J. Phys. Chem. 1987, 91, 2244-2249
2244
FEATURE ARTICLE Study of Low-Energy Electron-Molecule Interactions Using Rydberg Atoms F. B. Dunning Department of Space Physics and Astronomy and the Rice Quantum Institute, Rice University, Houston, Texas 77251 (Received: November 14, 1986)
Studies of collisions involving atoms in Rydberg states provide a novel means to investigate electron-molecule interactions at energies that extend below those readily accessible by alternate techniques. This application of Rydberg atom collision studies is discussed and is illustrated by results from experiments involving molecules that attach free low-energy electrons.
Introduction In recent years there has been increasing interest in the study of atoms in which one electron is excited to an orbital of large principal quantum number n. As illustrated in Table I, such atoms, termed high-Rydberg atoms, or more simply Rydberg atoms, possess physical characteristics quite unlike those of atoms in ground or low-lying excited states.’ In atomic terms, Rydberg atoms are enormous, and for values of n k 100 they approach the size of biological cells. The excited Rydberg electron is, on average, so far from the nucleus and inner electrons that it may be considered as an independent particle. Indeed, a Rydberg atom can be simply pictured as an excited electron in distant classical orbit about a compact core ion of unit net positive charge. This so-called “essentially free” electron model2is similar to that initially proposed by Bohr for the hydrogen atom, except that the proton is replaced by the larger core ion. At large orbital radii, however, the motion of the electron is governed primarily by the Coulomb field of the core and the properties of all Rydberg atoms are, therefore, similar to those of highly excited hydrogen atoms. Rydberg levels are closely spaced in energy and have binding energies of only a few millielectronvolts. This, coupled with their large physical size, makes Rydberg atoms extremely fragile. There is more than enough energy available in even a thermal-energy collision to ionize a Rydberg atom or to change its quantum state. Rydberg atoms are easily perturbed by collisions, resulting in a wide range of reaction processes, many unique to Rydberg The study of Rydberg atom collisions is facilitated by their relatively long natural lifetimes which, for atoms excited in the laboratory, may approach or exceed 1 ms. The essentially free electron model is frequently used in discussing Rydberg atom collision processes,2 analyzing them in terms of the separate interactions between the Rydberg electron, the core ion, and the target particle. For neutral targets, the ranges of the electrontarget and core-target interactions are small and, for large values of n, are much less than the size of the Rydberg orbit. Many Rydberg atom collision processes are dominated by the binary electron-target interaction, and hence by large impact parameters, with the core ion acting only as a distant spectator. Since the time-averaged kinetic energy of a Rydberg electron is equal to its binding energy, study of Rydberg atom collisions of this type can furnish information on electron-molecule interactions a t millielectronvolt energies. The application of Rydberg atom (1) Many properties of Rydberg atoms are discussed in: Rydberg States of Atoms and Molecules; Stebbings, R. F., Dunning, F.B., Ed.; Cambridge University Press: New York, 1983. (2) For a detailed discussion of this model,see articles by M. Matsuzawa and by A. P. Hickman, R. E. Olson, and J. Pascale in ref 1. (3) Dunning, F.B.; Stebbings, R. F.Adu. Electron. Electron Phys. 1982, 59. 79.
0022-3654/87/2091-2244$01.50/0
collision studies to the investigation of electron-molecule interactions is discussed in the present article and is illustrated by results from recent experiments involving molecules that attach free low-energy electrons. The independent-particle model also suggests that studies of Rydberg atom collision processes in which the interaction between the core ion and target is important might provide information on thermal-energy ion-molecule reactions. There is, however, a s i w i c a n t likelihood that core-target interactions will be perturbed by the Rydberg electron, which is, in contrast to the core ion, highly mobile. For values of n 5 2 0 the orbital period of the Rydberg electron (see Table I) is less than the characteristic times, s, typically associated with thermal-energy ion-molecule reactions. Thus, especially for states of low orbital angular momentum, the possibility exists that, during the core-target interaction, the Rydberg electron might approach sufficiently close as to modify the interaction by, for example, carrying off excess energy. Evidence of such behavior is provided by observations of the formation of stable product ions through associative ionization reactions4s5 of the type
-
K(nd)
+ H20
-
KH20++ e-
(1)
Although this reaction clearly involves the core ion, the Rydberg electron must also play an important role because KH20+ions formed in direct K+-H20 interactions are unstable and have very short lifetimes5
Experimental Techniques Many effects not normally considered in investigations involving atoms in ground or low-lying excited states are important in Rydberg atom studies.6 Thermal-energy collisions frequently lead to rapid mixing among Rydberg levels, resulting in a complex, time-dependent population distribution even if only a single Rydberg state is initially populated. The fraction of the total Rydberg population in other than the initial parent state increases rapidly in time due in part to the cumulative effect of collisions and in part to the longer natural lifetimes of the majority of the collision products. Thus, in order to determine reaction rate constants characteristic of a particular initial parent state, it is necessary either to limit the target gas density and perform measurements at sufficiently early times that no significant population of Rydberg atoms in other then the parent state has developed or, alternately, to take into account effects due to the (4) Weiner, J.; Boulmer, J. J. Phys. B 1986, 19, 599. (5) Zollars, B. G.;Walter, C. W.; Johnson, C. B.; Smith, K. A.; Dunning, F. B. J. Chem. Phys. 1986, 85, 3132. (6) Dunning, F. B.; Stebbings, R. F. Comments At. Mol. Phys. 1980, IO, 9.
0 1987 American Chemical Society
Feature Article
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 2245
TABLE I: Properties of Highly Excited Hydrogen Atoms
numerical values property Bohr radius binding energy separation between adjacent n levels root-mean-square velocity of the Rydberg electron period of electronic motion classical field ionization threshold
n = l
n = 30
n = 100
5.3 x cm (=ao) 13.6 eV (=R) 10.2 eV 2.2 x IO8 cm s-' (-00) 1.5 x S (=Ti) 3.2 X lo8 V cm-I (=E,)
4.8 X lod cm 1.5 x eV eV 1.0 X 7.3 x 106 cm s-l 4.1 X 10-l2s 395 V cm-'
5.3 X cm 1.4 x 10-3 eV 2.7 X eV 2.2 x 106 cm s-1 1.5 x 10-'0s 3.2 V cm-'
n dependence n2ao R/n2 2Rln' voln n3Tl E11n4
mixed Rydberg population by using suitable models and analysis procedures. Rydberg atoms also interact strongly with background 300 K blackbody radiation, and this interaction must be considered in the analysis of experimental data. For example, even in the absence of any target gas, blackbody-radiation-induced photoabsorption and stimulated emission can populate states other than the parent state at an appreciable rate.' It is also important to ensure that external fields are minimized because the presence of even a weak electric field can have a pronounced effect on Rydberg atom excitation and collision processes.8 Rydberg atoms can be generated by using a variety of techniques including electron impact excitation, electron capture, and photoexcitation. Only the latter technique, however, affords the resolution necessary to confine excitation to a single, well-defined Rydberg state. Indeed, it is the availability of tunable dye lasers that has been responsible for many of the experimental advances in the study of Rydberg species. In collision studies, Rydberg atoms are most frequently detected by electric field ionization or by observing the radiation they emit in spontaneous decay. With appropriate optical filtering, fluorescence measurements enable state-selective detection of Rydberg atoms with values of n 5 20. Atoms with larger values of n are difficult to study with fluorescence techniques because of their long natural lifetime^.^ Such atoms can, however, be easily detected by electric field ionization. This technique is convenient for use in absolute measurements because, for sufficiently large applied fields, Rydberg atoms are ionized with unit efficiency. Field ionization also permits state-selective detection. In a technique referred to as selective field ionization (SFI)'O," the Rydberg atoms are subjected to an increasing electric field and the resulting field ionization signal is measured as a function of applied field. Since Rydberg atoms in different quantum states ionize at different field strengths (see Table I), such SFI data enable, in principle, determination of the initial (zero-field) excited-state distribution. Electron Attaching Targets Collisions of Rydberg atoms with molecules that attach free thermal-energy electrons have been investigated in several laboratories, and these investigations have demonstrated the value of Rydberg atom studies in exploring electron-molecule interactions a t millielectronvolt energies. A large number of complex molecules, the majority halogenated, are known to efficiently attach free thermal-energy electrons, a process that is frequently accompanied by d i s s ~ c i a t i o n . ' ~Studies ~'~ of electron capture are of fundamental importance in understanding the properties of molecular negative ions and how these depend on the details of molecular structure. Attaching targets are also of considerable practical interest.14 For example, free electrons contribute to (7) Farley, J. W.; Wing, W. H. Phys. Rev. A 1981, 23, 2397. (8) Slusher, M. P.; Higgs, C.; Smith,'K. A,; Dunning, F. B.; Stebbings, R. F. Phys. Rev. A 1982, 26, 1350. (9) Theodosiou, C. E. Phys. Rev. A 1974, 30, 2881, 2910. (10) Jeys, T. H.; McMillian, G. B.; Smith, K. A,; Dunning, F. B.; Stebbings, R. F. Phys. Rev. A 1982, 26, 335. (1 1) Kellert, F. G.; Jeys, T. H.; McMillian, G. B.; Smith, K. A.; Dunning, F. B.; Stebbings, R. F. Phys. Rev. A 1981, 23, 1127. (12) Christophorou, L. G. Adu. Electron. Electron Phys. 1978, 46, 55. (13) Christophorou, L. G.; McCorkle, D. L.; Christodoulides, A. A. In Electron-Molecule Interactions and Their Applications; Christophorou, L. G., Ed.; Academic: Orlando, FL, 1984; Vol. I, p 477.
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