Stephen R. Lundeen

Professor
B.S., Trinity College (CT), 1969; Ph.D., Harvard University, 1975; Fellow, American Physical Society.
(970) 491-6647, lundeen@lamar.colostate.edu

Fast Beam Laser-RF Spectroscopy

Our major research effort is in the area of high precision spectroscopy of excited states of atoms and molecules. We use microwave resonance techniques to study fast beams of atoms and molecules, prepared and detected in excited states by lasers. Our measurements are in close contact with state-of-the-art atomic/molecular theory, and frequently stimulate or test improvements in these calculations.

The term "fast beam" refers to a beam of neutral atoms or molecules formed by charge exchange from an accelerated positive ion beam. In our laboratory, ion beams with energies in the range 1-100 KeV are used, making the neutral beams much faster than conventional thermal atomic or molecular beams, and fast enough that excited states can be studied carefully in a high vacuum apparatus before they eventually decay radiatively back to the ground state. We use a sensitive and efficient method of detecting particular excited states in the fast beam which we call Resonant Excitation Stark Ionization Spectroscopy (RESIS). It is especially well suited for detecting excited states with principal quantum numbers of 9 or 10, using a Doppler tuned CO2 laser, as illustrated below in Fig. 1. The laser excites atoms in a selected level (10G in this illustration) to a high-lying level which is then immediately Stark ionized. The resulting ion current is proportional to the population of the selected state. Other states, like the 10H state, are not excited because of the frequency resolution of the laser. Very high resolution microwave spectroscopy can be carried out based on the selective RESIS detection of different fine-structure levels.

An attractive feature of this experimental scheme is that it can be applied to a wide range of atoms and molecules simply be changing the identity of the positive ion which forms the fast beam. We have so far used it to study excited states of helium, H2, C, N, O, Ne, and S, but many other studies are also possible. The generality is due to the fact that neither the formation of the excited states nor their detection depend sensitively on the identity of the positive ion.

High-L Rydberg States

Many of the excited states which we study are "Rydberg states," states which have one electron highly excited. Among these, the excited states with large values of the orbital angular momentum (L > 4) are uniquely accessible with the RESIS technique. In these "high-L" Rydberg states, the excited electron is excluded virtually completely from the vicinity of the ion core by the repulsive contrifugal potential. As a result, the forces between the Rydberg electron and the ion core, aside from the hydrogenic Coulomb force, are so feeble that the appearance and behavior of these High-L Rydberg states are rather unusual. In simple systems like helium and H2, the high-L Rydberg structure can be predicted precisely from first principles, and measurements can pose tests of fundamental theory. In more complex systems, the high-L Rydberg electron can be regarded as a sensitive probe of many properties of the positive ion core which are otherwise difficult to measure.