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Penning and associative ionizations in collisions of laser-excited Na atoms with K atoms
Volume 84, number
CHEMICAL PHYSICS LE’ITERS
2
1 December 1981
PENNING AND ASSOCIATIVE NONAGONS IN COLLISIONS OF LASER-EXCITED
Na ATOMS WITH K ATOMS
M. KIMURA and S. SAIKAN of Physics. Fact&y of Science. Osaka University, Toyonoku, Osaka 564 Japan
Departntettt Received
31 July 1981
Collisional ionization has been observed in the collision of electronically excited sodium atoms with potassium atoms. By means of the crossed-beam and stepwise laser excitation technique and time-of-flight measurements, the folowing reactions hnve been invcstigarcd: Penning ionization, Na* + K - Na + K+ + e-, and associative ionization, Nn* + K -t NaK+ + el
some fraction
1. Introduction
by multiphoton
Ionization
reactions
of
electronically excited
species have received considerable attention. However, most works have been confined to collisions of long-lived metastable atoms, e.g. me&stable inert gases [l], since experiments to populate a large fraction of colliding atoms in specified short-lived excited levels are difficult. Recently, with the use of tunable lasers such experimental studies have become accessible, but still both experimental and theoreticai works are scarce. So far, associative ionizations by collisions between homonuclear alkali atoms both of which are in short-lived
first excited
of dimers which may come out as q absorption_
In our system,
Na + K,
such ambiguity WI be removed, because NaK does not exist as an initial stable moIecuie before coliision. We have started
this study in order to obtain ion-
ization cross sections in collisions of various excited Na atoms with K atoms in the presence of both intense and weak laser fields. The present report is a brief survey of the time-of-bitt (TOF) spectra of ions produced by the collision of an electronic~y excited Na beam with a K beam under single coflision conditions. The intensity of the laser Geld was reduced to suppress laser-induced processes.
IeveIs have
been studied with [2] and without [3] use of the laser excitation technique. When very intense radiation is applied to a collision system of two atoms, ionization in the field of the radiation, “laser-induced collisional ionization”, takes place. In this process photons are absorbed by a quasi-molecule formed by the two colliding atoms during the collision time, and then ionization takes place. Therefore it requires an intense photon density and the wavelength dependence shows a moiecular band structure. Laser-induced ionizations in Li + Li [4] and Na I- Na [!5l systems have been observed. Zn homonuciear systems of alkali atoms, X + X, the detected molecular ion X$ cannot be readily assigned to the product of asso;ative notation, be-
cause in the vapor of alkali atoms there always exists
2. Experimental The experimental apparatus employed is shown schematically in fig. 1. The collisional region of the
sodium and potassium beams was irradiated by two collinear tunable dye laser beams in order to excite Na atoms in two steps, 3s + 3p and 3p + tzs or 3p + nd. The two atomic and the two laser beams lie in the same vertical plane, and they overlap in the crossing region which is surrounded by a copper baffle in thermal contact with a liquid-nitrogen trap. The atomic densities were monitored by surface ioni~ation detectors and were of the order of lOlo atone
cm3. Product ions were extracted plane by a static electric 0 009-26~4/81/00~-0000/$02.75
at 90” to the beam field of 100 V/l5 mm and 0 1981 North-HoIJand
Volume 84, number 2
CHEMICAL
PHYSICS LETTERS
1 December 1981
Fig. 1. Schematic drawmg of the apparatus.
detected with an electron multrpher 19 cm apart from the collrsron center. Ion tune-of-flight measurements were employed to assign ion specres, where the laser pulse provrded a reference time marker. Two dye lasers opt~calty pumped by a pulsed N+ser delivered 4
ns light pulses at a repetition rate of 5 Hz. The line
wrdths were =0,3 cm-l and their peak intensities were of the order of several kW, dependmg on the selected wavelengths. One of the lasers (laser I) was tuned to the Na D, line and the other (laser II) was adjusted to one of the 3pns and 3p--nd tranntrons. The optogalvamc effect in a Na hollow cathode lamp was employed to adjust accurately the laser frequency to one of the Na transrtions mentioned above [6]. Focusmg of the laser beams on a sharp point results III an intense radiahon field and may cause the abovementroned “laser-induced process”. In order to reduce this effect and not to lower the resolution of TOF spectra in the present work, a cylindrical lens, mstead of a conventional spherical lens, was used to focus the laser beams only in the lrection of ton extractron.
3. Results
and discussion
The energes of the Na-K system are shown in fig. 2. The potential minimum of ground-state NaK+ was estrmated from the photoionization data and dissociation energy of NaK by Foster et al. [7] and Huber and Herzberg [8], respectively. Tlus value was used to modify the potential curve of NaK+ calculated theoretically by Valance [9]. The pumping actrons
yje
Ha(%)-K
Fg. 2. Potentral energies of the Na-K system. Two stepwise exatahons of Na by Iasers are shown by arrows.
of lasers I and 11 are also mdrcated rn fig. 2. In this experiment the followng reactions have been uwestigatedr Na*+K+Na+K++e-
Na*+K+NaK++e-
(Penning
ionrzation)
, (1)
(associative
ionization)
, (2)
where the excited levels of Na* are 6d, 7s, Sd, 6s, 4d and 5s. All of these levels have enough internal energy to induce reaction (2), but only the four levels 6d, 7s, 5d and 6s have sufficient energy for reaction (l), whose threshold energy is higher than that of (2) by 0.46 eV. The Stark effect of these levels induced by the applied dc field of 66.7 V/cm is negligrble compared to the distances to the adjacent levels. As expected from fig_ 2, K+ signals were not observed in the Na*-K collision systems with Na(4d) and Na(5.s) atoms, but it was really observed in the systems with Na(6d), Na(7s), Na(5d) and Na(6s) 277
CHEMICAL PHYSICS LETTERS
Volume 84, number 2 NaK+ Laser
I
r”
ii
4
!
-PC
4
Na(6d) _---
Cl
y-t-
-
i-15 3. TOf spectraof product ions. Tzme scale 2 &div. Iiiscr II was tuned to the follo~ng transrtions of Na (a) (b) 3p--4d and (c) 3p-5s.
c)
c----------i’
d)
I
t
FIS d_ TOF spectm of product tons. Tune scale 2 &dIv. (a) rour beams of Na, I;, laser I and laser II were crossed. The frequency of her II was tuned to the 3p-6d he. (b) Laser Ii waz tightly off-tuned by less than 5 A. (c) Na, K and laser fl on L;iser I off. (d) Elrher laser II off, and laser I, Na and K on, or Na off, and K, lasers I and II on. (e) Na, lasers I and il on, and K off.
278
atoms. On the other hand, the NaK+ signals were observed m all the colhsions studied except for the Na(4d)-K system. Typical TOF spectra observed on an oscdloscope are shown in fig. 3 for the Na(6d)-K, Na(4d)-K and Na(Ss)-K collision systems. In general, the NaK+ signals were less than 10-l of the K* slgnals when both s~gnak were observed_ Although the 4d level IS energetxaliy capable of inducmg associatlve lomzatlon, Its signal in tkus case was especially small and only a trace of NaKf signal was observed.
b)
3p-6d,
1 December 1981
Tlus suggests that the potentA curve of the Na(4d)K state IS highly repulsive or has a potent& bamer at a reiatwefy Iarge mternuclear separatron, thus preventmg the two atoms from conung cIose together. For a better understandmg of the formation mechanism of the lomc species, Further observations have been done for the Na(6dj--K collision system. Fig. 4a 1s the same as fig. 3a, where signals were observed under the condmons laser I and Iaser II on, Na and I( beams on. When the K beam was shut off (fig. 4e), only Na+ and Na; signals were observed. This indlcates that the K beam does not contnbute to formation of Na+ and Na$_ When the K beam was turned on and the Na beam was shut off, signals of all ~oruc species dlsappeared (fig. 4d). Thus means that the exlstence of Na is essential to produce the K+ signal. Next, consider the formation of Naz. When laser 11 was turned off, the Na$ signal disappeared (fig. 4d). However, when laser I was turned off, the Na$ agnat still appeared at the same height as in figs. 4a and 4e, provided that laser II was irra&ated (fig. 4~). Therefore, laser II contributes to the formation of Na$, but laser 1 does not. Even If laser II was offtuned by less than 5 A, Nai stti appeared (fig. 4b). These facts suggest that Na, comes from the lonizatron of Na2(B 1 ll) lying m the blue-green re@on above the drmer ground state (X I SC”), because the Na,(B I II) state can be effectively populated from the dimer ground state by laser II radiation and be Ionized by the same laser II ralatron. The formation mechanism of K$ m the Na(Ss)-K system may be similar to that of Naz u1 the Na(6d)-K system, because laser II ra&a-
bon tuned to the Na 3~4s transition is effective to excite the K2 molecules from the ground state to the B 1 II state. Na+ can be produced only when both lasers I and If are tuned to Na transitions (figs. 4a and 4e). Therefore, Na atoms are ionized predominantly through
Volume 84, number
2
CHEMICAL
resonance-enhanced three-photon photoronization. From the above observatrons It was concluded that under the present experimental conditions K+ and NaK+ ions were produced only through collisrons of highly excited Na(6d) atoms with ground-state K atoms. This conclusron was also confirmed for the other exerted levels of Na studied here. Our results should be mterpreted by means of the potential energy curves of electromcally exerted
(Na-K)* and ground-state (Na-K)+ systems, but informatron about them is insufficient at present. More detarled studies, especrally measurements of the cross sectrons for reactions (1) and (2) as a function of the relevant energy states of Na and the laser-induced romzation processes under a much more intense laser field, are in progress.
PHYSICS
PI G-H Bearman and J-J. Levcnthal, Phys. Rev. Letters41 (1978) 1227, A. de long and F. van der Valk, J. Pbys. B12 (1979) L.561. c31 A.N. KIyucharev and N.S. Ryazanov, Opt. Spectry. 31 (1971) 187; B.V. Dobrolezh, A.N. Klyucharev and V.Yu. Sepman, Opt. Spectry. 38 (1975) 630,
V.M. Borodrn. A_N. Klyuchsrev and V_Yu, Sepman. Opt Spectry. 39 (1975) 231; A.N. Klyucharev, V.Yu. Sepman and V. Vujnovic I. Phys BIO (1977) 715. [41 A. von HeMeld, J. Caddxk and J. Werner, Phys. Rev. Letters 40 (1980) 1369. 151 P. Polak-Dmgels, J.F. Delpech and J. Weiner, Pbys. Rev. Letters 44 (1978) 1663, F. Roussel, B. Carn5, P. Bregner and G. Sp:ess, J. Phys. B14 (1981) L313. [61 H. Wakata, S. Saikan and M. Kunuraa Opt. Commun. 38 (1981) 271. 171 P.J. Foster,
References [I]
S. Wevler and E.K. Parks, Ann. Rev. Phys. Chem. 30 (1979) 179. and references therem
1 December 1981
LETTERS
181
Pl
R E.
Leckenby
and
EJ.
Robbms,
J. Phyr
82 (1969) 478. H.P. Huber and G. Herzberg, Molecular spectra and mohXUh structllre, VoL 4. Constants of diatonuc cules (Van Nostrand, Prmceton, 1979) p_ 442. A. Valance, J. Chem. Phys. 69 (1978) 355.
mob
279
CHEMICAL PHYSICS LE’ITERS
2
1 December 1981
PENNING AND ASSOCIATIVE NONAGONS IN COLLISIONS OF LASER-EXCITED
Na ATOMS WITH K ATOMS
M. KIMURA and S. SAIKAN of Physics. Fact&y of Science. Osaka University, Toyonoku, Osaka 564 Japan
Departntettt Received
31 July 1981
Collisional ionization has been observed in the collision of electronically excited sodium atoms with potassium atoms. By means of the crossed-beam and stepwise laser excitation technique and time-of-flight measurements, the folowing reactions hnve been invcstigarcd: Penning ionization, Na* + K - Na + K+ + e-, and associative ionization, Nn* + K -t NaK+ + el
some fraction
1. Introduction
by multiphoton
Ionization
reactions
of
electronically excited
species have received considerable attention. However, most works have been confined to collisions of long-lived metastable atoms, e.g. me&stable inert gases [l], since experiments to populate a large fraction of colliding atoms in specified short-lived excited levels are difficult. Recently, with the use of tunable lasers such experimental studies have become accessible, but still both experimental and theoreticai works are scarce. So far, associative ionizations by collisions between homonuclear alkali atoms both of which are in short-lived
first excited
of dimers which may come out as q absorption_
In our system,
Na + K,
such ambiguity WI be removed, because NaK does not exist as an initial stable moIecuie before coliision. We have started
this study in order to obtain ion-
ization cross sections in collisions of various excited Na atoms with K atoms in the presence of both intense and weak laser fields. The present report is a brief survey of the time-of-bitt (TOF) spectra of ions produced by the collision of an electronic~y excited Na beam with a K beam under single coflision conditions. The intensity of the laser Geld was reduced to suppress laser-induced processes.
IeveIs have
been studied with [2] and without [3] use of the laser excitation technique. When very intense radiation is applied to a collision system of two atoms, ionization in the field of the radiation, “laser-induced collisional ionization”, takes place. In this process photons are absorbed by a quasi-molecule formed by the two colliding atoms during the collision time, and then ionization takes place. Therefore it requires an intense photon density and the wavelength dependence shows a moiecular band structure. Laser-induced ionizations in Li + Li [4] and Na I- Na [!5l systems have been observed. Zn homonuciear systems of alkali atoms, X + X, the detected molecular ion X$ cannot be readily assigned to the product of asso;ative notation, be-
cause in the vapor of alkali atoms there always exists
2. Experimental The experimental apparatus employed is shown schematically in fig. 1. The collisional region of the
sodium and potassium beams was irradiated by two collinear tunable dye laser beams in order to excite Na atoms in two steps, 3s + 3p and 3p + tzs or 3p + nd. The two atomic and the two laser beams lie in the same vertical plane, and they overlap in the crossing region which is surrounded by a copper baffle in thermal contact with a liquid-nitrogen trap. The atomic densities were monitored by surface ioni~ation detectors and were of the order of lOlo atone
cm3. Product ions were extracted plane by a static electric 0 009-26~4/81/00~-0000/$02.75
at 90” to the beam field of 100 V/l5 mm and 0 1981 North-HoIJand
Volume 84, number 2
CHEMICAL
PHYSICS LETTERS
1 December 1981
Fig. 1. Schematic drawmg of the apparatus.
detected with an electron multrpher 19 cm apart from the collrsron center. Ion tune-of-flight measurements were employed to assign ion specres, where the laser pulse provrded a reference time marker. Two dye lasers opt~calty pumped by a pulsed N+ser delivered 4
ns light pulses at a repetition rate of 5 Hz. The line
wrdths were =0,3 cm-l and their peak intensities were of the order of several kW, dependmg on the selected wavelengths. One of the lasers (laser I) was tuned to the Na D, line and the other (laser II) was adjusted to one of the 3pns and 3p--nd tranntrons. The optogalvamc effect in a Na hollow cathode lamp was employed to adjust accurately the laser frequency to one of the Na transrtions mentioned above [6]. Focusmg of the laser beams on a sharp point results III an intense radiahon field and may cause the abovementroned “laser-induced process”. In order to reduce this effect and not to lower the resolution of TOF spectra in the present work, a cylindrical lens, mstead of a conventional spherical lens, was used to focus the laser beams only in the lrection of ton extractron.
3. Results
and discussion
The energes of the Na-K system are shown in fig. 2. The potential minimum of ground-state NaK+ was estrmated from the photoionization data and dissociation energy of NaK by Foster et al. [7] and Huber and Herzberg [8], respectively. Tlus value was used to modify the potential curve of NaK+ calculated theoretically by Valance [9]. The pumping actrons
yje
Ha(%)-K
Fg. 2. Potentral energies of the Na-K system. Two stepwise exatahons of Na by Iasers are shown by arrows.
of lasers I and 11 are also mdrcated rn fig. 2. In this experiment the followng reactions have been uwestigatedr Na*+K+Na+K++e-
Na*+K+NaK++e-
(Penning
ionrzation)
, (1)
(associative
ionization)
, (2)
where the excited levels of Na* are 6d, 7s, Sd, 6s, 4d and 5s. All of these levels have enough internal energy to induce reaction (2), but only the four levels 6d, 7s, 5d and 6s have sufficient energy for reaction (l), whose threshold energy is higher than that of (2) by 0.46 eV. The Stark effect of these levels induced by the applied dc field of 66.7 V/cm is negligrble compared to the distances to the adjacent levels. As expected from fig_ 2, K+ signals were not observed in the Na*-K collision systems with Na(4d) and Na(5.s) atoms, but it was really observed in the systems with Na(6d), Na(7s), Na(5d) and Na(6s) 277
CHEMICAL PHYSICS LETTERS
Volume 84, number 2 NaK+ Laser
I
r”
ii
4
!
-PC
4
Na(6d) _---
Cl
y-t-
-
i-15 3. TOf spectraof product ions. Tzme scale 2 &div. Iiiscr II was tuned to the follo~ng transrtions of Na (a) (b) 3p--4d and (c) 3p-5s.
c)
c----------i’
d)
I
t
FIS d_ TOF spectm of product tons. Tune scale 2 &dIv. (a) rour beams of Na, I;, laser I and laser II were crossed. The frequency of her II was tuned to the 3p-6d he. (b) Laser Ii waz tightly off-tuned by less than 5 A. (c) Na, K and laser fl on L;iser I off. (d) Elrher laser II off, and laser I, Na and K on, or Na off, and K, lasers I and II on. (e) Na, lasers I and il on, and K off.
278
atoms. On the other hand, the NaK+ signals were observed m all the colhsions studied except for the Na(4d)-K system. Typical TOF spectra observed on an oscdloscope are shown in fig. 3 for the Na(6d)-K, Na(4d)-K and Na(Ss)-K collision systems. In general, the NaK+ signals were less than 10-l of the K* slgnals when both s~gnak were observed_ Although the 4d level IS energetxaliy capable of inducmg associatlve lomzatlon, Its signal in tkus case was especially small and only a trace of NaKf signal was observed.
b)
3p-6d,
1 December 1981
Tlus suggests that the potentA curve of the Na(4d)K state IS highly repulsive or has a potent& bamer at a reiatwefy Iarge mternuclear separatron, thus preventmg the two atoms from conung cIose together. For a better understandmg of the formation mechanism of the lomc species, Further observations have been done for the Na(6dj--K collision system. Fig. 4a 1s the same as fig. 3a, where signals were observed under the condmons laser I and Iaser II on, Na and I( beams on. When the K beam was shut off (fig. 4e), only Na+ and Na; signals were observed. This indlcates that the K beam does not contnbute to formation of Na+ and Na$_ When the K beam was turned on and the Na beam was shut off, signals of all ~oruc species dlsappeared (fig. 4d). Thus means that the exlstence of Na is essential to produce the K+ signal. Next, consider the formation of Naz. When laser 11 was turned off, the Na$ signal disappeared (fig. 4d). However, when laser I was turned off, the Na$ agnat still appeared at the same height as in figs. 4a and 4e, provided that laser II was irra&ated (fig. 4~). Therefore, laser II contributes to the formation of Na$, but laser 1 does not. Even If laser II was offtuned by less than 5 A, Nai stti appeared (fig. 4b). These facts suggest that Na, comes from the lonizatron of Na2(B 1 ll) lying m the blue-green re@on above the drmer ground state (X I SC”), because the Na,(B I II) state can be effectively populated from the dimer ground state by laser II radiation and be Ionized by the same laser II ralatron. The formation mechanism of K$ m the Na(Ss)-K system may be similar to that of Naz u1 the Na(6d)-K system, because laser II ra&a-
bon tuned to the Na 3~4s transition is effective to excite the K2 molecules from the ground state to the B 1 II state. Na+ can be produced only when both lasers I and If are tuned to Na transitions (figs. 4a and 4e). Therefore, Na atoms are ionized predominantly through
Volume 84, number
2
CHEMICAL
resonance-enhanced three-photon photoronization. From the above observatrons It was concluded that under the present experimental conditions K+ and NaK+ ions were produced only through collisrons of highly excited Na(6d) atoms with ground-state K atoms. This conclusron was also confirmed for the other exerted levels of Na studied here. Our results should be mterpreted by means of the potential energy curves of electromcally exerted
(Na-K)* and ground-state (Na-K)+ systems, but informatron about them is insufficient at present. More detarled studies, especrally measurements of the cross sectrons for reactions (1) and (2) as a function of the relevant energy states of Na and the laser-induced romzation processes under a much more intense laser field, are in progress.
PHYSICS
PI G-H Bearman and J-J. Levcnthal, Phys. Rev. Letters41 (1978) 1227, A. de long and F. van der Valk, J. Pbys. B12 (1979) L.561. c31 A.N. KIyucharev and N.S. Ryazanov, Opt. Spectry. 31 (1971) 187; B.V. Dobrolezh, A.N. Klyucharev and V.Yu. Sepman, Opt. Spectry. 38 (1975) 630,
V.M. Borodrn. A_N. Klyuchsrev and V_Yu, Sepman. Opt Spectry. 39 (1975) 231; A.N. Klyucharev, V.Yu. Sepman and V. Vujnovic I. Phys BIO (1977) 715. [41 A. von HeMeld, J. Caddxk and J. Werner, Phys. Rev. Letters 40 (1980) 1369. 151 P. Polak-Dmgels, J.F. Delpech and J. Weiner, Pbys. Rev. Letters 44 (1978) 1663, F. Roussel, B. Carn5, P. Bregner and G. Sp:ess, J. Phys. B14 (1981) L313. [61 H. Wakata, S. Saikan and M. Kunuraa Opt. Commun. 38 (1981) 271. 171 P.J. Foster,
References [I]
S. Wevler and E.K. Parks, Ann. Rev. Phys. Chem. 30 (1979) 179. and references therem
1 December 1981
LETTERS
181
Pl
R E.
Leckenby
and
EJ.
Robbms,
J. Phyr
82 (1969) 478. H.P. Huber and G. Herzberg, Molecular spectra and mohXUh structllre, VoL 4. Constants of diatonuc cules (Van Nostrand, Prmceton, 1979) p_ 442. A. Valance, J. Chem. Phys. 69 (1978) 355.
mob
279