Guanlin Shen

Molecular beam study of gateway-coupling N2(C 3Πu/a′ 1Σu−) and chemical quenching of the metastable N2(a′) state

Collisions of metastable N2 molecules in a beam with a variety of gases result in nitrogen “second positive” emission from a single, isolated level N2 (C 3Πu1, v=0, J=14). The highly selective collisional excitation was explained by a gateway-type coupling with the accidentally nearly degenerate level N2 (a′ 1Σu−, v=16, J=14). This identifies even the Λ component of the emitting level, namely “f”. Some additional, minor gateway emissions could in part be related to known N2(C) perturbations. Unlike gateway emissions reported earlier from this laboratory, the nature of the collision partner was important here. He, Ne, Ar, Kr, N2, CO, and CO2, all gave about equally strong emission, while almost none was observed with NH3, O2, and Xe. This is ascribed to the collisional quenching of N2(a′) by Penning ionization (for NH3) or by a harpooning mechanism (for Xe and O2).

Energy transfer reactions of the He(3 2S) atom with methane and chloromethane molecules under beam conditions

Abstract Energy transfer reactions of metastable He (2 3 S) with methane and chloromethane molecules were investigated by measuring the emission spectra of the electronically excited fragments under beam conditions. The formation rate constants of the fragments CH(A 2 Δ), CH(B 2 ∑ − ), CH(C 2 ∑ + ), H * and CCl(A 2 Δ) for the reactions He(2 3 S) + CH n Cl 4- n ( n = 1−4) have been determined. The nascent rovibrational distributions of CH(A 2 Δ, v ′ = 0−2) and CH(B 2 ∑ − , v ′ = 0) have been obtained. The experimental results show that the rotational distribution of CH(A 2 Δ, v ′ = 0) from the reaction of He(2 3 S) with methane is different from that of He (2 3 S) with chloromethanes. In the former case, the rotational distribution can be expressed by a single Boltzmann temperature, while in the latter it is not a complete equilibrium distribution and may be expressed by a double Boltzmann distribution. A discussion of the reaction mechanism based on statistical theory is presented.

Vibrational population inversion in CS(A) through the dissociative excitation of CS2 by Ar(3P2)

Abstract The CS(A 1 Π → X 1 Σ + ) emission spectra resulting from the energy transfer reaction of Ar( 3 P 2 ) + CS 2 under single collision condition have been obtained. The relative vibrational populations of the nascent product CS(A 1 Π, υ′) have been determined by means of spectral simulation. A population inversion is found at υ′ = 1. The population data are approximately represented by a distribution predicted from the impulsive half collision model. The dynamics and energetics of CS(A) formation has been discussed in detail.

Collision-induced intersystem crossing from NH(a 1Δ,b 1Σ+) to NH(A 3Π): Gateway-mediated and direct mechanisms

Metastable NH* radicals in a molecular beam, generated in a discharge, were allowed to collide with target particles (He through Xe rare gas atoms, and H2, CO, N2, NO, O2) in a cell or a crossed jet. Optical emission was observed issuing from the collision zone (and in the case of the jet also from different points along the primary beam). Spectral analysis (∼0.13 nm FWHM resolution) revealed two components; (a) a pair of sharp P, R lines (“spikes,” originating from the (perturbed) level NH(A 3Π, v=2, J=5, F3, Λ-component “e”; (b) broad NH(A 3Π→X 3∑−) emission in the (0, 0), (1, 1), and (2, 2) bands. Component (a) was shown to be due to a gateway coupling with the (perturbed) level NH(b 1∑+, v=5, J=5). From the collision gas pressure dependence of the “spike” intensity, relative cross sections were derived. They varied by less than a factor of 3 between He and NO. Weak spike emission was also observed issuing from the NH* beam without collisions. From the exponential decay of this “afterglow” intensity al...

Freezing of NO gateway emission by a magnetic field and very long field-free lifetimes of perturbed NO(a 4Π) levels

The collision-induced emission from perturbed NO(B 2Π/a 4Π) (“gateway”) levels, previously studied by us using a beam/target gas cell configuration, was re-examined by crossing the NO(a 4Π) beam with a target gas jet. Moving the observation point along the primary beam, spatially resolved NO(B,v=0 and 3) gateway emission profiles were recorded. Two types of measurements were made: (a) Applying a magnetic field at the collision zone, the B, v=0 emission was quenched within the field, but reappeared at the field exit. This “freezing” confirms the mechanism of the gateway quenching as formulated earlier. (b) In the absence of a magnetic field, anomalously long radiative lifetimes were determined from both the v=0 and v=3 emission profiles. This direct observation of the long-lived eigenstate resulting from the perturbation is consistent with earlier measurements on the other, short-lived component.

Observation of the electronic energy transfer reaction CO(a3Π) + NO(X2Π) → CO(X1Σ+) + NO(a4Π) in molecular beams

Abstract Metastable CO(a 3 Π) molecules in a molecular beam were allowed to interact with ground-state NO molecules under single collision conditions. Optical emission from the collision region was spectrally analyzed at a resolution of 1.5 A FWHM. In the wavelength range 290–400 nm, NO β bands having an anomalous contour were observed. Superimposed on a broad intensity distribution were pairs of narrow “spikes”. The broadband emission is due to NO(B) formed by energy transfer from CO(a) to NO(X). The line emission is evidence of another reaction branch, giving NO(a) products. Only those NO(a) molecules can emit which originate in specific “gateway” levels, exhibiting some NO(B) character as a result of S/O perturbation. The gateway emission was filtered out of the broadband background utilizing magnetic field quenching.

Study of the energy transfer reaction of N2(a1Πg, ν′) + CO(X1Σ, ν″ = 0) under molecular beam conditions

Abstract The energy transfer reaction of metastable N 2 ( a 1 Π g , ν′) prepared by a dc discharge of high-purity nitrogen with CO(X 1 Σ, ν″ = 0) was investigated by measuring the vacuum ultraviolet emission spectra of the electronically excited species CO(A 1 Π, ν′) and N 2 ( a 1 Π g , ν′) during the collision period under molecular beam conditions. The energy mismatch ≈ ΔE ≈ and q ν ′ ν ″ factors for both CO(A 1 Π, ν′ ← X 1 Σ, ν″) and N 2 ( a 1 Π g , ν′ → X 1 Σ, ν″) are the determining factors controlling the branching ratio from one N 2 ( a 1 Π g , ν′) donor to different vibrational levels of CO(A 1 Π, ν′) . A formula to calculate the branching ratio is presented by considering the energy mismatch ≈ ΔE ≈ and the q ν ′ ν ″ factors recalculated using a shifted value of the equilibrium internuclear distance r e of N 2 .