Population of electronically excited states

Important aspects of the interaction of strong laser fields with molecules can be missed in standard TOF experiments, most notably the population of electronically excited states. However, by studying vibrational excitation, the frequency and dephasing of the vibrational motion can be used to identify the electronic state undergoing the vibrational motion. In some cases, this turns out to be a ground state, and in others, an excited state. Once we have identified an excited state, we are left with the question of how and why the state was populated by the strong field. In one example above (the Ij A state discussed in Sect. 1.3.3), the excited state is formed by the removal of an inner orbital electron, in this case a iru electron. This correlates with the measured angular dependence for the ionization to this state. 

A possibility that has been suggested for the Co(NH3)63+(2+ reaction is that it could occur via initial thermal population of electronic excited states (equation 48a) followed by electron transfer between eg orbitals on Co11 and Com (equation 48b) followed by decay of Co(NH3)62+ (2Eg) to the ground state. 

Utilizing ionization efficiency curves to determine relative populations of vibrationally excited states (as in the photoionization experiments) is a quite valid procedure in view of the long radiative lifetime that characterizes vibrational transitions within an electronic state (several milliseconds). However, use of any ionization efficiency curve (electron impact, photon impact, or photoelectron spectroscopic) to obtain relative populations of electronically excited states requires great care. A more direct experimental determination using a procedure such as the attenuation method is to be preferred. If the latter is not feasible, accurate knowledge of the lifetimes of the states is necessary for calculation of the fraction that has decayed within the time scale of the experiment. Accurate Franck -Condon factors for the transitions from these radiating states to the various lower vibronic states are also required for calculation of the modified distribution of internal states relevant to the experiment.991 102... 

Kinetics of Population of Electronically Excited States in Plasma... 

This population of electronically excited states decreases exponentially with effective Boltzmann temperature 7 and has an absolute value corresponding to equilibrium with continuumy(ii) y y. In the opposite case, the population of electronically excited states far from continuum (E > Te) can be found as... 

From Equation [1] it is apparent that elevated temperatures promote population of electronic excited states. However, competing with this enhanced excited state population is ionization ... 

Fig. 2.1 Nuclear resonance absorption of y-rays (Mossbauer effect) for nuclei with Z protons and N neutrons. The top left part shows the population of the excited state of the emitter by the radioactive decay of a mother isotope (Z, N ) via a- or P-emission, or K-capture (depending on the isotope). The right part shows the de-excitation of the absorber by re-emission of a y-photon or by radiationless emission of a conversion electron (thin arrows labeled y and e , respectively)...

Principles and Characteristics The term luminescence describes the radiative evolution of energy other than blackbody radiation which may accompany the decay of a population of electronically excited chro-mophores as it relaxes to that of the thermally equilibrated ground state of the system. The frequency of the... 

As a result of ion-phonon interaction, the population of the excited state decreases via nonradiative transition from the excited state to a lower electronic state. The energy difference between the two electronic states is converted into phonon energy. This process of population relaxation is characterized by a relaxation time, xj, which depends on the energy gap between the two electronic states, the frequencies of vibration modes, and temperature (Miyakawa and Dexter, 1970 Riseberg and Moos, 1968). At room temperature, the excited state lifetime is dominated by the nonradiative relaxation except in a few cases such as the 5Do level of Eu3+ and 6P7/2 level of Gd3+ for which the energy gap is much larger than the highest phonon frequency of the lattice vibrations. 

First attempts to investigate the photodissociation dynamics of Fe(CO)5 used molecular beam technology coupled with high intensity femptosecond lasers [47, 48]. It is important to note that these experiments relied on multiphoton absorption to populate the electronic excited states of Fe(CO)5. This work built on the results of earlier experiments using nanosecond pulsed lasers which provided information on the photoproduct distribution and their energies [37-40, 49-56]. The energies of the various dissociation processes for Fe(CO)5 are presented in Fig. 18 for comparison with the excitation photon energies and the absorption profile of Fe(CO)5. 

The dipolar or induced dipolar natime of molecules means that the impacting electron can cause rotational excitation but, because of conservation of momentum, very little of the kinetic energy of the electron can be imparted and little direct vibrational excitation can occur (Cottrell, 1965). Further, although ion-sources frequently operate at fairly high temperatures, the population of vibrationally excited states of molecules even at 500°K is very low and the source of the large vibrational excitation of ions must be sought elsewhere. For illustrative... 

Enhanced fluorescence, or MEF, is a result of both a net system absorption and plasmon coupling and subsequently efficient emission, but to date, it has not been possible to quantify the relative contributions of enhanced emission and net increase in the system absorption to the MEF phenomena.(23) Due to the increase in the population of the singlet excited state or net system absorption, the very presence of MEP has also suggests an increase in the population of the triplet state.(23) The presence of Metal-Enhanced Fluorescence, Phosphorescence, Metal-Enhanced singlet oxygen and superoxide anion radical generation in the same system is an effect of the enhanced absorption and emission effects of the fluorophores near-to silver, although these processes are effectively competitive and ultimately provide a route for deactivation of electronic excited states. 

Absorption spectra of electronically excited states may be observed in flash photolysis studies. Porter has established the existence of the triplet state in a wide range of organic compounds in the liquid and gaseous phases. For example, the first triplet state of anthracene is populated by radiationless conversion from a photochemically excited singlet molecule, and may be observed by the absorption to the second triplet level. Absolute measurements of the triplet concentration may be made by determinations, from the absorption spectra, of the depletion of the singlet state. Similar results have been obtained with a variety of hydrocarbons, ketones, quinones and dyestuffs. 

The data obtained from the ECD and reduction potentials can be used to interpret PES data. Three examples for molecules are the lower values for the electron affinities of nitromethane, anthracene, and coronene. Based on the observation of excited states in anthracene and tetracene in the ECD data, it is reasonable to assume that the lower value for coronene derives from the population of an excited state. In Figure 6.8 the PES of coronene is shown with two sets of peaks. If the ground-state Ea for coronene is taken from the initial onset, it is much lower than the value obtained from reduction potential or electronegativity data. In addition, the second onset must be explained [38—42]. 

If some low-lying vibrational levels v > 0 of the electronic ground state are substantially populated, we must also consider contributions from Mn>v>0 m 0 to tJ = 0 are located to the red of the 0 0 transition (v 0 to v = 0). Hot bands can be identified on the basis of their temperature dependence given by the Boltzmann population of vibrationally excited states in thermal equilibrium. 

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