Neutral molecules, excited

Mass spectrometric studies yield principally three types of information useful to the radiation chemist the major primary ions one should be concerned with, their reactions with neutral molecules, and thermodynamic information which allows one to eliminate certain reactions on the basis of endothermicity. In addition, attempts at theoretical interpretations of mass spectral fragmentation patterns permit estimates of unimolecular dissociation constants for excited parent ions. 

Dependence of Flame Species Concentrations upon Additive Concentrations. A method of determining the dependence of various ionic, neutral molecule, and excited species concentrations on the concentration of hydrocarbon added to a hydrogen/oxygen or hydrogen/air flame (based on a principle similar to that of flame ionization detectors... 

There exist a series of beautiful spectroscopy experiments that have been carried out over a number of years in the Lineberger (1), Brauman (2), and Beauchamp (3) laboratories in which electronically stable negative molecular ions prepared in excited vibrational-rotational states are observed to eject their extra electron. For the anions considered in those experiments, it is unlikely that the anion and neutral-molecule potential energy surfaces undergo crossings at geometries accessed by their vibrational motions in these experiments, so it is believed that the mechanism of electron ejection must involve vibration-rotation... 

Vibrationally mediated photodissociation (VMP) can be used to measure the vibrational spectra of small ions, such as V (OCO). Vibrationally mediated photodissociation is a double resonance technique in which a molecule first absorbs an IR photon. Vibrationally excited molecules are then selectively photodissociated following absorption of a second photon in the UV or visible [114—120]. With neutral molecules, VMP experiments are usually used to measure the spectroscopy of regions of the excited-state potential energy surface that are not Franck-Condon accessible from the ground state and to see how different vibrations affect the photodissociation dynamics. In order for VMP to work, there must be some wavelength at which vibrationally excited molecules have an electronic transition and photodissociate, while vibrationally unexcited molecules do not. In practice, this means that the ion has to have a... 

No significant improvement for the vertical excitation energy of the 2 B (3p) state was found. From these results we have decided to describe the lowest states of B and A2 symmetries with the same set of molecular orbitals, optimized for the neutral molecule within the MCSCF/ 6422 expansion. 

Our best estimation for the vertical excitation energies for states of A, symmetry are reported in Table 12. They correspond to a ground state calculated at CI( 6) level using orbitals optimized for the neutral molecule with the MCSCF/SD expansion, and excited Rydberg states calculated at the level using orbitals optimized for the positive ion... 

Vertical excitation energies to states of B symmetry, calculated at the level using the orbitals optimized for the neutral molecule with the MCSCF/6422 expansion, are reported Table 12. The I Bi valence state and 2 B (3p) Rydberg state of C3H2 are respectively 5.2 eV and 7.5 eV above the ground state with large transition moments of... 

The upper state can also be formed in energetically excited ro-vibrational states. Most photoelectron experiments do not have enough resolution to observe rotational levels, except in rare cases, but vibrational resolution is commonly achieved. Therefore, it is possible to carry out limited vibrational spectroscopy of cations and reactive neutral molecules using this approach. 

Proton dissociation in the excited states commonly occurs much easier than in the ground states, and the great difference in proton dissociation constants by several orders of magnitude is characteristic for photoacids [47]. These dyes exist as neutral molecules and their excited-state deprotonation with the rate faster than the emission results in new red-shifted bands in emission spectra [48]. Such properties can be explored in the same manner as the ground-state deprotonation with the shift of observed spectral effect to more acidic pH values. 

The nature of the final state depends upon the energy, hv, of the exciting photons. In X-ray photoelectron spectroscopy (XPS) the exciting photons are provided by sources such as A1 Ka (1,486 eV) or Mg Ka (1,253 eV) and excitation of the core electrons of the molecules is observed. In UV photoelectron spectroscopy (UPS), Hel (21.2eV) or Hell (40.8 eV) radiation is used and excitation from the valence region of the neutral molecule is observed. XPS and UPS are surface-sensitive techniques, which are capable of providing extremely useful information on the chemical nature of a surface or interface and, in the case of the XPS, the conformational state of the molecules at the surface [64]. 

Not much is known about these processes, but they must be included to give a total picture. Emissions of Lyman and Balmer spectra of the H atom upon e-impact on hydrocarbons, H2, and H20, discussed in Sect. 4.3.2, fall in this category. Similarly, many of the excited states observed in dissociated radicals via electron impact on stable molecules (Polak and Slovetsky, 1976) also belong to this category. It is known from the dipole oscillator spectrum of H20 (Platzman, 1967) that most ionizations are accompanied by considerable excitation. Excitation transfer to the neighboring neutral molecule followed by fast dissociation cannot be ruled out. 

A different view of the OMT process is that the molecule, M, is fully reduced, M , or oxidized, M+, during the tunneling process [25, 26, 92-95]. In this picture a fully relaxed ion is formed in the junction. The absorption of a phonon (the creation of a vibrational excitation) then induces the ion to decay back to the neutral molecule with emission (or absorption) of an electron - which then completes tunneling through the barrier. For simplicity, the reduction case will be discussed in detail however, the oxidation arguments are similar. A transition of the type M + e —> M is conventionally described as formation of an electron affinity level. The most commonly used measure of condensed-phase electron affinity is the halfwave reduction potential measured in non-aqueous solvents, Ey2. Often these values are tabulated relative to the saturated calomel electrode (SCE). In order to correlate OMTS data with electrochemical potentials, we need them referenced to an electron in the vacuum state. That is, we need the potential for the half reaction ... 

For many years, investigations on the electronic structure of organic radical cations in general, and of polyenes in particular, were dominated by PE spectroscopy which represented by far the most copious source of data on this subject. Consequently, attention was focussed mainly on those excited states of radical ions which can be formed by direct photoionization. However, promotion of electrons into virtual MOs of radical cations is also possible, but as the corresponding excited states cannot be attained by a one-photon process from the neutral molecule they do not manifest themselves in PE spectra. On the other hand, they can be reached by electronic excitation of the radical cations, provided that the corresponding transitions are allowed by electric-dipole selection rules. As will be shown in Section III.C, the description of such states requires an extension of the simple models used in Section n, but before going into this, we would like to discuss them in a qualitative way and give a brief account of experimental techniques used to study them. 

FIGURE 30. Potential energy curves for a neutral molecule M, and its radical cation M in the ground and first excited state (equilibrium distances with respect to an arbitrary coordinate q along which the three geometries differ). Note the shift in the M+ /(M+ ) energy difference AE on going from e/eq of M (AE = A/v from the PE spectrum of M) to qeq of M (AE corresponds to /-IM ,X from the EA spectrum of M+")... 

FIGURE 32. Schematic representation of the geometry changes of a hypothetical model of two facing n-systems with HOMOs Ta and n t,. The neutral molecule is represented in the centre. Upon ionization (removal of an electron from the HOMO it ), the antibonding interactions which prevail in n are reduced, and the distance R decreases. As a consequence, the IT. /t overlap and cr increase. Conversely, upon electron ejection from n+ (or on 7T+ - %- excitation), the bonding interaction in 7T+ is diminished, which has the opposite effect on R and cr as described above... 

Such CR bands, which have been observed for many radical cations, usually manifest themselves by intense, broad bands in the visible or NIR part of the spectrum. The reason for the broadness is that, upon excitation of an electron from 7T+ to 7r, the antibonding interaction is greatly enhanced. Consequently, the equilibrium distance of the 7r-systems in the excited state is significantly larger than in the ground state of the radical cation (or that of the neutral molecule) which results in a Franck-Condon envelope for the EA band which may be even broader than that for the corresponding PE band. 

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