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Observation of field ionized Rydberg electrons from highly excited heavy ions after passage through foil targets
Nuclear Instruments and Methods in Physics Research B67 (1992) 146-147 North-Holland
Nuclear Instruments & Methods in Physics Research sect)0n 8
Observation of field ionized Rydberg electrons from highly excited heavy ions after passage through foil targets R. Schramrn and H.-D. Betz Sektion Physik, Unh'ersitiit Miinchen, 8046 Garching, Germany
The emerging fraction of Rydberg electrons after passing 125 MeV bromine ions through carbon foils has been measured by ionization in the motional electrical field (E = t,.B) of a magnetic analyzer. We analyzed excited states with principal quantum numbers, n, ranging from approximately 400 to 800. Contrary to earlier expectations, it was found that the lifetimes of these states in the ionizing field are long enough to be observable (ns range). In addition, an upper limit of the absolute number of electrons per projectile ion could be derived, given by P(n) = 24n -3.
After penetration through foil targets fast ions are found to be in a variety of excited electronic states. During the last decades both low-lying excited states and continuum states have received much attention. In the former case spectroscopic work was restricted mainly to principal quantum numbers n < 20 [1,2], for which decay rates and yields are not too small. The prcscn~ .,~ady deals with Rydberg states, and new resuits are presented with respect to their lifetimes in electrical fields. The first observation of ionization of hydrogen in electrical fields was made in 1930 by studies of the Stark effect in the Balmer spectrum utilizing a canal-ray light source [3]. Ion-foil-produced states with principal quantum numbers n < 25 and n < 120 have been detected in recent work with low energy ion beams ( < 100 keV) [4,5]! and 20 MeV oxygen ions [6], respectively, using relatively strong fields and ion detection. Higher n-states, by comparison, can be observed by measuring,, ionized Rydberg electrons. For a high beam velocity, orbital velocities of Rydberg electrons can be neglected;; as a consequence, the electrons move with the projectile velocity in the laboratory frame. It is easy to separate such electrons from the ion beam with moderate electrical or magnetic fields and to detect them with high efficiency. This procedure permits studies of high principal quantum numbers even for n > 100. In the present work we focus on the observation of Rydberg electrons which become ionized in the fringing field of a magnetic analyzer initially designed to detect convoy electrons. In the normal mode of operation, electron spectrometers do not allow to distinguish convoy electrons from field ionized Rydberg electrons, because both travel with beam velocity. Due to the finite spatial extension of the magnetic fringing field
and the high angular resolution of our spectrometer we were able to separate Rydberg electrons as an additional sharp electron peak at the high velocity tail of the convoy peak. In detail, distinction between the two types of electrons is possible because convoy electrons are deflected along their entire path in the magnet, while Rydberg electrons continue to travel a certain straight path length along with the projectile ions until they become liberated. Their trajectories within the spectrometer differ from the ones for convoy electrons. Therefore, both kinds of electrons are observed at different channel numbers in the electron velocity distribution and focussing conditions can be set for different locations of electron liberation points. As a consequence, there is a connection between the initial Rydberg state (characterized by the principal quantum number n), the location of field ionization (determined by the region where the field strength exceeds the minimum value required for liberation), and the detection geometry (such as maximum field, slit position and observation angle). In order to facilitate a quantitative interpretation, the magnetic fringing field strength has been measured as a function of the distance from the magnet and ray-tracing calculations taking into account the extended fringing field have been performed in order to calculate the position of Rydberg electron liberation. The study of electrons ionized from highly excited projectile states allows us to infer information on both the absolute number of electrons liberated from a certain range of n-states and the lifetimes of these excited states as a function of the mean quantum number n. For ionization we utilized the motional electrical field ( E = v . B ) which corresponds to the applied magnetic deflection field.
0168-583X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
147
R. Schramm, H.-D. Betz / Field ionized Rydberg electrons
,=experimental 1j- Rydberg intensity • theoretical =analyzing magnetic=. . ..=
1
.c_
~0.s o
I I I I -_ 50 6O 7O 8O distance [ram] Fig. 2. Relative experimental and theoretical intensities of Rydberg electrons, and the magnetic fringing field, as a function of the location where field ionization takes place. 3O
ve'/ocity [a.u.I Fig. 1. Electron velocity distributions in the forward direction, as a function of the observation angle. The two main features in the spectra refer to convoy (left) and Rydberg electrons (right), respectively. 125 MeV bromine ions ( % = 8 a.u.) are passed through a carbon foil with sufficient thickness to attain charge state equilibrium. The ions exit from the target and move through the magnetic spectrometer where Rydberg and convoy electrons are separated as mentioned above. The observation angle is still close to zero degrees but depends on the location where field ionization takes place and is, thus, a measure for the effective field strength and the corresponding n-levels (fig. 1). Fig. 2 shows the observed relative intensities of electrons as a function of the position along the trajectory where field ionization takes place. Furthermore, the experimentally determined magnetic fringing field is displayed on a relative scale. Finally, the intensity distribution is shown which one would expect theoretically, provided that two assumptions are valid. Firstly, the population P of Rydberg states should vary as P ( n ) = an -3 [6]; secondly the electrons must be emitted spontaneously. Comparison between theoretical and experimental intensities leads to a marked discrepancy. Since the n -3 law can hardly be debated we conclude that a lifetime effect is observed. Obviously, the field ionization process is not spontaneous but significantly delayed. The associated lifetimes depend on the observed n-level and increase with increasing n. In the investigated n-range (n = 400 to 800) the lifetimes of the excited bromine electrons can be estimated to vary
~0
between 0.2 and 2 ns. A theoretical discussion of this effect will be given elsewhere, but it is fair to state that the lifetimes correspond to orbital revolution times of the Rydberg electrons (above ionization threshold). An approximate upper limit for the absolute number of electrons carried by each projectile ion as a function of n, as obtained from our calibrated spectrometer, is given by P ( n ) = 24n-3. For the number of convoy electrons we derived Yc = 0.8-t-0.3 electrons per ion. Further experiments are under way which will allow a more accurate determination of both the observed level population and the inferred lifetimes with respect to field ionization for an increased range of n-values.
References [1] H.A. Bethe and E.E. Salpeter, Quantum Mechanics of One- and Two-Electron Atoms (Academic Press, New York, 1957). [2] J.R. Oppenheimer, Phys. Rev. 31 (1928) 349. [3] H. Rausch yon Traubenberg, R. Gebauer and G. Lewin, Naturwissenschaften 18 (1930) 417. [4] K.H. Berkner, I. Bomstein, R.V. Pyle and J.W. Stearns, Phys. Rev. A6 (1972) 278. [5] C.J. Latimer, R.G. McMahon and D.P. Murtagh, Phys. Lett. A87 (1982) 232. [6] K.P. Miiller, H. Kuiper, M. Sch6berl and R. Schmelzer, J. Phys. B20 (1987) 2803.
II. CAPTURE AND LOSS
Nuclear Instruments & Methods in Physics Research sect)0n 8
Observation of field ionized Rydberg electrons from highly excited heavy ions after passage through foil targets R. Schramrn and H.-D. Betz Sektion Physik, Unh'ersitiit Miinchen, 8046 Garching, Germany
The emerging fraction of Rydberg electrons after passing 125 MeV bromine ions through carbon foils has been measured by ionization in the motional electrical field (E = t,.B) of a magnetic analyzer. We analyzed excited states with principal quantum numbers, n, ranging from approximately 400 to 800. Contrary to earlier expectations, it was found that the lifetimes of these states in the ionizing field are long enough to be observable (ns range). In addition, an upper limit of the absolute number of electrons per projectile ion could be derived, given by P(n) = 24n -3.
After penetration through foil targets fast ions are found to be in a variety of excited electronic states. During the last decades both low-lying excited states and continuum states have received much attention. In the former case spectroscopic work was restricted mainly to principal quantum numbers n < 20 [1,2], for which decay rates and yields are not too small. The prcscn~ .,~ady deals with Rydberg states, and new resuits are presented with respect to their lifetimes in electrical fields. The first observation of ionization of hydrogen in electrical fields was made in 1930 by studies of the Stark effect in the Balmer spectrum utilizing a canal-ray light source [3]. Ion-foil-produced states with principal quantum numbers n < 25 and n < 120 have been detected in recent work with low energy ion beams ( < 100 keV) [4,5]! and 20 MeV oxygen ions [6], respectively, using relatively strong fields and ion detection. Higher n-states, by comparison, can be observed by measuring,, ionized Rydberg electrons. For a high beam velocity, orbital velocities of Rydberg electrons can be neglected;; as a consequence, the electrons move with the projectile velocity in the laboratory frame. It is easy to separate such electrons from the ion beam with moderate electrical or magnetic fields and to detect them with high efficiency. This procedure permits studies of high principal quantum numbers even for n > 100. In the present work we focus on the observation of Rydberg electrons which become ionized in the fringing field of a magnetic analyzer initially designed to detect convoy electrons. In the normal mode of operation, electron spectrometers do not allow to distinguish convoy electrons from field ionized Rydberg electrons, because both travel with beam velocity. Due to the finite spatial extension of the magnetic fringing field
and the high angular resolution of our spectrometer we were able to separate Rydberg electrons as an additional sharp electron peak at the high velocity tail of the convoy peak. In detail, distinction between the two types of electrons is possible because convoy electrons are deflected along their entire path in the magnet, while Rydberg electrons continue to travel a certain straight path length along with the projectile ions until they become liberated. Their trajectories within the spectrometer differ from the ones for convoy electrons. Therefore, both kinds of electrons are observed at different channel numbers in the electron velocity distribution and focussing conditions can be set for different locations of electron liberation points. As a consequence, there is a connection between the initial Rydberg state (characterized by the principal quantum number n), the location of field ionization (determined by the region where the field strength exceeds the minimum value required for liberation), and the detection geometry (such as maximum field, slit position and observation angle). In order to facilitate a quantitative interpretation, the magnetic fringing field strength has been measured as a function of the distance from the magnet and ray-tracing calculations taking into account the extended fringing field have been performed in order to calculate the position of Rydberg electron liberation. The study of electrons ionized from highly excited projectile states allows us to infer information on both the absolute number of electrons liberated from a certain range of n-states and the lifetimes of these excited states as a function of the mean quantum number n. For ionization we utilized the motional electrical field ( E = v . B ) which corresponds to the applied magnetic deflection field.
0168-583X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
147
R. Schramm, H.-D. Betz / Field ionized Rydberg electrons
,=experimental 1j- Rydberg intensity • theoretical =analyzing magnetic=. . ..=
1
.c_
~0.s o
I I I I -_ 50 6O 7O 8O distance [ram] Fig. 2. Relative experimental and theoretical intensities of Rydberg electrons, and the magnetic fringing field, as a function of the location where field ionization takes place. 3O
ve'/ocity [a.u.I Fig. 1. Electron velocity distributions in the forward direction, as a function of the observation angle. The two main features in the spectra refer to convoy (left) and Rydberg electrons (right), respectively. 125 MeV bromine ions ( % = 8 a.u.) are passed through a carbon foil with sufficient thickness to attain charge state equilibrium. The ions exit from the target and move through the magnetic spectrometer where Rydberg and convoy electrons are separated as mentioned above. The observation angle is still close to zero degrees but depends on the location where field ionization takes place and is, thus, a measure for the effective field strength and the corresponding n-levels (fig. 1). Fig. 2 shows the observed relative intensities of electrons as a function of the position along the trajectory where field ionization takes place. Furthermore, the experimentally determined magnetic fringing field is displayed on a relative scale. Finally, the intensity distribution is shown which one would expect theoretically, provided that two assumptions are valid. Firstly, the population P of Rydberg states should vary as P ( n ) = an -3 [6]; secondly the electrons must be emitted spontaneously. Comparison between theoretical and experimental intensities leads to a marked discrepancy. Since the n -3 law can hardly be debated we conclude that a lifetime effect is observed. Obviously, the field ionization process is not spontaneous but significantly delayed. The associated lifetimes depend on the observed n-level and increase with increasing n. In the investigated n-range (n = 400 to 800) the lifetimes of the excited bromine electrons can be estimated to vary
~0
between 0.2 and 2 ns. A theoretical discussion of this effect will be given elsewhere, but it is fair to state that the lifetimes correspond to orbital revolution times of the Rydberg electrons (above ionization threshold). An approximate upper limit for the absolute number of electrons carried by each projectile ion as a function of n, as obtained from our calibrated spectrometer, is given by P ( n ) = 24n-3. For the number of convoy electrons we derived Yc = 0.8-t-0.3 electrons per ion. Further experiments are under way which will allow a more accurate determination of both the observed level population and the inferred lifetimes with respect to field ionization for an increased range of n-values.
References [1] H.A. Bethe and E.E. Salpeter, Quantum Mechanics of One- and Two-Electron Atoms (Academic Press, New York, 1957). [2] J.R. Oppenheimer, Phys. Rev. 31 (1928) 349. [3] H. Rausch yon Traubenberg, R. Gebauer and G. Lewin, Naturwissenschaften 18 (1930) 417. [4] K.H. Berkner, I. Bomstein, R.V. Pyle and J.W. Stearns, Phys. Rev. A6 (1972) 278. [5] C.J. Latimer, R.G. McMahon and D.P. Murtagh, Phys. Lett. A87 (1982) 232. [6] K.P. Miiller, H. Kuiper, M. Sch6berl and R. Schmelzer, J. Phys. B20 (1987) 2803.
II. CAPTURE AND LOSS