Franck-Condon Process Research Articles

Franck-Condon processes in pentacene monolayers revealed in resonance Raman scattering

Franck--Condon processes in pentacene monolayers are revealed in resonance Raman scattering from intramolecular vibrations. The Raman intensities from a totally symmetric vibrational mode display resonance enhancement double peaks when incident or scattered photon energies overlap the free exciton (FE) optical emission. The two resonances are of about equal strength. This remarkable symmetry in the resonance Raman profile suggests that Franck--Condon overlap integrals for the respective vibronic transitions have the same magnitude, which could be explained by the small displacement of potential energy curves along the configuration coordinate upon the FE excitation. The interference between scattering amplitudes in the Raman resonance reveals quantum coherence of the symmetry-split states (Davydov doublet) of the lowest intrinsic singlet exciton in pentacene monolayers.

Raman Scattering Studies of the Ba2MnWO6 and Sr2MnWO6 Double Perovskites.

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Raman Scattering Studies of the Ba2MnWO6 and Sr2MnWO6 Double Perovskites

Polycrystalline Ba2MnWO6 (BMW) and Sr2MnWO6 (SMW) samples were studied between 80 and 1200 K by Raman scattering spectroscopy. In the case of BMW (space group Fmm), four Raman active vibrational modes, predicted by factor group analysis, were identified. Raman scattering studies with different wavelengths revealed a resonant bands between 300 and 800 cm-1. The origin of these bands was related to the Franck-Condon process. Line broadening versus temperature and phonon frequency were studied, and a qualitative explanation was proposed. SMW samples had considerably more complex Raman spectra. It was found that SMW transformed from tetragonal (room-temperature space group P42/n) to the cubic phase between 670 and 690 K; the phase transition temperature was dependent on sample preparation conditions, and it was considerably lower than in the case of large grain size powders. The role of grain size in phase transition is discussed. Mn ions were found to have a crucial role in the lattice dynamics of both materials.

Atom Scrambling of Linear C5 in the Gas Phase: a Joint Experimental and Theoretical Study

The radical anion [CC 1 3 CCC] - . , with known bond connectivity, has been synthesized in the ion source of a VG ZAB 2HF mass spectrometer by the reaction of (CH3) 3 Si-C≡C 1 3 C(=NNH-tosyl)-C≡C-Si(CH 3 ) 3 with HO - followedby F - (from SF 6 ). The collision-induced mass spectrum of [CC 1 3 CCC] - . shows only one fragmentation: exclusive loss of 1 2 C. Computational studies at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G-(d) level of theory indicate that this experimental observation means that [CC 1 3 CCC] - . does not rearrange under the conditions of collisional activation. The charge reversal [ - CR + , synchronous charge stripping of the radical anion (by a vertical Franck-Condon process) to the radical cation], and the neutralization/reionization [ - NR + , stepwise vertical oxidation of the radical anion to the neutral, then of the neutral to the radical cation] spectra of [CC 1 3 CCC] - . show partial and complete atom scrambling, respectively. Under the experimental conditions used, the neutral has a lifetime of 10 - 6 s before being converted to the corresponding radical cation. Computational studies at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory suggest that the major atom-scrambling pathway of the neutral involves cyclization of singlet CC 1 3 CCC to equilibrating and degenerate carbon-substituted rhombic structures that fragment to yield linear singlet 1 3 C 1 2 C 4 structures. Overall, the label is randomized. A similar mechanism is proposed to account for the partial atom scrambling of the radical cations formed in the - CR + experiment from [CC 1 3 CCC] - . .

Theoretical Study on the Electron Capture Dissociation Correlated with Proton Transfer Processes.

A novel approach was proposed for elucidating the reaction mechanism of the electron capture dissociation (ECD) recently introduced powerful technique to characterize peptides and proteins. Against a mechanism presented so far that amide bonds catch the hydrogen atom released from the protonated peptide through electron capture, we present another mechanism focusing the proton rearrangement among the sites of the nitrogen and the oxygen atoms. This proton transfer produces various types of protonated peptides that should be responsible for the cleavage after electron capture. We carried out MO calculations on the ECD processes of glycylserine as a model peptide to elucidate how the process of proton transfer correlates with backbone cleavages. Based on the calculated energy and the molecular geometry for various stages of species concerned with the ECD process, mechanisms for proton transfer and electron capture through the Franck-Condon process were discussed. Presented here are the correlation of protonated sites to types of product ions unique in ECD and the importance of unoccupied MO's to understand the bond cleavage reactions connected with the Franck-Condon process.

Rotational and vibrational excitation of the N 2+ (B) state in a He + N 2 electron-beam plasma

Emissions of the first negative system of N 2 + have been studied in a He + N 2(< 5%) mixture activated by a 10-keV electron beam. Flow in the free jet core was used as a gas target. Analysis of both the internal energy distributions in the N 2 +(B) state and the He density dependecies of the excitation rates have revealed three excitation mechanisms. One of them is the excitation by atomic ions He + going through the formation of long-living collisional complexes (He.N 2) +. This channel produces the vibrational and rotational heated ions N 2 +(B), with the rotational temperature independent of the rotational distribution in the ground state of the N 2 molecule before excitation. This temperature is estimated as T = (900 ± 60) K. The other excitation channels are evidently Franck-Condon processes with the population of vibrational levels according to Franck-Condon factors and with the rotational transition probabilities close to those governed by Hönl-London factors in the excitation.

Scattering angular distributions in collisionally activated dissociation of some high mass ions: Analysis of mass-analyzed ion kinetic energy peak shapes

A method has been developed to determine scattering angular distribution in collisionally activated dissociation (CAD) of high mass ions by analyzing translational energy spectra recorded by mass-analyzed ion kinetic energy spectrometry. Scattering angular distributions have been obtained for CAD of C6F5I+̇, Cr(CH3COCHCOCH3)+̇3, and Cs5I+4 with He. Large energy losses observed in the translational energy spectra are attributed to the elastic energy transfer to the collision gas accompanying collisional deflection. Deflections of the parent ions get larger as the translational energy for relative motion in the center-of-mass coordinate system decreases. This has been explained by the reactive line-of-centers model. Either vibrational excitation via momentum transfer or vibronic excitation via nonadiabatic interaction is compatible with the experimental data. On the other hand, electronic excitation via the Franck–Condon process is not a feasible mechanism to explain the excitation process in CAD of high mass ions at keV translational energy.

Photoelectron spectroscopy of small antimony cluster anions: Sb−, Sb2−, Sb3−, and Sb4−

We report the 351 nm photoelectron spectra of Sb−, Sb2−, Sb3−, and Sb4−. The electron affinity of atomic Sb is measured to be 1.046(5) eV. The Sb2− photoelectron spectrum displays rich vibrational and electronic structure. Low-lying electronically excited states are observed for both the anion and the neutral. Several features in both the 351 and 364 nm photoelectron spectra of Sb2− cannot be explained as Franck–Condon processes, indicating that we are accessing autodetaching resonances of the negative ion at these wavelengths. The adiabatic electron affinity of Sb2 is determined to be 1.282(8) eV. For the photoelectron spectra of Sb3− and Sb4−, the observed electronic structure is explained in terms of recently reported ab initio calculations. The adiabatic electron affinity of Sb3 is estimated to be 1.85(3) eV, and an upper bound on the electron affinity of Sb4 is reported, EA(Sb4)≤1.00(10) eV. The vertical detachment energies of Sb3− and Sb4− to the neutral ground states are determined to be 1.90(2) and 1.57(5) eV, respectively. We report photoelectron angular distributions for all the observed spectra, and find that the autodetaching resonance causes unusual angular distributions for Sb2− photodetachment. Finally, electron affinity trends for group V atoms, dimers, and small clusters are discussed in light of the present study.

Determination of non-Boltzmann vibrational distributions of N2(X,v″) in He/N2 microwave-discharge afterglows

We have extended a technique for studying the vibrational distributions of ground-electronic-state, molecular nitrogen in the afterglow of a microwave discharge through mixtures of helium and nitrogen. The technique is based upon adding metastable helium atoms to the afterglow. The He*(2 3 S) excites the N2(X,v) to N+2(B 2Σ+u) in a Penning-ionization reaction. Since Penning ionization is a Franck–Condon process, the vibrational distribution of the N+2(B) product is determined by that of the N2 (X,v) from which it was produced. The measurements show that the ground-state nitrogen distribution is highly non-Boltzmann, with vibrationally hotter distributions being produced with lower mole fractions of nitrogen in the discharge. We have also observed the production of N+2(C 2Σ+u) from He* Penning ionization of molecular nitrogen. This process is energetically allowed only if the vibrational energy in the ground-electronic-state nitrogen exceeds 3.8 eV or 15 vibrational quanta.

Product state and kinetic energy distributions in the ultraviolet photodissociation of the NO–Ar van der Waals molecule

The internal state and kinetic energy distributions of the X̃ NO fragments formed from the ultraviolet photodissociation of the NO–Ar van der Waals species were obtained by laser-excited fluorescence techniques. The initially excited à NO–Ar rapidly dissociates to form X̃ NO with little rotational excitation, with vibrational excitation determined by a Franck–Condon process, with a cos2 θ angular flux distribution (θ defined relative to the direction of polarization of the pump laser), and with a speed v∼4.4×105 cm/s.

Time dependent formulation of polyatomic photofragmentation: Application to H3+

We present a new approach to photofragmentation which through its explicit time dependence avoids the difficulties normally associated with calculation of Franck–Condon processes. No scattering eigenfunctions or Franck–Condon overlaps are required, yet the formulae are exact within the same assumptions as the usual time independent theory. Semiclassical implementation of the method is described in connection with a successful application to H+3, which involves, in our model, a multiple continuum breakup final state on one of two excited state surfaces treated. Full arrangement and energy partitioning information are given on both surfaces in a calculation whose main numerical effort involves running five classical trajectories.

Charge Transfer and Proton Transfer in Polyatomic Ion-Molecule Systems

Ion impact mass spectra of some hydrocarbon molecules have been investigated using mass analyzed beams of H+, D+, H2+, D2+,H3+, D3+, ArH+, ArD+, Ar+ and Ne+ ions. Charge-transfer and proton- or deuteron-transfer mechanisms suffice to account qualitatively for the observed spectra. The energetics of the respective processes in the systems studied provide a basis for assigning various ion fragments and parent molecule ions to either charge-transfer or ion-molecule reaction formation mechanisms. The role of the neutral molecule as an energy sink in molecular charge transfer has been examined in detail for H2+ charge-transfer reactions. The distribution of recombination energies experimentally observed in H2+ collisions with a variety of molecular species is rationalized with the aid of a model that considers the neutralization reaction as a Franck-Condon process. Estimates are made of the distribution of excited neutral product H2 molecules from the squares of the overlaps of H2+−H2 vibrational wavefunctions. This model also accounts for relatively low cross sections for H3+ ion-impact reactions because of the production of vibrationally excited H2 which renders some processes endothermic. The necessity of formation of vibrationally excited neutral molecular products in ion-molecule reactions that proceed via stripping mechanisms accounts for energy barriers not predicted from usual thermochemical considerations. Cyclopropane and propylene spectra were the subject of extensive study. With ArH+ projectile ions the velocity dependence of the component of ion-impact spectra attributed to ion-molecule reactions was found to be in good agreement with the 1 / υ dependence of Langevin collision cross sections, and the magnitude of experimental cross sections were found in good agreement with those calculated from simple classical theory. Significant differences were observed in the ion-impact spectra of cyclopropane and propylene. The ion-molecule reaction component was attributed primarily to proton- or deuteron-transfer reactions which initially produce excited C3H7+ ions. Processes which superficially appeared to be hydride ion-transfer processes were plausibly explained as the loss of H2 from excited C3H7+, and this argument was supported by the isotopic exchange observed with deuterium-substituted projectile ions. Studies on hydrocarbons up to and including neopentane showed further consistency with predictions made for ion-molecule reaction cross sections based on the Langevin model.

Ionization of acetylene and its electronic energy levels

The ionization processes in acetylene under photon, electron, ion, and metastable atom impact, including new results, are critically discussed in relation with the electronic structure of acetylene and with the known spectroscopic information. It is shown that the first ionization potential at 11.409 eV corresponds to a 1 πu electron removal and that Franck–Condon processes yield an adiabatic ionization potential in this particular case. The new production of ions around 13.25 eV and 14.5–15 eV is due to pre-ionized states probably optically forbidden. From the discussion it is suggested that these might be excited linear σg πu4 πg* or σg πu4 πu* states. The second ionization potential of acetylene, related to the removal of a 3 σg electron, is shown to be around 16.5 eV.