Adiabatic inversion for selective excitation

Method and apparatus for achieving isotopically selective adiabatic inversion, particularly for improved isotope separation efficiency. In a preferred embodiment for practicing the invention, chirped laser radiation induces photoionization of a vapor state material in isotopically selective excitation and ionization energy steps. A frequency sweep or "chirp" is provided in the excitation laser radiation at a controlled rate and over a range of frequencies which is limited to prevent loss of selectivity in the excitation. The frequency swept radiation has a theoretical capability of producing 100% inversion of ground state particles in the vapor. The features of the invention additionally permit excitation of a material to very high energy states useful in producing high frequency, ultraviolet lasing.

FIELD OF THE INVENTION 
This invention relates to photoexcitation and in particular to a method and 
apparatus for increasing the degree of photoexcitation in a material. 
BACKGROUND OF THE INVENTION 
A suitably tuned laser beam is a practical source of photons for producing 
isotopically selective excitation of the orbital electrons in a molecular 
or elemental state material, particularly in a vapor thereof. In one 
particular application for this technique, as is specifically disclosed in 
U.S. Pat. No. 3,772,519, specifically incorporated herein by reference, a 
system is described for using the radiant energy of lasers to produce 
selective photoionization of one uranium isotope, typically U.sub.235 , 
with respect to the other isotopes of uranium. For this purpose, the 
uranium is first produced in the form of a vapor, the U.sub.235 particles 
of which are then laser ionized. The photoionized particles of U.sub.235 
are then typically accelerated out of the vapor environment for separate 
collecting using magnetohydrodynamic forces. 
Theoretical analysis of the factors governing selective photoexcitation 
predicts that in the presence of constant frequency monochromatic 
radiation, 50% of the available, illuminated atoms in the uranium vapor 
will be in a photoexcited state, and 50% will be in the unexcited, 
typically ground state at any given moment. This theoretical limitation is 
of significance in the planning of production level enrichment processes 
because of its effect on enrichment yield. 
In a further application for the technique of photoexcitation employing 
laser radiant energy, it is common to cascade one or more stages of laser 
amplification on the output of a low power laser in order to boost the 
energy of the laser to higher levels. The lasing condition in each of the 
amplifying stages typically results from the presence of a "population 
inversion" wherein particles in a lasing medium have their orbital 
electrons excited to a predetermined energy level such that a greater 
percentage of the medium particles are excited to that particular energy 
level than the proportion of medium particles in a lower lying energy 
level. These conditions are theoretically necessary for the simultaneous 
decay of the excited particles to the lower lying energy state which in 
turn results in the production of laser radiant energy. The power 
generated by the lasing medium in these circumstances is directly related 
to the number of excited particles in the medium. In a two-level lasing 
system then, the same theoretical considerations as mentioned above would 
limit the number of excited particles to 50% of the available ones and 
thereby limit the laser output power accordingly. 
In addition, in applications where it is desired to selectively photoexcite 
particles by laser energy, a substantial frequency broadening may exist in 
the adsorption lines of particles which it is desired to selectively 
excite as, for example, by Zeeman splitting of the energy levels. The 
presence of this splitting, or the spreading of the original energy level 
into several levels covering a range of energies, may further tend to 
reduce the efficiency of excitation, particularly where very narrow 
bandwidth laser radiation is employed, as in the case of selective 
photoexcitation, by having the laser radiation cover a more narrow range 
of frequencies than the width of the absorption line for the particles 
being photoexcited. 
SUMMARY OF THE INVENTION 
In accordance with a preferred embodiment of the present invention, 
conditions of adiabatic population inversion are produced in particles of 
a selected isotope type to improve the efficiency of isotope separation. 
The technology of the present invention may also be applied to create 
adiabatic inversion to enhance laser efficiency in exciting a lasing 
medium. 
Adiabatic inversion is achieved according to the invention by sweeping the 
frequency of laser photoexcitation radiation over a frequency range at a 
controlled rate. The range, in the specific case of isotopically selective 
excitation, is selected so as not to exceed the isotope shifts in the 
material irradiated. The rate of sweep is selected so as to satisfy 
specific constraints related to excited state lifetimes and laser 
intensity. 
In the application to uranium enrichment, the isotopically selective 
adiabatic inversion depopulates ground energy levels for the desired 
uranium isotope in favor of an elevated or excited energy level from which 
further laser radiation produces photoionization in one or more energy 
steps. By depopulating the ground level, a greater yield of ionized 
particles is achieved. The selectively photoionized uranium ions may then 
be separated using the magnetohydrodynamic acceleration forces generally 
described above in U.S. Pat. No. 3,772,519. 
In a further application to laser amplification, the increased population 
of the excited state achieved through adiabatic inversion is a condition 
which increases the overall photon density in a laser amplifier. 
Depopulation of the ground levels also permits the practical realization 
of a high photon energy, ultraviolet, laser by exciting through additional 
energy steps to an excited level from which each transition to the ground 
level produces an ultraviolet photon.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention contemplates a photoexcitation system for creating an 
adiabatic inversion in the energy states of particles in a medium. The 
system for producing adiabatic inversion permits a theoretical 100% 
excitation of all illuminated medium particles from a ground or low lying 
energy level to an elevated, excited energy level. This adiabatic 
inversion permits higher productivity in isotope separation, specifically 
uranium enrichment, and permits higher amplifications in laser amplifiers, 
and contributes as well to the feasibility of high energy, typically 
ultraviolet, lasers. 
The adiabatic inversion is achieved by sweeping or "chirping" the frequency 
of an excitation laser through an absorption line for one isotope type in 
a medium or environment to which the laser radiation is applied. The width 
and rate of the frequency sweep or chirp to achieve adiabatic inversion 
are defined by the characteristic of the excitation laser frequency 
corresponding to a specific transition in the isotope to be excited, as 
well as the range of any significant splitting or broadening of degenerate 
levels in the ground and excited states. The swept frequency range is 
selected to encompass the broadened absorption line for the split 
degenerate levels. 
These theoretical considerations may best be explained with reference to 
FIG. 1 which is a diagram of exemplary energy levels and transitions 
between the levels which illustrate selective excitation without and with 
the teaching of the invention. 
With respect to FIG. 1, a set of transition or energy steps are illustrated 
which account for the theoretical distribution of atomic particles in 
excited and ground energy states 14 and 16 of a medium which is irradiated 
with laser radiation tuned to the frequency of that energy step. In the 
case of isotopically selective photoexcitation, the laser radiation is 
tuned to produce a transition 18 from the ground energy state 14 to the 
excited level 16. A transition 20 from the energy level 24 down to the 
ground state 22 will comprise an identical energy shift, assuming at this 
point, no degeneracy in the energy levels. In the presence of this laser 
radiation, the ground level particles of the appropriate isotope will be 
stimulated by the laser radiation through their absorption line for 
excitation to the energy level 16. The same photon energy, however, will 
be effective to produce stimulated emission which results in the opposite 
transition 20 from the elevated level 16 to the ground state 14. The 
probabilities for these two events will be equal for illuminated 
particles, resulting theoretically in a 50% population of the level 16, 
and 50% population of the level 14. As a result, only 50% of the original 
ground state atoms are available in the energy level 24 for 
photoionization to the continuum 24 in a subsequent transition 22. If 
photoionizing radiation from typically a second laser is applied to 
produce a transition 22 simultaneously with the radiation for the 
transition 18, the level 16 will be continually depleted by transitions 
into the ionization region 24 which will eventually deplete a larger 
percentage of the ground state atoms out of the level 14. 
A more direct and efficient system of achieving a higher percentage of 
excited state atoms and correspondingly photo-ionized atoms is to employ a 
transition 26 shown in FIG. 1 which produces an adiabatic inversion of 
ground level atoms to the elevated energy level 24. This adiabatic 
inversion is achieved by chirping or sweeping the frequency of the 
exciting laser radiation over a range including the frequency of the 
transition 26 at a predetermined rate. If the theoretical conditions to be 
described below are satisfied, then the frequency variation in the 
radiation forces each particle to switch its energy state rather than 
having a probability distribution of occupied energy states as described 
above. Thus, if all, or substantially all, atoms are originally in the 
ground state 14, the chirped excitation will produce a nearly complete 
shift to the state 16. Whatever particles may originally exist in the 
state 16 would correspondingly be switched to the state 14, but this may 
generally be assumed to be a negligible or insignificant fraction. With 
substantially all of the atoms excited to the level 16, nearly all of the 
illuminated atoms are thus in a condition to be photo-ionized from the 
level 16 to the continuum 24. 
In a further application for the present invention, a laser amplifier may 
be more efficiently operated with an adiabatic inversion. In that case, a 
population inversion may be created by the transition 26 to the energy 
level 16 from which spontaneous emission back to the ground level 14 
results in the generation of laser radiation. In a yet further 
application, the adiabatic inversion of the present invention makes 
feasible higher photon energy lasers of shorter wavelength as, for 
example, by permitting a transition 28 from ground level 14 to 
intermediate level 30, which substantially depopulates the ground level 14 
such that additional transition composed of energy steps 32 and 34 to 
respective energy levels 36 and 38, even though not adiabatic, will still 
create a population inversion between the levels 14 and 38. This 
population inversion may be induced to lead to one large energy step 40 
from energy level 38 to energy level 14 in a suitably tuned cavity in 
order to produce laser radiation of very high photon energy, corresponding 
to approximately the ultraviolet spectral region. 
As an additional consideration, a first set of energy levels 12 and 14, 
representing respectively low lying energy levels and an excited energy 
level, are illustrated in FIG. 1 as degenerate and having a plurality of 
discrete energy levels, greatly exaggerated for purposes of illustration. 
This degeneracy may result from a number of causes, typically Doppler 
effects from thermal motions of particles in the medium or Zeeman 
broadening in the presence of a magnetic field, particularly when used in 
association with a system for uranium enrichment employing 
magnetohydrodynamic forces. As a result of the broadening of the levels 42 
and 44, there will be a broadening of the absorption line for a particular 
transition 46 between the energy levels 42 and 44 since more than one 
allowed transition of slightly different energies may exist between the 
separate levels. Thus, if very narrow bandwith laser radiation is employed 
for selective photoexcitation of one particular isotope type between the 
energy levels 42 and 44, it may be too narrow to encompass all of the 
broadened absorption line. This would result in reduced excitation 
efficiency for that laser radiation. By sweeping the frequency of the 
applied excitation laser radiation, in accordance with the method and 
apparatus of the present invention described below, photoexcitation may be 
achieved generally over the entire, broadened absorption line. 
For purposes of establishing specific parameters for this invention, a 
mathematical analysis is presented below for a system with two energy 
levels, a and b, separated by an energy n.omega..sub.o The system of these 
energy levels is described by wave functions .psi.a and .psi.b. The 
unperturbed Hamiltonian of the system is defined as H.sub.o, and with 
energies measured from the point midway between the two levels, then in 
matrix notation, H.sub.o is 
##EQU1## 
where h is Plank's constant and .omega..sub.o the angular frequency 
corresponding to the energy between levels. The system is assumed to have 
no permanent electric dipole moment, but does possess a transition dipole 
moment connecting the states a and b. That is .mu.=0, where 
EQU .mu.=e&lt;a.vertline.x.vertline.b&gt;=e.psi..sub.a *x.psi..sub.b 
The phases of the states may always be arranged to make .mu. real, and this 
is assumed to be the case. 
In a static electric field, E an interaction Hamiltonian of the form 
##EQU2## 
mixes the states a and b. The wave functions of the stationary states in 
this case are linear combinations of .psi..sub.a and .psi..sub.b, namely 
just those which diagonalize the full Hamiltonian, H.sub.o +H.sub.int : 
##EQU3## 
The solution of the secular equation for .OMEGA. is then 
##EQU4## 
where .gamma.=(.mu..multidot.E)/h. Defining the angle .theta. by 
##EQU5## 
there are then two solutions for the eigenfunction: 
##EQU6## 
In a slowly turned-on electric field, if the atom started off in the 
ground state we have 
##EQU7## 
This solution is that corresponding to the minus sign in front of the 
square root, above. The effect of turning on the field is then to "rotate" 
.psi. into a final state 
##EQU8## 
If the atom had started off in the excited state, the situation described 
by the positive square root solution would have been appropriate. 
Now in the case of an atom interacting with a rapidly oscillating optical 
electric field, E(t) 
EQU E(t)=.epsilon.e.sup.i.omega.t, and 
the time dependence of the wave function is given by 
##EQU9## 
The solutions of this differential equation in matrix form is: 
EQU -h.omega..sub.a Ae.sup.i(.omega..sub.a.sup.-1/2.omega..sub.o.sup.)t 
=.mu..multidot..epsilon.Be.sup.i(.omega..sub.b.sup.-1/2.omega..sub.o.sup.) 
t 
EQU -h.omega..sub.b Be.sup.i(.omega..sub.b.sup.-1/2.omega..sub.o.sup.)t 
=.parallel..multidot..epsilon.Ae.sup.i(.omega..sub.a.sup.-1/2.omega..sub.o 
.sup.)t 
with A and B as constants to be determined. 
If .omega..sub.a -.omega..sub.b are selected such that 
.omega.-.omega..sub.o .tbd..DELTA..omega., the same time dependence exists 
on either side of this equation which can be then written 
##EQU10## 
where 
EQU .gamma.=(.mu..multidot..epsilon./h) 
The determinant of the matrix must vanish, of course, which fixes .omega.a 
and .omega.b: 
EQU .omega.a=1/2.DELTA..omega.+.lambda. 
EQU .omega.b=-1/2.DELTA..omega.+.lambda. 
where 
EQU .lambda.=.+-..sqroot.(1/2.DELTA..omega.).sup.2 +.gamma..sup.2 
Finally from the above relationship 
EQU (A/B)=-(.gamma./.omega.a)=-(.omega.b/.gamma.*) 
and 
##EQU11## 
there results 
EQU .vertline..gamma..vertline.=.+-..lambda. sin .theta.; 
1/2.DELTA..omega.=.+-..lambda. cos .theta. 
and as the solution 
##EQU12## 
By varying the optical frequency .omega. from one side of the resonance to 
the other, if the variation is "slow enough", the system will satisfy Eq. 
(1) at any given moment, and will be able to "follow" the change in 
.omega.. For 
.vertline..DELTA..omega..vertline.&gt;.vertline..gamma..vertline., .theta. is 
near zero, and the solution corresponds to having the atom initially in 
the ground state, b. As .vertline..DELTA..omega..vertline. decreases, 
.theta. approaches .+-.90.degree. (depending on the sign of 
.DELTA..omega.). At .DELTA..omega.=0,.theta.=.+-.90.degree. and 
.vertline.A.vertline..sup.2 =.vertline.B.vertline..sup.2. The probability 
of finding the atom in the excited state is then equal to that of finding 
it in the ground state. This situation corresponds to saturation, with an 
unchirped radiation pulse. As the frequency sweeps through .omega..sub.o ; 
in the same direction with .omega. finite, .theta. varies continuously 
toward .+-.180.degree.. As .vertline..DELTA..omega..vertline. becomes very 
large, compared to .vertline..gamma..vertline., .theta. approaches 
180.degree. and the atom becomes inverted, i.e., if .vertline.A.vertline. 
initial=0;.vertline.B.vertline. initial=1; then .vertline.A.vertline. 
final=1;.vertline.B.vertline. final=0. This is so regardless of the 
direction of the frequency chirp, as long as chirp direction remains the 
same throughout the chirp. This ensures that .DELTA..omega. will be a 
monotonic function, varying between -.infin. and .infin. and .theta. will 
vary between 0.degree. and 180.degree.. This is one condition on the 
chirp. 
There remains the question of how slow a variation is "slow enough" for the 
steady state solutions to be valid, or in other words, for the creation of 
the adiabatic inversion. The frequency sweep may be considered adiabatic, 
in the sense defined above, if the time in which the frequency changes 
from -.vertline..gamma..vertline./2 to +.vertline..gamma..vertline./2 is 
greater than (2.pi./.vertline..gamma..vertline.), which is the maximum 
time during which the atom makes a complete cycle, going from state a to b 
and back to a again. The "slowness" of the chirp depends, therefore, on 
the intensity of the light at the optical frequency. A chirp which would 
be too fast to invert an atom at one intensity, may very well do so at a 
higher intensity. 
The above theory identifies the system parameters for adiabatic inversion 
in the context of isotope separation or laser amplification. The apparatus 
which may be employed for this purpose is shown with reference to FIGS. 
2-7. In general, as shown in FIG. 2, the invention will employ a laser 
system 48 and a medium 49 to which radiation for an adiabatic inversion is 
applied FIG. 3 illustrates an embodiment of this apparatus for isotope 
enrichment, particularly of the uranium U.sub.235 isotope. With regard to 
FIG. 3, first and second laser systems 50 and 52 provide respective output 
beams of laser radiation 54 and 56, which are combined, for example in a 
dichroic mirror 58, for application to an isotope separation chamber 60 
through a window 62 on a pipe 64. In typical implementation, the laser 
system 50 may comprise the excitation laser whose output radiation in beam 
54 is employed and correspondingly tuned for selective photoexcitation. 
The laser beam 56 from laser system 52 may comprise one or more 
frequencies of laser radiant energy for producing selective 
photoionization from the excited state in one or more energy steps. The 
beams 54 and 56 are typically applied simultaneously in pulses having a 
duration of about a microsecond or less. Pulse repetition rates up to 50 
KH.sub.z are preferable but much lower rates can be used. The combined 
laser beam applied to the chamber 60 traverses its length and exits 
through a pipe 66 and window 68, typically for application to one or more 
similar chambers. The windows 62 and 68 may comprise optical quartz and 
the pipes 64 and 66 are provided in order to remove the windows 62 and 68 
from the vapor atmosphere within the chamber 60 so as to reduce deposits 
on the windows. Shutters may be added to isolate the windows except during 
illumination with laser radiation. 
A uranium vapor is generated in chamber 60 by a vapor source 70 and 
directed through the laser radiation beam into an ion accelerator 72. A 
vacuum pump 74 maintains a very low pressure within the chamber 60 to 
prevent atmospheric components from disturbing the selective 
photoexcitation and ionization process, as well as separate collection of 
the ionized particles within the chamber 60. 
A plurality of magnetic field coils 76 surround the chamber 60 
approximately coaxial to the applied laser beam. Coils 76 create an axial 
magnetic field within the region of the ion accelerator 72. The coils 76 
are excited from a current source 78. An orthogonal electric field is 
created within the ion accelerator from a voltage source 80 in order to 
generate crossed-field magnetohydrodynamic forces for the accleration of 
ionized particles onto separate collection surfaces. The voltage source 80 
is controlled for periodic application of the electric field, typically 
just subsequent to the application of each laser pulse in the beams 54 and 
56 by a timer 82. Timer 82 is also employed to activate the laser systems 
50 and 52 for typically simultaneous output. A cycle of laser radiation 
and applied voltage will typically occupy a period of several 
microseconds. 
With reference now to FIG. 4, there is shown an internal sectional view of 
a portion of the chamber 60 from FIG. 3. In particular, the vapor source 
70 is shown to include a crucible 90 containing a mass of elemental 
uranium 92 and having a plurality of cooling ports 94 supplied, for 
example, with water to remove the heat applied to the uranium 92 to 
produce vaporization. Vaporization is produced by energy from a beam 96 of 
electrons emanating from a filamentary source 98. The electron beam 96 is 
deflected by a magnetic field 100, produced by coils 76, to a focus along 
a line or series of spots on the surface of uranium mass 92. The energy in 
the incident beam is selected to be sufficient to produce vaporization of 
the uranium along the line of incidence in a radially expanding flow 102 
into the ion accelerator 72. 
The ion accelerator 72 is shown to include a plurality of chambers 104 
defined by an arcuate upper collection plate 106, generally concentric to 
the vapor line source, and having a plurality of radially extending plates 
108 which extend radially toward the vapor line source. Within each 
chamber 106, a central electrode 110 is placed. Each electrode 110 is 
electrically connected in common and applied through a switch 112 to one 
side of a voltage source 114. The other side of the voltage source 114 is 
connected to the structure of plates 106 and 108. The voltage between the 
plates 110 and 108 provided by the source 114 in conjunction with the 
magnetic field 100 is operative to provide crossed-field 
magnetohydrodynamic acceleration on ionized particles in the chambers 108. 
For this purpose, voltage source 114 is typically on the order of a few 
hundred volts and magnetic field 100 is typically in the range of a few 
hundred Gauss. The switch 112 is activated by the timer 92 to provide 
switch closure for a short duration, typically one or two microseconds, 
directly subsequent to each burst of laser radiation in the beams 54 and 
56. 
U.sub.235 particles in the chambers 104 are selectively ionized by 
irradiating a region 118 throughout the length of the chamber 60 by the 
combined beams 54 and 56. The particular shape of region 118 may be 
achieved through suitable masking of the beam or multiple reflections 
through the chamber 60. Once ions of the U.sub.235 isotope have been 
selectively produced through photoionization or otherwise, the voltage 
pulse applied between the plates 110 and 108 in conjunction with magnetic 
field 100 circulates the plasma electrons about the electrodes 110 and 
accelerates the ions for collection toward plates 108. Repeated 
applications of laser radiation and electric potential results in a 
build-up of enriched uranium on the plates 108 and depleted uranium on the 
plate 106. 
With reference to FIG. 5, there is shown a laser system for generating the 
frequency swept laser radiation for use in the separation system of FIGS. 
3 and 4 as, for example, laser system 50. It comprises a CW lasing medium 
120 which is typically a dye solution. The medium 120 is excited to a 
lasing condition by radiation from a further, Argon laser 122. The medium 
120 has a cavity defined by mirror/24 and a partially reflecting output 
mirror 126. The cavity may contain filters 128 or other means for mode 
selection or frequency control as necessary to select a narrow bandwidth 
in the absorption line for the U.sub.235 isotope but not the U.sub.238 
isotope. In addition, a crystal 130 which may be an electro-optic element 
is placed in the path of the laser beam from the medium 120 within the 
cavity. The crystal 130 is electrically modulated through a power 
amplifier 132 from an oscillator 134 which may be a sinewave oscillator in 
the embodiment of FIG. 5. The modulation effects a variation in the index 
of refraction of crystal 130 in correspondence with the impressed voltage. 
Crystals which may be used include lithium tantalate and potassium 
dideuterium phosphate. It is important that only a single frequency mode 
is present at any instant. 
The signal from the power amplifier 132 controls the optical properties of 
the crystal 130 so as to vary the resonant frequency of the cavity between 
the mirrors 124 and 126 in accordance with the sinewave output of the 
oscillator 134. This effect is illustrated in a curve 136 in FIG. 6. The 
output beam from the mirror 126 is applied to an amplifying medium 138 
which is in turn pumped to a lasing condition by an exciter 140 that may 
typically include a flashlamp. The exciter 140 is controlled by a voltage 
detector 142 which receives the output of the sinewave oscillator 134 and 
detects selected points such as points 144 and 146 on, for example, the 
falling portion of the sinewave 136. The voltage detector 142 activates 
the exciter 140 to commence pumping of the medium 138 at the point 144 and 
controls the exciter 140 to insure termination of output from the medium 
138 at the point 146. The resulting control of the amplifier insures that 
the amplified output of medium 138 exists only during the interval between 
points 144 and 146 which essentially correspond to a nearly linear sweep 
in frequency output. The detector 142 also provides a signal to timer 82 
to identify the cessation of laser radiation and commence activation of 
switch 112 immediately or within a fraction of a microsecond. 
With reference to FIG. 7, a further implementation of a laser system for 
providing a swept output is shown. A CW dye laser 150 having a single 
axial mode is excited from an Argon laser 152 to lase within a cavity 
defined by mirror 154 and partially reflecting output mirror 156. A 
crystal 158, similar to crystal 130, is provided outside the cavity in the 
beam path and is controlled by a power amplifier 160 which, in turn, 
receives the output of a waveform generator. The waveform generator 
includes a differential amplifier 162, having on its inverting input the 
signal at the junction of first and second resistors 164 and 166 which 
form a voltage divider between ground and the output of amplifier 162. A 
further resistor 168 leads from the output of amplifier 162 to the higher 
side of a grounded capacitor 170 which is, in turn, connected as the input 
of amplifier 160. A feedback resistor 172 connects the input to power 
amplifier 160 to the noninverting input of amplifier 162. A switch 172 
selectively connects the input of power amplifier 160 to a predetermined 
potential 173 under control of timer 82. The switch 172 is controlled to 
disconnect this potential from the input to amplifier 16 only during the 
interval when the excitation laser output is desired. The configuration of 
the waveform generator insures a voltage output which is a quadratic 
function of time which when applied to time vary the length of the optical 
path through crystal 158 to frequency modulate the radiation creates a 
linear time variation in radiation frequency. The output of the mirror 156 
is applied through a laser amplifier system 174 which, in turn, provides 
the chirped laser radiation output. 
The laser system 52 will not be frequency swept unless exciting lasers are 
included in it. Accordingly, no electro-optic crystal need be provided for 
that laser. The radiation frequency for laser 50 will be centered at an 
absorption line for U.sub.235 in the case of uranium U.sub.235 enrichment, 
for a transition to an elevated energy level. In the case where laser 
system 52 provides a single frequency, it will typically produce 
sufficient photon energy to ionize from the elevated energy state created 
by laser system 50. Specific absorption lines for excitation of U.sub.235 
are numerous and may be found in the literature. The lasers 50 and 52 may 
include Dial-A-Line laser system of the Avco Everett Research Laboratory, 
preferably with one or more stages of amplification. Where a Dial-A-Line 
laser is employed, laser system 50 will include the frequency modulating 
apparatus described above in FIGS. 5 and 7, in addition to the 
Dial-A-Line. 
Two specific examples are given below for laser radiation characteristics 
which, according to the above theory, will produce an adiabatic inversion 
of the ground state uranium U.sub.235 in three energy steps (E.sub.1, 
E.sub.2 and E.sub.3) to a level below ionization. In both cases, uranium 
vapor produced according to the above-described system is illuminated with 
three laser wavelengths tuned to produce three isotopically selective 
adiabatic inversions to a final excited level below ionization. The 
specific absorption lines may be selected from spectrographic observations 
or published tabulations for the initial step. The frequency range of the 
swept radiation for each wavelength is approximately 3 GH.sub.z, or about 
twice the broadening of the U.sub.235 absorption line due to Doppler and 
magnetic field induced Zeeman effects. The calculated excitation cross 
sections for the first two energy steps are approximately 
5.times.10.sup.-16 cm.sup.2. For the first case, the total energy density 
for the lasers for the first two steps are chosen at 1 
millejoule-cm.sup.-2 per pulse and 4 millejoule-cm.sup.-2 per pulse for 
the third step laser. Under these circumstances, 98% of the available 
atoms are photoexcited, inverted, in each of the first two steps, and 81% 
in the third step to produce a total excitation of 79% to the third level. 
In the second case, the energy densities are raised to 1.3 
millejoule-cm.sup.-2 in the first two lasers and 10 millejoule-cm.sup.-2 
in the third laser pulses. The percent excitation in the first two energy 
steps is essentially 100% and 98% in the third step for a total excitation 
yield of 98%. Ionization from the third level may be produced by a further 
laser's radiation. 
The calculations above assume a linear energy loss in laser beam intensity 
in passing through the uranium vapor and further specify laser energy 
densities in terms of the energy remaining in the laser beam after passing 
through the uranium vapor. Finally, frequency sweep disturbances from the 
uranium vapor have been ignored, a condition which may be satisfied by 
making the radiation path through the vapor sufficiently short. 
Having described above a preferred embodiment of the present invention, it 
will occur to those skilled in the art that alternatives and modifications 
can be designed within the spirit of the invention. It is accordingly 
intended to limit the scope of the invention only as indicated in the 
following claims.