Resonance Ionization Spectroscopy This article provides the author with an opportunity to express some personal views on resonance ionization spectroscopy (RIS). I would first like to give credit to the team of people a t Oak Ridge National Laboratory who have developed this subject and who have also found some very interesting and important applications of R E . Some recent reviews of the results of this effort are: an article in Reuiew of Modern Physics ( 1 ) that goes into a good hit of theoretical and experimental detail on the method; an article in Physics Today (2) that covers the highlights of the method as well as its applications on a less technical level; and a 1979 article in ANALYTICAL CHEMISTRY(3). Briefly, RIS is a photophysical process in which a laser beam is used to resonantly ionize atoms or molecules. In the process (for example, left part of Figure 1)a photon from a pulsed laser is tuned so that it can excite an atom in its ground state to some excited level. An additional photon can then complete an ionization process in which an electron is removed completely from an atom if and only if the first step of the process has been selected by having a laser of the correct frequency. There are many schemes (see right part of Figure 1for another example) for producing resonance ionization in an atom or in a simple molecule, but all of them have the common characteristic that only an atom (or molecule) of a given type is first selected by using a bound-bound transition and subsequently ionized. This builds spectroscopic selectivity into the ionization process. Using lasers 1448A
that are commercially available, one can carry out the RIS process [as embodied in five schemes (1)] for every known element except He and Ne. Selective ionization can be combined with radiation detectors that are sensitive to a single electron or to a single positive ion, incorporating in one measuring device the virtues of absorption spectroscopy with the extraordinary sensitivity that is afforded by radiation detectors. Furthermore, the resonance ionization process can be saturated, often in large volumes. When this is achieved, each atom of the selected type that was in its ground state before a laser beam was pulsed through it will he in the ionization continuum after the laser pulse. That is, each atom is reduced to one positive ion and one electron with unit efficiency. It is this ability to saturate the resonance ionization process that makes it possible to detect a single atom, as we have demonstrated in a number of applications. This article briefly summarizes the subject of RIS and some of its applications. Most of the applications of RIS are, in fact, analytical in nature. As a physicist, I am impressed with the broad view and the wide scope given to the field of analytical chemistry by my colleague Jack Young (3). Some of my personal views on the future of RIS as a tool in analytical chemistry will conclude the article. Brief Summary of RIS An accelerator experiment a t Oak Ridge National Laboratory was the beginning of RIS. For a number of years the Oak Ridge group had been
ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
interested in the energy pathways associated with the interaction of a charged particle with matter. Specifically, we were interested in knowing what happens when protons, or other swift particles, dissipate energy in a simple gas such as He. A number of studies on total ionization of He, as well as time-resolved emission of the vacuum ultraviolet radiation from He, were put together in an attempt to understand what types of states are ex. cited by the interaction of radiation with matter and to understand the consequence of these ezcitations. While the emission experiments and total ionization experiments revealed a large body of interesting information, there were still gaps concerning the population and the decay rates of metastable states that are created hy the interaction of protons with He. By definition, metastable states do not radiate directly; they are revealed in emission spectroscopy only as a consequence of rather complex collision phenomena. Thus, we had the idea of injecting photons that would interact with the metastable states of He and promote them to a higher level in a bound-hound transition, and of using other photons a t the same wavelength from the same laser pulse to ionize the intermediate state. Therefore, ionization could be measured to obtain the absolute population of the metastable levels. Instead of waiting for the metastable level to decay to the ground state through some complex emission process, we felt it was desirable to promote the metastable state selectively into the ionization continuum. We soon learned that commercial 0003-2700181lA35 1-1448501.0010
0 1981 American Chemical Soeiety
Report G. S.Hurst Chemical physics Section Oak Ridge National Labaatory Oak Ridge, Tenn. 37830
A
i Excited
I
State
Flgure 1. Examples of two schemes for the resonance ionization of atoms. With a total of five schemes ( l ) ,it is possible
to selectively ionize every atom in the periodic table except He and Ne
lasers were available that could provide the proper wavelength for the He metastable-state experiment; and we found that about 100 mJIcm2 in a laser pulse provided enough photons to cause each metastable state of He to be selectively ionized into the continuum. The absolute number of metastable states was measured, using the RIS scheme shown a t the left of Figure 1. By delaying the time between the Van de Graaff pulse that created these excited atoms and the laser pulse which selectively ionized these excited atoms, we were also able to get the lifetime of the He metastable state as a function of He pressure and thereby understand much more completely the energy pathways taking place in He. Along with the idea for the He metastable experiment we planned RIS experiments with atoms in their ground state. Alkali atgms are easily ionized from the ground state, and, in fact, the wavelength required is similar to the wavelength needed for the selective ionization of He metastable atoms. If we could do RIS with atoms in the ground state by bringing a laser beam directly into a proportional counter, then we could, in principle, measure the presence of a single atom. Because
of our interest in one-atom detection of Cs, we felt that it would also he of interest to do a number of very basic experiments on Cs atoms in their ground state. We proved, for example, that it was possible to saturate the process; that is, each atom of Cs could be ionized into the continuum. Some very interesting studies of collisional line broadening and excited state photoionization cross sections could also be done with the new resonance ionization process (I). Concurrent with this work on the excitation and ionization of Cs atoms in the ground state, we decided that it wduld be interesting to do other work with CsI molecules. Saturation of the molecular dissociation process led to a simple method of obtaining the absolute photodissociation cross section as a function of wavelength, without prior knowledge of the concentration of CsI molecules. By liberating free atoms a t time t = 0 and then detecting them at t > 0 with a second pulsed laser, the survival time of free atoms could he measured. When alkali atoms are created in a reactive atmosphere such that new chemical combinations occur, the rate a t which the free alkali atoms react with some other chemical species can be measured. For example,
we showed that it was possible to measure the reaction rate of Cs atoms with 0 2 molecules in the presence of AI. Diffusion coefficients for alkali atoms moving through noble gases could also be deduced from data on the survival time of atoms in a laser beam. Later on, these diffusion experiments were improved.by having two pqallel beams so that atoms would have to diffuse from one beam over to another to he counted. This proved to be a much better way for the measurement of diffusion coefficients because the effects of chemical reaction could essentially he eliminated by making the diffusion measurements a t two separations between the source and the detector laser beams. Following this work we found that it was possible to measure density fluctuations of atoms and also to obtain the Fano factor assoeiated with the ionization produced by the total absorption of an electron in a gas. Currently, many other aspects of statistical fluctuation work are continuing in the Chemical Physics Section a t Oak Ridge National Laboratory. One-atom detection was first demonstrated ( 4 ) experimentally in 1977. It was shown, for example, that one Cs atom could be detected as it diffused
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ANALYTICAL CHEMISTRY, VOL 53, NO. 13, NOVEMBER 1981
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