Study of singlet—triplet coupling in glyoxal by level anticrossing spectroscopy. V. Nature of singlet—triplet interaction

Probing Anion-Molecule Complexes of Atmospheric Relevance Using Anion Photoelectron Detachment Spectroscopy

Bimolecular reaction and collision complexes that drive atmospheric chemistry and contribute to the absorption of solar radiation are fleeting and therefore inherently challenging to study experimentally. Furthermore, primary anions in the troposphere are short lived because of a complicated web of reactions and complex formation they undergo, making details of their early fate elusive. In this perspective, the experimental approach of photodetaching mass-selected anion-molecule complexes or complex anions, which prepares neutrals in various vibronic states, is surveyed. Specifically, the application of anion photoelectron spectroscopy along with photoelectron-photofragment coincidence spectroscopy toward the study of collision complexes, complex anions in which a partial covalent bond is formed, and radical bimolecular reaction complexes, with relevance in tropospheric chemistry, will be highlighted.

Autodetachment over Broad Photon Energy Ranges in the Anion Photoelectron Spectra of [O2-]- ( = Glyoxal, Methylglyoxal, or Biacetyl) Complex Anions

Complexes of anion-neutral pairs are prevalent in chemical and physical processes in the interstellar medium, the atmosphere, and biological systems, among others. However, bimolecular anionic species that cannot be described as simple ion-molecule complexes due to their competitive electron affinities have received less attention. In this study, the [O₂-M]^- (M = glyoxal, methylglyoxal, or biacetyl) anion photoelectron spectra obtained with several different photon energies are reported and interpreted in the context of ab initio calculations. The spectra do not resemble the photoelectron spectra of M^- or O₂^- "solvated" by a neutral partner. Rather, all spectra are dominated by near-threshold autodetachment from what are likely transient dipole bound states of the cis conformers of the complex anions. Very low Franck-Condon overlap between the neutral M·O₂ van der Waals clusters and the partial covalently bound complex anions results in low-intensity, broad direct detachment observed in the spectra. The [O₂-glyoxal]^- spectra measured with 2.88 and 3.495 eV photon energies additionally exhibit features at ∼0.5 eV electron kinetic energy, which is more difficult to explain, though there are numerous quasibound states of the anion that may be involved. Overall, these features point to the inadequacy of describing the complex anions as simple ion-molecule complexes.

Excited-state dynamics of acetylene excited to individual rotational level of the V04K01 subband

Dynamics of the IR emission induced by excitation of the acetylene molecule using the (3(2)K(a) (0,1,2),A (1)A(u)<--4(1)l(a) (1),X (1)Sigma(g) (+)) transition was investigated. The observed IR emission was assigned to transitions between the ground-state vibrational levels. Acetylene fluorescence quenching induced by external electric and magnetic fields acting upon the system prepared using the (3(4)K(a) (1),A (1)A(u)<--0(0)l(a) (0),X (1)Sigma(g) (+)) excitation was also studied. External electric field creates an additional radiationless pathway to the ground-state levels, coupling levels of the A (1)A(u) excited state to the quasiresonant levels of the X (1)Sigma(g) (+) ground state. The level density of the ground state in the vicinity of the excited state is very high, thus the electric-field-induced transition is irreversible, with the rate constant described by the Fermi rule. Magnetic field alters the decay profile without changing the fluorescence quantum yield in collisionless conditions. IR emission from the CCH transient was detected, and was also affected by the external electric and magnetic fields. Acetylene predissociation was demonstrated to proceed by the direct S(1)-->S(0) mechanism. The results were explained using the previously developed theoretical approach, yielding values of the relevant model parameters.

Internal state distributions of fragment HCO via S0 and T1 pathways of glyoxal after photolysis in the ultraviolet region

The dynamics of photodissociation of glyoxal (HOC-COH) near the dissociation threshold on the triplet manifold are studied through measurement of distributions of nascent fragment HCO in various internal states. Three rotational levels 1(01) (*), 4(13) (*), and 3(21) (*)+3(22) (*) of vibrational state U (excitation wavelength approximately 394.4 nm, origin at 25,331.865 cm(-1)) of glyoxal in state A (1)A(u) and two other vibrational states at excitation wavelengths 390.33 and 382.65 nm are selected to produce fragment HCO. By means of fluorescence in the transition B (2)A(')-X (2)A(') of HCO, we determined the relative populations of internal states of that fragment. Rotational states of product HCO up to N=26 and K=2 are populated, and bimodal distributions of these rotational states are observed for the photolysis wavelengths used in this work. The high rotational part of the distribution with average energy near values calculated on the basis of the statistical model-phase-space theory is assigned to arise from glyoxal on its S(0) surface, and the low rotational part from the T(1) surface with an exit barrier. After photolysis near the threshold region on the triplet surface, HCO arising from the T(1) state appears to be a major component of products because these rotational levels 1(01) (*), 4(13) (*), and 3(2) (*) of U state selected are gateway states with an enhanced rate of intersystem crossing.

The Mechanism of Magnetically Tuned Singlet-Triplet Avoided Crossings in the Ã(1)A(2)-&Xtilde;(1)A(1) 4(1)(0) Band of Thioformaldehyde H(2)C&dbond;S

Magnetically tuned singlet-triplet perturbations in the 4(1)Ã(1)A(2)-2(1)3(1)ã(3)A(2) system of thioformaldehyde, found in ortho-rotational states (I = 1, the two hydrogen spins parallel) have been identified as being caused by vibronic spin-orbit coupling. This perturbation mechanism has been confirmed in several avoided crossings observed in this work for para states (I = 0, hydrogen spins antiparallel) which are much stronger. Parametrization of the theory has led to a quantitative understanding of the experimental frequency-field relations, and to an accurate prediction of the rovibrational energies of the triplet state. This in turn permitted the detection of about 100 Doppler-limited 2(1)3(1)ã(3)A(2)-0(0) &Xtilde;(1)A(1) rovibronic transitions which led into fine structure states. The combined data was then used to determine a set of rotational, fine, and hyperfine triplet-state parameters, the term value T(0)(2(1)3(1)ã(3)A(2)) = (16 685.385 +/- 0.002) cm(-1), and the spin-orbit vibronic singlet-triplet coupling constant, W(ST) = (0.0691 +/- 0.0016) cm(-1). A large number of frequency perturbations observed in the crossings, ranging from 2 to 300 MHz, can be explained with this single parameter. Copyright 2000 Academic Press.