the CN(B2Σ+ → X2Σ+) emission by Hg(63P0) metastables

The vibrational analysis of the CN(B²Σ⁺ → X²Σ⁺) emission sensitized by Hg(6³P₀) metastables has shown that the energy transfer process, Hg(6³P₀) + CN(X²Σ⁺) → Hg(6¹S₀) + CN(B²Σ⁺), populates the CN(B²Σ⁺) state in a non-Franck-Condon fashion. The relative vibrational populations for the ν = 0 to 4 states are 1.00, 0.56 ± 0.06, 0.26 ± 0.03, 0.11 ± 0.03 and 0.04 ± 0.01, respectively. Long-range attractive interaction between the Hg(6³P₀) atom and the CN(X²Σ⁺) radical is evidenced by the observed high rotational excitation of the CN(B²Σ⁺) radical following the energy transfer process.     

The energy balance and branching ratios associated with the chemiluminescent reaction Si(3P) + N2O(1Σ) → SiO*(a3Σ+, b3Π, A1Π) + N2(υ″ ⩾ 5) — possible formation of vibrationally excited N2

Silicon atoms react under single collision conditions with N₂O to yield chemiluminescent emission corresponding to the SiO a³Σ⁺?X¹Σ⁺ and b³Π?X¹Σ⁺ intercombination systems and the A¹Π?X¹Σ⁺ band system. A most striking feature of the SiN₂O reaction is the energy balance associated with the formation of SiO product molecules in the A¹Π and b³Π states. A significant energy discrepancy ( = 10000 cm^? = 1.24 eV) is found between the available energy to populate the highest energetically accessible excited-state quantum levels and the highest quantum level from which emission is observed. It is suggested that this discrepancy may result from the formation of vibrationally excited N₂ in a concerted fast SiN₂O reactive encounter. Emission from the SiO a³Σ⁺ (A¹Π) and b³Π(A¹Π, E¹Σ₀⁺) triplet-state manifold results primarily from intensity borrowing involving the indicated singlet states. Perturbation calculations indicate the magnitude of the mixing between the b³Π, A¹Π and E¹Σ₀⁺ states ranges between 0.5 and 2%. On the basis of these calculations, the branching ratio (excited triplet)/(excited singlet) is found to be well in excess of 500. An approximate vibrational population distribution is deduced for those molecules formed in the b³Π state. The present studies are correlated with those of previous workers in order to provide an explanation for diverse relaxation effects as well as observed changes in the ratio of a³Σ⁺ to b³Π emission as a function of pressure and experimental environment. Some of these effects are attributable to a strong coupling between the a³Σ⁺ and b³Π state. Based on the current results, there appears to be little correlation between either (1) the branching ratio for excited state formation or (2) the total absolute cross section for excited-state formation and (3) the measured quantum yield for the SiN₂O reaction. Implications for chemical laser development are considered.     

Theoretical study of the 2Σu+ and 2Σg+ states of Li2− and Na2−

Ab initio calculations are performed to obtain potential energy curves for the X¹Σ_g⁺ state of Li₂ and Na₂ and the X²Σ_g⁺ and A²Σ_g⁺ states of their anions. The A²Σ_g⁺ M₂^? curves are found to intersect the X¹Σ_g⁺M₂ curves at low energies and are expected to play a major role in the e^? + M₂ → M^? + M process.     

The rotational excitation and population distribution of OH(A2Σ+) produced by electron impact on water

The rotational excitation and population distributions in OH(A²Σ⁺, υ′ = 0) have been determined by analyzing the OH(A²Σ⁺ → X²Π_i) emission spectrum. The spectrum results from the impact of mon-energetic electrons (0–100 eV) on water vapour. It is shown that these rotational distributions of the OH(A²Σ⁺) state depend on the electron impact energy and have contributions from singlet and triplet states of water. The contribution from each dissociative state of water can be described by a Boltzmann distribution, both in the case of rotational excitation and population.Three distribution parameters (“temperatures”) for rotational excitation are obtained, namely 13800 K and 2900 K for the singlet contributions and 4000 K for the triplet contribution. The corresponding distribution parameters for the rotational population are 30000, 3300, and 4800 K, respectively. The results are discussed in view of recent theoretical calculations on water energy levels.     

SO+(A2Π-X2Πr) emission produced from a dissociative charge-transfer reaction of He+ with SO2 at thermal energy

The extensive bands observed from the helium afterglow reaction of SO₂ in the 250–540 nm region are assigned to the new SO⁺(A²Π-X²Π_r) system produced from the He⁺/SO₂ dissociative charge-transfer reaction at thermal energy. They had been erroneously interpreted as the SO⁺₂ (C?-X?) system produced from He(2³S)/SO₂ Penning ionization. The spectroscopic constants for the SO⁺A²Π) and SO⁺(X²Π_r) states were determined.     

Ionization of N2O by low-energy electrons and near-thermal rare-gas ions

Emission spectra between 185 and 600 nm have been investigated following near-thermal charge exchange between He⁺ and N₂O and ≤ 100 eV electron impact on N₂O. The charge exchange produces N₂O⁺Ã→X?and N₂⁺ B → X emission, but the two band systems account for at most 5% of all charge transfer products. These results and literature data on Ar⁺/N₂O are discussed in the light of Franck—Condon and energy resonance criteria as applied to low-energy charge exchange. The electron-impact experiments revealed a weak (≈ 10^?3) long-lived (≈ 50 × 10^?6 s) component in the N₂O⁺Ã→X?emission.     

Participation of Bao2* in chemiluminescence from low-pressure Ba(g) + O2 flames

New spectioscopic and vibronic population data support the essential correctness of BaO₂^* as the nascent polyatomic emitter and as the precursor to BaO (A ¹Σ⁺ → X¹Σ⁺) and (A' ¹Π → X¹Σ⁺) visible chemiluminescence from metal-rich Ba(g) + O₂ (+ Ar) diffusion flames at 2–350 mTorr- Absolute visible photon yields are reported over this pressure regime.     

Potential energy surfaces for the photodissociation H2O→O(1Dg + H2(X1Σ+g)

The minimum energy pathways for symmetrical dissociation of water into O(¹D_g + H₂(X¹Σ⁺_g) are calculated by the MRD Cl technique for various excited states of H₂O and possible mechanism for the photodissociation are discussed.     

Oscillator strength of the CN A 2Πi → B 2Σ+ (0,0) transition

The relative oscillator strength of the A ²H_i → B ²Σ⁺ transition has been measured by comparing the laser-induced fluorescence signal from excitation of a known distribution of CN A ²H_i and CN B ²Σ⁺ produced by the photodissociation of cyanogen at 158 nm. The oscillator strength of the A ²H_i → B ²Σ⁺ transition is 0.011 ± 0.006 times that of the X ²Σ⁺ → B ²Σ⁺ system. This leads to a value of (4.0 ± 2.2) × 10^?4 for the band oscillator strength.     

Lifetimes of the vibronic Ã2A1 states of H2O+ and of the 3Πi (υ′ = 0) state of OH+

Using the delayed coincidence technique, lifetimes have been measured for some Σ and Π vibronic òA₁ states of H₂O⁺ and for the ³Π_i (υ′ = 0) state of OH⁺ by analysing the decay curves of the òA₁(0, υ′₂, 0) ? X?²B₁ (0, υ″₂, 0) and the ³Π_i(υ′ = 0) ? ³Σ^?(υ″ = 0) emission intensities respectively. The excited molecular ionic states are produced via excitation of H₂O molecules by 200 eV electrons. For H₂O⁺(òA₁) the vibronic Σ levels with υ′₂ = 13 and 15 and the vibronic Π levels with υ′₂ = 12 and 14 have been considered. The radiative lifetimes obtained for these levels have about the same value, namely 10.5(±1) × 10^?6 s. The radiative lifetime for the OH⁺(³Π_iυ′= 0) state is 2.5(±0.3) × 10^?6 s. The lifetimes found in this work for H₂O⁺(òA₁) and OH⁺(³Π_i,υ′= 0) are about ten and three times longer respectively than the corresponding lifetimes given by other investigators [1,2]. The probable reason for this discrepancy is that in the other experiments no attention has been paid to the presence of a large space charge effect. This effect is caused by the positive ions which are created by the primary electron beam.     

Laser-induced fluorescence of CsH: The X1Σ+ state dissociation energy

Laser-induced fluorescence of CsH from the A¹Σ⁺ electronic state to the X¹Σ⁺ state was recorded using high-resolution Fourier transform spectrometry. Ground-state vibrational levels were observed from ν″ = 1 to the dissociation limit. These measurements showed anomalies in the X¹Σ⁺ potential energy curve due to the avoided crossing of ionic and covalent potential curves. Accurate molecular constants were derived for the lower X¹Σ⁺ vibrational levels. The observation of a quasibound level gave the first experimental determination of the dissociation energy (in cm^?1): 14802 ? D_c ? 14813.     

NH(A3Π → X3Σ−) Chemiluminescence from the CH(X2 Π) + NO reaction

NH(A³Π → X³Σ^?) and OH(A²Σ⁺ → X²Π) chemiluminescences from the reaction of CH(X²Π) with NO and O₂, respectively, have been observed at room temperature. From the decay of such emissions we have measured the rate constants for these two reactions: k_NO = (2.5 ± 0.5) × 10^?10 and k_O2 = (8 ± 3) × 10^?11 cm³ molecule ^?1 s^?1, which are in agreement with previously reported rates determined by direct CH(X) detection using, laser-induced fluorescence. This indicates that a four-centered mechanism generating these excited species is operative in both reactions. The CH generation from 266 nm photolysis of CHBr₃ has also been investigated via analysis of CH* emissions.     

Energy transfer processes in reactions of He(2 3S) and Ne(3P0,2) with CS2

A comparative spectroscopic study in the visible and ultraviolet ranges was conducted on the flowing afterglows resulting from the reactions of He(2 ³S) and Ne(³P_0,2) metastables with CS₂. Penning ionization was found to be the predominant energy transfer process. However, electron—ion recombination within the afterglows constitutes a major secondary process and gives rise to the most intense emitting system, CS(A ¹ Π → X ¹Σ⁺). Both afterglows were found to produce the CS⁺₂($\text{B}$²Σ⁺_u-$\text{X}$²Π_g), CS⁺₂($\text{A}$²Π_u-$\text{X}$²Π_g) and CS(a ³Π-X ¹Σ⁺) emission systems as well as some atomic sulfur emission lines. Some intensity differences were observed and are interpreted in terms of energetics and the formation mechanisms of the emitting species. A moderately strong CS⁺(A ²Π_i-X ²Σ⁺) emission system was also observed in the ehlium afterglow. In addition, a weak, sharp group of bands in the 390–420 nm range in the helium afterglow has been determined to be due to the presence of a small amount of He⁺ ions. This group of bands consists of two overlapping emission systems and are identified as CS(B ¹Σ⁺ → A ¹Π) and CS⁺(B ²Σ⁺ → A ²Π_i).     

A2Σ+ → X2Πi emission spectra of the phosphaethyne cations HCP+ and DCP+

Electron impact excited $\text{A}$²Σ⁺ → $\text{X}$²Π_i emission spectra of HCP⁺ and DCP⁺ have been observed. The spectra consist of short progressions in ν″₃. The 0 0⁰0 → 0 0¹0 bands have been studied under high resolution and rotational analyses carried out. Some of the more important derived constants are (in cm^?1) HCP⁺; ν^″₃ = 1150(10), A^″₀ = -146.97(3), B^″₀ = 0.6224(16), B^′₀ = 0.6690(17); DCP⁺; ν^″₃ = 1110(10), A^″₀ = -146.71(1), B^″₀ = 0.5284(2), B^′₀ = 0.5682(2).     

CS+(B2Σ+ → A 2Πi) fluorescence from penning excitation of CS

The energy transfer reation of He(2³S) + CS was studied spectroscopically in a flowing afterglow apparatus. The CS⁺(B²Σ⁺ → A ²Π_i) transition is identified via three members of the Δν = 0 sequence (406–415 nm). The spin-orbit splitting of the (0, 0) band of CS⁺(A ²Π_i) is 301 ± 5 cm^?1. A weak emitting system (280–340 nm) is tentatively identified as CS⁺(B²Σ⁺→ X²Σ⁺).     

Chemiluminescent ion-molecule reactions in the system C+ + H2

The optical emission resulting from collisions between C⁺ ions and H₂ gas was measured in the energy range 2 to 20 eV_c.m.. The observed spectrum consists mainly of the CH⁺ A ¹Π → X ¹Σ⁺ band system; CH⁺ (A ^fΠ) is shown to be formed in the chemiluminescent reactio: C⁺(²P⁰) + H₂ → CH⁺(A ¹Π) + H(²S). The energy dependence of the emission cross section was measured. The occurrence of this reaction is discussed in terms of a electronic state correlation diagram for the system.     

Energy transfer processes in reactions of He(23S) with triatomic molecules. II. H2O and H2S

The energy transfer reactions He(2³S) + H₂O and He(2³S) + H₂S were studied spectroscopically in the visible and ultraviolet ranges in a flowing afterglow apparatus. No primary triatomic ion emission was observed in this study. Only dissociative fragments were found to emit. In the He(2³S)/H₂O system intense OH(A²Σ⁺ → X²Π_i) emission bands and hydrogen Balmer series were observed while in the He(2³S)/H₂S system intense HS⁺(A³Π_i → X³ Σ^?), weak hydrogen Balmer series and some atomic sulfur lines were found. It is concluded that dissociative processes are competitive with Penning ionization in these energy transfer reactions with other possible reaction channels playing inferior roles. The post-ionization process of ion—electron recombination in the flowing afterglow dominates the emission results in the He(2³S)/H₂O system.