Excited-state Collisions Research Articles

Differential scattering of Na(3P) from HF. Reactive and non-reactive processes

Double differential cross-sections for the scattering of a laser-excited Na beam from an HF beam have been measured at collision energies between 0.29 and 0.86 eV. To the observed quantity ground-state collisions contribute with non-reactive events, and excited-state collisions contribute with reactive and non-reactive events. Therefore the analysis of the data allows us to determine sound estimates for characteristics of the potential-energy surfaces and the dynamics of the reaction. The effect of alignment of the attacking atom has also been studied. Our results show that this influence is small (2.5%) and is connected with the non-reactive channels.

Single and double electron capture into excited states in collisions between ions and neutrals

Recent experimental results on charge transfer into excited states in collisions between single and double charged ions with He, Ne, Ar, H2 and O2 are presented, respectively. Laboratory energy range of the ion beams of Heq+, Arq+, Ne+ (q=1,2)ions is (70-170)xq KeV. The optical measurement was performed by an optical multichannel analysis system (OMA).

Electron transfer into selective excited states in collisions of highly stripped C, N, O and Ne ions with H and H 2

The final state distributions have been determined by the method of translational energy spectroscopy in slow collisions of highly stripped C q+ ( q=4–6), N q+ ( q=5–7), O q+ ( q=6–8), and Ne q+ ( q=8,9) ions with atomic and molecular hydrogen. The observed state-selectiveness is discussed on the basis of the classical overbarrier model and the Landau-Zener model.

Charge exchange into excited states in collisions between noble gas ions and atoms

Experimental resutls obtained demonstrate that there are two processes of excitation in every collision system: one is an one-electron capture into excited states, the other is a direct excitation of projectile and target by projectile ion impact. From the comparison of the emission cross sections between the two collision systems, we find that the potential energy defect and the velocity of incident ion are very important factors in emission cross sections, and a competition process of these factors in collisions occurs.

Radiative lifetimes and depopulation cross-sections of excited rubidium2 S and2 D states

The radiative lifetimes of excited rubidiumn2S (n=8−13) andn2D (n=6−11) states have been measured using the time resolved laser induced fluorescence method. The cross-sections for depopulation of the excited states in collisions with the ground-state Rb atoms have been determined. The influence of the blackbody radiation on the measured values is discussed.

Excitation by impact of He+(2-20 keV) on Li(2s)

The formation of excited states in collisions of He+ with Li(2s) at impact energies of 2-20 keV has been investigated by means of photon spectroscopy. Absolute experimental excitation cross sections are compared with recent calculations based on a two-electron AO-expansion model by Bransden et al. (1984).

Collisional quenching and depolarization of NO2 Ã 2B2 state fluorescence as studied by Zeeman quantum beat spectroscopy

We have studied the effect of collisions on the time-resolved fluorescence arising from a single optically selected fine structure level (F1303) of the à 2B2 state of NO2 prepared via absorption near 5933 Å in the presence of a weak magnetic field. The fluorescence modulation induced by the magnetic field (Zeeman quantum beats) is used to monitor the time dependence of the alignment. The alignment decays exponentially and follows simple Stern–Volmer kinetics. The incoherent fluorescence intensity, used to determine population information, fits well a bi-exponential form at nonzero pressure. This is interpreted by a simple kinetic scheme involving cascade to additional fluorescing states. We find that only about 15% of the population removed from the initial level by collision produces subsequent emission that is detected (630–830 nm). We are unable to identify the exact nature of these emitting states. However, it is clear that pure rotationally inelastic collisions do not dominate the energy transfer process. Collisional population and alignment relaxation rate constants for the optically prepared level have been measured for excited state collisions with He, Ne, Ar, Kr, Xe, N2, CO, and NO2 at 295 K and low density (<1×1015 particle/cm3). In particular, we find that the rate for population removal of the initially excited level by collision is faster than the rate for elastic disalignment (depolarization within the same rotational level).

Inelastic collisions of virtually excited atoms

We report observations of inelastic collisions of atoms excited by a weak monochromatic laser field whose frequency is close to the resonance frequency of the transition from the ground state to an excited state. Our experiments use sodium atoms in a gas cell with the laser frequency detuning from resonance ($|\ensuremath{\Delta}|<6$ GHz) much smaller than the inverse of a collision duration. In this frequency region, our results may be interpreted in terms of inelastic collisions involving virtually excited atoms. The inelastic collision rate is determined by measuring the ratio of fluorescence from the collisionally populated level to emission from the laser populated level. By studying this emission ratio as a function of perturber pressure we show that the inelastic collision rate constant for virtually excited atoms is the same as the thermally averaged rate constant for level-to-level population transfer. Measurements of the emission ratio as a function of the laser detuning from resonance explicitly show how the correlation of collision rates with excitation frequency due to the Doppler effect is lost as the laser is tuned more than several Doppler widths from line center. The collisions of virtually excited atoms fundamentally limit experiments utilizing the Doppler effect to obtain velocity-dependent excited-state collision rates.

Electron Capture Into Excited States in Collisions Between Multiply Charged Ions and Atoms

We discuss some recent experimental results on charge transfer into excited states in collisions between multiply charged ions and Li atoms. Beams of He2+, Cq+, Oq+ (q = 1, 2, 3) ions in the energy range of (10-200)q keV were crossed with a thermal Li beam produced by an oven. From measurements of the radiation emitted by the decay of the excited ion states we were able to determine absolute cross sections for electron capture into specific n, l-states. The wavelength range covered is about 10-700 nm. For Cq+ and Oq+, we also used a molecular hydrogen target. Comparing the results for both targets, we can explain the differences as a result of the difference in ionization potential. The importance of metastable fractions in the Cq+, Oq+ ion beams is discussed.

Laser excited infrared fluorescence in COF2: Equilibration and relaxation of the C=O and C–F stretching modes

Over 30 different CO2 laser lines have been found to produce strong laser induced fluorescence at λ=5 μ from the ν1 (C=O stretch) and 2ν2 (C–F stretch) states in COF2. The observed fluorescence is of two distinct types consistent with two different primary pumping mechanisms: (1) laser pumping 0→ν2 followed by collisional filling of 2ν2 via excited state–excited state collisions, and (2) direct laser pumping of ambient molecules in ν2 via the ν2→2ν2 transition. The ν1 and ν2 manifolds reach vibrational steady state within 30 gas kinetic collisions, while approximately 500 collisions are required to relax this localized energy to the remaining vibrational modes.

Collisions of excited states and ionized media

The importance of the field is underlined. Essentially two classes of collisions are considered: bound-bound and bound-free transitions. In the first category, emphasis is put on transfer of excitation. Methods and results are reported, it is shown that electronic energy is more easily transferred during the collision of high excited states than of low excited stated. Explanation for this behaviour is given. The second category deals with ionization processes. Good agreement is obtained between theory and experiment for Penning and associative ionization processes. Different experimental methods used are described, they have contributed to deduce important data concerning probability of transition and potential curves. The last part is devoted to the collision of two excited states (metastables). The results obtained with the help of an afterglow technique seem to clear the way to various measurements concerning excited state chemistry.

Velocity Selection by Doppler Shift: A General Method for Studying Excited-State Collisions

We show that the Doppler shift can provide velocity resolution in excited-state molecular-collision experiments which is comparable to that obtainable in molecular-beam machines. In addition, the state of the quasimolecule formed in the collision can be selected.