Collision-induced Electronic Transitions Research Articles

The UV Spectrum of the Lyman‐Birge‐Hopfield Band System of N2 Induced by Cascading from Electron Impact

AbstractWe have measured in the laboratory the far ultraviolet (FUV: 125.0–170.0 nm) cascade‐induced spectrum of the Lyman‐Birge‐Hopfield (LBH) band system ( ) of N2 excited by 30–200 eV electrons. The cascading transition begins with two processes: radiative and collision‐induced electronic transitions (CIETs) involving two states ( and w1Δu → a1Πg), which are followed by a cascade induced transition at the single‐scattering pressures employed here. Direct excitation to the a‐state produces a confined LBH spectral glow pattern around an electron beam. We have spatially resolved the electron‐induced glow pattern from an electron beam colliding with N2 at radial distances of 0–400 mm at three gas pressures. This imaging measurement is the first to isolate spectral measurements in the laboratory of single‐scattering electron‐impact‐induced fluorescence from two LBH emission processes: direct excitation, which is strongest in emission near the electron beam axis; and cascading‐induced, which is dominant far from the electron beam axis. The vibrational populations for vibrational levels from v′ = 0–2 of the a 1Πg state are enhanced by radiative cascade and CIETs, and the emission cross sections of the LBH band system for direct and cascading‐induced excitation are provided at 40, 50, 100, and 200 eV.

Corrected Rate Constants for Collision-Induced Electronic Transitions from the N2 aΠg (υ = 0 and 1) Levels.

Corrected Rate Constants for Collision-Induced Electronic Transitions from the N₂ aΠ_g (υ = 0 and 1) Levels.

Corrected Rate Constants for Collision-Induced Electronic Transitions from the N2 aΠg (υ = 0 and 1) Levels

Corrected Rate Constants for Collision-Induced Electronic Transitions from the N₂ aΠ_g (υ = 0 and 1) Levels

Emissions from the N2 Lyman-Birge-Hopfield bands in the earth's atmosphere

Recent work on the N2 Lyman-Birge-Hopfield (LBH) band (a1Πg−X1Σg+) emissions from the Earth's aurora and daytime airglow has shown that models can give good agreement with observations by including cascading between the singlet (a1Πg, w1Δu and a′1Σu−) states. Both radiative and collision induced transitions contribute to the cascading. The collision induced transitions involve a process — sometimes referred to as collision-induced electronic transitions (CIET) — which has not been included in most models of the LBH bands from the Earth's atmosphere. Cascading not only improves the fit to the relative vibrational populations observed, it also increases the emission from the LBH bands, by a factor of ∼1.6 in the daytime airglow. Such an increase is consistent with recent work by Budzien et al. [1994] and Link et al. [1994]. In the aurora some calculations have included radiative cascading but not CIET, which is expected to be more important in typical auroras. However, the calculations involving CIET are based on extrapolation from laboratory measurements. These extrapolations are a source of significant uncertainty in the calculations.

State-resolved collision-induced electronic transitions.

Laser and molecular beam techniques have enabled researchers to determine the rovibrational levels populated in collision-induced electronic transitions from specified initial levels of several diatomic molecules. As exemplified by the N2+- and CN-rare-gas systems, such measurements, in combination with theoretical calculations of cross sections for these state-to-state collisional processes, provide a means to understand in detail the dynamics of these electronic quenching and energy transfer processes. The present article reviews state-to-state studies of collision-induced electronic transitions. The various collision systems studied provide examples of both perturbation-assisted "gateways" between the initial and final electronic states and perturbation-independent transitions enabled by nonadiabatic mixing induced by the collision partner.

Collision-induced electronic transitions in complexes between benzene and molecular oxygen

The collision-induced electric dipole transition moments for the O 2 + C 6H 6 complex have been investigated by the multi-configurational self-consistent field linear response method. Cooperative simultaneous transitions, ( 1Δ g , 3B 1 u ) ← ( 3Σ g −1, A 1 g ) and ( 1Δ g , 3E 1 u ) ← ( 3Σ g −1, A 1 g ) , as well as singlet-triplet and singlet-singlet transitions in each molecule have been studied in gas phase and in solvent phase as modelled by a self-consistent reaction field method. Strong intensity enhancement, respectively, reduction is predicted for the 3B 1 u ← 1A 1 g and 1E 1 u ← 1A 1 g transitions in benzene colliding with oxygen in gas or solvent environments. We find that transitions involving singlet molecular oxygen are greatly enhanced at intermolecular distances easily reached at normal temperature conditions; the b 1 Σ g + − a 1 Δ g, a 1 Δ g − X 3 Σ g − and b 1 Σ g + − X 3 Σ g − transition probabilities are predicted to become enhanced approximately by 10 5, 10 3 and 10 0 times in O 2 + C 6H 6 collisions. The enhancement is of such a magnitude that singlet molecular oxygen should be easy to detect by absorption and emission experiments tuned to the particular frequencies. It is argued that the benzene molecule is the most efficient choice among a set of organic colliders for promoting intensity of the Herzberg III band in molecular oxygen.

Collision‐induced transitions between the a1Πg, a′ 1Σu−, and w1Δu states of N2: Can they affect auroral N2 Lyman‐Birge‐Hopfield band emissions?

In the Earth's atmosphere bright ultraviolet (UV) emissions from the N2 a 1Πg state (Lyman‐Birge‐Hopfield bands) are seen, but the w 1Δu and a′ 1Σu− states, which have similar excitation cross sections, emit weakly if at all at UV wavelengths. Models of the singlet system (a, a′, and w states) have used radiative cascade, which produces transitions between excited states, and quenching, which removes molecules from them, to explain the lack of emission from the a′ and w states. Another process, which has never been properly included in such calculations, is collision‐induced electronic transitions (CIET) between the singlet states. Recent laboratory measurements of CIET involving N2 indicate that the rate constants are comparable to those for radiation, and recent measurements have yielded enough information to allow a preliminary calculation of CIET between the N2 a 1Πg, a′ 1Σu− and w 1Δu states. These calculations show that CIET can be more important than radiation for transitions between the a, a′, and w states in typical aurorae or up to the peak of typical dayglow emission profiles (∼130 km). The inclusion of CIET may allow self‐consistent modeling of observations of the Lyman‐Birge‐Hopfield (LBH) bands, one of the most prominent ultraviolet molecular emissions from the Earth's atmosphere.

Collision-Induced Electronic Transitions from the N2 aΠg(v = 1 and 2) Levels

We have used a double-resonance-enhanced multiphoton ionization (REMPI) technique to determine the quenching rates for collision-induced electronic transitions (CIETs) from the N2 a1Πg(v = 1) level as a function of the collision partners He, Ar, and nitrogen. The branching ratio from this level to the a‘1Σu-(v = 2 and 1) levels is determined experimentally and found to be in excellent agreement with an empirical model developed previously. This ratio is used to determine state-specific CIET rates from pressure-dependent radiative decay curves using a double-exponential model for collisional deactivation. The rates are determined for each of the collision partners, and the most efficient quenchant is determined to be nitrogen followed by Ar and He. In addition, the propensities for CIET have been observed from the a1Πg(v = 2) level to the a‘1Σu-(v = 3) level as a function of the rare-gas partners. There is a dependence of the propensities on the collision partner, and helium has a smaller ΔJ propensity tha...

State specific electronic quenching rates for the N2 a 1Πg(v=0) level from collisions with He, Ar, and N2

The technique of double, resonance enhanced multiphoton ionization (REMPI) spectroscopy has been used to observe collision induced electronic transitions (CIET) and rotational energy transfer (RET) from a selectively excited rotational level of the N2 a 1Πg (v=0) rotational manifold. A direct comparison of these two processes under the same experimental and single collision conditions is demonstrated for the first time and the results show clearly that CIET to the a′ 1∑−u(v=0) rotational levels occur prior to rotational equilibration in the a 1Πg manifold. The collision partners are He, Ar, and N2. There is a general propensity for ΔJ≊0 but detailed changes in the collisional selection rules for CIET are difficult to observe for the different colliding partners except for N2 where there is a fraction of E–E transfer that allows antisymmetric to symmetric transitions to be observable. The double REMPI experiments allow the determination of collisional deactivation paths and energy gaps. CIET from the a 1Πg(v=0) to the a′ 1∑−u(v=1) level which is endothermic by 295 cm−1 is observed for the first time at room temperature. From the determination of these deactivation paths a collisional quenching model is derived and state specific quenching rates are determined from pressure dependent radiative decay curves as a function of temperature for each of the collision partners.

Collisional deactivation of the isoelectronic 15N+2 A 2Πui (v=3) and CO+ A 2Πi (v=3 and 4) levels by helium atoms

An optical–optical double resonance (OODR) technique is used to selectively populate rotational levels of the 15N+2 A 2Πui (v=3) state and monitor collision induced electronic transitions (CIET) to the X 2Σ+g (v=6 and 7) vibronic manifolds. The branching ratio from the A (v=3) state to the X (v=6 and 7) levels is determined experimentally and used in a phenomenological model based on past results. This ratio is used to determine the state specific collisional quenching rates from observed double exponential decay curves of the A (v=3) level. Similar results are obtained for CIET from the CO+ A 2Π (v=4) level to the X 2Σ+ (v=12 and 13) levels. In this case, the branching ratio from the A (v=4) level to the X (v=12 and 13) levels is not as accurate as for the 15N+2 case, but the result agrees with the empirical model, and state specific quenching rates are also determined from double exponential decay curves. In addition, the deactivation rate is determined for the CO+ A (v=3) level from its observed single exponential laser induced fluorescence curves. These new data for quenching of the CO+ A (v=3 and 4) states continues the trend of increasing quenching cross sections with increasing vibrational quantum number observed in previously published values for the A (v=0, 1, and 2) levels. A comparison is also made between rotational relaxation within the A (v=4) state and the abovementioned electronic deactivation in CO+. These results show that CIET in CO+ compares favorably to analogous experiments with its isoelectronic partners N+2 and CN, including energy gap and Franck–Condon factor dependencies. The collider in all cases is helium.

Energy-gap dependence for inelastic collision-induced electronic transitions in N2+

An optical-optical double resonance procedure is used to compare the relative populations of e/f spin components of the $^{2}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$(v=7) rotational manifold resulting from the collision-induced electronic transitions in ${\mathrm{N}}_{2}^{+}$ for the cases of large and small energy gaps. Rotational levels of the $^{2}\mathrm{\ensuremath{\Pi}}_{\mathrm{ui}}$(v=3 or 4) vibrational state of ${\mathrm{N}}_{2}^{+}$ are selectively populated with a pump laser. Transitions to rotational levels of the $^{2}\mathrm{\ensuremath{\Sigma}}_{\mathrm{g}}^{+}$(v=7) level due to collisions with bath gas helium atoms at room temperature are monitored by scanning the lines of the B-X(5,7) band with a second, probe laser. The relaxation pathways A(v=3)\ensuremath{\rightarrow}X(v=7) and A(v=4)\ensuremath{\rightarrow}X(v=7) traverse energy gaps (\ensuremath{\Delta}E) of \ensuremath{\sim}0 and \ensuremath{\sim}1760 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$, respectively. Perturbation of the rotational levels of the ${\mathit{B}}^{2}$${\mathrm{\ensuremath{\Sigma}}}_{\mathit{u}}^{+}$(v=5) state by the ${\mathit{A}}^{2}$${\mathrm{\ensuremath{\Pi}}}_{\mathrm{u}1/2}$(v=17) manifold allows for the e/f, spin-rotation splitting of the lines of the B-X(5,7) probe laser scan to be resolved. The effects of these perturbations on the relative intensities of the B-X(5,7) lines have been taken into account. The intensity patterns for collision-induced transitions across the two widely different energy gaps are nearly identical, indicating that the propensities for this case have an insignificant energy-gap dependence. These detailed propensities from selectively excited rotational levels of the A(v=4 or 3) level to the individual, nearly degenerate rotational levels of the X(v=7) manifold are compared with theoretical models for inelastic scattering between $^{2}\mathrm{\ensuremath{\Pi}}$ and $^{2}\mathrm{\ensuremath{\Sigma}}$ electronic states.

Electronic transitions from the A 2Πu i(v=3) level of N+2 induced by inelastic collisions with helium atoms

An optical–optical double-resonance technique utilizing two pulsed lasers is used to study collision-induced electronic transitions from the N+2 A 2Πui(v=3) level by helium. Collisional deactivation paths are determined by this technique and found to be between this level and the X 2Σ+g (v=7 and 6) levels. The same propensity for ΔJ≊0 occurs for both of these paths in spite of an electronic energy gap size of approximately 0 cm−1 between the A(v=3) and X(v=7) levels and a large gap size of about 1950 cm−1 between the A(v=3) and X(v=6) levels. The electronic quenching rate from A(v=3) to X(v=7) is found to be only about three times larger than that to the X(v=6) level. We use this branching ratio in an electronic relaxation model to determine the collisional quenching rates between the A(v=3) and X(v=7 and 6) levels. These state specific rates are determined by fitting the model to observed radiative decay curves from the A(v=3) level obtained at various helium pressures. There is excellent agreement between the analytical and observed decay curves. The relatively efficient nature of the collision-induced electronic transition over the large energy gap is somewhat surprising in view of the fact that the nitrogen ions and helium atoms must remove most of this energy as translational kinetic energy. We have also revised our previous rate constants from the A 2Πui(v=4) level for 14N+2 and 15N+2.

Collision-induced electronic transitions.

We examine collision-induced electronic transitions mediated via long-range electrostatic fields such as those generated by ions and molecular dipoles. The chief characteristics of such processes are the relatively narrow widths and intense strengths of the collision-induced spectral lines. These features arise from and reflect the long-range nature of electrostatic forces which tend to generate long-lived collision-induced transition electric dipole moments. Of particular interest is the prediction that the collision-induced electromagnetic rates are proportional to the Raman cross section of the normally forbidden transition.

Comment on the relation between the nonadiabatic coupling and the complex intersection of potential energy curves

Simple relations are discussed that provide a correspondence between the complex intersection of two potential surfaces and the nonadiabatic coupling matrix element between those surfaces. These are key quantities in semiclassical and quantum mechanical theories of collision-induced electronic transitions. Within the two-state approximation, the complex intersection is shown to be directly related to the location and magnitude of the peak in the nonadiabatic coupling. Two cases have been considered: (i) the avoided crossing between two potential surfaces, and (ii) the spin—orbit interaction due to a 2P halogen atom. Comparisons are made between the results of the two-state model and the results of ab initio quantum chemical calculations.

Application of Baylis-Gombas pseudopotential to inelastic atomic collisions

Calculations are reported of matrix elements of the Baylis-Gombas model potential for Li-He and Li-Ne interactions. Included are comparisons of two computational approaches, one based upon a numerical double-integration procedure, the other an analytical, approximative method. The special emphasis of the calculation is the sensitivity of matrix elements that are important for collision-induced electronic transitions to the precise choice of the Baylis model parameter r0. Also reported and compared to the Baylis results are calculations of Li-He/Ne matrix elements for a Hartree-Fock frozen-core model potential and for two Bottcher-type model potentials.

Semiclassical theory of electronic transitions in molecular collisions: Combined effects of tunneling and energetically inaccessible electronic states

The semiclassical theory of collision-induced electronic transitions is extended to describe tunneling and the effect of energetically inaccessible states. The powerful technique of analytic continuation is utilized to yield very good agreement with exact quantum mechanical results on some model systems. The inaccessible states have a significant effect on transmission coefficients and thus can be important in semiclassical studies of molecular reactions.

Interpretation of the visible and near-infrared absorption spectra of compressed oxygen as collision-induced electronic transitions

The intensity profiles of some of the broad continuous absorption bands of oxygen in the near-infrared and visible regions were measured in the compressed gas over a range of pressures and temperatures. Three single electronic transitions (12 600, 10 600, 07620 Å) and three double transitions (6290, 5770, 4770 Å) were studied in detail. The asymmetry of the band profiles is shown to arise from a Boltzmann relation between the intensity distributions in the high and low frequency wings when the band origin is properly chosen. By assuming an appropriate rotational structure and broadening each rotational transition by a Boltzmann-modified dispersion curve the profiles of the bands could be reproduced with only minor discrepancies. These criteria, along with the well-known quadratic density dependence of the intensity, show that the bands are properly interpreted as collision-induced electronic transitions. The large width of the translational broadening functions required in the analysis indicates that the induction must be predominantly due to overlap interaction. No specific effects of (O2)2 complexes are identifiable in the spectra.

On Intensity Alterations in C2 “Swan” Emission Spectra

The presence of intensity alternations in C2 Swan emission spectra of flames is analyzed. It is argued that the nonequilibrium rotational energy distribution is caused by collision-induced electronic transitions between the radiating A 3Πg state and the vibrationally excited A′3Σg− or A″3Σu+ states of C2. The selection rules are derived on the basis of the parity properties of the relevant states and the various terms of the multipole expanded interaction potential. The selection rules lead to an unambiguous choice between the collision-induced transitions A″3Σu+→A 3Πg or A 3Πg→A′3Σg−. On the basis of available experimental information of the pressure dependence of the effect, additional evidence is given that a formation process is responsible for the occurrence of the intensity alternations. Examples of intensity alternations in Si2 and N2 are briefly discussed.