Excited ionic states, lifetimes

Additional details on some of these methods are described in other sections of this review. Attempts have also been made to determine excited-state populations in single-source mass-spectrometric experiments from an analysis of ionization efficiency curves.38ad There are several difficulties in applying such methods. For instance, it is now known from photoionization studies that ionization processes may be dominated by autoionization. Therefore, the onset of a new excited state is not necessarily characterized by an increased slope in the electron-impact ionization-efficiency curve, which is proportional to the probability of producing that state, as had been assumed earlier. Another problem arises because of the different radiative lifetimes that are characteristic of various excited ionic states (see Section I.A.4). 

Obviously, the various electronically excited states of an atomic or molecular ion vary in their respective radiative lifetime, t. The probability distribution applicable to formation of such states is thus a function of the time that elapses following ionization. Ions in metastable states, which have no allowed transitions to the ground state, are most likely to contribute to ion-neutral interactions observed under any experimental conditions since these states have the longest lifetimes. In addition, the experimental time scale of a particular experiment may favor some states over others. In single-source experiments, short-lived excited states may be of greater relative importance than in ion-beam experiments, in which there is typically a time interval of a few microseconds between ion formation and the collision of that ion with a neutral species, so that most of the short-lived states will have decayed before collision. There are several recent compilations of lifetimes of excited ionic states.lh,20 ,2,... 

Electronically excited ionic states, for which the transitions to the ground state are allowed, normally have very short radiative lifetimes, typically on the order of 10 nsec to 1 jLisec, Yet even these states are quite efficiently collisionally deactivated, particularly on interaction with the corresponding parent gases. Several such systems have been studied in detail, and the Stem-Volmer relation has been employed to determine rate coefficients for collisional deactivation.233-239 Some of these reactions and the pertinent kinetic data are displayed in the reactions that follow. 

In table I, only such metastable ionic states have been included for which the lifetime is at least 10 sec. If in applications the ions are produced in the gas itself, the time between the formation of the ion and the charge-exchange collision may be shorter. In such cases, other excited ionic states and other recombination processes must also be considered. 

Therefore, if the excited-state lifetimes r0 and Tq are known, the plot of ( / 0)/( / ) versus [H30+] yields the rate constants k3 and k i. However, it should be emphasized that corrections have to be made (i) the proton concentration must be replaced by the proton activity (ii) the rate constant k 3 must be multiplied by a correction factor involving the ionic strength (if the reaction takes place between charged particles), because of the screening effect of the ionic atmosphere on the charged reactive species. 

The behavior of practically all luminescent materials is sensitive to various parameters of physical and chemical origin. The excited state lifetimes and average intensities of the fluorescence and/or phosphorescence of these materials are modulated, for example, by temperature, oxygen, pH, carbon dioxide, voltage, pressure, and ionic strength. Consequently, the luminescence of various materials could be used, in principle, to monitor parameters of interest in medicine, industry, research, and the environment. 

From the standpoint of the electronic structure, Gadzuk et al. [62] show that the vibrational excitation of desorbed molecules is closely related to the lifetime of the negative ionic state. Hasselbrink [63] discusses that the translational energy obtained from the velocity distribution increases with increasing rotational energy of the desorbed molecules. Both calculations assume a form of the potential energy surface (PES) in the excited state and the assumed form plays an important role. 

The energy dependence of the vibrational excitation cross section depends on the lifetime of the intermediate ionic states (the so-called resonances). First let us consider the so-called short-lifetime resonanees (eg., H2, N2O, H2O, etc), where the lifetime of the autoionization states AB (bi) is much shorter than the period of oscillation (t 10 s). The... 

The excitation intensity can be increased if the excitation region is placed inside the resonator of a cw dye laser that is tuned to the selected transition. Before they reach the laser beam, the ions can be preexcited into highly excited long-living levels by gas collisions in a differentially pumped gas cell (Fig. 6.93a). This opens new transitions for the laser excitation and allows lifetime measurements of high-lying ionic states even with visible lasers [800]. 

The excited states in the ion beam must also be considered. To be able to survive from the time when the ion is formed until it enters the collision chamber, its lifetime must be of the order of 10 sec, i.e., the ionic state must be metastable. To identify a state as metastable, usual spectroscopic rules have to be used. 

N.P. Penkin Experimental determination of electronic transition probabilities and the lifetimes of the excited atomic and ionic states, in Atomic Physics 6, ed. by R. Damburg (Plenum, New York 1979) p.33... 

For atoms with several equivalent electrons excited states which are energetically equivalent to ionic states are possible. As a result there is a certain probability that they transform spontaneously into ions by emission of an electron. This transition can be investigated wave-mechanically and in this way a lifetime for the first state and a theoretical value for the probability We mentioned above can be obtained. If we compare this to the probability Wq of a transition to a nonexcited state of the atom by emission of a light quantum then the ratio of these two probabilities can be roughly tested with the help of the Auger experiment on the fluorescence yield of absorbed X-rays. Finally the influence of the spontaneous transformation on the dispersion is investigated. 

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