Method of producing population inversion and lasing at short wavelengths by charge transfer

A stimulated emission device operative in the ultraviolet and soft x-ray regions. A high-power infrared laser is focused on a slab target vaporizing the material and generating highly stripped target ions with varying velocities. Just prior to laser initiation, a gaseous environment of helium, hydrogen, argon or neon is injected to surround the target (e.g., carbon) at a pressure of from 1-10 Torr. The injected gas and associated electrons modify and mix the interacting particles originating from the vaporized target. Ion-atom resonance, charge-transfer reactions take place to form excited-state ions to produce amplified stimulated emission in the 300-800 Angstrom region.

BACKGROUND OF THE INVENTION 
This invention relates to lasers and more particularly to short wavelength 
lasers in the extreme ultraviolet x-ray spectral regions. 
Heretofore stimulated emission of radiation has been obtained from solid 
state, gaseous, and chemical lasers as well as from semiconductors whose 
outputs are in the infrared visible and near ultraviolet wavelengths. Many 
attempts have been made to obtain lasers with an output in the x-ray 
region. One such U.S. Pat. No. 4,053,783 has been issued in which 
stimulated x-ray emission at 304 A has been proposed. This arrangement 
makes use of a pulsed helium-ion beam, stripped of electrons to its 
nuclei, which is directed upon a jet of hydrogen gas to cause a population 
inversion due to the resonant charge exchange between the ions and the 
hydrogen atoms in the gas jet. 
Experimental results obtained in furthering the present invention are found 
in an article "Resonance Charge Transfer and Population Inversion 
Following C.sup.5+ and C.sup.6+ Interactions With Carbon in a 
Laser-Generated Plasma", by R. H. Dixon and R. C. Elton, Physical Review 
Letters, Vol. 38, pages 1072-1075, May 9, 1977, which is incorporated 
herein by reference. 
SUMMARY OF THE INVENTION 
This invention is directed to generating inverted excited-state population 
densities in highly-ionized atoms by resonance charge transfer at the high 
densities present in plasmas, which results in laser action in the extreme 
ultraviolet and soft x-ray spectral regions. The target material immersed 
in a background gas is vaporized by a laser to produce the plasma from 
which population inversion is obtained. The partially ionized background 
gas and electrons act as a catalyst for the reaction since they neutralize 
and localize the initial target ions and attenuate the following target 
ions to an optimum velocity for the charge transfer process.

DETAILED DESCRIPTION 
FIG. 1 illustrates a schematic drawing of an x-ray laser system made in 
accordance with the teaching of this invention. The output of a pulsed Nd 
glass laser 10 or any other suitable high power laser is focused by a lens 
system 11 to an elongated image with an aspect ratio of approximately 10 
to 1. The laser beam is focused on the surface of a rotatable target 12, 
such as carbon, to an irradiance of at least 2.times.10.sup.11 W/cm.sup.2. 
The target is secured within a vacuum chamber 13 to which is added a 
surrounding gas, for example, helium. hydrogen, argon, or neon at a 
pressure of from 1 to 10 Torr. The gas is admitted from a pressurized tank 
14 by a valve 15 just prior to the irradiation by the laser pulse. The 
admitted gas and target contaminants are removed subsequent to each 
operation through a valve 16 and vacuum exhaust pump 17. The operation of 
the system as an x-ray amplifier depends upon the surrounding gas and 
associated electrons/ions. The gaseous medium and the associated free 
electrons produced by ionization surrounding the carbon contribute both to 
the formation of neutral target atoms from fast ions and to the 
attenuation of both the expansion velocity of the reacting ions. This 
results in the creation of a vital ion-atom mixing layer about 5-25 mm 
from the target in which appropriate relative velocities exists. 
In operation, the system is set up with the target in a housing which is 
evacuated. The laser is aligned to direct its output through the focusing 
optics onto the target. Just prior to activating the laser, the gaseous 
medium is admitted at a pressure of from 1-10 Torr. The laser is activated 
and the incident radiation vaporize the target to generate highly stripped 
target ions which accelerate at various velocities. The partially ionized 
gaseous medium and associated charged particles contribute to the 
formation of neutral atoms and to the attenuation of the expansion 
velocity of the following target ions. This results in the creation of a 
vital ion-atom mixing layer about 5-25 mm from the target. Subsequently, 
ions attenuated by the atom/ion/electron mixture enter the mixing layer 
with the appropriate relative velocity. In this environment, 
ion-atom-resonance charge transfer reactions take place. In particular, 
for a carbon slab target charge transfer reaction to take place between 
carbon atoms and C.sup.5+ and C.sup.6+ ions, to form excited state 
(asterisk) C.sup.4+ and C.sup.5+ ions, respectively, C+C.sup.5+,.sup.6+ 
.fwdarw. C.sup.+ +(C.sup.4+,.sup.5+)*+.DELTA.E.sub.if. 
.DELTA.E.sub.if is the difference in binding energies of the initial and 
final states at infinite separation. 
FIG. 2 illustrates the example of the binding energy for an electron in the 
initial carbon atom and either a C.sup.4+ or C.sup.5+ final (product) 
ion. Electron collisional mixing of the states for n.gtoreq.4 is indicated 
by "c" with several collisionally mixed levels indicated for C.sup.4+. The 
relative populations of the excited states in the C.sup.4+ and C.sup.5+ 
product ions were determined from the np.sup.1 P.sub.1 .fwdarw.1s.sup.1 So 
resonance series spectral lines in the 30-40 A spectral region. 
Preferential population of the n=4 and n=5 excited states occurs with 
inverted population densities relative to the n=3 state, with stimulated 
emission at 350-760 A. The cross-section resonance for n=4 state 
population occurs at a velocity of about 5.times.10.sup.6 cm/sec. Direct 
population of the n=3 state requires velocities of greater than 10.sup.8 
cm/sec. Likewise n.gtoreq.5 state population requires velocities less than 
10.sup.6 cm/sec with very narrow resonances at correspondingly large 
pseudo-crossing diameters. 
An ion density of about 10.sup.18 cm.sup.-3 is necessary in the plasma 
region for significant amplification. Electron collisional excitation and 
recombination explain the distribution of the n=4 population density among 
higher-lying states, and charge transfer into the n=5 levels cannot be 
completely ruled out if some ions are slowed sufficiently at late times. 
The selective excitation takes place from 5-25 mm from the surface of the 
vaporized target approximately 100 ns after the primary (infrared) laser 
irradiation. 
A vital phenomenon is the early formation of a significant density of 
neutral target atoms in the 5 to 25 mm region away from the target. These 
atoms are formed by the rapid neutralization of initial ions. Momentum 
transfer and electrostatic forces during and after neutralization leave 
the target atoms in this region in static equilibrium. Also, of importance 
is the velocity attenuation for the sheath of ions moving into the 
preformed atomic atmosphere to velocities appropriate for the charge 
transfer reaction. With a carbon target, relative population densities for 
n=5, 4 and 3 levels in the C.sup.4+ and C.sup.5+ product ions have been 
determined to be in the ratios of 3.8 to 3.4 to 1 for C.sup.4+ ions and 
3.8 to 2.6 to 1 for C.sup.5+ ions. These ratios indicate a definite 
presence of population inversion for the transitions 4-3 and 5-3 of each 
species. The ratios between the n=5 and n=4 levels are consistent with 
collisional excitation of the ions as they move into a stationary electron 
cloud with the measured velocity. The inversions continue for the duration 
of the reaction region, i.e., in a quasi-cw state, since the rate of the 
n=3 decay exceeds the population rate. This extended inversion, not 
limited by excited state lifetimes, is an important feature of this 
device. With sufficient density, amplification of spontaneous emission 
exists at wavelengths extending from 350-760 Angstroms depending on the 
excited ion density. 
Anomalous intensities for spectral lines in, for example, the resonance 
series of C.sup.4+ and C.sup.5+ ions associated with transactions 
originating on n.gtoreq.4 levels are directly related to enhanced 
preferential population of the n=4 level. Population inversions occur on 
levels associated with very short wavelengths. The detailed processes 
generating the interaction zone in the gaseous atmosphere are not as yet 
fully understood. 
Obviously many modifications and variations of the present invention are 
possible in light of the above teachings. It is therefore to be understood 
that within the scope of the appended claims the invention may be 
practiced otherwise than as specifically described.