`
`1. M. Beterov, A. V. Eletskii, and B. M. Srnirnov
`
`Usp. Fiz. Nault 155, 265-298 (June i988)
`
`A plasma formed by the action on a gas of monochromatic radiation whose frequency
`corresponds to the energy of a resonance transition in the atom is studied. The elementary
`methods of creating and studying a plasma ofthis type are analyzed. The kinetics offormation ofa
`photoresonance plasma is studied. including collision processes with participation of excited
`atoms leading to formation of molecular ions and highly excited atoms, processes of stepwise
`ionization and triple recombination, and radiative processes. A photoresonance plasma is
`characterized by a high electron density with a relatively low electron temperature; for this reason
`the condition of id eality is more easily violated in a plasma of this type. Some ways of utilizing a
`photoresonance plasma are presented.
`
`TABLE OF CONTENTS
`
`1. Introduction .....................................................................................................535
`2. Types of photoresonance plasmas and methods of preparing them .............. ..536
`2.1. Photoresonance non-laser plasmas. 2.2. Photoresonance laser plasmas.
`2.3. Quasiresonance plasmas. 2.4. Beam and jet photoresonance plasmas.
`3. Elementary processes in a photoresonance plasma ........................................ .541
`3.1. Photoprocesses. 3.2. Collision of electrons with excited atoms. 3.3. ioniza-
`tion with participation of excited atoms.
`4. Properties of photoresonance plasmas ............................................................ ..545
`4.l. Establishment of equilibrium in photoresonance plasmas. 4.2. Nonideal
`photoresonance plasmas.
`5. Optogalyanic spectroscopy ............................................................................. .551
`5.1. The optogalvanic effect. 5.2. Laser isotope analysis.
`6. Conclusion ....................................................................................................... ..552
`References ........................................................................................................ ..553
`
`1. lNTFl0DUCTlON
`
`One of the methods of creating a plasma involves the
`action of optical resonance radiation on a gas. This method
`was first realized by Mohler and Boecl<ner,' who observed
`the formation of ions upon irradiating cesium vapor with
`resonance radiation. Thus they established the possible oc-
`currence in the gas of the process of associative ionization, in
`which an electron and a molecular ion are formed by colli-
`sion of excited and unexcited atoms, so that the energy need-
`ed for ionizing the atom is released through formation of a
`molecular ion. Studies of photoresonanee plasmas (PRPS)
`began with the study of Morgulis, Korchevoi, and
`Przhonskiiz in 1967. By illuminating cesium vapor with res-
`onance radiation to obtain a gas with a high concentration of
`excited atoms, they found as a result that a plasma is formed
`with a rather high concentration of charged particles. Since
`the ionization energy of the cesium atom (3.89 eV) exceeds
`by more than twofold the energy ofa resonance photon ( 1.39
`or 1.45 eV), this result indicated a complex, multistep char-
`acter of the kinetics of the ionization of cesium atoms under
`the conditions studied. The subsequent detailed studies of
`this kinetics3“5 have permitted obtaining rich information on
`the mechanisms and rates of processes involving excited
`atoms.
`
`The formation of a photoresonancc plasma is accompa-
`nied by various phenomena that occur in gases. Thus, the
`ionization of a gas under the action of resonance optical radi-
`ation is one of the fundamental mechanisms of formation of
`
`an ioni'1.at.ion wave in the gas, which propagates upon apply-
`
`ing an external electric field." This same mechanism plays
`the decisive role in the phenomenon of ionization of a gas
`ahead of the front ofa strong shock wave in the gas.7 Irradia-
`tion ofa gas with optical resonance radiation is used as one of
`the methods of preliminary ionization of the active medium
`of high—pressure molecular lasers? This enables one to cre-
`ate a plasma homogeneous throughout the volume, while
`avoiding the factors that favor the development of instabili-
`ties and spatial inhomogeneities of the active medium? The
`stated method of creating a high-density plasma homoge-
`neous throughout the volume has attracted the attention of
`investigators also in connection with the problem of heating
`thermonuclear targets with beams of light ions.“ In this case
`the ionization of the gas under the action of resonance radi-
`ation enables one to create for a short time an extended plas-
`ma channel, which serves for transport of the ion beam to the
`target, while hindering electrostatic repulsion of the ions.”
`The potentialities of‘ study of photoresonance plasmas,
`as Well as the set of their applications, have been expanded by
`the invention of frequency-tunable lasers. On the one hand,
`this has enabled considerable increase in the fluxes of reso-
`nance radiation transmitted through the gas, and on the oth-
`er hand, study of the processes that occur upon optical exci-
`tation of various states of the atom. The photoresonance
`plasma formed by using tunable lasers is used as a nonlinear
`element in frequency transformation of laser radiation, "‘ as
`a source of ions of a given type,‘ "'3 etc.
`The set of phenomena that occur in a photoresonance
`plasma is closely bordered by the optogalvanic effect, which
`
`535
`
`Sov. Phys. Usp. 31 (6), June 1983
`
`0038-5670/88/O60535—20$D1.80
`
`© 1969 American Institute oi Physics
`
`535
`
`~» --
`
`(cid:34)(cid:52)(cid:46)(cid:45)(cid:1)(cid:18)(cid:17)(cid:20)(cid:18)
`ASML 1031
`
`
`
`
`
`Te ~ 103 K was formed upon irradiating Cs vapor at a pres-
`sure of IO‘3—l0“ Torr with a cesium gas-discharge lamp.
`The radiation of the lamp corresponded to the spectral range
`/l>600 nm. The bulk of the energy of the radiation of the
`lamp was contained in the lines at/l = 894.3 and 852.1 nm,
`corresponding to the 63S~6 3P resonance transitions. A de-
`tailed mass-spectrometric analysis showed‘ that the main
`type of ions in this plasma is the atomic ion Cs+. This indi-
`cates a complex character of the kinetics of ionization in the
`photoresonance cesium plasma; in particular, this implies
`that the process of associative ionization in the collision of
`two resonance-excited Cs atoms ( 621’) is not the fundamen-
`tal ionization channel.’ As is implied by the results of de-
`tailed experimental studies of recent years,3”5 the complex
`kinetics of ionization of atoms in the cesium photoresonance
`plasma includes processes of collision of two resonance-
`excited atoms,
`
`2C3 (6 2P) --> C3 (6 28) + C3 (8 2P),
`
`(2.1)
`
`processes of quenching ofthe excited Cs atoms (63P, 82!’) by
`electron impact, processes of ionization of excited atoms in
`collisions with fast electrons formed as a result of quenching,
`processes of associative ionization, etc.
`
`As the source of optical radiation for creating the pho-
`toresonance cesium plasma, not only cesium lamps, but also
`helium gas-discharge lamps have been successfully em-
`ployed. This possibility ariscs from the coincidence of the
`wavelength of one of the effective transitions in the spectrum
`of He (xi = 388.8 nm) with the wavelength of the 628-821’
`transition of the cesium atom. We note that this coincidence
`
`is the basis of one of the first schemes for excitation of a gas
`laser with optical pumping, which was proposed by F. A.
`Butaeva and V. A. Fabrikant” and realized experimentally
`in Ref. l7. Using this scheme, a photoresonance cesium plas-
`ma was obtained with the parameters N32,» z 10’ cm”,
`Ne -103-109 cm’3, Te -0.3 eV, PCS ~ lO”3~l()" Torr.”
`In a plasma of this type, processes of stepwise excitation of
`highly excited levels from the BZP level by electron impact
`play an essential role. Figure 1 shows the dependences of the
`parameters ofthis plasma on the density of the Cs vapor with
`fixed intensity of resonance irradiation.
`
`tr",/z,,7;, rel. units
`
` 0
`
`25
`
`5.9
`
`FIG. 1. Dependence of the parameters of a cesium plasma on the density
`of Cs vapor at fixed intensity of resonance irradiation.” I-density rt" of
`excited atoms (Cs, 8"P) ; 2——density :2, of electrons; 3--temperature Tc of
`electrons.
`
`Beterov er al.
`
`536
`
`consists in a change in the electrical properties of a gas—dis—
`charge plasma or flame (e.g., the volt-ampere characteris-
`tics) when acted on by optical resonance radiation." The
`optogalvanic effect is used for determining trace impurities
`of elements in a gas, in studying the mechanisms of elemen-
`tary processes in a gas-discharge plasma, in controlling the
`parameters of a gas-discharge plasma for transmission and
`processing of information, and as a detection method in laser
`spectroscopy with superhigh sensitivity.
`The process of multistep ionization of atoms opens up
`broad possibilities. The use for this purpose simultaneously
`of several frequency-tunable lasers enables one to transfer an
`appreciable number of atoms of a certain type to a given
`highly excited state. This technique, usually based on using
`atomic beams, is applied for detecting individual atoms, for
`detecting submillimeter radiation, for generating coherent
`radiation in the UHF range (maser), and in experiments on
`laser isotope separation. ’5 The identification of highly excit-
`ed atoms is performed by ionizing them in an external elec-
`tric field. Here one uses the sharp dependence of the ioniza-
`tion probability in a field of given intensity on the effective
`value rt‘ of the principal quantum number of a highly excited
`atom. The ions formed by ionization are extracted from the
`system by applied fields.
`The properties and specifics of a photoresonance plas-
`ma are associated with the processes that occur in them.
`Thus a photoresonance plasma whose properties are deter-
`mined by elementary collisiorrradiation processes, is natu-
`rally distinguished from a laser plasma, in which the trans-
`formation of the energy of the laser radiation into the energy
`of plasma particles results from the excitation of collective
`motions in the plasma.
`At the same time it seems natural to classify as a photo-
`resonance plasma one formed by the action on a gas of radi-
`ation having a frequency that does not necessarily corre-
`spond to a resonance transition, but also to transitions
`between ground and highly excited states, or transitions
`between two excited states. In all objects of this type, the
`fundamental ionization mechanism is collisional processes
`involving excited atoms ( more rarely—molecules).
`This review will analyze the current status of studies of
`photoresonance plasmas, information will be presented on
`the properties and parameters of this object, and problems
`will be discussed involving the application of photoreson-
`ance plasmas in the technique of physical experimentation
`and in applied fields.
`
`2. TYPES OF PHOTORESONANCE PLASMAS AND METHODS
`OF PREPARING THEM
`
`2.1. Photoresonance non-laser plasmas
`
`The most convenient method of obtaining a photore-
`sonance plasma not involving the use of laser radiation con-
`sists in irradiating a gaseous substance with a gas-discharge
`lamp filled with the same substance. Here the parameters of
`the photoresonance plasma are determined by the intensity
`of the resonance radiation emitted by the lamp. The most
`interesting results have been obtained in cases in which the
`lamp is characterized by a high coeflicient of transformation
`of electrical energy into energy of resonance radiation. Thus,
`in the pioneer study,‘ a quasistationary plasma with an elec-
`tron density Ne ~10” cm“ and an electron temperature
`
`536
`
`Sov. Phys. Usp. 31 (6). June 1988
`
`
`
`
`
`UHF diagnostics, at a concentration of Cs ~ 3 X 10"‘ cm‘3,
`Hg-3 X 10" cm‘3, and a pressure ofbutfer gas (Ar) ~ 100
`Torr, the photoresonance plasma was characterized by a
`density N, ~10” cm‘3 and a temperature T, z 2000 K. The
`role of the buffer gas consists in reducing the effectiveness of
`the diffusion losses of charged particles, and hence, in main-
`taining the density of the electrons of the photoresonance
`plasma at a sufliciently high level. An analogous scheme for
`creating a photoresonance plasma was realized in Ref. 23,
`where a mixture of Cd and Cs vapors was irradiated with the
`resonance light of a cadmium lamp.
`
`2.2. Photoresonance laser plasmas
`
`The invention and wide spread of frequency-tunable
`narrow-band lasers based on dyes has stimulated to a consid-
`erable degree the study of the properties and possible appli-
`cations of photoresonance plasmas. The set of studied ob-
`jects has considerably expanded to encompass all the alkali
`metals, and also a number of metals of the second and third
`groups of the periodic table. The object of the studies was the
`mechanisms of ionization and recombination of particles of
`a plasma, the elucidation of the role of the buffer gas, the
`possibility of more complete extraction of ions in a photore-
`sonance plasma and identifying them, etc.
`Among the large number of experimental studies (see
`the review ofthe early studies“) on the creation and study of
`photoresonance laser plasmas, primary attention is owed to
`a series of publications reporting the practically 100% ioni-
`zation of metal vapors irradiated with the resonance radi-
`ation of a pulsed laser of relatively low power. Figure 3
`shows a diagram of the first of these experiments, which was
`performed by Lucatorto and McIlrath.2" The radiation of a
`dye laser pumped with a flash lamp was tuned to a line at
`/1 = 589.6 nm, which corresponds to the 33S,/z—32P,/3 tran-
`sition ofthe Na atom, and was focused on a 10-cm column of
`Na vapor with addition of He to a total pressure about
`1
`Torr. The pulses of laser radiation of duration 500 ns had an
`energy of 300 mJ, which corresponds to a pulse power of0.6
`MW. The degree of ionization of the vapor was determined
`with a vacuum-ultraviolet spectrograph, which enabled
`measuring the absorption coeflicient in the region /1 = 15-
`42 nm. Figure 4 shows typical densitograms of the spectrum
`obtained without (a) and with (b) laser irradiation. As is
`shown by comparison of the absorption coefficients in the
`region of 21:32.21 nm corresponding to transitions of the
`Na* ion, the degree of ionization of Na during the laser
`pulse reaches 100%. The practically complete ionization of
`the Na vapor is also indicated by the sharp (by a factor of
`
`FIG. 3. Diagram ofan experiment to produce and study a photore-
`sonance sodium plasma with a high degree ofionization. 1—radi-
`ation source with a continuous spectrum; 2—anode; 3-—toroidal
`mirror; 4—capillary rings; 5—vacuum pump; 6—furnace; 7—
`three-meter reflecting spectrograph; 8—difi'raction grating; 9—
`photoplate; 10—cylindrical lens; II—laser; 12—delay generator;
`13—pulse shaper.
`
` 2
`
`4 68/0
`
`/mm . A
`
`FIG. 2. Dependence ofthe concentration (I) and temperature (2) ofthe
`electrons of a photoresonance plasma in l-lg vapor on the value of the
`discharge current in the optical-excitation source.“
`
`Mercury-vapor lamps are also an intense source of reso-
`nance radiation, and have been successfully used to create a
`photoresonance plasma in mercury vapor.'°'Z" Investiga-
`tions in this direction were stimulated by practical problems
`of optical separation of mercury isotopes? ' Upon irradiating
`a gas-discharge mercury lamp with the resonance line corre-
`sponding
`to
`the
`transition
`Hg(63P‘," —>6'S0)
`(/l. = 253.7 nm), an increased concentration and a reduced
`temperature of the electrons was observed. This was asso-
`ciated with an increase in the efficiency of ionization under
`the action of the resonance radiation. The mercury lamp
`used as the source of resonance radiation had the form of a
`
`jacket arranged coaxially around the cylindrical cell being
`irradiated.” With a pressure of mercury vapor in the cell of
`~0.05 Torr, the concentration ofHg atoms in the 6“P‘,’ state
`reached 10' ' cm ‘ 3. The electrical characteristics of the pho-
`toplasma that was formed are shown in Fig. 2. Owing to the
`coaxial excitation geometry used in this experiment. a high
`degree of homogeneity of the photoresonance plasma was
`attained.
`
`An interesting variety of photoresonance plasma was
`realized in Ref. 22, where a mixture of Hg and Cs vapors was
`irradiated with the resonance radiation of a mercury lamp
`(/1 = 253.7 nm ). The ions were formed by the Penning reac-
`tion
`
`Hg(5‘*P.)+Cs —> Hg+Cs*+e.
`
`(2.2)
`
`According to the measurements performed using probe and
`
`
`
`537
`
`Sov. Phys. Usp. 31 (6), June 1988
`
`Beterov er al.
`
`537
`
`Ht
`
`-op
`
`
`
`
`
`
`
`Transmission.rel.units
`
` FIG. 4. Densitograms of the absorption spectrum of sodi-
`
`32
`
`30
`
`28
`
`um.“ a—Absorption of sodium vapor without irradiation;
`the dots indicate the emission lines ofthe vacuum arc, and the
`dotted line the absorption in He. b—Absorption of Na vapor
`irradiated with resonance laser radiation. The solid squares
`indicate the absorption lines ofneutral Na.
`
`25 /l,I'1|'h
`
`pors under the action of resonance laser radiation has been
`observed in subsequent experiments with vapors of Li,“
`Cs,” Ca,”, Sr,” Ba,2“‘“ Na,” and Mg.“ The properties of
`the PRP formed thereby have been studied in greatest detail
`in Ref. 36, where Mg vapor was irradiated with radiation
`corresponding to the resonance transition 3'S(,—»3‘P,.
`(/1 = 285.2 nm) of the singlet system oflevels. A pulsed liq-
`uid dye laser with frequency doubling pumped with the sec-
`ond harmonic of a neodymium laser with a pulse repetition
`frequency of 3 Hz was used as the radiation source. The
`pulses of ultraviolet radiation had a line width ~0.l cm’ ',
`and duration ~10 ns at a power ~l kW. Another liquid
`laser (2. = 280.3 nm) tuned to the transition 32P,,2—32S,,2
`of the Mg+ ion was used to measure the concentration of
`Mg”’ ions. In addition, the experiment measured the lumi-
`nescence of the Mg vapor and the time-dependence of the
`photocurrent. The pulse of the probe radiation had a delay
`with respect to the pump pulse variable in the range up to
`100 its. Figure 7 shows a diagram of the experiment.“
`The experimental chamber, which was made of stain-
`less steel and fitted with quartz windows and internal plane
`electrodes to measure the photocurrent, was filled with an
`inert gas at a pressure ~ 1 Torr. The pressure of metal vapor
`(Mg) was varied in the range 0.1-1.0 Torr. Upon tuning the
`pump radiation to the frequency of the resonance transition
`
`
`
`Time, ps
`
`FIG. 6. Dependences ofthe electron temperature on the time elapsed after
`cessation of the laser radiation pulse.” The curves are marked with the
`diflerent values of the concentration of Na vapor.
`
`Beterov er al.
`
`538
`
`10“) decline in the absorption coefiicient of the resonance
`laser radiation owing to the formation of the photoresonance
`plasma. Even the first rough estimates of the authors“ indi-
`cated a complex, multistep mechanism of ionization of the
`vapor in the described experiments. Neither three-photon
`ionization nor radiation collision
`
`2Na* (3p)—i-fio)—>-Na*(2p°)+Na (3s)+e,
`
`(2.3a)
`
`nor multistep collisional excitation
`
`2Na (3p) —» Na (55) + Na (35)
`
`(2.3b)
`
`with subsequent photoionization of the excited Na (5s)
`atoms possess values of the rate constants high enough to
`explain the observed 100% ionization of the atoms.
`Analogous results were obtained in Ref. 25, where a cell
`filled with a mixture of Na vapor and Ar at a pressure of
`several Torr was irradiated with pulses of radiation of a dye
`laser based on rhodamine 6G pumped with a nitrogen laser.
`~ The radiation pulses of 100 kW power had a duration of 10
`ns and a line width ~O.l nm. The ionization of the gas was
`measured by the magnitude of the photocurrent, for which a
`‘ typical dependence on the Na vapor density is shown in Fig.
`5. Figure 6 shows the dependence of the electron tempera-
`ture on the time elapsed since the end of the laser pulse, as
`obtained by the Langmuir double-probe method. The results
`of the probe measurements of the electron density performed
`at a point separated by 2 mm from the focus of the laser beam
`are shown in Table I.
`The effect described above of intense ionization of va-
`
`r a kW
`65'
`3:
`‘-5
`
`fig 70’
`=
`3
`jg 70?
`1?(U
`E m
`P.’
`
`5 85
`
`7
`E a
`
`7” 75
`M 14
`7013
`Density of Na vapor, cm ‘3
`
`7016‘
`
`FIG. 5. Dependences of the photocurrerit on the density of Na vapor
`obtained at difierent levels of laser power."
`
`538
`
`Sov. Phys. Usp. 31 (6), June 1988
`
`
`
`
`
`iv,.,~a,cm'3l
`
`TABLE I. Values of the electron density IV, in a photoresonance plasma measured at different
`values of the density of Na vapor.”
`1,n.1m»
`5,9-10“
`i,s.1m:~ 5,340“ [
`
`
`
`‘
`1.0.1018
`i
`5,540»
`9.5.1012
`1
`7.0-10”
`
`Ne, cm”
`
`of Mg, an emission from the Mg vapor arose in the region of
`the lascr—hearn focus at frequencies corresponding to transi-
`tions rz 'D,—3 ‘P, (11 = 4-10) ofthe singlet system and n “S-
`3 3P (:1 = 4, 5) of the triplet system of Mg levels. Here the
`absence was noted of radiation at the strong line 5 ‘S.,—3 ‘P,
`(/1 = 57 l .1 nm) and several other strong lines. As was
`shown by measurements of the density of Mg ions performed
`with probe radiation, the maximum concentration of ions
`( ~ 2 X 10” cm’ 3 ) was observed about 30 ns after the end of
`the pulse of pump radiation, while the total time of existence
`of the PRP amounted to ~ 10 ps. The maximum degree of
`ionization of the plasma reached 5%. The absence of satura-
`tion in the dependence of the ion density on the intensity of
`pump radiation allows one to expect an increase in the PRP
`density upon using more intense resonance pump radiation.
`Another detailed study worthy of attention on the char-
`acter of formation and physical properties of a PRP with a
`rather high degree of ionization was performed by the auth-
`ors of Ref. 35, where a dye laser (rhodamine C) was used as
`the source of resonance radiation. It was pumped with the
`radiation of the second harmonic of a solid-state pulsed laser
`based on yttrium aluminum garnet of type LTIP4-5 with a
`pulse-repetition frequency of 12.5 Hz. To narrow and tune
`smoothly the emission line of the dye laser, a diffraction
`grating ( 1200 lines/mm) was used and was set up in a glanc-
`ing—incidence system. The width of the emission line of this
`laser amounted to ~ 1 nm, and the emission power was 340
`kW at a pulse duration of ~ 10”‘ s. The laser was tuned
`either to the resonance transition of Na (2. = 589.0 nm) or
`to the wavelength 578.’? nm corresponding to two-photon
`absorption to the excited state of Na (4-d 2D5,; ).
`The Na vapor diluted with inert gases filled the dis-
`charge tube, which was made of Pyrex and had niobium tu-
`bular electrodes. The pressure of the vapor in the tube was
`maintained by using a heating element. Under the conditions
`
`
`
`FIG. 7. Diagram of an experimental arrangement to form a quasireson-
`ance laser plasma in Mg vapor. ii’ 1-neodymium garnet laser; 2, 4-fre-
`quency doublers based on KDP; 3——<iye laser; 5-—delay circuit; 6 ~-cell
`with Mg vapor; 7—— monochromator with photomultiplier (PM); 8-os-
`cillograph.
`
`of the experiment one pulse of laser radiation contained
`about 5 X 10“ photons. About the same number of Na atoms
`was contained in the irradiated volume. Here conditions
`were selected such that, on the one hand, practically com-
`plete absorption of the laser radiation was attained, and on
`the other hand, the intensity of absorption varied weakly
`along the laser beam. This was made possible by detuning
`the frequency of the laser beam from the center of the ab-
`sorption line by four widths of the Doppler absorption con-
`tour (equal to 0.0024 nm). Whereas at the center of the
`absorption line the optical density of the medium amounted
`to ~ 10“, at the detuned frequency it was close to unity.
`The formation of a PRP was measured from the change
`in electrical conductivity of the irradiated volume. To do
`this, a voltage was applied to the electrodes of the discharge
`tube smaller than the excitation voltage of the discharge, and
`the current was measured that arose in the electrical circuit
`
`under the action of the laser illumination. Figure 8 shows an
`oscillogram of this current obtained at a voltage on the elec-
`trodes of 100 V and a vapor pressure of Na of 3 X 10" 3 Torr.
`The pressure of the buffer gas amounted to 1 Torr. As the
`pressure of Na vapor was increased from 3 X 10“‘ Torr to
`0.2 Torr, the magnitude of the signal and its duration in-
`creased by more than an order of magnitude. Upon detuning
`from the resonance at xi = 589 nm, no plasma formation was
`observed.
`
`Plasma formation was also observed in two-photon la-
`set‘ excitation of the level 4p3D5 ,2. The oscillograms of the
`current obtained here at a pressure of Na vapor ~ 5 X l()’‘'3
`Torr are shown in Fig. 9. The rapidly growing advance front
`of the current pulse was associated” with the phenomenon
`of three-photon ionization of atoms via a two-photon reso-
`nance, and the subsequent, smoother increase in electrical
`conductivity—-with a supplementary mechanism of forma-
`tion of free electrons (heating of electrons by superelastic
`processes and subsequent ionization of atoms by electron
`impact).
`As an analysis of the experiments to create and study a
`laser PRP shows, a plasma of rather high density is formed
`using very-low-power laser radiation. This arises from the
`high absorption power of gases for resonance radiation, and
`also the high efiiciency of conversion of the energy of reso-
`nance-excited atoms into ionization energy.
`
`2.3. Quasiresonance plasmas
`
`As has been established in a number of experiments of
`recent years,“”"*37 to form a photoresonancc plasma one
`need not use radiation whose frequency corresponds to a
`resonance transition between the ground and excited states
`of the atom. Efficient ionization of the atoms of a metal va-
`por has also been observed using radiation corresponding to
`a transition between two excited states of the atom. Here the
`intensity of the laser radiation was not so great that one
`
`539
`
`Sov. Phys. Usp. 31 (6), June 1988
`
`Beterov etal.
`
`539
`
`out
`
`.n
`
`--
`
`
`
`
`
`)‘jI—--.-..........
`
`‘
`bE"_
`
`FIG. 3. Oscillcgram of the photocurrent that arises
`upon irradiating a discharge tube containing a mix-
`ture of Na + N: with a pulse of laser radiation
`(Ii :: 589 nm),” in the presence ofadischarge {a},
`and at a voltage below that for ignition of the dis-
`charge Eb).
`
`One of the atoms formed as a result of photodissociation of
`the dimer exists in the excited state (in the case of Cs this
`state is 6p2PW‘3,2 ), and is already capable of resonance ab-
`sorption of laser radiation. This leads to formation of highly
`excited atoms, whose ionization results from subsequent
`collisional processes.
`
`2.4. Beam and jet photoresonance plasmas
`
`The most productive way to study the primary starting
`mechanisms responsible for the formation of a PRP involves
`using atomic beams. In this case one can reduce to a mini-
`mum the role of secondary collisional processes and isolate
`in pure form the process that occurs with participation of
`optically excited atoms and leads to their ionization. A char-
`acteristic example ofsuch an experiment is Ref. 39, where an
`effusion beam of Na atoms was irradiated with a dye laser
`tuned to a resonance transition while pumped with an argon
`ion laser. The density of atoms in the irradiated zone
`amounted to 109 cm“'‘, the intensity of the laser radiation
`was 0.5 W/cm”, and the line width was 20 MHz. The posi-
`tive ions were extracted from the interaction region with a
`pair of electrodes to which a potential was applied, and
`which created an electrical field in the cell. Then the ions
`
`entered the input of a quadrupole mass spectrometer. This
`made it possible to establish that only molecular Na; ions
`are formed under the conditions of the experiment. As was
`shown by comparing the results of the mass-spectrometric
`measurements with those of relative measurements of the
`concentration of resonance-excited atoms based on the flu-
`
`orescence of the beam, a proportional relation is observed
`between the yield of Na{” ions and the square of the density
`of excited Na atoms. This enabled concluding that the deci-
`sive role is played by the process of associative ionization
`
`2Na(3p)—>Na;‘-E-e
`
`(2.4)
`
`and establishing the value of the rate constant of this process
`(~ 1.5x 10‘ '3 crn3,«’s) and its cross section (--O.5><1O“‘7
`crnz). "
`A higher intensity of ionization using resonance radi-
`ation was obtained in ajet experiment,“" in which a beam of
`monoenergetic Cs*'
`ions was formed in this way. A glass
`
`Continuous
`
`dye laser
`Cur-mace
`
`
`FIG. 10. Diagram of an experiment to form a quasiresonance laser plasma
`using a o0ntinuous—wave dye laser. ‘3
`
`Beterov er al.
`
`540
`
`could ascribe such an unexpected result to effects of multi-
`photon ionization of atoms, including the nonresonance ex-
`citation of a real level.”
`A plasma of the type being studied has been called a
`“quasiresonance laser plasma.” As has been established by
`detailed experimental studies, to form a quasiresonance
`plasma one can use radiation at one of many frequencies
`corresponding to transitions between excited states of the
`atom. Thus, a quasiresonance cesium plasma was efficiently
`formed upon irradiating Cs vapor with laser radiation hav-
`ingfit = 583.9, 601.0, 603.5, and 621.3 nm, corresponding to
`the transitions
`28. ,, 10s -» 2P?” 6p, ZDM 8d -» 2P?,2 6p,
`3S,,3lOs—-3O§,2 6p, and 2D3,28d-»2P‘§,2 6p, as well as when
`using radiation at several other transitions of the atom.”
`Here the formation of the quasiresonance plasma has a
`threshold character—this phenomenon is observed only
`upon exceeding certain values of the pressure of the vapor
`and intensity of the laser radiation.
`Another interesting feature of a quasiresonance laser
`plasma involves the relatively low level of intensity of radi-
`ation used to maintain it. Thus, in the series of studies cited
`above,” continuous irradiation of cesium vapor of density
`~3 X 10” cm‘ 3 with quasiresonanee radiation of power up
`to ~ 100 mW yielded a plasma with a degree of ionization of
`1Cl"3. A diagram of the experiment is shown in Fig. 10. As is
`shown by analyzing photographs of the plasma column
`formed upon focusing the beam of a continuous laser of 10
`mW power, the extent of the column amounts to about 4 mm
`and the diameter to less than 1 mm. Even weaker radiation
`was used in Ref. 37 to maintain a plasma in Na vapor. This
`radiation having a power of ~ 2 mW was tuned in resonance
`with the 3p-4d transitions (/1 = 568.8 or 568.2 nm) of the
`Na atom. The pressure of the Na vapor amounted to ~10
`Torr. Detailed spectral studies of the plasma that was
`formed led the authors to conclude that the decisive role in
`
`forming the plasma was played by the process of associative
`ionization involving a ( 4d) Na atom.
`The mechanism of ionization of a gas under the action
`of quasiresonance radiation ‘3 includes the process of photo-
`dissociation of dimers of a metallic vapor, which are always
`present in the system:
`
`A.3+?ioJ—>~A+A“".
`
`Q@
`
`-
`FIG. 9. Oscillogram of the photocurrent observed in two-photon laser
`excitation of Na vapor (/1 : 578.9 nm).-‘5
`
`540
`
`Sov. Phys. Usp. 31 (6). June 1988
`
`
`
`
`
`/,nA
`
`2::
`
`+-7,5W,488nm
`--0,7W,4.9D‘,.5nm
`n—a,4w, J0f,7rim
`
`-:
`
`'3
`
`n
`Q :
`5 E
`
`+
`u
`
`Electrode potential, V
`
`'45
`
`Nozzle potential, \/
`
`7,0
`
`0.5
`
`-7,5
`
`chamber maintained at a constant temperature in the range
`400-500 K (pressure of Cs vapor ~2>< lO‘3—0.2 Torr) and
`equipped with a nozzle 0.12 mm in diameter was used as the
`source of the cesium vapor. Thus ajet of vapor was formed
`that was concentrated in a solid angle 6 = 2.7 X l0'2 stera-
`dian and characterized by an intensity of 8.6>< 10" atoms/s
`and a mean energy of the atoms of ~0.06 eV. The energy
`spread ofthe atoms in thejet was ofthe same order ofmagni-
`tude. This corresponds to a density of Cs atoms near the
`critical cross section of the nozzle of ~ 10” cm”. The ioni-
`zation of the Cs atoms was carried out in two stages. In the
`first stage the resonance radiation ofa semiconductor injec-
`tion CaAlAs laser was used (/1 = 852.1 nm), which convert-
`ed the Cs atoms to the 62P3,2 state. In the second stage the
`radiation of an argon ion laser was used (/1 = 488.0, 496.5,
`and 501.7 nm), which enabled the photoionization of the
`resonance-excited atoms. Figure l 1 shows the dependence of
`the photocurrent on the extracting potential. According to
`the estimates the flux ofions in thejet reached values of 10“
`s‘ ’. A substantial increase in this parameter was obtained in
`a subsequent study by this same author,“ where values were
`reached of ~10” s
`‘with an energy spread ofthe ions at the
`level of0. 15 eV.
`
`Two-stage and multistage photoionization ofatoms has
`been used successfully to obtain beams ofthe following ions:
`Ca+ ,"' In+ (under the action ofdye-laser radiation),“ Al+
`(using the radiation of an excimer XeCl l