MPI photoelectron spectroscopy of ungerade excited states of acetylene: Intermediate state mixing and ion state selection

Analysis of low-lying gerade Rydberg states of acetylene using two-photon resonance fluorescence excitation spectroscopy

The np gerade Rydberg states of acetylene were analyzed using two-photon resonance fluorescence excitation spectroscopy in the 72,000-93,000 cm(-1) energy region. The npπ(1)Σ(g)(+) and npπ(1)Δ(g) Rydberg series (n = 3-5) were identified in the fluorescence excitation spectrum measured by monitoring the C(2) d(3)Π(g)-a(3)Π(u) Swan system. Some vibronic bands were assigned to the npπ(1)Δ(g)-X̃(1)Σ(g)(+) transition on the basis of rotational analysis. The 5pσ(1)Π(g) state was observed, which is the first such observation in an npσ(1)Π(g) series. Rotational analysis of the 5pσ(1)Π(g)-X̃(1)Σ(g)(+) transition showed e/f-symmetry dependent predissociation of acetylene in the 5pσ(1)Π(g) state. The 0(0)(0) band of the deuterated acetylene (C(2)D(2)) 4pπ(1)Σ(g)(+)-X̃(1)Σ(g)(+) transition exhibits an atypical structure, which was satisfactorily reproduced by a simple model of quantum interference between the discrete and quasi-continuum states. The predissociative lifetimes of the npπgerade Rydberg states were estimated from the spectral profiles. The predissociation mechanism of acetylene in the Rydberg states is discussed.

Reaction of C2H2(+) (n · ν2, m · ν5) with NO2: reaction on the singlet and triplet surfaces

Integral cross sections and product recoil velocity distributions were measured for reaction of C(2)H(2)(+) with NO(2), in which the C(2)H(2)(+) reactant was prepared in its ground state, and with mode-selective excitation in the cis-bend (2ν(5)) and CC stretch (n · ν(2), n = 1, 2). Because both reactants have one unpaired electron, collisions can occur with either singlet or triplet coupling of these unpaired electrons, and the contributions are separated based on distinct recoil dynamics. For singlet coupling, reaction efficiency is near unity, with significant branching to charge transfer (NO(2)(+)), O(-) transfer (NO(+)), and O transfer (C(2)H(2)O(+)) products. For triplet coupling, reaction efficiency varies between 13% and 19%, depending on collision energy. The only significant triplet channel is NO(+) + triplet ketene, generated predominantly by O(-) transfer, with a possible contribution from dissociative charge transfer at high collision energies. NO(2)(+) formation (charge transfer) can only occur on the singlet surface, and appears to be mediated by a weakly bound complex at low energies. O transfer (C(2)H(2)O(+)) also appears to be dominated by reaction on the singlet surface, but is quite inefficient, suggesting a bottleneck limiting coupling to this product from the singlet reaction coordinate. The dominant channel is O(-) transfer, producing NO(+), with roughly equal contributions from reaction on singlet and triplet surfaces. The effects of C(2)H(2)(+) vibration are modest, but mode specific. For all three product channels (i.e., charge, O(-), and O transfer), excitation of the CC stretch fundamental (ν(2)) has little effect, 2 · ν(2) excitation results in ∼50% reduction in reactivity, and excitation of the cis-bend overtone (2 · ν(5)) results in ∼50% enhancement. The fact that all channels have similar mode dependence suggests that the rate-limiting step, where vibrational excitation has its effect, is early on the reaction coordinate, and branching to the individual product channels occurs later.

Low-energy electron-stimulated desorption of cations and neutrals from Si(111)-(7x7):C2D2

The interactions of low-energy (5-50 eV) electrons with acetylene-d(2) (C(2)D(2)) adsorbed on the Si(111)-(7x7) surface have been examined by monitoring the stimulated desorption products. These include primary cation desorbates, D(+) and C(2)D(2)(+) (C(2)HD(+)), the fragment ion C(2)D(+), smaller amounts of C(2)(+), CDH(+) (CH(3)(+)), and neutral D((2)S). The approximately 23-25 eV threshold energies for D(+) and hydrocarbon fragment ion detection indicate involvement of two-hole or two-hole one electron final states that Coulomb explode. These multihole states can be created via Auger decay of single holes in shallow core levels localized on C or Si surface atoms. The approximately 12 eV appearance threshold for the C(2)D(2)(+) molecular ion can be correlated with direct excitation of an adsorbate-induced surface state, which may initially possess character of the A(3) surface state of Si. The 18 eV threshold for C(2)D(+) correlates with decomposition of C(2)D(2)(+) with excess vibronic energy. C(2)D(+) desorption via direct excitation of the dissociative (2)Sigma(u)(+)-type state of the C(2)D(2)(+) ion is also possible. The approximately 8 eV threshold energy for production and desorption of neutral D((2)S) may correlate with excitation of the perturbed/mixed F (1)Sigma(u)(+)<--X (1)Sigma(g)(+) and E (1)Sigma(u)(+)<--X(1)Sigma(g)(+) dissociative transitions of adsorbed acetylene molecules. Time-of-flight distributions of D((2)S) indicate both nonthermal (557 and 116 meV; 4300 and 900 K) and thermal (17 meV; 130 K) components. The two fast components can be related to the geometry of di-sigma bonded acetylene on the Si(111)-(7x7) surface.