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To achieve large population transfer to high vibrational levels in a selected ground-state mode of a polyatomic molecule [(Cr(CO)[SUB6]], we apply chirped femtosecond mid-infrared laser pulses at 2000 cm[SUP-1] to optimize vibrational ladder climbing as an energy deposition mechanism, which in turn controls the outcome of a unimolecular dissociation process. Its dependence on excitation parameters (frequency, intensity, chirp) is investigated and found to be in excellent agreement with a theoretical calculation. In particular, it is shown that optimizing vibrational ladder climbing allows for coherently controlled excitation even in a polyatomic molecule. [ABSTRACT FROM AUTHOR]

Studies the charge-transfer process, a relevant link in the chain of reactive events which occur in the E and F-regions of the atmosphere. Overview of ion-molecule interactions with atmospheric relevance; Transient evidence of a van der Waals molecule in the reaction; Impact of the vibrational quantum state of the reagent ions on the cross section of reactions.

This article presents detailed internal and kinetic energy dependent cross sections and reaction rates for the hydrogen atom transfer processes N+2(X 2Σ+g, v+=0–4, J+=2)+H2→N2H++H, which were obtained under single-collision conditions in a guided-ion beam/scattering gas experiment. Preparation of ions in specific states relied on single-color excitation within a resonantly enhanced (2+1) multiphoton ionization scheme. The translational energy of the ions, Elab, was varied from 0.1 eV to approximately 30 eV. A small activation barrier impedes the reaction. Vibronic state preparation of the nitrogen ion is influential on the nature of the energy surface—N+2+H2 or H+2+N2—along which the H atom transfer proceeds. Calculations of model potential energy surfaces suggest that the reaction pathway must involve several exoergic and endoergic channels which open successively as the collision energy increases. A purely collision determined cross section—as would be evidenced by the E-1/2 dependence formulated in the Langevin–Gioumousis–Stevenson model—is observed only within a narrow window of kinetic energies. © 1995 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report vibrational level channeling and photoelectron angular distributions which characterize the single-step photoionization of the E 1Π Rydberg state of CO. Particular attention is paid to the influence of the intermediate state’s rotational preparation and its spatial alignment. The two-photon excitation is tuned to populate the lowest possible J values of E, which permits us to isolate a single magnetic sublevel of this intermediate state. Vibrational branching is found to be insensitive to the choice of the excitation line and does not match the predictions suggested by the Franck–Condon factors for a CO E 1Π(v1)→CO+ X 2Σ+(v+) direct ionization. A competitive autoionization from the three-photon-energy state S1 may be responsible for this anomalous behavior. The phenomenological pattern of photoelectron distributions can in all cases be rationalized on the basis of the intermediate state alignment. A separate chapter of the discussion is devoted to P(2) pumping, since it provides a particularly simple situation where all excited molecules are oriented with their axes parallel to the light vector E, which makes this case tractable to a quantitative calculation. Its results show qualitative agreement with the experimental observations. The remaining discrepancy can be attributed to the autoionization which has not been considered in the calculation. [ABSTRACT FROM AUTHOR]

We present vibrational branching ratios and photoelectron angular distributions for resonantly enhanced (2+1)-photon ionization of CO. The excitation ladder involves the B 1Σ+(vi=0 or 1) Rydberg state. Contrary to expectations fostered by the Franck–Condon principle, ionization via vi=0 branches into vibrational states v+=0–4. Such phenomena are also observed in the case of vi=1, but only to a minor extent. The angular emission patterns of the photoelectrons ejected during the ionizing step are of distinct character in that they are highly anisotropic for Δv=vi-v+=0 processes, but show isotropy when due to Δv≠0 transitions. The photoelectron angular distribution which accompanies the Δv=0 ionization of B(vi=1) shows p-wave character, and hence we may postulate a spherical potential of the Rydberg ion core. The same approximation should hold for the vi=0 state. The dissimilar appearance of the angular distributions when ionizing from this level invites the hypothesis of two individually different ionization channels. [ABSTRACT FROM AUTHOR]

This paper reports the results of two related experiments: (A) The state-to-state rotational transition probabilities of N2(a 1Πg) in collisions with rare gas atoms (Ar or He) were measured by the technique of two-step multiphoton ionization. Results show that the selection rule antisymmetric–symmetric is obeyed. The transition probability drops rapidly with increasing |ΔJ|. A propensity rule related to the Π+ or Π- symmetry conservation of the electronic wave function during the collision induced rotational transition holds. (B) The cross section for the rovibronic energy transfer between N2(1Πg) and N2(X 1Σ+g) is found to be ∼28 Å2. N2(a 1Πg) product populations show a Boltzmann-like distribution with a rotational temperature suggesting an equipartition of the available energy among the rotational and translational degrees of freedom of the products. A mechanism invoking an intermediate collision complex along with intermolecular electron exchange may explain the results. [ABSTRACT FROM AUTHOR]

The collision-induced electronic energy transfer N2(a 1πg,v′) +CO(X 1Σ+,v″=0) →N2(X 1Σ+g,v″) +CO(A 1πg,v′)+ΔE is studied in a gas cell. N2(a 1πg,v′, J′) is prepared by two-photon (2hν1) absorption from the ground state. CO(A 1πg,v′, J′) is probed by two-photon (hν1+hν2) ionization via CO(B 1Σ+) as the resonant intermediate state. Experiments show that the overall energy transfer cross sections exceed that of gas kinetic collisions by a factor of 3–4. The energy mismatch ΔE is the determining factor controlling the branching ratio from one N2(1πg,v′) donor to different vibrational levels of CO(A 1π,v′). For small values of ΔE, CO(A 1πg,v′, J′) shows a Boltzmann-like rotational level population. Its rotational temperature scales with ΔE. About 28% of the excess energy funnels into the rotation of CO(A 1π). An explanation for the observed rotational distribution of CO* and the energy transfer mechanism is given. The rate constants are analyzed in terms of the surprisal. [ABSTRACT FROM AUTHOR]

Investigates the hydrogen atom abstraction reaction between gaseous ions and neutral molecules. Role of translational and internal energy dependence of the integral cross section; Details on the characterization of carbon monoxide ions; Analysis on the occurrence of charge transfer and dissociative ionization; Description of ion-molecule capture.

The photoelectron spectra are reported from resonantly enhanced multiphoton ionization of N2 via the a 1Πg(v=10–14) states. Ionization from these levels is found to follow a (2+1) pathway resulting in N+2 X 2Σ+g(v+=0–4) ions. The observed branching ratios in the photoelectron spectra reveal a strong propensity for forming ground state ions in a single vibrational state. The results are explained qualitatively by favorable Franck–Condon overlap in the ionizing X 2Σ+g–a 1Πg transition. [ABSTRACT FROM AUTHOR]

A piezoelectric valve is described which delivers intense gas pulses limited by the nozzle for diameters of up to 1.0 mm. Minimum pulse lengths between 0.15 and 0.25 ms (fwhm) are obtained at full opening, depending on the nozzle size. The upper limit of the repetition rate is near 1 kHz. The valve and its modifications meet almost all experimental requirements. In particular, ways of operating in a wide temperature range are outlined. Reliable operation with a remote nozzle at 77 K was recently demonstrated. Nozzle temperatures of more than 1000 K should be possible with a similar setup. [ABSTRACT FROM AUTHOR]