Curve-crossing Mechanism Research Articles

Theoretical study of the Na(3s-3p) excitation in Na+He collisions

The Na(3s to 3p) excitation in the Na+He collision, in the 30 eV<ECM<10 keV energy range, is investigated theoretically along the same lines as those of a previous study on the Na+Ne collision by Courbin-Gaussorgues et al. (1983). Calculations are first performed on the curve-crossing mechanism (i) at low energy using a five-state molecular basis that involves the diabatic SCF orbitals obtained by Courbin-Gaussorgues et al. Contrary to the Na+Ne case the threshold of the experimental Na (3s-3p) cross section and its magnitude are not reproduced by this calculation. On the other hand the direct 3s to 3p transition mechanism (ii), treated in an eight-state basis that involves orthogonalised frozen atomic orbitals, accounts for the gross features of the experimental data in the whole energy range. Finally an eight-state model that combines both mechanisms (i)+(ii) is proposed and its results are discussed and compared with the available total and differential experimental data.

Tunable laser photodissociation: quantum yield of I ★( 2P [formula omitted]) from Ch 2I 2

Tunable laser I ★( 2P 1 2 ) quantum-yield measurements are presented for CH 2I 2 in the wavelength range 248–340 nm. The results suggest that a curve-crossing mechanism is operative in the dissociation.

Collisional quenching of Cl[3p5(2P½)] by noble gases

A kinetic study of the collisional quenching of Cl[3p5(2P½)](2P–½= 881 cm–1) by the noble gases is presented. Cl(32P½) was generated by the repetitive pulsed irradiation of CCl4 and monitored photoelectrically in absorption in the vacuum ultraviolet at λ= 136.34 nm [3p44s(2P)â†� 3p5(2P0½)] by time-resolved attenuation of atomic resonance radiation, coupled with signal averaging. The decay of the electronically excited atom was monitored in the presence of the noble gases He, Ne, Ar, Kr and Xe, yielding the following absolute second-order rate constants (kQ/cm3 molecule–1 s–1 at 300 K): kHe= 3.8 ± 0.6 × 10–15, kNe= 4.0 ± 0.5 × 10–14, kAr= 1.1 ± 0.3 × 10–12, kKr= 1.4 ± 0.2 × 10–12 and kXe= 1.8 ± 0.2 × 10–11. These data are considered within the context of non-adiabatic transitions based on a curve-crossing mechanism, spectroscopic data on noble gas–Cl diatomic molecules and a simplified Landau–Zener formalism.

Generalization of the Rosen-Zener model of noncrossing interactions. I. Total cross sections

The formalism developed previously to study electronic transitions by the curve-crossing mechanism is used to obtain a general model for transitions which take place due to the strong coupling of curves which do not cross. This exactly solvable model is a generalization of a model developed by Rosen and Zener. The model is shown to give respectable agreement with exact calculations at low velocities, and to go to the correct resonant-charge-exchange limit at high velocities. Predictions of the model are shown to give good agreement with the measured total cross sections for charge exchange in the sytem ${\mathrm{Li}}^{+}$ + Na\ensuremath{\rightleftarrows}Li+${\mathrm{Na}}^{+}$.

Excitation of XeF* by reactions of XeF2 with Ar(3P0,2),Kr(3P2) and Xe(3P2)

The reactions of the lowest metastable states of Ar, Kr and Xe with XeF2 were studied in a flowing afterglow apparatus; XeF emission (from D2Π12 and B 2Π+ states) was observed in all cases. The total rate constants (cm3 molecule−1 s−1) for XeF* formation were determined as 75 × 10−11 − Xe(3P2);64 × 10−11 − Kr(3P2) and 20 × 10−11 − Ar(3P0,2). The reactions of Ar(3P0,2) and Kr(3P2) with XeF2 also gave ArF* and KrF*, respectively. Analysis of these emissions indicates that at least two different mechanisms are operative: reactive quenching by the ionic—covalent curve-crossing mechanism and excitation transfer. The Ar(3P0,2 + XeF2 reaction is a sufficiently strong source of XeF(D—X) emission that the main features of the XeF(D2Π12 − X2Σ+) system could be photographed and tentative assignments of these vibrational bands are given. The XeF(D → B) emission could not be observed and the ratio of the D—X versus the D—B transition probability must be > 1000 : 1.

Systematics of the Landau–Zener rate constant

The binary rate coefficient for atomic collision processes governed by a potential curve-crossing mechanism is obtained from the closed-form Landau–Zener (LZ) cross section approximation. The results are presented in the form of a table (and graphs) of reduced LZ rate constants vs reduced temperature T* as a function of a reduced ’’system-parameter’’ ζ*. The dependence of the rate coefficient upon the curve-crossing characteristics of the system and the temperature is readily understood in terms of the structure of the parameters ζ* and T* and the scaling of the reduced rate constant.

Electronic energy transfer between metastable argon atoms and ground-state oxygen atoms

The transfer of electronic energy between metastable argon and ground-state oxygen atoms has been studied in a discharge-flow apparatus. The excitation energy of the argon metastables is transferred to the 3p 3P state of atomic oxygen with a cross section of 3 A 2. The energy transfer is discussed in terms of an ionic-intermediate, curve-crossing mechanism for which the calculated cross section is 13 A 2.

Electronic Excitation in Fast Collisions of Na with SO2 and NO2

A sputtered Na beam has been used to investigate collisionally induced electronic excitation in scattering with SO2 and NO2. Light from the radiative decay of excited electronic states was observed at several wavelengths, and measurements were made of the excitation cross sections as a function of the center-of-mass collision energy in the range 1–30 eV. In both scattering systems, the most intense light measured originated from the Na (32P→32S) D line. Light from the (42D→32P) Na transition was observed, but its intensity was more than an order of magnitude smaller than that of the D line. Emission also occurred from transitions with long radiative lifetimes (∼40 μsec). This emission is probably due to the B1→A1 molecular transitions of SO2 and NO2. The results have been discussed in terms of a potential curve-crossing mechanism which seems to satisfactorily explain collisionally induced alkali atom excitation. For the molecular emission, an alternate mechanism has also been considered which involves vibrational coupling of the A1 and B1 states.

Study of electronically excited carbon atoms, C(21D2), by the attenuation of atomic emission, (31P°1→21D2). Part 1.—Collisional deactivation of C(21D2) by the noble gases

Electronically excited carbon atoms C(21D2) generated by the flash photolysis of carbon suboxide in the vacuum ultra-violet, have been monitored photoelectrically by attenuation of the atomic emission at λ= 193.1 nm (31P°1→21D2) from a microwave-powered atomic lamp. The decay of these optically metastable atoms has been measured in the presence of the noble gases, and the following rate constants for the collisional deactivation to the electronic ground state C(23PJ) are reported (k in cm3 molecule–1 s–1; 300 K): Xe, 1.1±0.3 × 10–10; Kr, 9.4±1.6 × 10–13; Ar, ≲ 10–15; Ne, 1.1±0.4 × 10–15; He, < 3 × 10–15. The probabilities for the electronic to translational energy transfer are discussed in terms of a curve-crossing mechanism and the influence of the spin-orbit interaction on collision.