Resonance radiation plasma (photoresonance plasma)
`
`I. M. Beterov, A. V. Eletskii, and B. M. Smirnov
`
`Usp. Fiz. Nauk 155, 265-298 (June 1988)
`
`A plasma formed by the action on a gas ofmonochromatic radiation whose frequency
`corresponds to the energy ofa resonance transition in the atom is studied. The elementary
`methods ofcreating 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 ofideality is more easily violated in a plasma ofthis type. Some ways of utilizing a
`photoresonance plasma are presented.
`
`TABLE OF CONTENTS
`1. Introduction .......................... ..
`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.
`............................... ..54l
`3. Elementary processes in a photoresonance plasma ....... ..
`3.1. Photoprocesses. 3.2. Collision of electrons with excited atoms. 3.3. Ioniza-
`tion with participation of excited atoms.
`.....545
`.
`4. Properties of photoresonance plasmas .............................. ..
`4.]. Establishment of equilibrium in photoresonance plasmas. 4.2. Nonideal
`photoresonance plasmas.
`5. Optogalvanic spectroscopy ............................................ ..
`5.1. The optogalvanic efi"ect. 5.2. Laser isotope analysis.
`
`............ ..552
`6. Conclusion ................ ..
`............ ..553
`References
`
`............................... ..55l
`
`1. INTRODUCTION
`
`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 Boeckner,' who observed
`the formation of ions upon irradiating cesium vapor with
`resonance radiation. Thus they established the possible oc-
`currence in the gas ofthe process ofassociative 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 photoresonance 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 ofthe 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 ofthe kinetics ofthe 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 photoresonance 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 ofthe fundamental mechanisms offormation of
`an ionization 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 ofthe 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 ofheating
`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 ofthe 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 ofions ofa 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 1988
`
`0038-5670/88 /060535-20$01 .80
`
`l\ u-
`
`(9 1989 American Institute of Physics
`53
`ASML11§4
`(cid:36)(cid:54)(cid:48)(cid:47)(cid:3)(cid:20)(cid:20)(cid:21)(cid:23)
`
`

`
`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.” 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 n"‘ 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 collision-radiation 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 piasma 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 PHOTOFIESONANCE 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 coefiicient 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
`
`T: ~ 103 K was formed upon irradiating Cs vapor at a pres-
`sure of lO‘2—lO“‘ Torr with a cesium gas-discharge lamp.
`The radiation of the lamp corresponded to the spectral range
`/12600 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 62S~6 2P 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 (62P) is not the fundamen-
`tal ionization channel.‘ As is implied by the results of de-
`tailed experimental studies of recent years,” the complex
`kinetics of ionization of atoms in the cesium photoresonance
`plasma includes processes of collision of two resonance-
`excited atoms,
`
`2CS(6 2P) —-> C5 (6 2S)+Cs (8 2P),
`
`(2.1)
`
`processes ofquenching ofthe excited Cs atoms (62P, 82P) 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 arises from the coincidence of the
`wavelength of one of the effective transitions in the spectrum
`of He (/1 = 388.8 nm) with the wavelength of the 62S—82P
`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. 17. Using this scheme, a photoresonance cesium plas-
`ma was obtained with the parameters N32,, : I07 cm‘3,
`Ne ~ 108-109 cm‘3, Te ~O.3 eV, PCS ~ 10‘3-1O‘2 Torr.”
`In a plasma of this type, processes of stepwise excitation of
`highly excited levels from the SIP level by electron impact
`play an essential role. Figure I shows the dependences of the
`parameters of this plasma on the density of the Cs vapor with
`fixed intensity of resonance irradiation.
`
`Iz'jng,7,, rel. units
`
`
`
`320
`
`2,5
`/70, 10 3cm‘3
`FIG. 1. Dependence of the parameters ofa cesium plasma on the density
`ofCs vapor at fixed intensity of resonance irradiation.” ]4density n‘ of
`excited atoms (Cs, 83P);24density I1: ofelectrons; 3--temperature T, of
`electrons.
`
`Beterov e! 2/.
`
`536
`
`
`
`

`
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`
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`
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`
`O
`
`A
`
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`
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`
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`
`A
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`
`FIG. 2. Dependence ofthe concentration (1) and temperature (2) ofthe
`electrons of a photoresonance plasma in Hg vapor on the value of the
`discharge current in the optical-excitation sourcer”
`
`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.'°*1° Investiga-
`tions in this direction were stimulated by practical problems
`ofoptical separation ofmercury isotopes.“ Upon irradiating
`a gas-discharge mercury lamp with the resonance line corre-
`sponding
`to
`the
`transition
`I-lg(63P‘,’ —»6‘S,,)
`(xi. = 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 63F? state
`reached 10‘ ' cm”. The electrical characteristics ofthe 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 ofHg 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-
`11011
`
`Hg(6”P.)+Cs —> Hg+Cs*+e.
`
`(2.2)
`
`According to the measurements performed using probe and
`
`
`
`UHF diagnostics, at a concentration of Cs ~ 3 X 10"‘ cm”,
`Hg~3>< 10” cm‘~‘, and a pressure ofbuffer gas (Ar) ~ 100
`Torr, the photoresonance plasma was characterized by a
`density NC ~10” cm” and a temperature T, z2000 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 sufficiently high level. An analogous scheme for
`creating a photoresonance plasma was realized in Ref. 23,
`where a mixture ofCd and Cs vapors was irradiated with the
`resonance light ofa 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 ofthe 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.“ The radiation ofa
`dye laser pumped with a flash lamp was tuned to a line at
`A = 589.6 nm, which corresponds to the 33S , V,/3—32P,,~2 tran-
`sition ofthe Na atom, and was focused on a l0-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 of 0.6
`MW. The degree of ionization of the vapor was determined
`with a vacuum-ultraviolet spectrograph, which enabled
`measuring the absorption coefficient in the region /1 == 15-
`42 nm. Figure 4 shows typical densitograms ofthe spectrum
`obtained without (a) and with (b) laser irradiation. As is
`shown by comparison of the absorption coefficients in the
`region of /1:32.21 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 of an experiment to produce and study a photore-
`sonance sodium plasma with a high degree of ionization. I—radi-
`ation source with a continuous spectrum; 2—anode; 3~—toroidal
`mirror; 4—capillary rings; 5-—vacuum pump; 6—furnat:e; 7—
`three-meter reflecting spectrograph: 8—-ditfraction grating; 9-
`photoplate; 10—cylindrical lens; 11-—-laser; 12—delay generator;
`I3—pulse shaper.
`
`537
`
`Sov. Phys. Usp. 31 (6), June 1988
`
`Beterov er al.
`
`537
`
`

`
`
`
`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. l}—Absorption ofNa vapor
`irradiated with resonance laser radiation. The solid squares
`indicate the absorption lines of neutral Na.
`
`25 A, nm
`
`pors under the action of resonance laser radiation has been
`observed in subsequent experiments with vapors of Li,“
`Cs,“ Ca,”, Sr,” Ba,3“‘3‘ 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,.
`(xi. = 285.2 nm) of the singlet system oflevels. A pulsed liq-
`uid dye laser with frequency doubling pumped with the sec-
`ond harmonic ofa 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 ~O.l cm‘ ',
`and duration ~10 ns at a power ~l kW. Another liquid
`laser (,1 = 280.3 nm) tuned to the transition 33P1,2—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 ps. 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, /18
`
`FIG. 6. Dependences ofthe electron temperature on the time elapsed after
`cessation of the laser radiation pulse.” The curves are marked with the
`different values ofthe concentration of Na vapor.
`
`10‘) decline in the absorption coeflicient 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)+hw—>-Na*(2p°)+Na(3s)+e,
`
`(2.3a)
`
`nor multistep collisional excitation
`
`2iVa (3p) —>- Na (55) + Na (35)
`
`(2.3b)
`
`with subsequent photoionization of the excited Na ( 55)
`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-
`
`/no kw
`$3
`‘.5
`
`§ 70’
`3
`570?
`1E(H
`E 70
`
`958‘
`
`7
`5
`E a
`
`m” M”
`m“ M”
`Density of Na vapor. cm‘?
`
`FIG. 5. Dependences of the photocurrent on the density of Na vapor
`obtained at different levels oflaser power.”
`
`538
`
`Sov. Phys. Usp. 31 (6). June 1988
`
`Beterov er a/.
`
`538
`
`
`
`

`
`TABLE 1. Values of the electron density N: in a photoresonance plasma measured at different
`values of the density of Na vapor.”
`5,9401:
`1
`1,n.m1s
`E
`13.1015
`*
`53.1015
`I
`1,n.1ma
`E
`7,r).1m2
`6,640”
`I
`9.5.1012
`1
`
`NNa,cm"3i
`l
`
`
`Ne, Cm‘)
`
`ofMg, an emission from the Mg vapor arose in the region of
`the laser-beam focus at frequencies corresponding to transi-
`tions n 'D3—3 ‘P, (n = 4-10) ofthe singlet system and n 3S-
`3 3P (n = 4, 5) of the triplet system of Mg levels. Here the
`absence was noted of radiation at the strong line 5 ‘S0—3 ‘P,
`(xi. = 571.1 nm) and several other strong lines. As was
`shown by measurements ofthe density ofMg ions performed
`with probe radiation, the maximum concentration of ions
`(~ 2 X 10” cm”) was observed about 30 ns after the end of
`the pulse ofpump radiation, while the total time of existence
`of the PRP amounted to ~10,us. The maximum degree of
`ionization ofthe plasma reached 5%. The absence ofsatura-
`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 ofattention 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 ofthe second harmonic ofa solid-state pulsed laser
`based on yttrium aluminum garnet of type LTIP4-5 with a
`pulse-repetition frequency of l2.5 Hz. To narrow and tune
`smoothly the emission line of the dye laser, a ditfraction
`grating ( 1200 lines/mm) was used and was set up in a glanc-
`ing-incidence system. The width of the emission line ofthis
`laser amounted to ~ 1 nm, and the emission power was :40
`kW at a pulse duration of ~10’ “ s. The laser was tuned
`either to the resonance transition of Na (xi = 589.0 nm) or
`to the wavelength 578.7 nm corresponding to two-photon
`absorption to the excited state of Na (4d 2D5,z).
`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.” 1-neodymium garnet laser; 2, 4-fre-
`quency doublers based on KDP; 3—dye laser; 5-—delay circuit; 6——cell
`with Mg vapor; 7~monochromator with photomultiplier (PM); 63-05-
`cillograph.
`
`539
`
`Sov. Phys. Usp. 31 (6), June 1988
`
`of the experiment one pulse of laser radiation contained
`about 5 X 10“ photons. About the same number ofNa 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 ofa PRP was measured from the change
`in electrical conductivity of the irradiated volume. To do
`this, a voltage was applied to the electrodes ofthe discharge
`tube smaller than the excitation voltage ofthe discharge, and
`the current was measured that arose in the electrical circuit
`under the action of the laser illumination. Figure 8 shows an
`oscillogram ofthis current obtained at a voltage on the elec-
`trodes of 100 V and a vapor pressure of Na of3 X 10”‘ Torr.
`The pressure of the butter 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 ofmagnitude. Upon detuning
`from the resonance at /7. = 589 nm, no plasma formation was
`observed.
`
`Plasma formation was also observed in two-photon la-
`ser excitation of the level 4p2D5,2. The oscillograms of the
`current obtained here at a pressure of Na vapor ~ 5 X 10‘-‘
`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 efficiency of conversion of the energy of reso-
`nance-excited atoms into ionization energy.
`
`2.3. Ouasiresonance plasmas
`7
`As has been established in a number of experiments of
`recent years,”'*‘”7 to form a photoresonance plasma one
`need not use radiation whose frequency corresponds to a
`resonance transition between the ground and excited states
`of the atom. Eflicient ionization of the atoms ofa 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
`
`Beterov et a/.
`
`539
`
`

`
`r
`
`—j-:--__.....__
`
`b 1#8
`
`FIG. 8. Oscillogram ofthe photocurrent that arises
`upon irradiating adischarge tube containing a mix-
`ture of Na + Ne with a pulse of laser radiation
`(A = 589 nm),” in the presence ofa discharge (a),
`and at a voltage below that for ignition of the dis-
`charge (b).
`
`could ascribe such an unexpected result to effects of multi-
`photon ionization of atoms, including the nonresonance ex-
`citation ofa 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 efliciently
`formed upon irradiating Cs vapor with laser radiation hav-
`ing/7. = 583.9, 601.0, 603.5, and 621.3 nm, corresponding to
`the
`transitions
`2S,,2lOs—~ ZPfi’,.2 6p,
`2D3,2
`8d—~ 3
`‘(,2 6p,
`2S,,2lOs—~3O§,2 6p, and 3D3,28d-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 quasiresonance radiation of power up
`to ~ 100 mW yielded a plasma with a degree ofionization of
`10”. A diagram ofthe experiment is shown in Fig. 10. As is
`shown by analyzing photographs of the plasma column
`formed upon focusing the beam ofa continuous laser of 10
`mW power, the extent ofthe 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 (/7. = 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 ofionization of a gas under the action
`of quasiresonance radiation ‘3 includes the process of photo-
`dissociation of dimers ofa metallic vapor, which are always
`present in the system:
`
`A-2-l-71w—>A -1- A*.
`
`
`
`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,m ), and is already capable of resonance ab-
`sorption oflaser radiation. This leads to formation ofhighly
`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/cmz, 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 held in the cell. Then the ions
`entered the input of a quadrupole mass spectrometer. This
`made it possible to establish that only molecular Nag’ 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
`
`2i\'a(3p)—>1\'8,'-4.-9
`
`(2.4)
`
`and establishing the value ofthe rate constant ofthis process
`(~ 1.5x l0’ '3 cm3/s) and its cross section (~0.5><10“7
`Crnl) ' I}
`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
` Furnace
`
`FIG. 9. Oscillogram of the photocurrent observed in two—photon laser
`excitation of Na vapor (1. = 578.9 nm).”
`
`FIG. 10. Diagram of an experiment to form a quasiresonance laser plasma
`using a continuous-wave dye laser. "‘
`
`540
`
`Sov. Phys. Usp. 31 (6), June 1988
`
`Beterov er a/.
`
`540
`
`
`
`

`
`+—-I,5W,488nn‘
`--a,7w,4a5,5nm
`n—o,4w,5o/,7m~
`
`20
`
`L
`
`F
`
`:1
`Q :
`53
`
`El‘:
`
`:
`
`+
`u
`
`"-7
`
`Electrode potential, V
`
`7,17
`
`0,5
`
`0,5
`
`-05
`
`Nozzle potentia’. V
`
`chamber maintained at a constant temperature in the range
`400-500 K (pressure of Cs vapor ~ 2 X l0”‘l—O.2 Torr) and
`equipped with a nozzle 0.12 mm in diameter was used as the
`source of the cesium vapor. Thus a jet of vapor was formed
`that was concentrated in a solid angle 6 = 2.7 X 10‘: 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 ofthe nozzle of ~10” cm “3. 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 (2 = 852.1 nm), which convert-
`ed the Cs atoms to the 63PM state. In the second stage the
`radiation of an argon ion laser was used (A 2 488.0, 496.5,
`and 501.7 nm), which enabled the photoionization of the
`resonance-excited atoms. Figure 11 shows the dependence of
`the photocurrent on the extracting potential. According to
`the estimates the flux ofions in the jet 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 of 0. 15 eV.
`
`Two-stage and multistage photoionization ofatoms has
`been used successfully to obtain beams ofthe following ions:
`Ca’' ,4‘ In’' (under the action ofdye—laser radiation),“ Al+
`(using the radiation of an excimer XeCl laser)_.“ etc.”"5
`The ion beams thus formed were characterized by being
`highly monoenergetic and by a degree ofpurity unattainable
`when using other methods of forming an ion beam. Here, as
`in the studies cited above, the obtainable ion fluxes were
`relatively small, so that problems of extracting the positive
`ions from the phot