2Π Emission Research Articles

Vibronic structure of the cyanobutadiyne cation. II. Theoretical exploration of the complex energy landscape of HC5N.

The results of an extensive ab initio study of the cyanobutadiyne cation, initially motivated by threshold-photoelectron spectroscopy experiments [see the study by Gans et al., J. Chem. Phys. 150, 244304 (2019)], are reported in the present paper. Calculations at the internally contracted multireference configuration interaction level of theory have been performed to derive the rovibronic properties of the seven lowest electronic states of HC5N+. Equilibrium geometries, rotational constants, vibrational frequencies, electric dipole moments, and spin-orbit constants have been calculated and compared with experimental data when available. Adiabatic and vertical ionization energies from the neutral ground state as well as transition energies within the cation electronic manifold are predicted, using the convergence to the complete basis set limit. The accurate description of the complex energy landscape up to 32 000 cm-1 above the ionization potential allows us to perform Franck-Condon simulations of the photoionization spectrum to the X+ 2Π, A+ 2Π, B+ 2Σ+, and C+ 2Π states and allows us to simulate the A+ 2Π → X+ 2Π emission spectrum. The vibronic perturbations occurring on the excited potential energy surfaces are revealed and discussed, in particular, for the 3 2Π surface, which presents a double-well topography.

Gateway-type intramolecular collisional transfer in15NO(a4π, b4Σ−→ B2π)

Abstract Excitation of 15NO molecules into the B 2π state by collisional intramolecular energy transfer from the metastable a 4π state was studied under single-collision conditions. Passing a beam of 15NO (a4π) through a cell filled with argon at a few mTorr, B 2π → × 2π emission was observed. It originated from only three specific B state levels, namely 2π1/2, J = 21·5 and 2π3/2, J = 21·5 in v = 2 and 2π1/2, J = 10·5 in v = 3. This selectivity is characteristic of a gateway-type process mediated by spectroscopic perturbations. The analysis showed that in the last case there is a direct spin-orbit interaction of the emitting B state level with a well-specified level of NO(a). The other two perturbations are caused by interactions of the respective NO(B) levels with uniquely identifiable levels of the short-lived NO(b4Σ−) state, which in turn are collisionally populated from a broad range of NO(a 4π) levels. This latter mechanism is thus a novel type of gateway process. The results explain the anomalous spectral distribution observed in an earlier isotopic study of the 15N16O afterglow, which showed an enhanced population of the vibrational levels B, v = 2 and 3 [14].

OH A 2∑+→X 2Π chemiluminescence measurements of N+2, Ar++H2O hydrogen-atom-transfer reactions at suprathermal energies

Chemiluminescence attributable to OH A 2∑+→X 2Π emission is observed in hydrogen-atom transfer reactions of N+2 and Ar+ with H2O. High-resolution OH A→X spectra [0.5 nm full width at half maximum (FWHM)] are presented as a function of collision energy in the range Ec.m.=5–20 eV. The spectra clearly show an increase in rotational broadening with collision energy. To reasonably fit the spectral data, simulations require contributions from two Boltzmann distributions with different rotational temperatures. It is proposed that the OH A state is formed via two channels. In the dominant channel, A state OH is formed in both v′=0 and v′=1, with a collision energy-dependent rotational temperature ranging from 1000 K at Ec.m.=6 eV to 3400 K at Ec.m.=19 eV. The rotationally excited products are proposed to be formed in a two-step reaction involving charge transfer, followed by the transfer of a proton to the ‘‘primary’’ species. The second channel is characterized by formation of A state OH primarily in the v′=0 vibrational level with a rotational temperature of 300 K. Comparison of low-resolution (14 nm FWHM) OH A→X and H2O+ Ã 2A1→X̃ 2B1 emissions provides an estimate for the OH A state cross sections of σ<0.3×10−16 cm2 for N+2+H2O at Ec.m. between 6 and 20 eV, representing less than 10% of the hydrogen atom pickup channel at these energies. For Ar++H2O at Ec.m. between 11 and 32 eV, the estimated OH A state cross section is σ<0.5 Å2, which may represent all of the hydrogen atom pickup channel at the higher energies.

Chemiluminescence measurements of the N+2, N++H2O charge transfer systems at suprathermal energies: Direct probe of the dynamics of large cross section charge transfer processes

The chemiluminescence from suprathermal N+2–H2O and N+–H2O collisions is studied using a new experiment. Intense H2O+Ã 2A1–X̃ 2B1 emission is observed from the N+2 +H2O charge transfer whereas no emissions are detected from the N+–H2O system. OH A 2Σ+–X 2Π emission originating from the N+2+H2O atom pickup channel is also observed.

ChemInform Abstract: Photochemistry of BrNCO and INCO

The absorption spectra of BrNCO and INCO in the region 160–400 nm are continuous in nature, and photolysis of these molecules with pulsed excimer lasers leads to dissociation. Photolysis of BrNCO and INCO at 308 and 351 nm, respectively, produces vibrationally cold ground state NCO radicals. Weak NCO A 2Σ+→X 2Π emission results from two‐photon absorption processes at these wavelengths. Photolysis of BrNCO and INCO at 193 and 222 nm, respectively, produces intense NCO A 2Σ+→X 2Π emission. The NCO(A) is a direct one‐photon photofragment in each case, and has considerable vibrational excitation. The yield of NCO(A) from BrNCO photolysis at 193 nm is approximately 16%; the yield of NCO(A) from INCO photolysis at 222 nm is near 13%. From the production of NCO(A) at these wavelengths and the short wavelength limit of the NCO A 2Σ+→X 2Π emission, limits on the heats of formation of these molecules are found to be ΔHf(BrNCO)≥−1.7 kcal mol−1 and ΔHf(INCO)≥10.9 kcal mol−1. These results demonstrate the preference o...

Photochemistry of BrNCO and INCO

The absorption spectra of BrNCO and INCO in the region 160–400 nm are continuous in nature, and photolysis of these molecules with pulsed excimer lasers leads to dissociation. Photolysis of BrNCO and INCO at 308 and 351 nm, respectively, produces vibrationally cold ground state NCO radicals. Weak NCO A 2Σ+→X 2Π emission results from two-photon absorption processes at these wavelengths. Photolysis of BrNCO and INCO at 193 and 222 nm, respectively, produces intense NCO A 2Σ+→X 2Π emission. The NCO(A) is a direct one-photon photofragment in each case, and has considerable vibrational excitation. The yield of NCO(A) from BrNCO photolysis at 193 nm is approximately 16%; the yield of NCO(A) from INCO photolysis at 222 nm is near 13%. From the production of NCO(A) at these wavelengths and the short wavelength limit of the NCO A 2Σ+→X 2Π emission, limits on the heats of formation of these molecules are found to be ΔHf(BrNCO)≥−1.7 kcal mol−1 and ΔHf(INCO)≥10.9 kcal mol−1. These results demonstrate the preference of the halogen isocyanates for dissociation to doublet fragments by scission of the nitrogen–halogen bond, in contrast to the analogous halogen azides which photodissociate to singlet or triplet fragments.

Mechanisms of the L′ 2Φ–X 2Π emission of NO in Ar and Kr matrices

New time and energy resolved spectra of the L′ 2Φ–X 2Π (0,v″) bands of NO in Ar and Kr matrices are reported. The L′(0,v″) bands are excited exclusively via the B′ 2Δ valence state and their lifetime is 3000±500 ns in both matrices. The quantum efficiency for L′(v=0) emission is estimated to be ≤0.04. The 2Φ–2Π transition is discussed in terms of a statically induced transition moment involving spin–orbit mixing with B′ 2Δ, but also in terms of nonadiabatic matrix elements due to the coupling to the lattice. Relaxation down to L′(v=0) is discussed in terms of matrix-induced interstate cascading with the b 4Σ− state.

Emission spectra of CH(A 2Δ) from methane in an argon flowing afterglow

The CH A 2ΔX 2Π emission was observed in the flowing afterglow reaction of charged species of argon with CH 4. Energetic considerations show that metastable argon ions, Ar +M( 4D, 4F, 2F, 2G, are plausible candidates, although the contribution of Ar 2+ cannot be completely excluded. The CH A 2ΔX 2Π spectrum was analyzed; the ratio of vibrational populations for the υ′ = 0 and 1 states, P vib(υ′ = 0), is estimated to be 0.56 ± 0.17, and the effective rotational temperatures are (3.0 ± 1.5) × 10 3 and (1.7 ± 0.4) × 10 3 K, respectively.

Dissociative excitation of benzene by electron impact

In the wavelength region 1850–9000 Å radiation from H, C and CH fragments is observed as a result of the dissociative excitation of benzene by electron impact (0–1000 eV). Emission cross sections and threshold energies have been determined for the Balmer series of the hydrogen atom and the A 2Δ - X 2Π emission of the CH fragment.

Predictions for the A2∑- X2Π system of LiO

Theoretical potential well constants for the 7Li 16O molecule were used to predict the spectral and intensity distribution (equal level population assumption) of the A 2∑- X 2Π emission band system, so far not experimentally detected. Morse Franck-Condon factors were computed and are given. The system should be sought in the infrared, via the Δ v = 0 sequence (0, 0 at 4.3 μ) and stronger but overlapped sequences Δ v = -1 (3.2 μ), Δ v = -2(2.5 μ), and Δ v = -3 (2.1 μ).

Luminescence of Systems Containing Hydroxyl Ion

A fluorescence of alkali halide single crystals doped with the corresponding alkali hydroxide has been observed and is attributed to the A 2Σ+ → X 2Π emission of a photoproduced hydroxyl radical. A phosphorescence of systems containing the hydroxide ion has also been noted at λ ∼ 400 mμ and is attributed to a Σ−4 → X 2Π emission of the OH radical. Photoprocesses in systems containing the OH− ion, as well as in aqueous systems containing certain salts and acids, are discussed in a qualitative way.